Chapter 12. Central Nervous System

Arachnoid Mater

"Spidery mother' - The middle, web-like meninx. Subarachnoid space - a fluid-filled cavity beneath the arachnoid mater. It's filled with cerebrospinal fluid and it contains the large blood vessels that supply the brain. Arachnoid Villi - projections of the arachnoid mater that protrude upward, through the dura mater, into the superior sagittal sinus, (the space at the top of the head between the cerebral hemispheres). Cerebrospinal fluid passes from the subarachnoid space through the arachnoid villi and into the venous blood of the superior sagittal sinus.

Anatomy of the Cerebellum

- Bialteral symmetry - The worm-like vermis connects the right and left cerebellar hemispheres. - Folia - the transverse pleat-like gyri on the surface of the cerebellum - Thin cerebellar cortex of gray matter, with internal white matter, and small, deep, paired masses of gray matter called the denticulate nuclei. -The cerebellar cortex consists of three layers: the molecular layer, the granule cell layer, and the Purkinje cell layer in the middle - Purkinje Cells - Large neurons with extremely branched dendrites that are located in the cerebellar cortex. These are the only cortical neurons that send axons through the white matter to synapse with the central nuclei of the cerebellum. - Arbor Vitae - "tree of life" the distinctive pattern of white matter in the cerebellum the resembles a branching tree - Cerebellar Penduncles - three paired fiber tracts that connect the cerebellum to the brainstem. Links to cerebrum through medulla, pons, and midbrain.

Three types of functional areas of the cerebral cortex

- Motor areas: control voluntary movement - Sensory areas: conscious awareness of sensation - Association areas: deal with complex, integrative functions such as memory, emotions, reasoning, will, judgment, personality traits, and intelligence. b. Each hemisphere concerned with contralateral side of body c. The conscious mind is in this area. It enables us to be aware of ourselves and our sensations, to communicate, remember, understand, and initiate voluntary movements.

Cerebrospinal Fluid

1) Acts as a shock-absorber and protects the brain and spinal cord from jolts. 2) Maintains the appropriate chemical environment for accurate neuron functioning 3) Is a medium for exchange of nutrients and waste products between blood and nervous tissue Cerebrospinal Fluid is produced by ependymal cells that cover choroid plexuses in the ventricles of the brain

Examples of Tracts

1. Ascending a. Spinothalamic Tracts - the axons run up the spinal cord to the thalamus b. Spinocerebellar Tracts - the axons run from the spinal cord up to the cerebellum 2. Descending a. Corticospinal Tracts - run from the cerebral cortex down to the anterior gray horns of the spinal cord. b. Rubrospinal Tract 0 runs from the red nucleus of the mid brain down to the anterior horns of the spinal cord.

Spinal Cord Functions

1. Conduction - Provides two-way communication to and from the brain. 2. Locomotion - contain groups of neurons (central pattern generators) that produce the sequence of flexions and extensions involved with movements that allow the body to move from place to place 3. Reflexes - involuntary stereotyped responses to stimuli. Involves the brain, spinal cord, and peripheral nerves. - The spinal cord extends from the foramen magnum, through the vertebral foramen, down the vertebral column. - The spinal cord is protected by bone, cerebrospinal fluid, and meninges. - Denticulate ligaments - extensions of the pia mater that attach the spinal cord to the inside of the vertebrae.

Parts of the Spinal Cord

1. Enlargements Two Enlargements of the Spinal Cord a. Cervical Enlargement - in the lower neck b. Lumbar Enlargement - in the lower thoracic to upper lumbar region. 2. Conus Medullaris - the tapered, inferior end of the spinal cord 3. Fillum Terminale - a fibrous extension of the pia mater that extends downward from the conus medullaris and attaches to the posterior surface of the coccyx. The fium terminale anchors the spinal cord. 4. Cauda Equina - a collection of nerve roots at the inferior end of the spinal cord. Resembles a horses tail.

