Ch. 11 Nervous System 2

Subdural Hematoma

- A blow to the head may rupture some blood vessels associated with the brain, and the escaping blood may collect beneath the dura mater. This condition, called subdural hematoma, can increase pressure between the rigid bones of the skull and the soft tissues of the brain. Unless the accumulating blood is promptly removed, compression of the brain may lead to functional losses or even death.

Plexuses

- Anterior branches of the spinal nerves form complex networks called plexuses instead of continuing directly to the peripheral body parts, except in the thoracic regions T2 through T12. In a plexus, the fibers of various spinal nerves are sorted and recombined in a way that enables fibers associated with a particular peripheral body part to reach it in the same peripheral nerve, even though the fibers originate from different spinal nerves

Association Areas of the Cortex

- Association areas are neither primarily sensory nor motor. They connect with each other and with other brain structures. - Association areas occupy the anterior portions of the frontal lobes and are widespread in the lateral portions of the parietal, temporal, and occipital lobes. - Association areas analyze and interpret sensory experiences and help provide memory, reasoning, verbalizing, judgment, and emotions

Long Term Memory

- Long-term memory can hold much more information than short-term memory and lasts a lifetime. Some long-term memory establishes new synaptic connections through increased branching of axons and dendrites. In another mechanism, long-term potentiation, very rapid repeated stimulation of the same neurons increases the number of postsynaptic neurotransmitter receptors and causes physical changes at the synapse that make synaptic transmission more effective

Partition: Falax Cerebelli

- Separates the right and left cerebellar hemispheres

Cervical Plexuses

- The anterior branches of the first four cervical nerves form the cervical plexuses, which lie deep in the neck on either side. Fibers from these plexuses supply the muscles and skin of the neck. In addition, fibers from the third, fourth, and fifth cervical nerves pass into the right and left phrenic nerves, which conduct motor impulses to the muscle fibers of the diaphragm.

The Brain - Development

- The basic structure of the brain reflects the way it forms during early (embryonic) development. It begins as the neural tube that gives rise to the central nervous system. - The portion that becomes the brain has three major cavities, or vesicles, at one end—the forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon) - Later, the forebrain divides into anterior and posterior portions (telencephalon and diencephalon, respectively), and the hindbrain partially divides into two parts (metencephalon and myelencephalon). - The resulting five cavities persist in the mature brain as the fluid-filled ventricles and the tubes that connect them. Cells of the tissue surrounding the spaces differentiate into the structural and functional regions of the brain.

The Brain

- The brain contains neural centers associated with sensory functions and is responsible for sensations and perceptions. It issues motor commands to skeletal muscles and carries on higher mental functions, such as memory and reasoning. - The brain also contains neural centers and pathways that coordinate muscular movements, and others that regulate visceral activities. - In addition to overseeing the function of the entire body, the brain is responsible for characteristics such as personality.

Energy Supply of Brain

- The brain is highly dependent on aerobic respiration for production of ATP, and must constantly receive oxygen from its blood supply or risk injury. In a "stroke," or cerebrovascular accident, brain tissues downstream from a blockage of blood flow die, and some loss of function may result.

The Brainstem

- The brainstem connects the brain to the spinal cord. It consists of the midbrain, pons, and medulla oblongata. These structures include many tracts of nerve fibers and masses of gray matter called nuclei

The CNS

- The central nervous system (CNS) consists of the brain and the spinal cord. The brain is the largest and most complex part of the nervous system. - It oversees many aspects of physiology, such as sensation and perception, movement, and thinking. - The brain includes the two cerebral hemispheres, the diencephalon, the brainstem, and the cerebellum - areas of the nervous system containing mostly neuron cell bodies (and unmyelinated axons) appear gray and are called gray matter, whereas areas containing myelinated axons appear white and are called white matter.

Segmental Innervation

- skeletal muscles typically are innervated by motor neurons at more than one level of the spinal cord, although one or two levels of the spinal cord may predominate. This is called segmental innervation. For example, the biceps brachii is innervated by the musculocutaneous nerve with axons originating from C5, C6, and C7. Similarly, the gastrocnemius is innervated by the tibial nerve with axons originating from S1 and S2. At the same time, some of the incoming sensory impulses stimulate interneurons that inhibit the action of the antagonistic extensor muscles (reciprocal innervation). This inhibition of antagonists allows the flexor muscles to effectively withdraw the affected part.

Glossopharyngeal Nerve

The ninth pair of cranial nerves, the glossopharyngeal nerves (IX), are associated with the tongue and pharynx (the region posterior to the nasal cavity, the oral cavity and the larynx). These nerves arise from the medulla oblongata. They are mixed nerves, with predominant sensory fibers that conduct impulses from the lining of the pharynx, tonsils, and posterior third of the tongue to the brain. Fibers in the motor component of the glossopharyngeal nerves innervate certain salivary glands and a constrictor muscle in the wall of the pharynx that functions in swallowing.

Spinal Nerves

The sensory neurons that are associated with the spinal cord have cell bodies that are not in the spinal cord at all. Rather, the cell bodies of these unipolar neurons are clustered in the posterior root ganglia (singular, ganglion) associated with the spinal nerves at each segment of the spinal cord. The central processes of their axons enter the spinal cord through the posterior roots. Axons of motor neurons in the anterior horns leave the spinal cord through anterior roots. The posterior roots and anterior roots combine to form spinal nerves A horizontal bar of gray matter in the middle of the spinal cord, the gray commissure, connects the wings of the gray matter on the right and left sides. This bar surrounds the central canal, which is continuous with the ventricles of the brain and contains CSF. The central canal is prominent during embryonic development, but it becomes almost microscopic in adulthood. The gray matter divides the white matter of the spinal cord into three regions on each side—the anterior, lateral, and posterior funiculi. Each funiculus consists of longitudinal bundles of myelinated nerve fibers called tracts that compose the major pathways.

cephal

head: encephalitis—inflammation of the brain.

Facial Nerves

- The seventh pair of cranial nerves, the facial nerves (VII), are mixed nerves that arise from the lower part of the pons and emerge on the sides of the face. Their sensory branches are associated with taste receptors on the anterior two-thirds of the tongue, and some of their motor fibers conduct impulses to muscles of facial expression. Still other motor fibers of these nerves function in the autonomic nervous system by stimulating secretions from tear glands and certain salivary glands (submandibular and sublingual glands).

Abducens

- The sixth pair of cranial nerves, the abducens nerves (VI), are small and originate from the pons near the medulla oblongata. They enter the orbits of the eyes and supply motor impulses to the remaining pair of external eye muscles, the lateral rectus muscles. This nerve is considered motor, with some proprioceptive fibers.

Cranial Nerves

- mixed nerves, but some of those associated with special senses, such as smell and vision, have only sensory fibers. Other cranial nerves that innervate muscles and glands are primarily composed of motor fibers. The first pair, which is sensory, has fibers that begin in the nasal cavity and synapse in the frontal lobe of the cerebrum. The second pair, also sensory, originate in the eyes, and their fibers synapse in the thalamus. The remaining cranial nerves attach to the the brainstem, and the direction of their fibers depends on whether they are sensory, motor, or mixed. All of the cranial nerves pass from their sites of attachment through foramina of the skull and lead to areas of the head, neck, and trunk. The cranial nerves that are described as primarily motor do have limited sensory functions because they contain neurons associated with certain receptors (proprioceptors) that respond to the changes in length and force of contraction of skeletal muscles. However, because these proprioceptive fibers contribute directly to motor control, cranial nerves whose only sensory component is from such proprioceptors are usually considered motor nerves. This pertains to cranial nerves III, IV, VI, XI, and XII.

Ascending Pathways

- Ascending pathways typically involve three neurons between a sensory receptor and the final destination of impulses within the brain—the peripheral sensory neuron and two interneurons. Among the major ascending tracts of the spinal cord are the following: 1. Fasciculus Gracilis and Fasciculus Cuneatus 2. Lateral and Anterior Spinothalamic Tracts 3. Posterior and Anterior Spinocerebellar Tracts Descending Tracts: 1. The lateral and anterior corticospinal 2. Lateral Reticulospinal Tracts 3. The fibers of the rubrospinal tracts

The Basal Nuclei

- The basal nuclei, also called the basal ganglia, are masses of gray matter deep within the cerebral hemispheres. They include the caudate nucleus, the putamen, and the globus pallidus, and they develop from the anterior portion of the forebrain The basal nuclei produce the neurotransmitter dopamine. The neurons of the basal nuclei interact with other brain areas, including the motor cortex, thalamus, and cerebellum. These interactions, through a combination of stimulation and inhibition, facilitate voluntary movement.

