Holes Anatomy ch 9 Nervous System

Define refractory period.

After an action potential passes, the fiber needs time to return to its resting potential. This time is called the refractory period. The refractory period has two parts. The first is the absolute refractory period. This lasts about 1/2,500th of a second, and is the period when no new impulses can be sent. The second is called the relative refractory period. During this time, the fiber still has not returned to its resting potential, but enough sodium and potassium ions have moved to allow an action potential to occur to very strong stimuli.

Distinguish between alpha adrenergic and beta adrenergic receptors.

Alpha adrenergic receptors are responsible for smooth muscle contraction causing vasoconstriction. Stimulation of the beta receptors, however, will cause smooth muscle relaxation leading to bronchodilation in the lungs. In essence, stimulation of alpha receptors is constrictive in nature, which stimulation of beta receptors will cause dilation.

Explain hemisphere dominance.

Although both hemispheres participate in basic functions, in most people, one hemisphere is dominant over the other. For instance, in over 90 percent of the population, the left hemisphere controls language activities such as reading, speech, and writing as well as complex intellectual functions requiring verbal, analytical, and computational skills. The non-dominant hemisphere seems to be more in control of the nonverbal activities such as spatial orientation, interpreting musical patterns, visual experiences, and emotional and intuitive thought.

Explain how cerebrospinal fluid is produced and how it functions.

Cerebrospinal fluid (CSF) is secreted by tiny reddish cauliflower-like masses of specialized capillaries in the pia mater called choroid plexuses that project into the ventricles. CSF is important in the protection and support of the CNS by absorbing the forces of impact, maintaining a stable ion concentration, and providing a route for waste products to be removed. Humans secrete nearly 500 milliliters of CSF daily. However, only about 140 milliliters are in the nervous system at any time.

Explain the consequences of nerve fibers crossing over.

Crossing over causes the impulses from one side of the body to be received and controlled by the opposite side of the brain.

Distinguish between excitatory and inhibitory postsynaptic potentials.

Different neurotransmitters cause distinctly different responses in the postsynaptic neuron. If a neurotransmitter binding to the postsynaptic neuron causes sodium ion channels to open, the ions move inward and depolarize the membrane, possibly causing an action potential. Because this reaction causes the membrane to be closer to the threshold potential, it is said to be an excitatory postsynaptic potential (EPSP). If the neurotransmitter causes the potassium receptors to open, the postsynaptic membrane becomes hyperpolarized in response to an influx of potassium ions. This reaction makes an action potential less likely and is called an inhibitory postsynaptic potential (IPSP).

Distinguish between neurons and neuroglia.

Neurons are the structural and functional cells reacting to the physical and chemical changes in their environment. Neuroglia is the supporting cells necessary for nourishing and maintaining the neurons, among other functions.

Describe the three types of neurons classified on the basis of function.

Nerve fibers can be classified into three groups: Sensory—sensory (afferent) neurons sense changes inside or outside the body by means of receptors ends or nearby receptor cells. They send impulses to the CNS in response to these changes. Most of these neurons are unipolar, with some bipolar. Interneurons—interneurons (association or internuncial neurons) are multipolar neurons found in the CNS. They link with other neurons and send impulses from one part of the CNS to another. Motor—motor (efferent) neurons are multipolar, and send impulses from the CNS to muscles or glands. There are two types of motor neurons that control smooth or cardiac muscle. Accelerator neurons increase muscle activity, while inhibitory neurons decrease muscle activity.

Describe the three types of neurons classified on the basis of structure.

Nerve fibers can be classified into three main groups: Bipolar—bipolar neurons have only two nerve fibers, one is the axon and one is the dendrite. They are from opposite sides of the cell body. Unipolar—unipolar neurons have a single nerve fiber extending from the cell body. From there it branches in two directions; one branch extends into a peripheral body part and serves as a dendrite. The other extends into the CNS and acts like an axon. Multipolar—multipolar neurons have one axon and many other extensions from the cell body that serve as dendrites.

Explain how information is passed from a presynaptic neuron to a postsynaptic cell.

Nerve impulses pass from neuron to neuron (or to other cells) at synapses. A presynaptic neuron brings the impulse to the synapse and, as a result, stimulates or inhibits a postsynaptic neuron (or a muscle or gland). A narrow space, or synaptic cleft, separates the two cells, which are connected functionally, not physically.