Two types of Descending (Motor) Tracts

1. Pyramidal or Direct Tracts - the axons descend from the pyramidal cells of the primary motor cortex of the cerebrum, all the way to the spinal cord without synapsing Ex: Lateral and Anterior Corticospinal Tracts. - The lateral corticospinal tracts decussate, or cross over each other, in the medulla oblongata - The anterior corticospinal tracts decussate in the spinal cord, just before synapsing. 2. Extrapyramidal or Indirect Tracts - The neurons originate in different motor nuclei of the brain stem. Ex: Rubrospinal Tract - red nucleus to spinal cord.

The 3 primary brain vesicles form 5 secondary brain vesicles:

1. Telencephalon - becomes the cerebrum: cortex, white matter, and basal nuclei 2. Diencephalon - becomes the thalamus, hypothalamus, and epithalamus 3. Metencephalon - becomes the brain stem: pons and cerebellum 4. Metencephalon - becomes the brain stem: pons and cerebrum 5. Myelencephalon - becomes the brain stem: medulla oblongata

Spinal Nerves

31 pairs of spinal nerves arise from the spinal cord. The spinal nerves serve as pathways of communication between the spinal cord and specific regions of the body.

Corpora Quadrigemina

4 rounded elevations, the superior and inferior colliculi.

Reticular Formation

A broad region, where white and gray matter exhibit a net-like arrangement. (A system of small nuclei scatters through the core of the brain stem. - The Reticular Formation extends from the lower part of the diencephalon throughout the brainstem and into the upper part of the spinal cord - The Reticular Formation helps maintain consciousness, causes awakening from sleep, filters repetitive sensory input, and contributes to regulation of muscle tone.

Cerebral Peduncles

A pair of tracts that contain axons of motor and sensory neurons. - Each peduncle contains a large pyramidal (corticospinal) motor tract descending towards the spinal cord.

Infundibulum

A stalk-like structure that connects the pituitary gland to the hypothalamus.

Cerebrospinal Fluid (CSF)

A watery fluid, located in and around the brain and spinal cord, that acts as a liquid cushion for the brain and helps nourish the brain.

Gray Comissure of Spinal Cord

Bridge of gray matter the connects the two lateral masses of gray matter in the spinal cord.

Ventricles

Cavities in the brain that are filled with CSF and are lined by ependymal cells. The ventricles are continuous with each other and with the central canal of the spinal cord. Ventricles of the brain: a. 2 Lateral Ventricles - Large C-shaped cavities. One is located in each cerebral hemisphere. The Interventricular Foramen - connects the lateral ventricles to the third ventricle. b. Third Ventricle - a narrow ventricle in the diencephalon The Cerebral Aqueduct - connects the third and fourth ventricle c. Fourth Ventricle - is inferior to the 3rd ventricle and is continuous with the central canal of the spinal cord. - Tissue fluid is filtered continuously from the capillaries of the choroid plexuses. - The tissue fluid filtrate passes through and is processed by the later of ependymal cells surrounding each choroid plexus. - The filtrate (+ water) then enters the ventricles of the brain as cerebrospinal fluid. CSF circulates in the ventricles of the brain, the central canal of the spinal cord, and the subarachnoid space around the brain and spinal cord.

4 Major Parts of the Brain

Cerebrum, Diencephalon, Brainstem, and Cerebellum

White Matter of the Spinal Cord

Comprised of both myelinated and unmyelinated nerve fibers or axons that allow communication between different parts of the spinal cord and between the cord and the brain. These fibers run in three directions. 1. Ascending - run from lower parts of the spinal cord to upper parts of the spinal cord or to the brain. Sensory nerve fibers are ascending. 2. Descending - run from the brain and upper spinal cord to lower parts of the spinal cord. Motor neurons are descending. 3. Transversely. Run from one side of the spinal cord to the other. Interneurons. The ascending and descending tracts make up most of the white matter of the spinal cord. The white matter on each side of the spinal cord is divided into three white columns or funiculi; the dorsal, lateral, and ventral columns. Anterior Median Fissure - deep groove along the anterior of the spinal cord that separates anterior funicluli (tracts) Posterior Median Sulcus - Shallower groove along the posterior of the spinal cord that divides posterior funiculi *tracts) - Each white column contains several nerve fiber tracts. - Each tract is made up of axons with similar destinations and functions - These tracts cross each other or decussate just after they enter and exit the brain, so the left side of the brain controls the right side of the body and vice versa.