The Frontal Lobe

- The frontal lobe forms the anterior portion of each cerebral hemisphere. It is bordered posteriorly by a central sulcus (fissure of Rolando), which passes out from the longitudinal fissure at a right angle, and inferiorly by a lateral sulcus (fissure of Sylvius), which exits the undersurface of the brain along its sides.

The Insula

- The insula (island of Reil) is a lobe deep within the lateral sulcus of each hemisphere and is so named because it is covered by parts of the frontal, parietal, and temporal lobes. A circular sulcus separates the insula from the other lobes.

Lateral and Anterior Spinothalamic Tracts

- The lateral and anterior spinothalamic tracts are in the lateral and anterior funiculi, respectively. The lateral tracts conduct impulses from various body regions to the brain and give rise to sensations of pain and temperature. Impulses conducted on fibers of the anterior tracts are interpreted as touch and pressure. Impulses in these tracts cross over in the spinal cord

Lateral Reticulospinal Tracts (Descending)

- The lateral reticulospinal tracts are in the lateral funiculi, whereas the anterior and medial reticulospinal tracts are in the anterior funiculi. Some fibers in the lateral tracts cross over, whereas others remain uncrossed. Those of the anterior and medial tracts remain uncrossed. Motor impulses conducted on the reticulospinal tracts originate in the brain and control muscular tone and activity of sweat glands.

Structure of the Spinal Cord

- The spinal cord consists of thirty-one segments, each of which gives rise to a pair of spinal nerves. Spinal nerves are grouped according to the level of the vertebra with which they are associated. Within each group, the pairs of nerves are numbered in sequence from superior to inferior. Why are there eight pairs of cervical spinal nerves when there are only seven cervical vertebrae? C1 passes superior to the vertebra C1, and each of the remaining seven pairs of cervical spinal nerves, C2 through C8 pass below the vertebra above them, through the intervertebral foramina. Therefore, although there are seven cervical vertebrae, there are eight pairs of cervical nerves (numbered C1 to C8). Below the cervical region, spinal nerves are named for the vertebra above them. There are twelve pairs of thoracic nerves (numbered T1 to T12), five pairs of lumbar nerves (numbered L1 to L5), five pairs of sacral nerves (numbered S1 to S5), and one pair of coccygeal nerves (Co). These nerves branch to various body parts and connect them with the CNS.

Functions of the Spinal Cord

- The spinal cord has two main functions. It is a center for spinal reflexes, and it is a conduit for impulses to and from the brain.

The Spinal Cord

- The spinal cord is a slender column of nervous tissue that is continuous with the brain and extends downward through the vertebral canal. The spinal cord originates where nervous tissue leaves the cranial cavity at the level of the foramen magnum. The cord tapers to a point and terminates near the intervertebral disc that separates the first and second lumbar vertebrae

The Temporal Lobe

- The temporal lobe lies inferior to the frontal and parietal lobes and is separated from them by the lateral sulcus.

plex

interweaving: choroid plexus—mass of specialized capillaries associated with spaces in the brain.

mening

membrane: meninges—membranous coverings of the brain and spinal cord.

funi

small cord or fiber: funiculus—major nerve tract or bundle of myelinated axons within the spinal cord.

gangli

swelling: ganglion—mass of neuron cell bodies.

The Cerebral Cortex

- All lobes of the cerebrum have a thin layer of gray matter (2 to 5 millimeters thick) called the cerebral cortex. The cortex constitutes the outermost portion of the cerebrum. It covers the gyri, dipping into the sulci and fissures. The cerebral cortex contains nearly 75% of all the neuron cell bodies in the nervous system. Just beneath the cerebral cortex is a mass of white matter that makes up the bulk of the cerebrum. This mass contains bundles of myelinated axons that connect neuron cell bodies of the cortex with other parts of the nervous system. Some of these fibers pass from one cerebral hemisphere to the other by way of the corpus callosum, and others carry sensory or motor impulses from the cortex to areas of gray matter deeper in the brain or to the spinal cord.

NTD

- Anencephaly is a type of neural tube defect (NTD). It occurs at about the twenty-eighth day of prenatal development, when a sheet of tissue that normally folds to form the neural tube remains open at the top. - A fetus or newborn with anencephaly has a face and lower brain structures but lacks most higher brain structures. - A newborn with this anomaly typically survives only a day or two. In spina bifida, an opening is farther down the neural tube, and in severe cases may cause paralysis from that point downward. In some cases surgery, before or shortly after birth, can partially correct spina bifida. - Taking folic acid supplements just before and during pregnancy can lower the risk of an NTD.

Nerve Fiber

- Another term for the Axon of a Neuron.

The Withdrawal Reflex

- Another type of reflex, called a withdrawal reflex, happens when a person touches something painful (and potentially damaging), as in stepping on a tack. Activated skin receptors send impulses to the spinal cord along the axons of sensory neurons. There the sensory neurons synapse with interneurons, which in turn synapse with motor neurons. In this case, the motor neurons activate fibers in the flexor muscles of the thigh, which contract in response, pulling the foot away from the painful stimulus.

Spinal Nerve Injuries

- Birth injuries, dislocations, vertebral fractures, stabs, gunshot wounds, and pressure from tumors can all injure spinal nerves. Suddenly bending the neck, called whiplash, can compress the nerves of the cervical plexuses, causing persistent headache and pain in the neck and skin, which the cervical nerves supply. If a broken or dislocated vertebra severs or damages the axons reaching the phrenic nerves from the cervical plexuses, partial or complete paralysis of the diaphragm may result. Intermittent or constant pain in the neck, shoulder, or upper limb may result from prolonged flexion of the arm, such as in painting or typing. This is due to too much pressure on the brachial plexus. Called thoracic outlet syndrome, this condition may also result from a congenital skeletal malformation that compresses the plexus during upper limb and shoulder movements. Degenerative changes may compress an intervertebral disc in the lumbar region, producing sciatica, which causes pain in the lower back and gluteal region that can radiate to the thigh, calf, and foot. Sciatica is most common in middle-aged people, particularly distance runners. It usually compresses spinal nerves between L2 and S1, which contain fibers of the sciatic nerve. Rest, drugs, or surgery are used to treat sciatica. In carpal tunnel syndrome, repeated hand movements, such as typing or weeding, inflame the tendons that pass through the carpal tunnel, which is a space between bones in the wrist. The swollen tendons compress the median nerve in the wrist, sending pain up the upper limb. Surgery or avoiding repetitive hand movements can relieve symptoms.

Hemisphere Dominance

- Both cerebral hemispheres participate in basic functions, such as receiving and analyzing sensory impulses, controlling skeletal muscles on opposite sides of the body, and storing memory. However, one side usually acts as a dominant hemisphere for certain other functions. - In most people, the left hemisphere is dominant for the language-related activities of speech, writing, and reading. It is also dominant for complex intellectual functions requiring verbal, analytical, and computational skills. In other people, the right hemisphere is dominant, and in some, the hemispheres are equally dominant. Nerve fibers of the corpus callosum, which connect the cerebral hemispheres, enable the dominant hemisphere to control the motor cortex of the nondominant hemisphere. These fibers also transfer sensory information reaching the nondominant hemisphere to the general interpretative area of the dominant one, where the information can be used in decision making.

Grey and White Matter in Brain and Spinal Cord

- Both the brain and the spinal cord have gray matter and white matter. - In the brain, the outer layers of the cerebral hemispheres and cerebellum are largely gray matter. - White matter, representing interconnecting axons, is found deeper, with islands of gray matter located throughout. - In the spinal cord, in contrast, gray matter (the cell bodies of neurons) is found more centrally, with white matter more peripheral and consisting of axons extending up to the brain or down from the brain.

