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Central Nervous System

Richard L. Trader MMsc PA-C FCCM

Anatomy and Physiology CNS

Institute of Health SciencesTimonium, Maryland 21093

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The Nervous System

• The nervous system is divided into two

parts, the central nervous system and the

peripheral nervous system. The central

nervous system consists of the brain and the

spinal cord. The peripheral nervous system

is made up of 12 pairs of cranial nerves and

all the remaining nerves of the body

(including the autonomic system).

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Skull

• The skull is formed by the bones of calvaria and

the skull base. The calvaria is comparatively large

to accommodate the brain. In the newborn, it is

about 25% of its adult size. It reaches about 75 %

of its adult size by the end of the first year. The

newborn skull is made up of thin, pliable bones

due to incomplete ossification (the process of

hardening of the bone). They are separated by

fibrous or membranous tissue (the sutures).

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• The membranous gaps are called fontanelles and are larger at the corners of the parietal bones. The centerally located anterior frontanelle is the largest and diamond-shaped. It does not fully ossify until 18 to 24 months of age while the other frontanelles usually close by 2 to 3 months of age. The sutures where most of the bone growth occurs also do not completely obliterate until approximately 8 years of age. The skull base consists of maxilla, frontal, ethimoid, sphenoid, temporal, parietal, and occipital bones.

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• The skull contains various foramina (canals

through the bone) which permit the major

arteries, veins and cranial nerves to pass

through. The largest is the foramen magnum

through which the brainstem continues

down to the spinal cord.

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Spinal Column

• The bony spinal column is built from alternating

bony vertebrae (the individual spinal segments)

and fibrocartilaginous discs (spongy tissue

between each vertebra) which are intimately

connected by strong ligaments and supported by

powerful surrounding muscles. There are 33

vertebrae consisting of 7 cervical (neck), 12

thoracic (chest), 5 lumbar (small of back), 5

sacral (hip)and 4 coccygeal (tail of spine).

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• A typical vertebra consists of multiple

pieces of bone, a cylindrical body (base of

the canal), and a posterior arch composed

of a pair of pedicles (side walls of the

canal) and laminae (roof of the canal)

which fuse to form a spinous process (a

posterior projection which can be felt as a

bump through the skin).

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• The body and the arch create a vertebral

canal through which the spinal cord courses

down to the level of the second lumbar

vertebra. The intervertebral disks between

the vertebral bodies act as elastic buffers to

absorb mechanical shocks.

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Brain

• The brain is divided into five parts on an

embryological basis.

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1. Telencephalon

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• The telencephalon is the area of brain most

developed in the human species and

considered to be the center of the highest

functions. It is composed of two major

structures, the cerebrum and the basal

ganglia.

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• The cerebrum comprises most of the

visible brain surface and is divided into

right and left hemispheres (the two halves

of the brain) by a longitudinal fissure (a

major cleft between two pieces of the

brain).

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• Its surface is made up of convolutions

called gyri, which are separated by shallow

and deep grooves called sulcus and fissure

respectively. Although every human brain

shares the presence of certain sulcus and

fissure, no two brains have exactly the same

pattern.

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GYRUS

SULCUS

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• The fissures and sulci divide each

hemisphere into four main areas called

frontal, parietal, temporal and occipital

lobes. Seen on a horizontal, cross-sectional

view of the brain is the outer layer of the

cerebral hemisphere, the gray matter or the

cortex, which is composed of neuron

(nerve)cell bodies.

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• The inner layer, the white matter, is made

up of long axons (strands which conduct the

nerve's electrical signals throughout the

brain and the rest of the body) projected

from the cell bodies in the gray matter.

Some axons pass from one hemisphere to

the other in bundles such as the large

corpus callosum.

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CORPUS CALLOSUM

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• Some pass from lobe to lobe from gyrus to

gyrus in the same hemisphere. Many axons ,

however, descend from the cortex to other

areas of the central nervous system such as

the spinal cord passing through an area

called the internal capsule.

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LATERAL VIEW

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VENTRALVIEW

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DORSAL VIEW

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LATERAL VIEW

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• The basal ganglia, the area for crude motor

activity, are buried deep in the cerebral

hemisphere. When the brain is cut in a

horizontal (axial) plane, one can see some

of the basal ganglia, such as caudate

nucleus, globus pallidus, putamen, and

claustrum.

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2. Diencephalon

• The diencephalon is located in the middle of the brain, between the two cerebral hemispheres. The diencephalon is divided into the thalamus, the main relay center for the various sensory and motor functions and the hypothalamus. The hypothalamus is the area concerned with temperature, appetite and various other hormonal control. In addition, the diencephalon includes the medial and lateral geniculate bodies associated with auditory and visual relay centers respectively, as well as the subthalamic nucleus and the pineal body.

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Stalk of the pituitary

gland

Pineal

Thalamus

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3. Mesencephalon

• The mesencephalon along with the pons and

the medulla oblongata together form a

wedge-shaped structure, the brainstem.

The mesencephalon(midbrain) is located

between the diencephalon and the pons. The

aqueduct of Sylvius, carrying cerebral

spinal fluid from the third ventricle to the

fourth, passes through the mesencephalon.