Explain how malfunctioning neuroglia can harm health.

Neuroglias form more than half the volume of the brain and are critical to neuron function. Abnormal neuroglia is associated with certain disorders.

List three ways that the nervous system changes as we age.

1. Neuron death accelerates to a certain degree. 2. Decreased levels of neurotransmitters. 3. Fading memory and slower reflexes.

List six functions of neuroglia.

1. Structural support 2. Transport of substances 3. Communication between neurons 4. Induce synapse formation 5. Speed neurotransmission 6. Phagocytosis

Distinguish among polarized, hyperpolarized, and depolarized.

A cell membrane is usually electrically charged, or polarized, so that the inside is negatively charged with respect to the outside. This polarization is due to an unequal distribution of positive and negative ions on either side of the membrane. If, in response to a stimulus, the resting potential becomes more negative, the cell is said to be hyperpolarizing. If the stimulus causes the resting potential to become less negative, the cell is said to be depolarizing.

Distinguish between myelinated and unmyelinated axons.

A myelinated nerve fiber is one, which is bound by Schwann cells longitudinally along its length. The Schwann cells wrap tightly around the nerve fiber and form a myelin sheath. Unmyelinated nerve fibers lack these sheaths. In this case, these Schwann cells are not wound around the axons but simply form a grove or valley in which the axon sits. Myelinated (medullated) nerve fibers appear white. Unmyelinated nerve fibers appear gray.

Describe the connective tissue and nervous tissue making up a peripheral nerve.

A peripheral nerve consists of nerve fiber bundles surrounded by connective tissue. Each bundle of nerve fibers (fascicle) is encased in a sleeve of connective tissue called the perineurium, which is in turn, enclosed by dense collagenous fibers called the epineurium. The individual nerve fibers are surrounded by loose connective tissue called the endoneurium within the perineurium.

Define plexus, and locate the major plexuses of the spinal nerves.

A plexus is the main portion of the spinal nerves that have combined to form complex networks. Except in the thoracic region, anterior branches of the spinal nerves provide the network for the plexus. In a plexus, the fibers of various spinal nerves are sorted and recombined so that the fibers associated with a particular peripheral body part reach it in the same nerve, even though the fibers originate from different spinal nerves. There are three main plexuses: Cervical plexuses—supply the muscles of the skin and neck and the phrenic nerves innervate the diaphragm. Brachial plexuses—supply the muscles and skin of the arm, forearm, and hand. Lumbosacral plexuses—give rise to motor and sensory fibers associated with the muscles and skin of the lower abdominal wall, external genitalia, buttocks, thighs, legs, and feet.

Distinguish between a reflex arc and a reflex.

A reflex arc is the simplest response to a stimulus. It begins with a receptor at the end of sensor nerve fibers. It travels to a reflex center in the CNS and an impulse is sent to an effector along a motor nerve fiber. A reflex is an automatic, subconscious response to stimuli inside or outside the body.

Explain why the "trigger zone" of a neuron is named as such.

A trigger zone is an area at the proximal end of an axon at the axonal hillock that starts a nerve impulse from the action potential.

Explain the relationship between an action potential and a nerve impulse.

An action potential occurs at a specific site. When an action potential occurs at the trigger zone of a nerve cell, it sends an electrical impulse to the adjacent membrane. This causes an action potential at the next site. This occurs in a wavelike sequence, without losing amplitude, from the beginning of the fiber to the end, and is known as a nerve impulse.

Explain the effects of apoptosis on the developing brain.

Apoptosis begins before birth as a form of programmed cell death. This process helps to carve out the structures that will remain in the brain. When brain apoptosis fails, disease results.

Explain how autonomic neurotransmitters influence the actions of effector cells.

Autonomic transmitters act by binding to protein receptors of effector cell membranes. This receptor binding alters the membrane in certain ways to produce the desired effect. Muscarinic receptors are found in the membranes of all effector cells at the end of postganglionic parasympathetic and cholinergic sympathetic nerve fibers. Nicotinic receptors are found in the synapses between the pre- and postganglionic neurons of the postganglionic neurons of the sympathetic and parasympathetic pathways.

Define facilitation.

