XII - Functional Anatomy of the Nervous System

A. Organization of the nervous system - The nervous system is divided into two major divisions.

1. Central nervous system (CNS) - This division consists of the brain and the spinal cord.

2. Peripheral nervous system (PNS) - This division is made up of the spinal nerves and the cranial nerves. It is further divided into two subdivisions.

a. Afferent subdivision - This consists of the nerve fibers which conduct information toward the CNS. It is the sensory division.

b. Efferent subdivision - These are the nerve fibers that conduct impulses away from the CNS. This is the sensory division, and it is subdivided into two additional compartments.

(l) Somatic division - These are the nerve fibers that supply the skeletal muscles. These fibers are usually under conscious control.

(2) Autonomic division - This consists of the nerve fibers that supply smooth muscle, heart muscle, and glands. These fibers are not under conscious control.

B. Embryology of the nervous system - It is easier to understand the structure of the adult nervous system if the development of the system is first studied. The CNS begins as a dorsal hollow tube. At the anterior end of this tube are three swellings which represent the embryonic brain. These three regions give rise to the five adult regions of the brain.

1. Embryonic regions Adult regions

a. Prosencephalon (forebrain) a. Telencephalon

b. Diencephalon

b. Mesencephalon (midbrain) c. Mesencephalon

c. Rhombencephalon (hindbrain) d. Metencephalon

e. Myelencephalon

2. The remainder of the tube gives rise to the spinal cord. During development the walls of the tube thicken to form the various regions of the brain and cord. Eventually outgrowths from the brain and cord give rise to the PNS.

C. Definitions

1. Nerve - A bundle of neuronal processes found in the PNS. The structure is similar to that of a skeletal muscle. Individual processes are surrounded by a layer of connective tissue termed the endoneurium. These processes are grouped together into bundles termed fasciculi. The fasciculus are bounded by more connective tissue, the perineurium, and finally, the entire nerve has a connective tissue envelope surrounding it known as the epineurium.

2. Tract - This is the same type of structure as the nerve but it is found in the CNS instead of the PNS.

3. Ganglion - This is a collection of cell bodies and synapses which is found in the PNS.

4. Nucleus - This is a collection of cell bodies and synapses found in the CNS.

5. Center - This is a synaptic area in the CNS with a restricted function such as the vision center, speech center, etc.

6. Gray matter - A general term for unmyelinated nervous tissue. It usually consists of centers and nuclei, but there are some gray tracts.

7. White matter - A general term for myelinated nervous tissue. It usually consists of nerves and tracts.

D. Protection of the CNS - There are three distinct systems which protect the delicate CNS.

l. There is the bony case formed by the skull and the vertebral column.

2. Meninges - This is a series of three separate membranes that surround the CNS. They are as follows.

a. Dura mater - This is a tough outer layer which is fused with the periosteum of the cranial bones. In the vertebral column there is a venous plexus that separates this meninges from the bone.

b. Arachnoid - This is the second or middle membrane. It is a very delicate and is separated from the inner meninges by a large space known as the subarachnoid space. This space is filled with cerebrospinal fluid (CSF).

c. Pia mater - This is the innermost meningeal membrane. It is very thin and delicate, and is tightly attached to the surface of the brain and cord.

Inflammation of the meninges is known as meningitis.

3. Cerebrospinal fluid (CSF) - This is the third of the protective layers. It consists of a fluid, derived from the blood plasma, which fills the subarachnoid space and the cavities of the CNS. By filling the subarachnoid space the CSF forms a fluid barrier completely around the CNS which serves as an excellent shock absorber.