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• The area above the aqueduct is the tectum, which

contains the the quadrigeminal plate made up of

2 pair of rounded projections called colliculi. The

upper projections form the superior colliculi and

the lower form the inferior colliculi. The area just

below the aqueduct is the body or the tegmentum

of the midbrain, containing various fiber tracts and

relay centers such as red nucleus and substancia

nigra.

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• Also located in the tegmentum are the cell

bodies of two important cranial nerves,

oculomotor(3rd) and trochlear(4th)

nuclei, which control the movement of the

eyes. Finally, situated at the base of the

midbrain are a pair of huge fiber bundles,

the crus cerebri, which are a continuation

of motor fibers from the internal capsule.

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4. Pons and Cerebellum

• The pons and cerebellum together make up the

fourth division of the brain. The cerebellum is a

multiply-folded structure located under the

occipital lobe and is concerned with equilibrium

and the coordination of motor activity. The

cerebellum communicates with the underlying

brainstem via three pairs of fiber bundles:

superior, middle and inferior cerebellar

peduncles.

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• The pons is located between the midbrain and the medulla and is separated from the overlying cerebellum by the fourth ventricle, filled with cerebrospinal fluid (CSF). The pons contains various ascending and descending fiber tracts. Also in the pons are the cell bodies of 3 cranial nerves, trigeminal(5th), abducens(6th), and facial(7th) nerves.

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5. Medulla Oblongata

• The medulla oblongata is the last division of the brain. It becomes continuous with the spinal cord at the foramen magnum. It contains various ascending and descending fiber tracts like the midbrain and pons. It also harbors nuclei of the following cranial nerves, the vestibulocochlear(8th), glossopharyngeal(9th), vagus(10th), accessory(11th) and hypoglossal(12th). The respiratory and cardiac centers are also situated in the medulla.

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Spinal Cord

• The spinal cord is the downward continuation of

medulla starting at the foramen magnum. It

descends to about the level of the second lumbar

vertebra, tapering to a structure called the conus

medullaris. There is a thin thread-like

continuation of the conus, the filum terminale,

which extends as far as the coccyx and is

composed of non-nervous tissue.

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• The cord serves as a conduit for the

ascending and descending fiber tracts that

connect the peripheral and spinal nerves

with the brain. The cord projects 31 pairs of

spinal nerves on either side (8 cervical, 12

thoracic, 5 lumbar, 5 sacral and 1

coccygeal) which are connected to the

peripheral nerves.

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• A cross section of the spinal cord

demonstrates a butterfly-shaped gray matter

in the middle, surrounded by white matter.

As in the cerebrum, the gray matter is

composed of cell bodies. The white matter

consists of various ascending and

descending tracts of myelinated axon fibers

with specific functions.

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Cranial Nerves

• There are 12 pairs of cranial nerves.

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• 1. The Olfactory Nerve (Cranial Nerve I)

arises in the nasal mucosa and conveys

smell to the brain

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• 2. The Optic Nerve (Cranial Nerve II)

deliveres images of sight from the eye's

retina to the brain.

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• 3. The Oculomotor Nerve (Cranial Nerve III) runs from the midbrain to some of the muscles of the eye to move the eye upward, downward and medially. It also goes to the muscles within the pupil and is responsible for constriction of the pupil. Finally, it travels to the muscles of the eyelid and is partially responsible for raising the upper eyelid

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• 4. The Trochlear Nerve (Cranial Nerve

IV) This motor (a nerve having only fibers

traveling to muscles to cause their

movement) nerve also arises in the midbrain

and goes to some of the muscles involved in

eye movement. It causes the eye to turn

downward and outward.

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• 5. The Trigeminal Nerve (Cranial Nerve

V) The trigeminal nerve arises within the

Pons and travels to the jaw's muscles to

power chewing. The nerve also contains

nerves bringing sensation from the face to

the Pons.

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• 6. The Abducens Nerve (Cranial Nerve VI)

This nerve arises in the Pons and goes to

eye muscles which rotate the eye outward.

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• 7. The Facial Nerve (Cranial Nerve VII) This

nerve also arises in the Pons and innervates (gives

nerve supply to) the muscles of facial expression,

the eyelids, as well as some of the muscles which

assists speech and mastication. It also is involved

in the control of saliva secretion. The nerve also

contains fibers which bring taste sensation from

anterior two-thirds of tongue back to the

brainstem.

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• 8. The Vestibulo-Cochlear Nerve (Cranial

Nerve VIII) This sensory (a nerve carrying

sensation information to the brain) nerve

arises in the inner ear and goes to the Pons.

The vestibular component conveys

equilibrium and position sense and

coordinates movement of head and neck.

The cochlear component coveys hearing.

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• 9. The Glosso-Pharyngeal Nerve (Cranial Nerve

IX) This mixed (a nerve having both sensory and

motor fibers) nerve brings sensation from the

pharynx (back of the throat), senses blood

pressure from the carotid artery (one of the main

blood delivery pathways to the brain), taste from

posterior one-thirds of tongue to the Medulla. It

also sends motor nerve fibers to the throat to

power swallowing.