Because a neuron in a region of a neuronal pool may receive excitatory and inhibitory impulses at the same time, the net effect may be either excitatory or inhibitory. If the net effect is excitatory enough to pass the threshold potential, an action potential will occur. If the net effect is excitatory but subthreshold, the neuron becomes more excitable and easier to push over the threshold. This state is called facilitation.

Define resting potential.

Because the movement of sodium ions into the cell is slower than the movement of potassium out of the cell, there are more positive ions (cations) outside and more negative ions (anions) inside. The difference in the charges between the two sides of the cell's membrane is about -70 millivolts (in relation to the interior charge). This difference in charge is defined as the resting potential because it shows a potential to do the work (send a message).

What do Schwann cells and oligodendrocytes have in common; and how do they differ?

Both cell types are neuroglial cells. Schwann cells are found in the PNS, oligodendrocytes are part of the CNS. Schwann cells encase large axons in lipid-rich layers composed of myelin. Oligodendrocytes produce myelin. In the brain and spinal cord, unmyelinated axons lack neurilemmae.

Describe the events of brain development.

During embryonic development, the brain begins as a neural tube that gives rise to the CNS. At one end there are three major cavities or vesicles: the forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon). The forebrain divides into the anterior (telencephalon) and posterior (diencephalon) portions. The hindbrain partially divides into the metencephalon and myelencephalon. These five cavities in the mature brain become the ventricles and the tubes that connect them. The tissue of the telencephalon becomes the cerebrum and basal ganglia while the diencephalon remains unchanged. The midbrain continues to mature and is still called the midbrain in the adult structure. The hindbrain matures into the cerebella, pons, and medulla oblongata. The brain stem is comprised of the midbrain, pons, and medulla oblongata and connects the brain to the spinal cord.

Describe the parts and their functions of a spinal nerve.

Each spinal nerve emerges from the spinal cord by two short branches that lie within the vertebral column. The dorsal root is also called the posterior or sensory root. It can be identified by the dorsal root ganglion. This root conducts sensory impulses inward from the peripheral body parts. The ventral root is also called the anterior or motor root. It consists of axons from the motor neurons. The roots unite to form a spinal nerve, which extends outward from the vertebral canal through an intervertebral foramen. Each spinal nerve splits into three parts called the meningeal, posterior, and anterior branches. Spinal nerves in the thoracic and lumbar regions have a fourth or visceral branch, which supplies the autonomic nerve fibers.

Explain how enzymes within synaptic clefts and reuptake of neurotransmitter prevents continuous stimulation of the postsynaptic cell.

Enzymes in synaptic clefts and on postsynaptic membranes rapidly decompose some neurotransmitters. These enzymes include acetylcholinesterase, which decomposes acetylcholine, and monoamine oxidase, which inactivate epinephrine and norepinephrine after reuptake.

Explain how an injured neuron may regenerate.

If the axon of a peripheral nerve is separated from the cell body, the distal portion deteriorates and the fragments are removed by macrophages. The proximal end then develops new sprouts, and nerve growth factors from surrounding neuroglia cause the sprouts to grow. At the same time, remaining Schwann cells proliferate and surround the new axon. If a sprout grows into the remaining basement membranes of the original tract, the new fiber may rejoin with its original connection. If the injured axon is from a neuron in the CNS, the lack of a myelin sheath prevents the new fiber from being guided to its original connection. Therefore, regeneration in the CNS is very unlikely. An injury to the cell body of a neuron usually causes death to the entire fiber, and no regeneration will occur.

Distinguish between normal and paradoxical sleep.

Normal sleep (slow wave or non-REM) occurs when a person is very tired and is caused by decreased activity of the reticular formation. It is restful, dreamless, and accompanied by reduced blood pressure and respiratory rate. Paradoxical sleep (REM sleep) is so named because some areas of the brain are active. It is identified by dreaming, rapid eye movement beneath the eyelids, and irregular respiratory and heart rates.

Define paravertebral ganglion.

Paravertebral ganglia are two groups of ganglia whose preganglionic fibers split from the spinal nerves of the thoracolumbar division at branches called white rami. They are located as chains along the sides of the vertebral column and comprise part of the sympathetic trunks.

Describe sleep problems that may accompany aging.

Problems may include transient difficulty in getting to sleep and staying asleep, more frequent movements while sleeping, insomnia, diminished REM sleep, and daytime sleepiness.

Distinguish a sensory receptor from an effector.