Gray Matter

Consists of cell bodies, dendrites, and glia.

Superior Colliculi

Contain reflex centers the coordinate head and eye movements when we visually follow a moving object if not consciously looking at it

Dura Mater

Dura Mater - 'Tough mother" the outer, strongest layer of the meninges. The Dura mater consists of 2 layers of fibrous connective tissue (1 around the spinal cord) - The outer layer is attached to the inner surface of the skull - The 2 layers of the dura mater of the brain are fused together except where they separate to form the dural sinuses. -Dural Sinuses - collect venous blood and direct it into the internal jugular veins. -Dural Septa - Inward extentions of the dura mater that form partitions in the cranial cavity. These septa limit the movement of the brain within the cranium. -Subdural Space - A thin cavity between the dura mater and arachnoid mater. - Epidural space - the area between the dura mater and the vertebral wall. Filled with fat and a network of veins. - Anesthetic and steroid injections are injected into the epidural space of the spinal cord.

Diencephalon

Forms the central core of the forebrain and is surrounded by the cerebral hemispheres. Parts of diencephalon: thalamus, hypothalamus, epithalamus.

Epithalamus

Forms the roof of the third ventricle - Helps regulate the sleep-wake cycle - Contains the Pineal gland

Meningitis

Inflammation of the meninges caused by a bacterial or viral infection.

Association Areas

Integrate sensory experiences to generate meaningful patterns of recognition and awareness. - A person with damage to part of the primary visual field will be blind in part of his visual field. A person with damage to a visual association area may be able to see normally, but might not be able to recognize someone they know. - Association areas receive input from primary sensory areas and from other brain regions. i. Somatosensory Association Cortex - Parietal Lobe - Integrates and interprets sensory input from touch sensations (pressure, temperature, texture) and stores memories of past sensory experiences. - The Somatosensory Association Area enables you to recognize an object by feeling it. - You can recognize your car keys when you feel them in your pocket. If you have damage to this area, you will need to see the keys to recognize them. ii. Prefrontal Cortex - or Anterior Association Area - Anterior Frontal Lobe. Involved with personality, intellect, learning, recall, initiative, judgment, foresight, reasoning, conscience, intuition, mood, planning for the future, and abstract ideas - Wernike's Area - interprets the meaning of speech by recognizing spoken words, translates words to thoughts, and adds emotions to speech. iv. Limbic Association Area (cingulate gyrus, parahippocampal gyrus, and hippocampus) - Provides the emotional impact that makes a scene important to us. Ex: Sense danger v. Visual Association Area - Occipital Lobe - uses past visual experiences to interpret visual stimuli, enabling us to recognize images. vi. Auditory Association Area - Temporal Lobe - allows you to realize whether a sound is speech, music, or noise. Memories of past sounds are stored here.

Pia Mater

Intimate mother - the inner meninx. The pia mater directly lines the surface of the brain and spinal cord. It contains lots of blood vessels.

Insula (Central lobe)

Involved in gastrointestinal and other visceral activities

Temporal Lobe

Involved in hearing, equilibrium, emotion and memory.

Frontal Lobe

Involved with control of voluntary movements, emotional expressions and moral behavior

Parietal Lobe

Involved with general senses (touch, pressure...) and taste.

Basal Nuclei

Large cores of gray matter located within each cerebral hemisphere. i. The basal ganglia help coordinate muscle movements ii. Receive input from midbrain and motor areas of cerebral cortex and send signals back to both of these locations. iii. Include the caudate, putamen, and globus pallidus. iv. Collectively called the corpus striatum.