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• The Vagus Nerve (Cranial Nerve X) This

nerve is also mixed. Its motor fibers come

from the medulla and are involved in

swallowing, and regulation of cardiac,

pulmonary, and part of gastrointestinal

activities. It brings sensation from the

gastrointestinal tract back to the medulla.

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• 11. The Accessory Nerve (Cranial Nerve

XI) arises in the medulla supplies the

muscles which elevate the shoulder as

occurs with a "scrug".

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• 12. The Hypoglossal Nerve (Cranial Nerve

XII) This motor nerve comes from the

medulla and goes to the muscles of the

tongue.

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Cerebral Cortex

• The cerebral cortex is highly developed in

man and certain areas are associated with

specific neurological functions.

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• The Frontal Lobe

• Prefrontal cortex (the front tips of the

hemispheres) is concerned with higher intellectual

functions and is involved in the many behavioral

aspects of man. It inhibits certain primitive

behaviors. Bilateral destruction of this area results

in a loss of concentration, a decreased intellectual

ability and a lack of judgment.

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• Primary motor cortex is located just in

front of the central sulcus in the frontal

lobe. The area controls the movement of the

rest of the body while the premotor cortex

just adjacent to it is concerned with the

initiation, activation, and performance of

the actual movement.

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• The Parietal Lobe is primarily concerned

with the interpretation and integration of

sensory inputs. Destruction of the parietal

lobe may result in clumsiness, defective

recognition of sensory inputs and lack of

interpretation of spatial relationships.

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• The Somatosensory cortex is located just

behind the central sulcus in the parietal

lobe. It is associated with reception and

perception of touch, vibration, and position

sense of the body.

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• The posterior part of the Temporal Lobe, known as the auditory cortex, is concerned with the reception and interpretation of sound information, while the medial part, the olfactory cortex, is concerned with the smell information. A part of the superior temporal lobe and the inferior part of parietal lobe on the dominant hemisphere (usually the left hemisphere)is called the language cortex and participates in recognition and interpretation of language.

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• The Occipital Lobe contains the primary

visual cortex(the striate cortex). Lesions in

this area may produce a loss of vision on the

opposite side or a lack of ability to interpret

visual inputs.

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Physiology of Cerebrospinal Fluid

• Cerebrospinal fluid (CSF) flows within the

ventricles of the brain, the central canal of the

spinal cord and out to the subarachnoid spaces

surrounding the brain and spinal cord, effectively

floating these two structures. It serves as a

medium for the transfer of substances between the

blood and the nervous tissues as well as a liquid

buffer, absorbing mechanical shocks to the brain

or the cord.

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• Most of CSF is provided by the choroid plexuses

that reside in lateral, third and fourth ventricles. In

adults, the volumn of this fluid has been calculated

to be from 125 to 150 ml (4-5 oz). It is in

continuous formation, circulation and absorption .

Approximately 430 to 450 ml (nearly 2 cups) of

CSF are produced every day, or 0.35 ml per

minute in adults and 0.15 per minute in infants.

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• The choroid plexuses of the lateral ventricles are the largest and produce the majority of CSF. The fluid then flows through the foramina of Monro into the third ventricle. The fluid is augmented by the production from this ventricle and continues down through a narrow passage called the aqueduct of Sylvius to the fourth ventricle.

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• After addition of more CSF from the fourth

ventricle, it escapes into the subarachnoid

space through the foramina of Magendie

and Luschka. The CSF then circulates

throughout the base of the brain, down

around the spinal cord as well as upward

over the cerebral hemispheres.

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• The CSF is then absorbed primarily through arachnoid villi into the superior sagittal sinus and joins the blood circulation.

• The obstruction of the normal CSF flow or overproduction of CSF from a choroid plexus papilloma (a benign tumor of the choroid plexus) can lead to a condition known as hydrocephalus. It is defined as a disproportionate enlargement of part or all of the ventricular system due to an excess of CSF and is typically accompanied by serious increase in intracranial pressure.

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Increased Intracranial Pressure

• The normal values for intracranial pressure (ICP)

at the level of foramen of Monro are

approximately 90-210 mm of CSF in adults and

15-80 mm of CSF in infants. Increased ICP can

occur as a result of an increased mass within the

limited volume of the cranium. Examples include

an increase in CSF volume, cerebral edema, and

growing mass lesions such as tumors and

hematomas.

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• Cerebral edema is the increase in brain tissue water causing swelling. It may occur secondary to head injury, infarction or a response to adjacent hematoma or tumor. Uncorrected increased ICP can lead to further brain damage due to the pressure and inadequate blood perfusion of neurological tissues. The treatment for increased ICP includes removing the mass(tumor, hematoma) by surgery, draining CSF from the ventricles by a drain or a shunt, hyperventilation, steroids, osmotic dyhydrating agents, and barbituates.

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Circulation

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VENTRAL SURFACE

Vertebral a. Basillar a.

Internal

carotid a.

Internal

carotid a.

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Cerebral Angiogram

Vertebral

Basillar

Internal

carotid

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The end