Sensory neurons carry nerve impulses from peripheral body parts into the brain or spinal cord. Interneurons lie entirely within the brain or spinal cord. Motor neurons carry nerve impulses out of the brain or spinal cord to effectors outside the nervous system.

Distinguish between short-term and long-term memory.

Short-term memories are thought to be electrical in nature such that the neurons are connected in a circuit so that the last in the series stimulates the first. As long as the stimulation continues, the thought is remembered. When it ceases, so does the memory, unless it enters long-term memory. Long-term memories appear to change the structure or function of certain neurons that enhance synaptic transmission. The synaptic patterns must meet two requirements of long-term memory. First, there must be enough synapses to encode an almost infinite number of memories. Second, the pattern of synapses can remain unchanged for years.

What carries sensory impulses to the CNS from receptors in muscle or skin?

general somatic afferent fibers

Explain how Schwann cells encase large axons including the formation of myelin, the neurilemma, and the nodes of Ranvier.

Surrounding larger axons and dendrites of peripheral nerves are sheaths of neuroglial cells called Schwann cells. These cells are wound tightly around the fibers and, as a result, the cell membranes are layered closely together with little or no cytoplasm between them. The layers are composed of a lipoprotein called myelin, which forms a myelin sheath on the outside of the fibers. The outermost Schwann cells contain most of the cytoplasm and their nuclei remain outside the myelin sheath. This layer is known as neurolemma or neurolemmal sheath.

Define sympathetic tone.

Sympathetic tone is a maintained state of partial contraction of muscles stimulated by only the sympathetic division.

Describe the neuroglia of the CNS.

The CNS has four different types of neuroglial cells. They are: Astrocyte—Astrocytes are star-shaped cells located between neurons and blood vessels. They provide structural support and transport substances between the neurons and blood vessels. Astrocytes are joined together by gap junctions, providing a channel for circulating calcium ions. Other duties include metabolism of substances (such as glucose), keeping the synaptic clefts free of excess ions and neurotransmitters, and scar tissue formation after injury. Oligodendrocytes—Oligodendrocytes function in myelin production. By extending numerous cellular processes, an oligodendroctye can provide myelin sheaths to several different axons. Because of this arrangement, no neurilemmal sheaths are formed. Microglia—Microglia are small cells found throughout the CNS. They provide support and phagocytize bacteria and debris. Ependyma-Ependyma are ciliated cuboidal or columnar cells found as the inner lining of the central canal and as a single-layered membrane covering the ventricles of the brain. They are joined together by gap and tight junctions, and provide a porous layer for substances to diffuse between the interstitial fluids of the brain and cerebrospinal fluid in the ventricles.

Describe the neuroglia of the PNS.

The PNS has only one type of neuroglial cell: the Schwann cell. They are cells with abundant, lipid-rich membranes that wrap tightly around the axons of peripheral neurons. They function to speed neurotransmission.

Explain the importance of the nodes of Ranvier and conduction in myelinated fibers as opposed to conduction in unmyelinated fibers.

The Schwann cells surrounding a myelinated nerve fiber serve as an insulator and prevent most ions from passing through. Between adjacent Schwann cells is a small gap called a node of Ranvier, where the nerve fiber is exposed. When a nerve impulse is conducted along a myelinated fiber, it "jumps" from node to node. This type of conduction is called saltatory conduction. An unmyelinated axon conducts an impulse over its entire surface, whereas, myelin prevents almost all flow of ions through the membrane that it encloses.

Explain the function of the association areas of the lobes of the cerebrum.

The association areas are found in the anterior frontal lobes, and in the lateral areas of the parietal, temporal, and occipital lobes. These function to analyze and interpret sensory experiences involving memory, reasoning, verbalizing, judgment, and emotions. The association areas of the frontal lobes deal with concentration, planning, problem solving, and judging the consequences of behavior. The areas of the parietal lobes deal with understanding speech and word choice for thought expression. The areas of the temporal lobes deal with complex sensory interpretation, such as reading, music, and memories of visual scenes. The areas of the occipital lobes deal with visual pattern analysis and combining these images with other sensory experiences.

Explain the general function of the brain, spinal cord, and brainstem, and their interrelationship.

The brain oversees many aspects of physiology, including sensation and perception, movement, and thinking. The brainstem connects the brain and spinal cord and allows two-way communication between them. The spinal cord provides two-way communication between the CNS and the PNS.