Brain Waves

- Brain waves are recordings of fluctuating electrical changes in the brain. To obtain such a recording, electrodes are positioned on the surface of a surgically exposed brain (an electrocorticogram, ECoG) or on the outer surface of the head (an electroencephalogram, EEG). These electrodes detect electrical changes in the extracellular fluid of the brain in response to changes in potential among large groups of neurons. The resulting signals from the electrodes are amplified and recorded. Brain waves originate from the cerebral cortex but also reflect activities in other parts of the brain that influence the cortex, such as the reticular formation. Brain waves vary markedly in amplitude and frequency between sleep and wakefulness. Brain waves are classified as alpha, beta, theta, and delta waves (fig. 11F). Alpha waves are recorded most easily from the posterior regions of the head and have a frequency of 8-13 cycles per second. They occur when a person is awake but resting, with the eyes closed. These waves disappear during sleep, and if the wakeful person's eyes open, higher-frequency beta waves replace the alpha waves. Beta waves have a frequency of more than 13 cycles per second. Most are recorded in the anterior region of the head. They occur when a person is actively engaged in mental activity or is under tension. Theta waves have a frequency of 4-7 cycles per second and occur mainly in the parietal and temporal regions of the cerebrum. They are normal in children but do not usually occur in adults. However, some adults produce theta waves in early stages of sleep or at times of emotional stress. Delta waves have a frequency below 4 cycles per second and happen during sleep. They originate from the cerebral cortex when the reticular formation is not activating it.

CSF

- CSF is a clear, somewhat viscous liquid that differs in composition from the fluid that leaves the capillaries in other parts of the body. - Specifically, it contains a greater concentration of sodium and lesser concentrations of glucose and potassium than do other extracellular fluids. - Its function is nutritive as well as protective. CSF helps maintain a stable ionic concentration in the CNS and provides a pathway to the blood for waste - Most CSF forms in the lateral ventricles, from where it slowly circulates into the third and fourth ventricles. Small amounts enter the central canal of the spinal cord, but most CSF circulates through the subarachnoid space of both the brain and the spinal cord by passing through openings in the wall of the fourth ventricle near the cerebellum. - Humans secrete nearly 500 milliliters of CSF daily. However, only about 140 milliliters are in the nervous system at any time, because CSF is continuously reabsorbed into the blood. - Most CSF reabsorption occurs through tiny, fingerlike structures called arachnoid granulations that project from the subarachnoid space into the blood-filled dural sinuses

Ventricles

- CSF is formed in four interconnected cavities called ventricles that lie in the cerebral hemispheres and brainstem. - These spaces are filled with CSF and are continuous with the central canal of the spinal cord, which extends the full length of the cord (although in most adults it is at least partially closed). - The two lateral ventricles are the largest. The first ventricle is in the left cerebral hemisphere and the second ventricle is in the right cerebral hemisphere. They extend anteriorly and posteriorly into the cerebral hemispheres. - A narrow space that constitutes the third ventricle is in the midline of the brain beneath the corpus callosum, which is a bridge of axons that links the two cerebral hemispheres. - This ventricle communicates with the lateral ventricles through openings (interventricular foramina) in its anterior end. - The fourth ventricle is in the brainstem, just anterior to the cerebellum. A narrow canal, the cerebral aqueduct (aqueduct of Sylvius), connects the third ventricle to the fourth ventricle and passes lengthwise through the brainstem. - This ventricle is continuous with the central canal of the spinal cord and has openings in its roof that lead into the subarachnoid space of the meninges.

CSF Pressure

- Cerebrospinal fluid (CSF) is secreted and reabsorbed continuously, which keeps the fluid pressure in the ventricles relatively constant. - However, infection, a tumor, or a blood clot can interfere with the fluid's circulation, increasing pressure in the ventricles (intracranial pressure or ICP). - The pressure can collapse cerebral blood vessels, slowing blood flow. Brain tissues forced against the skull may be injured. - A lumbar puncture (spinal tap) measures CSF pressure. - A physician inserts a fine, hollow needle into the subarachnoid space between the third and fourth or between the fourth and fifth lumbar vertebrae—below the end of the spinal cord. - An instrument called a manometer measures the pressure of the fluid, which is usually about 130 millimeters of water (10 millimeters of mercury). At the same time, samples of CSF may be withdrawn and tested for abnormal constituents. - Red blood cells in the CSF, for example, may indicate a hemorrhage in the central nervous system. A temporary drain inserted into the subarachnoid space between the fourth and fifth lumbar vertebrae can relieve pressure.

Reflex Arcs

- Communication in the nervous system combines a series of action potentials along the axon of a neuron and synaptic transmission between that neuron and a postsynaptic cell. Two or more neurons involved in such communication constitute a nerve pathway. The simplest of the nerve pathways begins with a sensory receptor and ends with an effector, and includes as few as two neurons. Such a nerve pathway is called a reflex. All reflexes share the same basic components, which together are known as a reflex arc. A reflex arc begins with a sensory receptor at the dendritic end of a sensory neuron. Impulses on these sensory neurons enter the CNS and constitute a sensory or afferent limb of the reflex. The CNS is a processing center. Afferent neurons may synapse with interneurons, which may in turn connect with other parts of the CNS. Afferent neurons or interneurons ultimately connect with motor neurons, whose fibers pass outward from the CNS to effectors. (It may help to remember that efferent neurons control effector organs.)

Dermatome

- Each spinal nerve below C1 contains sensory fibers that reach the skin, and the region innervated is called a dermatome. (Similarly, the collection of muscles innervated by motor nerve fibers of a particular spinal nerve is called a myotome.) Dermatomes are highly organized, but they vary considerably in size and shape. A map of the dermatomes is useful in localizing the sites of injuries to dorsal roots or to the spinal cord.

Memory Consolidation

- Eventually memories are stored in various parts of the cerebral cortex in a process called memory consolidation. The hippocampus plays an important role in directing memory information to the appropriate location in the cortex, although it does not actually store memories. Another area of the temporal lobe, the amygdala, assigns value to a memory, such as whether it was pleasant. ex: Unusual behaviors and skills of people who have damaged the hippocampus have taught researchers much about this intriguing part of the brain. In 1953, a surgeon removed parts of the hippocampus and the amygdala of a young man to relieve his severe epilepsy. His seizures became less frequent, but he suffered a profound loss in the ability to consolidate short-term memories into long-term ones. As a result, events in his life faded quickly from his memory. He was unable to recall any events that took place since surgery, living always as if it was the 1950s. He would read the same magazine article repeatedly with renewed interest each time.

Intercostal Nerves

- Except for T1, the ventral branches of the thoracic spinal nerves do not enter a plexus. Instead, they extend into spaces between the ribs as intercostal nerves (T2-T11) or under the ribs as subcostal nerves (T12). These nerves supply motor impulses to the intercostal muscles and the upper abdominal wall muscles. They also receive sensory impulses from the skin of the thorax and abdomen.

Fasciculus Gracilis and Fasciculus Cuneatus

- Fasciculus gracilis and fasciculus cuneatus are tracts in the posterior funiculi of the spinal cord. Together they constitute the posterior column. Their fibers conduct sensory impulses from the skin, muscles, tendons, and joints to the brain, where they are interpreted as sensations of touch, pressure, and body movement. At the base of the brain in the medulla oblongata most of the fasciculus gracilis and fasciculus cuneatus fibers cross (decussate) from one side to the other—that is, those ascending on the left side of the spinal cord pass across to the right side, and vice versa. As a result, the impulses originating from sensory receptors on the left side of the body reach the right side of the brain, and those originating on the right side of the body reach the left side of the brain

PD (Parkinson's Disease)

- Fox kept his diagnosis private, but by the late 1990s his co-workers began to notice symptoms that emerged when medication wore off—rigidity, a shuffling and off-balance gait, and poor small-motor control. Fox's face became masklike, a characteristic called hypomimia. It took effort to speak, a symptom called hypophia. Even though his brain could string thoughts into sentences, the muscles of his jaw, lips, and tongue could not utter them. He also developed micrographia, or small handwriting. Fox founded the Michael J. Fox Foundation for Parkinson's Research, and he continues to act on television. In PD, neurons degenerate in the substantia nigra area of the brainstem. Substantia nigra means "large black area," named for the dark pigment that the neurons release as a by-product of synthesizing the neurotransmitter dopamine. When these neurons degenerate, less dopamine reaches synapses with neurons in the striatum of the basal nuclei. The decrease in dopamine causes the motor symptoms of PD. Some patients develop other symptoms, including depression, dementia, constipation, incontinence, sleep problems, and orthostatic hypotension (dizziness upon standing).