E. Cavities of the CNS - Not all of the hollow embryonic tube which gave rise to the CNS becomes filled with cells. Parts of it still remain hollow in the adult and form a series of cavities which are known as the ventricles in the brain and the central canal in the spinal cord.

l. Ventricles - There are four ventricles. Ventricles I and II are known as the lateral ventricles and are found in the cerebral hemispheres. They are separated from one another medially by a membrane known as the septum pellucidum. Each of the lateral ventricles communicates with ventricle III which is located in the diencephalon region of the brain. A canal, the cerebral aqueduct(mesencephalic aqueduct), connects the third ventricle to ventricle IV which is located in the hindbrain region. Foramen (lateral and medial apertures) in the roof of ventricle IV permits communication with the subarachnoid space. The fourth ventricle then becomes continuous with the central canal which runs through the center of the spinal cord.

2. CSF production - Each ventricle contains a capillary complex known as a choroid plexus, which produces CSF. The CSF flows through the ventricles and into the subarachnoid space. Within the subarachnoid space are capillary tufts which are known as arachnoid villi. They reabsorb the CSF into the blood from which it originated.

F. Brain - This is the largest organ of the nervous system. It is divided into five regions in the adult. The regions, from anterior to posterior, along with there major structures, are described below.

l. Telencephalon - This is the most anterior region of the brain. Along with the diencephalon it makes up the forebrain. It is the largest region of the brain and its most obvious structure is the cerebrum.

a. Cerebrum - This is composed of two hemispheres. They are the most anterior part of the brain and in humans the largest. The two hemispheres are separated by a longitudinal fissure. Fissures are deep depressions in the brain surface. Lesser depressions are known as sulci and the elevations between the sulci are known as convolutions or gyri.

(1) The outer surface of the cerebrum is covered by several millimeters of gray matter known as the cerebral cortex. The cortex consists of millions of unmyelinated cell bodies and processes synapsing together in an infinitely complex manner.

(2) The bulk of the inner mass is made of myelinated tracts and is therefore white. The fibers in these tracts connect different parts of the cortex together as well as connecting the cortex with other parts of the CNS. There are three types of fibers found here.

(a) Projection fibers - These connect the cortex to other regions of the CNS.

(b) Commissural fibers - Corresponding parts of the two hemispheres are connected by these fibers.

(c) Association fibers - These connect the gyri and lobes of the same hemisphere.

(3) Each hemisphere is divided into four lobes, named for the skull bones under which they lie. These lobes are the frontal, parietal, temporal, and occipital. The lobes are separated from one another by sulci and fissures.

(a) The frontal lobe is separated from the parietal lobe by the central sulcus.

(b) The lateral fissure separates the temporal lobe from the frontal and parietal lobes.

(c) The parieto-occipito sulcus separates the parietal lobe from the occipital lobe.

The insula is an area of the cortex which lies deep within the lateral fissure. It is sometimes considered a fifth lobe.

(4) Functional areas of the cortex - There are three general functional divisions of the cortex: sensory, motor, and association.

(a) Sensory areas - These localize and interpret sensory impulses.

/1/ Primary sensory area - This is located on the post central gyrus of the parietal lobes. It interprets and localizes the origins of touch, pressure, temperature, and pain.

/2/ Visual area - This region interprets visual patterns and it located on the occipital lobe.

/3/ Auditory area - Interpretation of sound occurs here. It is located on the temporal lobe.

/4/ Gustatory area - Taste interpretation occurs here, on the parietal lobes.

/5/ Olfactory area - This area is located on the temporal lobe and frontal lobe. It interprets smell.

(b) Motor areas - These areas control movement.

/1/ Primary motor area - This is located pm the precentral gyrus of the frontal lobe. It controls specific muscles and groups.

/2/ Premotor area - Located anterior to the primary motor area, this region controls the sequencing of muscle contraction in skilled movements, such as typing.

/3/ Frontal eye field area - This region is located on the frontal lobes and controls the voluntary scanning movement of the eyes.

/4/ Motor speech area (Broca's area) - This translates thoughts into speech. It is located on the frontal lobe and sends signals to the premotor area to initiate speech.

(c) Association areas - These connect motor and sensory areas together. Each sensory and motor has an association area. They are concerned with the higher functions, memory, judgement, intelligence, and personality. For example, damage to the visual association area would not result in blindness, but would result in an inability to understand what is being seen. Especially important is the prefrontal cortex which is where most of the higher functions seem to be coordinated.