Define cauda equina.

The cauda equina is so named because in the adult, the spinal cord ends between the first and second lumbar vertebrae. Because of this the lumbar, sacral, and coccygeal nerves must descend down the spinal column to the exit points resembling a horse's tail.

Explain the difference between the central nervous system (CNS) and the peripheral nervous system PNS.

The central nervous system (CNS) is composed of the brain and the spinal cord. The peripheral nervous system (PNS) is composed of all of the peripheral nerves that connect all of the parts of the body with the CNS.

Define cerebral cortex.

The cerebral cortex is the outermost layer of the cerebrum and is a layer of gray matter that contains 75 percent of all neuron bodies in the nervous system.

Describe the structure of the cerebrum.

The cerebrum consists of two cerebral hemispheres separated by a layer of dura mater called the falx cerebri and connected deeply by a nerve fiber bundle called the corpus callosum. The hemispheres are marked by many convolutions separated by shallow grooves called sulci (sing. sulcus) and deep grooves called fissures. These grooves form distinct patterns. For instance, the longitudinal fissure separates left and right hemispheres, and the transverse fissure separates the cerebrum from the cerebellum. Various sulci divide each hemisphere into lobes named after the skull bones they underlie. They are: Frontal lobe—the frontal lobe forms the anterior portion of each cerebral hemisphere, and lies in front of the central sulcus (fissure of Rolando) and above the lateral sulcus (fissure of Sylvius). Parietal lobe—the parietal lobe lies behind the central sulcus and frontal lobe. Temporal lobe—the temporal lobe lies below the frontal and parietal lobes, separated by the lateral sulcus. Occipital lobe—the occipital lobe is the posterior portion of each hemisphere separated from the cerebellum by the tentorium cerebelli. There is no clear boundary between the temporal, parietal, and occipital lobes. Insula—the insula (island of Reil) is found deep in the lateral sulcus and is separated from the frontal, parietal, and temporal lobes by a circular sulcus.

Name the layers of the meninges, and explain their functions.

The layers of the meninges surround the brain and spinal cord. They are, from the outermost to the innermost layers: Dura mater—the dura mater is a tough, fibrous connective tissue layer containing many blood vessels and nerves. It functions as a protective layer, surrounding the brain and spinal cord. Arachnoid mater—the arachnoid mater is a thin web-like membrane that lacks blood vessels and nerves. It is attached to the pia mater by thin strands. Pia mater—the pia mater is a thin membrane containing many nerves and blood vessels that provide nourishment to the underlying brain cells and spinal cord. It is attached directly to the surface of the brain and spinal cord.

List the parts of the limbic system, and explain its functions.

The limbic system controls emotional experience and expression. It produces feelings of fear, anger, pleasure, and sorrow. It apparently recognizes upsets in a person's physical or psychological condition that could be life threatening. By relating pleasant or unpleasant feelings about experiences, it guides behaviors that may increase the chance of survival. It also interprets sensory impulses from the olfactory receptors.

Name the functions of the midbrain, pons, and medulla oblongata.

The midbrain joins the lower parts of the brain stem and spinal cord with the higher parts of the brain. It also contains certain reflex centers. Two bundles of nerve fibers called the cerebral peduncles lie on the underside of the midbrain and form the corticospinal tracts, which are the main motor pathways between the cerebrum and lower parts of the nervous system. Two pairs of rounded knobs called the corporal quadrigemina provide centers for certain visual reflexes and the auditory reflex centers. In the center of the midbrain is a mass of gray matter called the red nucleus, which provides posture-maintaining reflexes. The pons is a rounded bulge on the inferior side of the brain stem where it separates the midbrain from the medulla oblongata. The dorsal portion of the pons relays impulses between the medulla oblongata and the cerebrum. The ventral portion relays impulses from the cerebrum to the cerebellum. The pons also relays impulses from the peripheral nerves to higher brain centers. It also works with the medulla oblongata to regulate rate and depth of breathing. The medulla oblongata is an enlarged continuation of the spinal cord at its superior end. It extends from the foramen magnum to the pons. Because of its location, all ascending and descending nerve fibers connecting the brain and the spinal cord must pass through it. Some of the nuclei in the gray matter relay ascending impulses to the other side of the brain stem and higher brain centers. Other nuclei control vital visceral activities and are called the cardiac center, the vasomotor center, and the respiratory center.