More Structures of Spinal Cord

- In the neck region, a thickening in the spinal cord, called the cervical enlargement, supplies nerves to the upper limbs. A similar thickening in the lower back, the lumbar enlargement, gives off nerves to the lower limbs. Just inferior to the lumbar enlargement, the spinal cord tapers to a structure called the conus medullaris. From this tip, nervous tissue, including axons of both motor and sensory neurons, extends downward within the vertebral canal to become spinal nerves at the remaining lumbar and sacral levels. The resulting structure resembles a horse's tail, and is called the cauda equina A thin cord of connective tissue originating from the pia mater and dura mater descends to the upper surface of the coccyx. This cord, called the filum terminale, anchors the spinal cord. Two grooves, a deep anterior median fissure and a shallow posterior median sulcus, extend the length of the spinal cord, dividing it into right and left halves. A cross section of the cord reveals that it consists of white matter surrounding a core of gray matter. The pattern the gray matter produces roughly resembles a butterfly with its wings spread. These posterior and anterior "wings" of gray matter are called the posterior horns and the anterior horns, respectively. Between them on either side in some regions is a protrusion of gray matter called the lateral horn.

Frontal Eye Field

- In the superior part of the frontal lobe is a region called the frontal eye field. The cortex in this area controls voluntary movements of the eyes and eyelids. Nearby is the cortex responsible for movements of the head that direct the eyes. Another region just anterior to the primary motor area controls the learned movement patterns of the hands and fingers that make such skills as writing possible

Spinal Nerve Branches

- Just beyond its foramen, each spinal nerve branches. One of these parts, the small meningeal branch, reenters the vertebral canal through the intervertebral foramen and supplies (innervates) the meninges and blood vessels of the cord, as well as the intervertebral ligaments and the vertebrae. A posterior branch (posterior ramus) of each spinal nerve turns posteriorly and innervates the muscles and skin of the back. The main portion of the nerve, the anterior branch (anterior ramus), continues forward to supply muscles and skin on the front and sides of the trunk and limbs. The spinal nerves in the thoracic and lumbar regions have a fourth branch, or visceral branch, which is part of the autonomic nervous system. Motor fibers associated with this branch form synapses in ganglia (paravertebral ganglia) adjacent to the vertebral column.

Memory

- Memory, one of the most astonishing capabilities of the brain, is the consequence of learning. Whereas learning is the acquisition of new knowledge, memory is the persistence of that learning, with the ability to access it at a later time. Two stages of memory, short term and long term, have been recognized for many years. They differ in characteristics other than duration. - Both mechanisms—new synaptic connections and repeated stimulation of the same neurons—fulfill two requirements of long-term memory. First, sufficient synapses form to encode an almost limitless number of memories. Each of the 10 billion neurons in the cortex can make tens of thousands of synaptic connections to other neurons, forming 60 trillion links. Second, a certain pattern of synapses can persist for years.

Meningitis

- Meningitis, an inflammation of the meninges usually caused by bacterial or viral infection of the CSF, affects the arachnoid and pia maters and sometimes the dura mater, mostly in children. - Complications include visual loss, hearing loss, paralysis, and intellectual disability. - Meningitis may be fatal. - Children are vaccinated against Haemophilus influenza type b, which was once the most common bacterial cause of meningitis.

Nerve and Nerve Fiber Classification

- Nerves that have only fibers of sensory neurons, conducting impulses into the brain or spinal cord, are called sensory nerves. Nerves that have only fibers involved in motor control are motor nerves. Most nerves include both sensory and motor fibers and are called mixed nerves. Nerves originating from the brain that communicate with other body parts are called cranial nerves, whereas nerves originating from the spinal cord that communicate with other body parts are called spinal nerves. The nerve fibers in the cranial and spinal nerves can be subdivided further into four groups as follows: General somatic efferent fibers conduct motor impulses outward from the brain or spinal cord to skeletal muscles and stimulate them to contract. General visceral efferent fibers conduct motor impulses outward from the brain or spinal cord to smooth muscle and glands associated with internal organs. General somatic afferent fibers conduct sensory impulses inward to the brain or spinal cord from receptors in the skin and skeletal muscles. General visceral afferent fibers conduct sensory impulses to the CNS from blood vessels and internal organs. The term general in each of these categories indicates that the fibers are associated with general structures such as the skin, skeletal muscles, glands, and viscera

Cerebellum - Overall...

- Overall, the cerebellum integrates sensory information concerning the position of body parts and coordinates skeletal muscle activity and maintains posture. It receives sensory impulses from receptors in muscles, tendons, and joints (proprioceptors) and from special sense organs, such as the eyes and ears. For example, the cerebellum uses sensory information from the semicircular canals of the inner ears concerning the motion and position of the head to help maintain equilibrium. Damage to the cerebellum can cause a specific type of tremor (cerebellar tremor), inaccurate movements of voluntary muscles, loss of muscle tone, an uneven walk, and loss of equilibrium (balance). Recent studies suggest other possible roles for the cerebellum. These include interaction with other parts of cerebral cortex, such as the limbic system and the auditory areas.

Reflex Behavior

- Reflexes are automatic responses to changes (stimuli) inside or outside the body. They help maintain homeostasis by controlling many involuntary processes such as heart rate, breathing rate, blood pressure, and digestion. Reflexes also carry out the automatic actions involved in swallowing, sneezing, coughing, and vomiting.

Reticular Formation

- Scattered throughout the medulla oblongata, pons, and midbrain is a complex network of nerve fibers associated with tiny islands of gray matter. This network, the reticular formation, or reticular activating system, extends from the superior portion of the spinal cord into the diencephalon. Its neurons connect centers of the hypothalamus, basal nuclei, cerebellum, and cerebrum with all of the major ascending and descending tracts. When sensory impulses reach the reticular formation, it responds by activating the cerebral cortex into a state of wakefulness. Without this arousal, the cortex remains unaware of stimulation and cannot interpret sensory information or carry on thought processes. Decreased activity in the reticular formation results in sleep. If the reticular formation is injured and ceases to function, the person remains unconscious, even with strong stimulation. This is called a comatose state. The term ascending reticular activating system refers to this functional aspect of the reticular formation. Without it the cerebral cortex cannot function consciously.

Sensory Areas of Cerebral Cortex

- Sensory areas in several lobes of the cerebrum receive and interpret impulses from sensory receptors, producing feelings or sensations. For example, the sensations of temperature, touch, pressure, and pain in the skin arise in the postcentral gyri of the parietal lobes (somatosensory cortex) posterior to the central sulcus. The posterior parts of the occipital lobes receive visual input (visual cortex), whereas the superior posterior portions of the temporal lobes contain the centers for hearing (auditory cortex). The sensory areas for taste (gustatory cortex) are near the bases of the lateral sulci and include part of the insula. The sense of smell arises from centers deep in the temporal lobes (olfactory cortex) --> Sensory fibers from the PNS cross over in the spinal cord or the brainstem. Thus, the centers in the right central hemisphere interpret impulses originating from the left side of the body, and vice versa. -->However, the sensory areas concerned with vision receive impulses from both eyes, and those concerned with hearing receive impulses from both ears.

Short Term Memory

- Short-term memory is thought to involve neurons connected in a circuit and stimulated so rapidly that the likelihood of neurons in the circuit reaching threshold is temporarily increased. As long as the circuit is facilitated, the memory persists, but if the stimulus is removed, the circuit becomes inactive. Working memory is related to short-term memory. It refers to the application of short-term memory to performing a specific task, such as remembering a term that you want to look up long enough to type it into a search window.

Types of Sleep

- Sleep is a normal, recurring, and reversible state of unconsciousness characterized in part by limited sensory input and inhibited skeletal muscle activity. The two types of normal sleep are non-rapid eye movement (non-REM) and rapid eye movement (REM). Non-REM sleep occurs when a person is very tired, and it reflects decreasing activity of the reticular formation. It is restful, dreamless, and accompanied by reduced blood pressure and respiratory rate. Non-REM sleep may range from light to heavy and is described in three stages. The third stage is also known as slow-wave sleep. It may last from seventy to ninety minutes. Non-REM and REM sleep alternate. REM sleep is also called "paradoxical sleep" because some areas of the brain are active. As its name implies, the eyes can be seen rapidly moving beneath the eyelids. Cats and dogs in REM sleep sometimes twitch their limbs. In humans, REM sleep usually lasts from five to fifteen minutes. This "dream sleep" is important.