Cerebrum

Largest part of the brain. Controls consciousness 1. Cerebral Hemispheres - the right and left cerebral hemispheres (or sides of the cerebrum) are separated by a groove called the longitudinal cerebral fissure. Each cerebral hemisphere is composed of a cortex, white matter, and basal ganglia.

Embryonic Development of the Brain

Neural Tube - the brain and spinal cord begin as an embryonic structure. The anterior end of the neural tube expands and constricts to form three primary brain vesicles: 1. Prosencephalon - the forebrain 2. Mesencephalon - the midbrain 3. Rhombencephalon - the hindbrain

Thalamus

Paired oval masses of gray matter organized into nuclei. A bridge of gray matter called the intermediate mass connects the two halves of the thalamus. - The thalamus is the relay station for information coming into the cerebral cortex. The thalamus: - Relays and filters all ascending (sensory) information - Relays a small proportion to cerebral cortex (conscious perception) - Relays most information to basal nuclei and brain stem centers - Coordinates voluntary and involuntary motor behavior.

Gray Matter of the Spinal Cord

Projections of the gray matter masses. The anterior, posterior, and lateral Anterior Horns - mainly contain nerve cell bodies of somatic motor neurons. The axons of these neurons extend outwards through the anterior roots of the spinal nerves to skeletal muscles The lateral Horn Neurons - are autonomic motor neurons that send impulses to visceral organs. Their axons leave the spinal cord via the anterior roots. The Posterior Gray Horns - contain interneurons that are located between the motor and sensory neurons. Central Canal - a canal that runs the length of the spinal cord through the gray commissure. It contains cerebrospinal fluid.

Blood Brain Barrier

Protective mechanism that helps maintain a stable environment for the brain and restricts harmful substances from reaching the brain. - The brain is highly dependent on a constant internal environment (so neurons don't fire constantly) Blood borne substances are separated from neurons by: 1. Continuous endothelium of capillary walls 2. Relatively thick basal lamina 3. Bulbous feet of astrocytes The brain barrier is highly permeable to water, glucose, and lipid-soluble substances such as oxygen, CO2, alcohol, caffeine, nicotine, and anesthetics. Slightly permeable to Na, K, Cl, Urea, and Creatine.

Fissures

Really deep grooves of the cerebral cortex.

Hydrocephalus

The abnormal accumulation of CSF in the brain, usually resulting from a blockage in its route of flow and reabsorption. Good recovery can be achieved if a tube (shunt) is inserted to drain fluid from the ventricles into a vein of the neck.

Projection Pathways

The asecnding pathways in the spinal cord to the brain conduct sensory impulses upward through chains of three successive neurons to areas of the brain. 1. First Order Neurons - sensory neurons whose cell bodies are located in a ganglion (dorsal root or cranial). First order neurons conduct impulses from cutaneous receptors and proprioceptros to the spinal cord or brainstem where they connect with second order neurons 2. Second Order Neurons - Interneurons whose cell bodies are located in dorsal horns of the spinal cord or in nuclei of the medulla oblongata. These neurons transmit impulses to the thalamus or the cerebellum where they synapse 3. Third Order Neurons- Interneurons that have cell bodies in the thalamus. They relay impulses to the somatosensroy cortex of the cerebrum.

Brain

The control center of the central nervous system. - The brain controls our body, receives information, analyzes information, and stores information (memories). It allows us to think, move, feel, see, hear, taste and smell. - The brain produces impulses, which enable parts of the body to communicate. Nerves send these impulses throughout the body. - The brain is located in the cranial cavity of the skull and is surrounded by meninges. - The brain is protected by the skull, meninges, cerebrospinal fluid and the blood brain barrier

Corpus Callosum

The corpus callosum is a bridge of nerve fivers that relay impulses between the two cerebral hemispheres. The braod band of white matter internally connects the two hemispheres. White matter, in general consists of myelinated axons and neuroglia.