(d) General interpretive area (Gnostic) - This is located among the various sensory association areas. It integrates these association areas so that a common thought can be formed from the various sensory inputs. This thought pattern can then be transmitted to the brain areas so that an appropriate response can be produced. Damage to this are results in becoming an imbecile as it is impossible to convert sensory stimuli into a logical thought/action.

(5) Basal ganglia - These are really basal nuclei, islands of gray matter found deep in the cerebrum. They include the caudate nucleus and the lentiform nucleus. The basal ganglia are very important as motor coordinating centers. They seem to inhibit certain motor activity and thereby yield smooth coordinated movement. Damage to the basal ganglia usually results in some type of tremor. Parkinson's disease is a case in point.

(6) Internal capsule - This structure consists of projection fibers to and from the cortex. It is the major connecting point between the cortex and the other parts of the CNS.

b. Hemisphere specialization - While the two hemispheres appear anatomically similar, there is a definite specialization.

(l) Left hemisphere - In most people this is where the general intrepetive area and the speech center are located. Consequently verbal and sequential processing, logic, language, analysis, and problem solving are all localized in this hemisphere. This is the categorical (dominant) hemisphere for most people. Most left dominant individuals are right handed while those individuals in which the categorical hemisphere is the right one are usually left handed.

(2) Right hemisphere - This hemisphere processes complex visual patterns and mental visualization of objects in three dimensional space. Artistic abilities and emotional responses seem to reside here. Because of its ability to deal with spatial relationships it is referred to as the representational hemisphere.

2. Diencephalon - This part of the forebrain is difficult to visualize as most of it is covered by the cerebrum. It consists of three regions plus the third ventricle. As a rule, the diencephalon consists of paired structures which lie on either side of the third ventricle. It is also referred to as the thalamic region of the brain.

a. Epithalamus - This upper part of the diencephalon is also known as the pineal gland. It is an endocrine gland that secretes a hormone known as melatonin. Melatonin is derived from the neurotransmitter serotonin. The hormone seems to effect reproductive processes and circadian rhythms, regular daily patterns of physiological changes.

b. Thalamus - This is the largest gray matter region found deep within the brain stem. It is a major relay center for all of the sensory pathways except the olfactory pathway. All of these sensory pathways synapse here before proceeding on to the sensory areas of the cortex. It is often compared to a switchboard due to its relay and switching functions.

c. Hypothalamus - This is the lower region of the diencephalon. It has the hypophysis (pituitary gland) suspended from it. It coordinates much of the autonomic nervous system and controls a large part of the endocrine system. As a result, most of the internal functions such as respiration, body temperature, water balance, etc. are regulated by this area.

d. Limbic system - This area includes parts of the diencephalon and telencephalon. Parts of the cerebral cortex, the basal ganglia, and the hypothalamus all contribute. This system seems to qualitatively evaluate stimuli as to whether they are pleasurable or not pleasurable. It plays a major role in the emotions. Stimulation of this area can produce a large variety of emotional responses including rage, fear, hunger, and sexuality. The limbic system also is important in establishing long term memories. One limbic system nucleus, the hippocampus, is especially important in long term memory. Much of the limbic system is derived from the rhinencephalon, the primitive smelling brain. This explains why odors can have such a strong emotional impact and bring back vivid memories.

3. Mesencephalon - This region consists of the cerebral peduncles, corpora quadrigemina, and it surrounds the cerebral canal.

a. Cerebral peduncles - These carry fiber tracts to and from the cerebral hemispheres.

b. Corpora quadrigemina - These four bodies are found dorsal to the cerebral canal. They form two paired structures. The upper and larger pair are known as the superior colliculi and coordinate visual reflexes. The lower pair are the inferior colliculi and are involved with auditory reflexes.