Choricoid plexus

- Structures called choroid plexuses secrete CSF. Choroid plexuses are tiny, reddish, cauliflower-like masses of specialized capillaries from the pia mater, covered by a single layer of specialized ependymal cells - The ependymal cells are joined closely by tight junctions. Choroid plexuses project into the cavities of the ventricles. - In much the same way that astrocytes provide a barrier between the blood and the brain interstitial fluid (blood-brain barrier), ependymal cells in the choroid plexuses block passage of water-soluble substances between the blood and the CSF (blood-CSF barrier). - At the same time, these cells selectively transfer certain substances from the blood into the CSF by facilitated diffusion and transfer other substances by active transport, regulating CSF composition.

The Limbic System

- Structures in the region of the diencephalon also are important in controlling emotional responses. Parts of the cerebral cortex in the medial parts of the frontal and temporal lobes connect with the hypothalamus, thalamus, basal nuclei, and other deep nuclei. These structures form a complex called the limbic system. It controls emotional experience and expression and can modify the way a person acts, producing such feelings as fear, anger, pleasure, and sorrow. The limbic system reacts to potentially life-threatening upsets in a person's physical or psychological condition. By causing pleasant or unpleasant feelings about experiences, the limbic system guides behavior that may increase the chance of survival. In addition, parts of the limbic system interpret sensory impulses from the receptors associated with the sense of smell (olfactory receptors).

Brachial Plexuses

- The anterior branches of the lower four cervical nerves and the first thoracic nerve give rise to brachial plexuses. These networks of nerve fibers are deep in the shoulders between the neck and the axillae (armpits). The major branches emerging from the brachial plexuses include the following: Musculocutaneous nerves supply muscles of the arms on the anterior sides and the skin of the forearms. Ulnar nerves supply muscles of the forearms and hands and the skin of the hands. Median nerves supply muscles of the forearms and muscles and skin of the hands. Radial nerves supply muscles of the arms on the posterior sides and the skin of the forearms and hands. Axillary nerves supply muscles and skin of the anterior, lateral, and posterior regions of the arm. The lateral and medial pectoral nerves supply the pectoralis major and pectoralis minor muscles. The dorsal scapular nerve supplies the rhomboid major and levator scapulae muscles. The lower subscapular nerve supplies the subscapularis and teres major muscles. The thoracodorsal nerve supplies the latissimus dorsi muscle. The suprascapular nerve supplies the supraspinatus and infraspinatus muscles.

Anterior Root

- The anterior root (ventral, or motor, root) of each spinal nerve consists of axons from the motor neurons whose cell bodies lie within the gray matter of the cord. The posterior root (dorsal, or sensory, root) can be identified by an enlargement called the posterior root ganglion. This ganglion contains the cell bodies of the sensory neurons whose axons (peripheral processes) conduct impulses inward from peripheral body parts. The axons (central processes) of these neurons continue through the posterior root and into the spinal cord, where they form synapses with other neurons or ascend to the brain. An anterior root and a posterior root unite to form a spinal nerve, which extends outward from the vertebral canal through an intervertebral foramen (the posterior root is usually absent from the first pair of spinal nerves).

Arachnoid Matter

- The arachnoid mater is a thin, weblike membrane that does not have blood vessels and is located between the dura and pia maters. - It spreads over the brain and spinal cord but generally does not dip into the grooves and depressions on their surfaces. - Many thin strands extend from its undersurface and attach to the pia mater --> A subarachnoid space between the arachnoid and pia maters contains the clear, watery cerebrospinal fluid, or CSF.

Association Areas of various structures in the Brain

- The association areas of the frontal lobes provide higher intellectual processes, such as concentrating, planning, and complex problem solving. The anterior and inferior portions of these lobes (prefrontal areas) control emotional behavior and produce awareness of the possible consequences of behaviors. These abilities are also collectively called executive function. The parietal lobes have association areas that help interpret sensory information and aid in understanding speech and choosing words to express thoughts and feelings. Awareness of the form of objects, including one's own body parts, stems from the posterior regions of these lobes. The association areas of the temporal lobes and the regions at the posterior ends of the lateral sulci interpret complex sensory experiences, such as those needed to understand speech and to read. These regions also store memories of visual scenes, music, and other complex sensory patterns. The occipital lobes have association areas adjacent to the visual centers. These are important in analyzing visual patterns and combining visual images with other sensory experiences, such as when one person recognizes another. The functions of the two lobes of the insula are not as well known as those of the other lobes because their location deep within the cerebrum makes them impossible to study with surface electrodes. - insula serves as a crossroads for translating sensory information into appropriate emotional responses, such as feeling disgust at the sight of something unpleasant, or a feeling of joy when hearing music or when biting into a slice of pizza. Some researchers hypothesize that in some complex way the insula is responsible for some of the qualities that make us human.

The Autonomic Nervous System

- The autonomic nervous system (ANS) is an efferent (motor) part of the PNS that functions independently (autonomously) and continuously, without conscious effort. This system controls visceral activities by regulating the actions of smooth muscles, cardiac muscles, and various glands. It oversees heart rate, blood pressure, breathing rate, body temperature, and other visceral activities that aid in maintaining homeostasis. Portions of the autonomic nervous system also respond during times of emotional stress and prepare the body to meet the demands of strenuous physical activity. While some consider visceral afferents to be part of the ANS, historically the ANS involves only the motor (efferent) pathways, as described here. Nonetheless, it is important to remember that such "autonomic decisions" require sensory information from visceral receptors.

General Characteristics of ANS

- The autonomic nervous system includes two divisions, the sympathetic and parasympathetic divisions. Many organs receive input from both divisions. Impulses from one division may activate an organ, and impulses from the other division inhibit it. Thus, the divisions may function antagonistically, regulating the actions of some organs by alternately activating or inhibiting them. One example of this, concerns the baroreflex control of heart rate. In that reflex, parasympathetic activity decreases heart rate and sympathetic activity increases it. Moment to moment changes between the two keep heart rate at a level appropriate to maintaining homeostasis. The sympathetic division is always operating, usually without our noticing, but is most active during energy-expending, stressful, or emergency situations (so-called "fight or flight"). Conversely, the parasympathetic division is most active under ordinary, restful conditions (often described as "rest and digest"). It also counters the effects of the sympathetic division and restores the body to a resting state following a stressful experience. For example, during an emergency the sympathetic division increases heart rate. Following the emergency, the parasympathetic division decreases heart rate.

Meninges

- The brain lies in the cranial cavity of the skull, and the spinal cord occupies the vertebral canal in the vertebral column. - Beneath these bony coverings, membranes called meninges, located between the bone and the soft tissues of the nervous system, protect the brain and spinal cord - The meninges (sing., meninx) have three layers—dura mater, arachnoid mater, and pia mater. The dura mater is the outermost layer. - It is primarily composed of tough, white, dense connective tissue and contains many blood vessels and nerves. - The dura mater attaches to the inside of the cranial cavity and forms the internal periosteum of the surrounding skull bones - In some regions, the dura mater extends inward between lobes of the brain and forms supportive and protective partitions. - In other areas, the dura mater splits into two layers, forming channels called dural sinuses, Venous blood flows through these channels as it returns from the brain to vessels leading to the heart.

Cerebellum Communication

- The cerebellum communicates with other parts of the CNS by means of three pairs of nerve tracts called cerebellar peduncles For example, one pair, the inferior peduncles, brings sensory information concerning the position of body parts such as limbs and joints to the cerebellum via the spinal cord and medulla oblongata. The middle peduncles send information from the cerebral cortex about the desired position of these body parts. After integrating and analyzing the information from these two sources, the cerebellum sends correcting impulses from the dentate nucleus via the superior peduncles to the thalamus and eventually to the motor cortex

The Cerebellum

- The cerebellum is a large mass of tissue inferior to the occipital lobes of the cerebrum and dorsal to the pons and medulla oblongata. It consists of two lateral hemispheres partially separated by a layer of dura mater called the falx cerebelli. A structure called the vermis connects the cerebellar hemispheres at the midline. Like the cerebrum, the cerebellum is primarily composed of white matter with a thin layer of gray matter, the cerebellar cortex, on its surface. This cortex doubles over on itself in a series of complex folds that have myelinated nerve fibers branching into them. A cut into the cerebellum reveals a treelike pattern of white matter, called the arbor vitae, surrounded by gray matter. A number of nuclei lie deep within each cerebellar hemisphere. The largest and most important is the dentate nucleus.