List three general functions of the nervous system.

The nervous system functions in three ways: Sensory—the sensory function is accomplished by means of sensory receptors that note changes in their environment. Integrative—the CNS can take the impulses from all of the sensory receptors and combine them to make perceptions and sensations about the environment. Motor—the CNS can send impulses along some peripheral nerves to effectors in the muscles and glands in response to changes in the internal and external environment.

Trace a sympathetic nerve pathway through a ganglion to an effector.

The pathway begins with the neuron in the lateral horn of the spinal cord. Its preganglionic fiber exits through the ventral roots of spinal nerves. It branches off in segments called white rami and enters the paravertebral ganglia (forming sympathetic trunks). Some fibers synapse with these ganglia while others pass through to other paravertebral ganglia or on to, or beyond, the collateral ganglia. The postganglionic fibers extend out to the visceral effectors. The fibers leaving the paravertebral ganglia usually pass through gray rami and return to a spinal nerve before synapsing with an effector.

Distinguish between a preganglionic fiber and a postganglionic fiber.

The preganglionic fiber is the axon of the first neuron in the two neuron autonomic system. Its cell body is located in the CNS and forms a synapse with one or more nerve fibers whose cell bodies are housed within an autonomic ganglion. The axon of the second neuron is called the postganglionic fiber, because it extends from the ganglia to a visceral effector

Describe the location and function of the motor areas of the cortex.

The primary motor areas of the cerebral cortex lie in the frontal lobes along the anterior wall of the central gyrus. Large pyramidal cells are responsible for nerve impulses sent through the corticospinal tracts to voluntary muscles. Impulses from the upper parts of the motor areas control muscles in the legs and thighs; the middle portion control muscles in the shoulders and arms; and the lower portions control the muscles of the head, face, and tongue.

Describe the location and function of the reticular formation.

The reticular formation is scattered throughout the medulla oblongata, pons, and midbrain as a complex network of nerve fibers associating with small islands of gray matter. It extends from the superior portion of the spinal cord through to the diencephalon and connects the hypothalamus, basal ganglia, cerebellum, and cerebrum with fibers in all the major ascending and descending tracts. Because the cerebral cortex is totally dependent on sensory impulses for its awareness of the external environment, the reticular formation is responsible for activating it into a state of wakefulness. Decreased activity in the reticular formation causes sleep. The reticular formation also filters incoming sensory impulses to prevent the cortex from being constantly bombarded by sensory stimulation, and allows it to concentrate on the significant information. The cerebra cortex can also activate the reticular formation during intense cerebral activities, keeping a person awake.

Describe the location and function of the sensory areas of the cortex.

The sensory areas for temperature, touch, pressure, and pain in the skin are found in the anterior portion of the parietal lobes along the central sulcus. Vision sensory areas are found in the posterior portion of the occipital lobes. The sensory areas for hearing are found in the dorsal posterior portion of the temporal lobes. The sensory areas for taste are found near the base of the central sulci along the lateral sulci and the sense of smell arises from deep in the cerebrum.

Distinguish between the somatic and autonomic nervous systems.

The somatic nervous system is a division of the peripheral nervous system (PNS) and consists of cranial and spinal nerves that oversee conscious activities. The autonomic nervous system is the other division of the PNS and includes the fibers that connect the central nervous system (CNS) to the viscera. It controls unconscious activities.

Distinguish between the types of activities that the somatic and autonomic nervous systems control.

The somatic nervous system oversees conscious (voluntary) activities. The autonomic nervous system controls viscera and the subconscious (involuntary) actions.

Describe the structure of the spinal cord.

The spinal cord is a long slender column of nerve fibers that begins at the foramen magnum of the skull and extends downward to a point near the first and second lumbar vertebrae. The cord is actually a group of thirty-one segments that give rise to pairs of spinal nerves. These nerves connect all of the body to the CNS. A thickening in the neck region, called the cervical enlargement, supplies the nerves to the arms and a similar thickening, the lumbar enlargement, supplies the nerves to the legs. Inferior to the lumbar enlargement, the spinal cord tapers into a structure (conus medullaris) that is connected to the coccyx by a thin cord of connective tissue (filum terminale). Along the length of the cord are two grooves, the anterior median fissure and posterior median sulcus, which divide the cord into left and right halves. A cross section of the cord shows a gray matter core surrounded by white matter. The gray matter resembles a butterfly. The upper wings are called the posterior horns and the lower wings are called the anterior horns. Between these horns is a small protuberance called the lateral horn. A horizontal bar of gray matter surrounds the central canal and connects the wings on both sides. The white matter is divided on each side into three regions, the anterior, lateral, and posterior funiculi.