Medulla Oblongata - Myencephalon

The most inferior part of the brainstem. Relays motor and sensory impulses between parts of the brain and spinal cord. Contains vital centers: cardiovascular centers - rate & force of heartbeat. Vascomotor center - blood pressure (blood vessel diameter) - Respiratory centers - rate & depth of breathing - Regulates vomiting, sneezing, coughing, hiccupping, and swallowing - Pyramids - bulges on the anterior aspect (front) of the medulla oblongata. - Decussation of the pyramids (crossing the axons) occurs in the medulla oblongata. - This decussation is why each cerebral hemisphere controls the voluntary muscle movements on the opposite side of the body - Olives of the Medulla Oblongata - oval bulges containing neurons that relay impulses for relay sensory information on the degree of stretch in muscles and joints to the cerebellum.

Brainstem

The part of the brain between the spinal cord and the diencephalon - The brainstem relays messages between the spinal cord and brain - All of the cranial nerves except for the olfactory and optic nerves emerge from the brainstem. - The brainstem consists of the Midbrain, Pons, and Medulla Oblongata. Parts of the Brainstem: 1. Midbrain - Mesencephalon - relays motor impulses from the cerebral cortex to the pons. - Relays sensory impulses from the spinal cord to the thalamus. - Contains the occulomotor nerves, which are involved with focusing and constricting the pupils - Contains the cerebral peduncles and Corpora Quadrigemina the substantia Nigra

Cerebellum

The second largest part of the brain (-11%) - Located inferior to the occipital lobe of the cerebrum and dorsal to the pons and medulla oblongata Function of the cerebellum: The cerebellum refines and coordinates muscular movements. - Is involved in producing smooth, coordinated muscle activity. - Regulates posture and balance - Cerebellar control occurs subconsciously - May also play a role in thinking, language, and emotion - The cerebellum is important in motor coordination. It compares the intent (from the cerebrum) with the performance based on the positions of various parts of the body (from propioreceptors) - Lesions of the cerebellum can result in problems walking (ataxia) and make some tasks like climbing stairs impossible.

Meninges

Three connective tissue membranes that surround the brain and spinal cord - Cover and protect the CNS, protect blood vessels and enclose venous sinuses, contain cerebrospinal fluid and form partitions within the skull.

Roots

Two bundles of axon that connect each spinal nerve to the spinal cord. a. Posterior (Dorsal) Root - contains only sensory axons, which conduct impulses from sensory receptors in the skin, muscles, and internal organs, to the spinal cord. Dorsal Root Ganglion - a swelling on the posterior root that contains the cell bodies of sensory neurson. b. Anterior (Ventral) Root - contains axons of motor neurons which conduct impulses from the CNS to effector organs

Red nuclei

Two red nuclei that are deep to the substantial nigra. Are red due to rich blood supply and iron pigment in neurons. Some descending motor pathways that affect limb flexion are relayed by the red nuclei.

Cerebrospinal fluid consists of:

Water, glucose, proteins, lactic acid, urea, Na+, K+, Ca2+, Mg2+, Cl-, and HCO3, and some white blood cells.

The white matter of the cerebrum consists of 3 types of fibers or axons

i. Association fibers/tracts - link one area of the cortex to another area of the cortex within the same hemisphere ii. Commissural fibers/tracts - link one area of a cerebral hemisphere to the same area of the other hemisphere iii. Projection fibers - link the cerebral cortex to other structures of the brain.

Motor Areas

i. Primary Motor Cortex - Precentral Gyrus of Frontal Lobe - sends impulses to specific voluntary muscles or muscle groups. Is composed of large neurons called pyramidal cells ii. Premotor Cortex - Posterior Frontal Lobe - Located anterior to the precentral gyrus - Controls learned, repetitious or patterned motor skills - Coordinates simultaneous or sequential actions - Involved in the planning of movements Ex: The premotor area would control my typing these notes wheres the Primary motor area might control swatting at a fruit fly with my hand. iii. Broca's Area - Frontal Lobe - A motor speech area that directs the muscles involved in speech production - Is present in only one hemisphere iv. Frontal Eye Field Area - Frontal Lobe - Controls voluntary movements of the eyes (like when you're reading these notes