Functions of Cerebral Cortex

- The cerebral cortex provides higher brain functions: interpreting impulses from sense organs, initiating voluntary muscular movements, storing information as memory, and retrieving this information in reasoning. The cerebral cortex is also the part of the brain responsible for intelligence and personality. - Researchers have divided the cerebral cortex into sensory, association, and motor areas that overlap somewhat. Each of these areas is a collection of neurons that work together to provide a particular brain function.

Structures of the Cerebrum

- The cerebrum, which develops from the anterior portion of the forebrain, is the largest part of the mature brain. - It consists of two large masses, or cerebral hemispheres, which are essentially mirror images of each other. - A broad, flat bundle of axons called the corpus callosum connects the cerebral hemispheres. A layer of dura mater called the falx cerebri separates them. - Many ridges or convolutions, called gyri (sing., gyrus), separated by grooves, mark the cerebrum's surface. Generally, a shallow to somewhat deep groove is called a sulcus, and a very deep groove is called a fissure. The pattern of these elevations and depressions is complex, but, despite individual variations, is similar in all normal brains. For example, a longitudinal fissure separates the right and left cerebral hemispheres; a transverse fissure separates the cerebrum from the cerebellum; and sulci divide each hemisphere into lobes Most of the five lobes of the cerebral hemispheres are named after the skull bones that they underlie. The lobes of each hemisphere include the following: - Frontal - Occipital - Parietal - Temporal - Insula

The Diencephalon

- The diencephalon develops from the posterior forebrain and is located between the cerebral hemispheres and superior to the brainstem - It surrounds the third ventricle and is largely composed of gray matter. In the diencephalon, a dense mass called the thalamus bulges into the third ventricle from each side. Another region of the diencephalon that includes many nuclei is the hypothalamus. It lies inferior to the thalamic nuclei and forms the lower walls and floor of the third ventricle - Other parts of the diencephalon include (1) the optic tracts, which originate from the optic chiasma (formed by some of the optic nerve fibers crossing over); (2) the infundibulum (pituitary stalk), which is a conical process behind the optic chiasma to which the pituitary gland is attached; (3) the posterior pituitary gland, which hangs from the floor of the hypothalamus, attached to the infundibulum; (4) the mammillary bodies, which are two rounded structures behind the infundibulum; and (5) the pineal gland, which is a cone-shaped projection from the roof of the diencephalon

Dura Matter

- The dura mater continues into the vertebral canal as a strong, tubular sheath that surrounds the spinal cord. It is attached to the cord at regular intervals by a band of pia mater (denticulate ligaments) that extends the length of the spinal cord on either side. - The dural sheath ends as a closed sac at the level of the second sacral vertebra, below the tip of the spinal cord. - The sheath around the spinal cord is not attached directly to the vertebrae but is separated by an epidural space, which lies between the dural sheath and the bony walls --> Located just deep to the skull - Top to bottom: Dura Matter, Arachnoid matter, Pia matter.

Olfactory Nerves

- The first pair of cranial nerves, the olfactory nerves (I), are associated with the sense of smell and include only sensory neurons. These bipolar neurons, located in the lining of the upper nasal cavity, serve as olfactory receptor cells. Axons from these receptor cells pass upward through the cribriform plates of the ethmoid bone, conducting impulses to the olfactory neurons in the olfactory bulbs, which are extensions of the cerebral cortex just beneath the frontal lobes. Sensory impulses move from the olfactory bulbs along olfactory tracts to cerebral centers where they produce the sensation of smell.

Trochlear Nerves

- The fourth pair of cranial nerves, the trochlear nerves (IV), are the smallest. They arise from the midbrain and conduct motor impulses to a fifth pair of external eye muscles, the superior oblique muscles, which are not supplied by the oculomotor nerves. The trochlear nerve is considered motor, with some proprioceptive fibers.

The Midbrain

- The midbrain (mesencephalon) is a short section of the brainstem between the diencephalon and the pons. It contains bundles of myelinated nerve fibers that join lower parts of the brainstem and spinal cord with higher parts of the brain. The midbrain includes several masses of gray matter that serve as reflex centers. It also contains the cerebral aqueduct that connects the third and fourth ventricles - Two prominent bundles of nerve fibers on the anterior of the midbrain comprise the cerebral peduncles. These fibers include descending tracts and are the main motor pathways between the cerebrum and lower parts of the nervous system. Beneath the cerebral peduncles are large bundles of sensory fibers that carry impulses upward to the thalamus. - Two pairs of rounded knobs on the superior surface of the midbrain mark the location of four nuclei, known collectively as corpora quadrigemina. The upper masses (superior colliculi) contain the centers for certain visual reflexes, such as those responsible for fixing the eyes on a stationary object as the head turns. The lower ones (inferior colliculi) contain the auditory reflex centers that operate when it is necessary to move the head to hear sounds more distinctly Near the center of the midbrain is a mass of gray matter called the red nucleus. This nucleus communicates with the cerebellum and with centers of the spinal cord, and it plays a role in reflexes that maintain posture. It appears red because it is richly supplied with blood vessels.

The Occipital Lobe

- The occipital lobe forms the posterior portion of each cerebral hemisphere and is separated from the cerebellum by a shelflike extension of dura mater called the tentorium cerebelli. The occipital lobe and the parietal and temporal lobes have no distinct boundary.

The Parietal Lobe

- The parietal lobe is posterior to the frontal lobe and is separated from it by the central sulcus.

The Patellar Reflex

- The patellar reflex (knee-jerk reflex) is an example of a simple monosynaptic reflex, so-called because it uses only two neurons—a sensory neuron communicating directly to a motor neuron. Striking the patellar ligament just below the patella initiates this reflex. The quadriceps femoris muscle group, attached to the patella by a tendon, is pulled slightly, stimulating stretch receptors in the muscle group. These receptors, in turn, trigger impulses that pass along the peripheral process of the axon of a unipolar sensory neuron, continuing along the central process of the axon into the lumbar region of the spinal cord. In the spinal cord, the sensory neuron axon synapses with a motor neuron. An impulse is then triggered on the motor neuron and is conducted along its axon to the neuromuscular junctions in that motor unit of the quadriceps femoris. The muscle fibers involved respond by contracting, and the reflex is completed as the leg extends The patellar reflex helps maintain an upright posture. For example, if a person is standing still and the knee begins to bend in response to gravity, the quadriceps femoris is stretched, the reflex is triggered, and the leg straightens again. Adjustments within the stretch receptors keep the reflex responsive at different muscle lengths.

Peripheral Nervous System

- The peripheral nervous system consists of the nerves that branch from the CNS, connecting it to other body parts. The PNS includes the cranial nerves that arise from the brain and the spinal nerves that arise from the spinal cord. The PNS can also be subdivided into somatic and autonomic nervous systems. Generally, the somatic nervous system consists of the nerve fibers that connect the CNS to the skin and skeletal muscles, so it plays a role in conscious activities. The autonomic nervous system includes fibers that connect the CNS to viscera such as the heart, stomach, intestines, and various glands. The autonomic nervous system controls subconscious actions.

Pia Matter

- The pia mater is thin and contains many nerves, as well as blood vessels that nourish the underlying cells of the brain and spinal cord. - The pia mater is attached to the surfaces of these organs and follows their irregular contours, passing over the high areas and dipping into the depressions.

The Pons

- The pons occupies the full thickness of the brainstem, but it is most visible ventrally as a rounded bulge where it separates the midbrain from the medulla oblongata The ventral portion of the pons consists mostly of longitudinal nerve fibers, which relay information between the medulla oblongata and the cerebrum. Its ventral portion also contains large bundles of transverse nerve fibers that wrap around to the back and connect with the cerebellum. They conduct impulses from the cerebrum to centers within the cerebellum. Several nuclei of the pons relay sensory information from peripheral nerves to higher brain centers. Other nuclei may function with centers of the medulla oblongata to control breathing.