Contrast the sympathetic and parasympathetic divisions of the autonomic nervous system.

The sympathetic division of the autonomic nervous system is concerned primarily with preparing the body for energy-expending, stressful, or emergency situations. The parasympathetic division is active under ordinary, restful conditions. It counterbalances the effects of the sympathetic division and restores the body to a resting state following a stressful experience.

Explain the two main functions of the spinal cord.

The two main functions of the spinal cord include serving as a center for spinal reflexes and serving as a conduit for nerve impulses to and from the brain.

Trace a parasympathetic nerve pathway.

These fibers arise from neurons in the midbrain, pons, and medulla oblongata of the brainstem and from part of the sacral region of the spinal cord. From there, they lead outward on cranial or sacral nerves to ganglia near or in various organs. Some continue on to specific muscles or glands in these organs.

Explain how the spinal nerves are grouped and numbered.

They are grouped according to the level from which they arise, and each nerve is numbered in sequence. There are eight pairs of cervical spinal nerves, twelve pairs of thoracic spinal nerves, five pairs of sacral spinal nerves, and one pair of coccygeal nerves.

Describe how the nervous system detects change associated with the body and reacts to that change to maintain homeostasis.

Through a vast communicating network of cells and the biochemicals that they send and receive, the nervous system can detect changes in the body, make decisions based on the basis of the information received, and stimulate muscles or glands to respond. These responses counteract the effects of the changes, thus helping to maintain homeostasis.

Describe the relationship among the cerebrospinal fluid, the ventricles, the choroid plexuses, and arachnoid granulations.

Tiny masses of specialized capillaries, called choroid plexuses, secrete CSF. These structures project into the cavities of the ventricles. CSF is continuously reabsorbed into the blood through tiny, fingerlike structures called arachnoid granulations that project from the subarachnoid space into the blood-filled dural sinuses.

Explain the conversion of short-term to long-term memory.

Understanding how neurons in different parts of the brain encode memories and how short-term memories are converted to long-term memories is at the forefront of research into the functioning of the human brain. This process is referred to as memory consolidation. Some theorists believe this conversion is explained by near simultaneous repeated stimulation of the same neurons.

Describe a withdrawal reflex

When a person touches something painful, receptors in the skin send impulses to interneurons in a reflex center in the spinal cord. The reflex center sends impulses to the flexor muscles of the affected part causing the part to be moved away. At the same time this is happening, impulses to the extensor muscles of the affected part are inhibited, so that the flexors can work more effectively. A phenomenon, called a crossed extensor reflex, occurs simultaneously with the initial reflex that causes the extensors of the opposite limb to contract.

List in correct order the changes that occur during an action potential.

When enough stimuli have accumulated to cause the threshold potential to be released, the area stimulated opens its sodium channels. As the sodium ions rush in, the inside of the cell becomes momentarily positive. At the same time, potassium channels open to allow the potassium ions out. This causes the inside of the cell to return to a negative charge (repolarization). The entire sequence takes less than 1/1,000th of a second.

Distinguish between convergence and divergence.

When input nerve fibers from different areas of the body send impulses to the same neuron, they are said to converge. When a neuron sends impulses out on an output (efferent) nerve fiber, the impulse may stimulate others. These, in turn, may stimulate still others, and so on. Divergence works with input nerve fibers as well. For instance, the impulse from a sensory receptor may diverge and reach several different areas of the CNS for processing.

Distinguish between cholinergic and adrenergic nerve fibers.

With a few exceptions, the preganglionic fibers of both the sympathetic and parasympathetic divisions, and the postganglionic fibers of the parasympathetic division, secrete acetylcholine, and are thus called cholinergic fibers. The postganglionic sympathetic fibers secrete norepinephrine (noradrenalin), and are thus called adrenergic fibers.

Explain what determines the output of a neuronal pool in terms of input neurons, excitation, and inhibition.