Sensory Areas of Cerebral Cortex

i. Primary Somatosensory Area - Post Central Gyrus of Parietal Lobe - receives information from the somatic sensory receptors in the skin and from proprioceptors in the skeletal muscles, joints, and tendons. - Receives impulses for touch, proprioception (general sense of where your body parts are in relation to one another), pain, itch, tickle, and temperature. ii. Primary Visual Cortex - Posterior of Occipital Lobe - receives impulses for vision (Eye --> Thalamus ---> Primary Visual Area) iii. Primary Auditory Cortex - Temporal Lobe - Receives information from receptors of the inner ear for hearing. iv. Vestibular Corex - Insula and Parietal Lobe - conscious awareness of balance. v. Primary Gustatory Cortex - Insula - perceiving taste stimuli. vi. Primary Olfactory Cortex - Temporal Lobe - receives impulses for smell from the olfactory tracts. vii. Visceral Sensory Area - Insula - upset stomach, full bladder, holding breath.

Choroid Plexuses

Clusters of capillaries that hang from the roof of each ventricle and produce CSF. - Each Choroid plexus is surrounded by a single layer of ependymal cells. - Choroid plexuses also remove wastes from cerebrospinal fluid

Gyri

Convolutions (lumps) on the surface of the cerebral cortex.

Cerebral Lateralization

Difference in function of the 2 hemispheres. Left - categorical hemisphere - specialized for spoken and written language and for the sequential and analytical reasoning employed in such fields as science and mathematics. Right - representational hemisphere - perceives information in a more integrated, holistic way. Major location of imagination and insight, musical and artistic skill, perception of patterns and spatial relationships, and comparison of sights, sounds, smells, and tastes.

Lobes of the Cerebrum

Frontal, Parietal, Temporal, Occipital, Insula (Central lobe)

Sulci

Grooves of the cerebral cortex, between the gyri.

Occipital lobe

Involved in vision.

Hypothalamus

Located inferior to the thalamus and is composed of a dozen or so nuclei. The hypothalamus is the main visceral control center of the body. It is vitally important to overall homeostasis. - Controls the Endocrine System - Secretes hormones that regular the pituitary gland - Is the chief integration center of the autonomic (involuntary) nervous system (bp, rate and force of heart beat, digestive tract motility, etc.) - Regulates body temperature - Regulates food intake - Regulates water balance and thirst - Regulates sleep-wake cycles - Initiates responses to emotions - Is involved with memory - via mammillary body

Pons

Mostly conduction tracts - Relays impulses between the higher brain centers and the spinal cord - Relays information between the motor cortex and the cerebellum - The trigeminal, abducens, and facial nerves arise from the pons - The pons and medulla oblongata help control breathing

Cerebral Cortex

Outer layer of gray matter

Inferior Colliculi

Relay information from hearing receptors of the ear to the sensory cortex - Also involved in auditory reflexes - Ex: the startle reflex

Effects of Cerebral Lesions

Reveal some functions of association areas: - Parietal lobe lesions - can cause people to become unaware of objects or event their own limbs on the other side of the body. Contralateral Neglect Syndrome. The sufferers commonly ignore the side of the body affected. - Temporal Lobe Lesions - Often result in agnosia, the inability to recognize, identify, and name familiar objects. In prosopagnosia, a person cannot remember familiar faces, even their own in a mirror. - Frontal Lobe Lesions - Are devastating to personality. May lead to profound personality disorders and socially inappropriate behaviors.

Pineal Gland

Secretes the hormone melatonin, which contributes to the setting of the body's biological clock.

Substantia Nigra

Two pigmented nuclei that contain a lot of melanin. Melanin is the precursor for dopamine, which is released by neurons of the substantia nigra. Parkinson's Disease is ultimately caused by the degeneration of the dopamine-releasing neurons of the substantia nigra.