Posterior and Anterior Spinocerebellar Tracts

- The posterior and anterior spinocerebellar tracts lie near the surface in the lateral funiculi of the spinal cord. Fibers in the posterior tracts remain uncrossed, whereas those in the anterior tracts cross over in the medulla. Impulses conducted on their fibers originate in the muscles of the lower limbs and trunk and then travel to the cerebellum. These impulses are used by the cerebellum to coordinate muscular movements.

Primary Motor Area (Cortex)

- The primary motor areas (motor cortex) of the cerebral cortex lie in the precentral gyri of the frontal lobes just in front of the central sulcus and in the anterior wall of this sulcus (fig. 11.8). The nervous tissue in these regions contains many large pyramidal cells, named for their pyramid-shaped cell bodies. Impulses from the pyramidal cells move downward through the brainstem and into the spinal cord on descending tracts. Most of the nerve fibers in these tracts cross over from one side of the brain to the other within the brainstem. Impulses conducted on these pathways in special patterns and frequencies are responsible for movements in skeletal muscles cells in the upper portions of the motor areas send impulses to muscles in the thighs and legs; those in the middle portions control muscles in the arms and forearms; and those in lower portions activate muscles of the head, face, and tongue. The relative (distorted) size of each area in the figure reflects the extent of the cortex devoted to it.

Optic Nerves

- The second pair of cranial nerves, the optic nerves (II), are sensory and lead from the eyes to the brain. They are associated with vision. The cell bodies of these neurons form ganglion cell layers in the eyes, and their axons pass through the optic foramina of the orbits and continue into the visual nerve pathways of the brain

Vagus Nerve

- The tenth pair of cranial nerves, the vagus nerves (X), originate in the medulla oblongata and extend downward through the neck into the chest and abdomen. These nerves are mixed, including both somatic and autonomic branches, with the autonomic fibers predominant. Among the somatic components of the vagus nerves are motor fibers that conduct impulses to muscles of the larynx and pharynx. These fibers are associated with speech and swallowing reflexes that use muscles in the soft palate and pharynx. Vagal sensory fibers conduct impulses from the linings of the pharynx, larynx, and esophagus and from the viscera of the thorax and abdomen to the brain. Autonomic motor fibers of the vagus nerves supply the heart and many smooth muscles and glands in the viscera of the thorax and abdomen

Thalamus

- The thalamus is a selective gateway for sensory impulses ascending from other parts of the nervous system to the cerebral cortex. It receives all sensory impulses (except some associated with the sense of smell) and channels them to appropriate regions of the cortex for interpretation. The thalamus relays sensory information by synchronizing action potentials. Consider vision. An image on the retina stimulates the lateral geniculate nucleus (LGN) region of the thalamus, which then sends action potentials to a part of the visual cortex. Those action potentials are synchronized—fired simultaneously—by the LGN's neurons only if the stimuli come from a single object, such as a bar. If the stimulus is two black dots, the resulting thalamic action potentials are not synchronized. The synchronicity of action potentials, therefore, may be a way that the thalamus selects which stimuli to relay to higher brain structures. Therefore, the thalamus is not only a messenger but also an editor. Pathways connect the hypothalamus to the cerebral cortex, thalamus, and parts of the brainstem so that it can receive impulses from them and send impulses to them. The hypothalamus maintains homeostasis by regulating a variety of visceral activities and by linking the nervous and endocrine systems. The hypothalamus regulates: heart rate and arterial blood pressure body temperature water and electrolyte balance control of hunger and body weight control of movements and glandular secretions of the stomach and intestines

Oculomotor Nerves

- The third pair of cranial nerves, the oculomotor nerves (III), arise from the midbrain and pass into the orbits of the eyes. One component of each nerve connects to a number of voluntary muscles, including those that raise the eyelids and four of the six muscles that move the eye. A second portion of each oculomotor nerve is part of the autonomic nervous system, supplying involuntary muscles inside the eyes. These muscles help adjust the amount of light that enters the eyes and help focus the lenses. This nerve is considered motor, with some proprioceptive fibers.

Ascending and Descending Tracts

- The tracts of the spinal cord together with the spinal nerves provide a two-way communication system between the brain and body parts outside the nervous system. The spinal tracts that conduct sensory information to the brain are called ascending tracts; those that conduct motor commands from the brain via motor neurons reaching muscles and glands are descending tracts The ascending and descending tracts are comprised of axons. Many of the names that identify nerve tracts reflect common origins and terminations. For example, a spinothalamic tract begins at various levels of the spinal cord and conducts sensory impulses associated with the sensations of pain and touch to the thalamus. A corticospinal tract originates in the cerebral cortex and conducts motor impulses on so-called upper motor neurons downward through the spinal cord. These impulses control lower motor neurons at various levels of the spinal cord whose cell bodies are in the anterior horn and whose axons lead to skeletal muscles

Hypoglossal Nerve

- The twelfth pair of cranial nerves, the hypoglossal nerves (XII), arise from the medulla oblongata and pass into the tongue. They primarily consist of fibers that conduct impulses to muscles that move the tongue in speaking, chewing, and swallowing. This nerve is considered motor, with some proprioceptive fibers.

Spinal Cord Injuries

- The two most common causes of spinal cord injury are accidents in the workplace and motor vehicle accidents. Third most common are sports injuries. A spinal cord injury may result from a sudden and unexpected movement. For example, one man suffered a severe spinal cord injury after a powerful wave knocked him down while he was standing in just a foot of water at a shoreline. Regardless of the cause, if nerve fibers in ascending tracts are cut, sensations arising from receptors below the level of the injury are lost. Damage to descending tracts results in loss of motor functions below the level of the injury. Problems of this type in fibers of the descending tracts produce upper motor neuron syndrome, characterized by spastic paralysis in which muscle tone increases, with little atrophy of the muscles. A hemi-lesion of the spinal cord (severed on one side) affecting the corticospinal and spinothalamic tracts can cause Brown-Séquard syndrome. Ascending tracts cross over at different levels, so the injured side of the body becomes paralyzed and loses touch sensation. The other side of the body retains movement but loses sensations of pain and temperature. Injury to motor neurons in the anterior horns of the spinal cord results in lower motor neuron syndrome. It produces flaccid paralysis, a total loss of muscle tone and reflex activity, and the muscles atrophy.

Lumbosacral Plexus

- The ventral branches of the lumbar and first four sacral nerves form the lumbosacral plexuses. The lumbar portions are in the lumbar regions of the abdomen and the sacral portions are in the pelvic cavity. These networks of nerve fibers give rise to a number of nerves associated with the lower abdominal wall, external genitalia, buttocks, thighs, legs, and feet. The major branches of these plexuses include the following: The obturator nerves supply the adductor muscles of the thighs. The femoral nerves divide into many branches, supplying motor impulses to muscles of the anterior thighs and receiving sensory impulses from the skin of the thighs and legs. Other nerves associated with the lumbosacral plexus that innervate various skeletal muscles include the following: The pudendal nerve supplies the muscles of the perineum. The inferior and superior gluteal nerves supply the gluteal muscles and the tensor fasciae latae muscle.

The Brain - Brain Stem

- The wall of the anterior portion of the forebrain gives rise to the cerebrum and basal nuclei, whereas the posterior portion forms a section of the brain called the diencephalon. - The region the midbrain produces continues to be called the midbrain in the adult structure, and the hindbrain gives rise to the cerebellum, pons, and medulla oblongata. - Together, the midbrain, pons, and medulla oblongata comprise the brainstem, which attaches the brain to the spinal cord.

Spinal Nerves in Detail

- Thirty-one pairs of spinal nerves originate from the spinal cord. The first pair is purely motor. The remaining pairs are mixed nerves that provide two-way communication between the spinal cord and parts of the upper and lower limbs, neck, and trunk Each spinal nerve, except for the first pair, emerges from the cord by two short branches, or roots, which lie within the vertebral column. The roots arising from the superior part of the spinal cord pass outward almost horizontally, whereas those from the inferior portions of the spinal cord descend at sharp angles. This anatomical feature is a consequence of growth. In early life, the spinal cord extends the entire length of the vertebral column, but with age, the column grows more rapidly than the cord. Thus, the adult spinal cord ends at the level between the first and second lumbar vertebrae, so the roots associated with the lumbar, sacral, and coccygeal nerves descend to their exits beyond the end of the cord, still within the vertebral canal. These descending roots form a structure called the cauda equina (horse's tail)

Specialized Structures

- Three other groups of fibers, found only in cranial nerves, are associated with more specialized, or special, structures: Special somatic efferent fibers conduct motor impulses outward from the brain to the muscles used in chewing, swallowing, speaking, and forming facial expressions. Special visceral afferent fibers conduct sensory impulses inward to the brain from the olfactory and taste receptors. Special somatic afferent fibers conduct sensory impulses inward to the brain from the receptors of sight, hearing, and equilibrium.