Within the CNS, neurons are grouped together into specialized regions called neuronal pools. Incoming impulses are processed according to the special characteristics of the pool and any resulting impulses are carried away on output fibers. Each input fiber divides many times as it enters, and the branches spread over a certain region of the pool. These branches form hundreds of terminal synapses with the dendrites of the neurons in the pool.

The autonomic portion of the PNS functions _________.

are varied. The general functions of the autonomic system are to bring in sensory signals from the visceral organs and skin, interpret them, and send out the appropriate motor response. The autonomic nervous system includes two interacting divisions, the sympathetic and parasympathetic divisions.

Indicate whether each nerve tract is ascending or descending: Fasciculus gracilis

ascending

Indicate whether each nerve tract is ascending or descending: Spinocerebellar

ascending

Indicate whether each nerve tract is ascending or descending: Spinothalamic

ascending

Diffusion of which ions into the synaptic knob triggers the release of neurotransmitters?

calcium ions

Indicate whether each nerve tract is ascending or descending: Corticospinal

descending

Indicate whether each nerve tract is ascending or descending: Reticulospinal

descending

Indicate whether each nerve tract is ascending or descending: Rubrospinal

descending

lists the parts of the brainstem

midbrain, pons, and medulla oblongata

Broca's area controls _________.

movements used in speaking

Vestibulocochlear nerves

sensory nerves that arise from the medulla oblongata. There are two distinct parts: Vestibular branch—located in the ganglia associated with the parts of the inner ear and serve to help to maintain equilibrium. Cochlear branch—located in parts of the inner ear that house the hearing receptors. Impulses from this branch pass through the pons and medulla oblongata on their way to the temporal lobes for interpretation.

What is the sequence of events in a reflex arc?

sensory receptor to CNS to interneurons to motor neurons to effectors.

The ________ transmits sensory information from other parts of the nervous system to the cerebral cortex.

thalamus

Trigeminal nerves

these are the largest and arise from the pons. These are mixed nerves that have three major branches: Opthalmic division—brings sensory impulses to the brain from the surface of the eyes, the tear glands, and the skin of the anterior scalp, forehead, and upper eyelids. Maxillary division—carries sensory impulses from the upper teeth, upper gum, and upper lip, as well as from the mucous lining of the palate and the skin of the face. Mandibular division—transmits impulses from the scalp behind the ears, the skin of the jaw, the lower teeth, the lower gum, and the lower lip. It has motor branches that supply the muscles of mastication, and certain muscles in the floor of the mouth.

Facial nerves

these arise from the lower part of the pons and emerge on the sides of the face. The sensory branches are associated with taste receptors on the tongue. The motor fibers transmit impulses to the muscles of facial expression while others function in the autonomic nervous system and stimulate secretions from the tear glands and salivary glands.

Glossopharyngeal nerves

these arise from the medulla oblongata and are associated with the tongue and pharynx. These are mixed nerves but are predominantly sensory. They carry impulses from the linings of the pharynx, tonsils and posterior third of the tongue to the brain. The motor portion innervates muscles of the pharynx that function in swallowing.

Hypoglossal nerves

these arise from the medulla oblongata and pass into the tongue. These work on tongue muscles that function in speaking, chewing, and swallowing.

Trochlear nerves

these arise from the midbrain and carry motor impulses to certain voluntary muscles that move the eyes but are not supplied by the oculomotor nerves.

Oculomotor nerves

these arise from the midbrain and pass into the orbits of the eyes. These function to raise the eyelid, innervate muscles that move the eye, and allow the eye to adjust the amount of light entering the eyes and allow the lens to focus.

Optic nerves

these lead from the eyes to the brain and are associated with the sense of sight.

Abducens nerve

these originate from the pons and enter the orbits of the eyes and supply motor impulses to a pair of muscles that move the eyes.

Vagus nerves

these originate in the medulla oblongata and extend downward into the chest and abdomen. These are mixed nerves containing both autonomic and somatic branches. The autonomic are the predominate ones, associated with speech, swallowing, and motor activity of the smooth muscles and glands in the thorax and abdomen.

Accessory nerves

these originate in the medulla oblongata and the spinal cord. The cranial branch joins a vagus nerve and carries impulses to muscles of the soft palate, pharynx and larynx.

Olfactory nerves

this pair serves as olfactory receptor nerve fibers which pass through the cribiform places as olfactory tracts to cerebral centers for interpretation as sensations of smell.