Crossed Extensor Reflex

- While flexor muscles on the affected side (ipsilateral side) contract, the flexor muscles of the limb on the other side (contralateral side) are inhibited. Furthermore, the extensor muscles on the contralateral side contract, helping to support the body weight shifted to that side. This phenomenon, called a crossed extensor reflex, is due to interneuron pathways in the spinal cord that allow sensory impulses arriving on one side of the cord to pass across to the other side and produce an opposite effect Reflexes like these can be found at different levels of the spinal cord and in the brain, depending on which body parts are involved. Concurrent with the withdrawal reflex, other interneurons in the spinal cord carry sensory impulses upward to the brain. The person becomes aware of the experience and may feel pain.

Motor Speech Area

- a region called the motor speech area, also known as Broca's area, is in the frontal lobe, usually in the left hemisphere, just anterior to the primary motor cortex and superior to the lateral sulcus. Broca's area is important in generating the complex pattern of motor instructions for the muscular actions of the mouth, tongue, and larynx, which make speech possible. Bundles of axons directly and indirectly connect Broca's area with Wernicke's area. - Broca's area provides motor instructions for written or spoken communication, whereas Wernicke's area ensures that the communication makes sense. For example, a person with an injury to Broca's area may be able to understand spoken words but be unable to speak. A person with damage to Wernicke's area could initiate speech, but without control of its content.

ALS

In amyotrophic lateral sclerosis (ALS, Lou Gehrig's disease, or motor neuron disease), motor neurons in the spinal cord, brainstem, and cerebral cortex degenerate. The disease may be due to an inability of the motor neurons or associated astrocytes to counter buildup of oxygen free radicals, or to overreactive microglia that kill motor neurons. The first symptoms of ALS may be vague and affect only a small part of the body: difficulty speaking, a dragging foot, clumsiness, fatigue, or problems with fine coordination such as turning a key or pulling a zipper. Muscle twitches (fasciculations) may prompt the person to seek medical attention. Diagnosis may take more than a year as a neurologist observes spreading weakness and rules out other conditions, such as multiple sclerosis or a spinal cord tumor. The average age at diagnosis is 55, but adolescents as well as the very elderly have developed ALS. ALS affects the upper and lower parts of the body, and progresses faster if symptoms begin in the face or neck (bulbar onset) compared to the arms and legs (limb onset). Usually the battle is lost two to five years after diagnosis, typically from respiratory failure, but about 10% of patients live more than a decade with the disease. ALS has no cure, but a drug (Rilutek [riluzole]) may extend time until respiratory difficulty. Assisted breathing devices may be used, including a ventilator to sustain life. Although about 50% of people with ALS experience some cognitive decline, the mind remains sharp in many people. One patient wrote a novel during his last months, and another remained a brilliant songwriter. About 10% of ALS cases are inherited, due to mutations in any of several genes. Because prevalence has been increasing over the past few years at a rate faster than the aging of the population can explain, an environmental trigger may combine with an inherited susceptibility to cause the disease.

Structure of PNS

Nerves are essentially bundles of axons, but they have specific levels of organization. A small amount of loose connective tissue called endoneurium surrounds individual axons. Axons are organized in bundles called fascicles. Each fascicle is enclosed in a sleeve of loose connective tissue called the perineurium. A group of bundled fascicles, surrounded by an outermost layer of dense connective tissue called the epineurium, constitutes a nerve Blood vessels in the epineurium and perineurium give rise to a network of capillaries in the endoneurium that provides oxygen and nutrients to the neurons.

Vestibulocochlear Nerve

The eighth pair of cranial nerves, the vestibulocochlear nerves (VIII, acoustic, or auditory, nerves), are sensory nerves that arise from the medulla oblongata. Each of these nerves has two distinct parts—a vestibular branch and a cochlear branch. The neuron cell bodies of the vestibular branch fibers are located in ganglia near the vestibule and semicircular canals of the inner ear. These structures contain receptors that sense changes in the position of the head and, in response, initiate and send impulses to the cerebellum, where they are used in reflexes that maintain equilibrium.

Accessory Nerves

The eleventh pair of cranial nerves, the accessory nerves (XI, spinal accessory), originate in the medulla oblongata and the spinal cord. Therefore, these nerves have both cranial and spinal branches. Each cranial branch of an accessory nerve joins a vagus nerve and conducts impulses to muscles of the soft palate, pharynx, and larynx. The spinal branch descends into the neck and supplies motor fibers to the trapezius and sternocleidomastoid muscles. This nerve is considered motor, with some proprioceptive fibers.

The fibers of the rubrospinal tracts

The fibers of the rubrospinal tracts cross over in the brain and pass through the lateral funiculi. They conduct impulses from the brain to synapses with lower motor neurons, and help to coordinate muscle actions.

Trigeminal Nerves

The fifth pair of cranial nerves, the trigeminal (tri-jem′i-nal) nerves (V), are the largest and arise from the pons. They are mixed nerves, with more extensive sensory portions. Each sensory component includes three large branches, called the ophthalmic, maxillary, and mandibular divisions The ophthalmic division of each the trigeminal nerve consists of sensory fibers that conduct impulses to the brain from the surface of the eye; the tear gland; and the skin of the anterior scalp, forehead, and upper eyelid. The fibers of the maxillary division conduct sensory impulses from the upper teeth, upper gum, and upper lip, as well as from the mucous lining of the palate and facial skin. The mandibular division includes both motor and sensory fibers. The sensory branches conduct impulses from the scalp behind the ears, the skin of the jaw, the lower teeth, the lower gum, and the lower lip. The motor branches supply the muscles of mastication and certain muscles in the floor of the mouth.

The Medulla Oblongata

The medulla oblongata is an enlarged continuation of the spinal cord, extending from the level of the foramen magnum to the pons Its dorsal surface flattens to form the floor of the fourth ventricle, and its ventral surface is marked by two longitudinal enlargements called the pyramids. These contain descending tracts, most of whose fibers cross over at this level. On each side of the medulla oblongata is an oval swelling called the olive, from which a large bundle of nerve fibers arises and passes to the cerebellum. Peripheral nerves conduct impulses originating in the cardiac center to the heart, where they increase or decrease heart rate. Certain cells of the vasomotor center initiate impulses that affect smooth muscle in the walls of blood vessels and stimulate them to contract, constricting the vessels (vasoconstriction) and thereby raising blood pressure. A decrease in the activity of these cells can produce the opposite effect—dilation of the blood vessels (vasodilation) and a consequent drop in blood pressure. The respiratory center maintains the basic rhythm of breathing and works with other brainstem areas to adjust the rate and depth of breathing to meet changing needs. Some nuclei in the medulla oblongata are centers for certain nonvital functions, such as coughing, sneezing, swallowing, and vomiting. However, because the medulla also contains vital control centers, injuries to this part of the brainstem are often fatal.

CH. 11 QUIZ

Which of the following terms and definitions is correct? - Cerebral cortex—a thin layer of gray matter forming the outermost part of the cerebrum If the left corticospinal tract is severed in the neck near the first cervical vertebra,...? --> muscles in the right arm and leg are paralyzed. Which lobe of your brain are you using when you answer this question? -->Frontal Lobe Basal ganglia are located in the __________ and __________. --> deep regions of the cerebral hemispheres; aid in control of motor activities Melinda has Parkinson disease. Her movements are slowing and she has difficulty initiating voluntary muscular actions. The region that is affected in her brain is the...? --> Basal Ganglia The brain waves most closely associated with mental activity are...? --> Beta Waves After a particularly bad forearm break, a person loses motor function and feeling in their 4th and 5th phalanges, along with the side of the hand directly above these fingers. Which nerve(s) has/have been damaged? --> The Ulnar Nerve Most cerebrospinal fluid is secreted from the choroid plexuses in the...? --> Lateral Ventricles The pattern of gray matter in the spinal cord is divided into...? --> Horns