4. Metencephalon - This is the first area of the hindbrain. It consists of two major structures known as the pons and the cerebellum.

a. Pons - This is a conspicuous segment that extends transversely across the brain stem. It serves to connect the two hemispheres of the cerebellum together and to connect the cerebellum to the medulla. In addition, tracts to and from other regions of the brain pass through the pons.

b. Cerebellum - This is the second largest region of the brain. It is separated from the brain stem by the fourth ventricle, but it is connected to the brain stem by three paired fiber bundles known as the cerebellar peduncles.

(1) Anatomy - The cerebellum is composed of two hemispheres plus a small central portion known as the vermis. There is a thin outer layer of gray matter (cortex) and an inner mass of white matter (medulla).

(2) Function - The cerebellum functions in two areas both of which are related.

(a) Motor coordination - The cerebellum functions as a high speed computer, coordinating signals initiated by the primary motor area of the cerebrum. For this reason it has been referred to as the "secretary" to the cerebrum.

(b) Equilibrium - The cerebellum receives inputs from the eyes, inner ear, and proprioreceptors all of which are involved in maintaining balance. The cerebellum integrates these inputs and coordinates the necessary muscle contractions necessary to maintain balance.

Damage to the cerebellum results in movements which are stiff and robot-like coupled with a loss of ability to maintain balance.

5. Myelencephalon - This is the last region of the hindbrain and consists of the medulla oblongata. This is essentially an upward extension of the spinal cord. It contains many important reflex centers such as those regulating heart rate, blood pressure, respiration, swallowing, digestion, salivation, coughing, sneezing, etc. In addition there are many tracts leading t and from the brain and cord.

a. Reticular formation - This is a mass of gray matter that extends the entire length of the brain stem from the medulla to the diencephalon. Pathways from the basal ganglia, the cerebellum, various brain stem nuclei, and the spinal cord synapse in this formation. The reticular formation sends neurons to these areas as well.

(1) A major function of this area is the maintenance of the alert and aroused state. This is known as the reticular activating system. When we wake up from sleep is because of the RAS. Damage to the RAS can lead to permanent sleep or coma. It is probably here that most general anesthetics work.

(2) A second function is sensory filtration. Insignificant sensory signals are filtered out by this system. It is estimated that probably 99% of all sensory input is filtered out either by the reticular formation or the cerebral cortex.

(3) The third major function is coordinating motor reflexes both in the somatic and autonomic division.

G. Spinal cord - This is the second major structure of the CNS.

l. Function

a. The fibers of the cord conduct impulses between the brain and the rest of the body.

b. It integrates sensory and motor responses for fixed, stereotyped actions (spinal reflexes).

2. Anatomy - The cord extends from the foramen magnum to the level of the first or second lumbar vertebra. The tip of the cord is conical in shape and is referred to as the conus medullaris. A strip of the meninges called the filum terminale extends from the conus medullaris to the coccyx.

a. Thirty one pairs of spinal nerves arise from the cord. Each portion of the cord that gives rise to a pair of nerves is termed a spinal segment.

b. During embryology, the spinal nerves are pulled downward and form a mass of nerve roots at the bottom of the cord. This is termed the cauda equina (horse tail).

c. Each spinal nerve has a dorsal (posterior) root and a ventral (anterior) root. The dorsal root is composed entirely of sensory fibers and the ventral root is composed entirely of motor fibers. The dorsal root has a ganglion associated with it which contains the cell bodies of the sensory neurons.

d. The cord consists of both gray and white matter. White matter consists of myelinated fibers while the gray matter consists of cell bodies, synapses, and neuroglial cells. Unlike the brain, gray matter in the cord is located inside and the white matter is on the outside.

(1) Gray matter in the cord is in the shape of an H. The various parts of the "H" are as follows.

(a) Posterior horns or columns

(b) Anterior horns or columns

(c) Gray commissure

(2) White matter consists of myelinated processes. In each half of the cord the white matter is divided by the gray matter into three regions.

(a) Posterior funiculus (white column)

(b) Lateral funiculus (white column)

(c) Anterior funiculus (white column)

The funiculi are further subdivided into tracts, some of which will be discussed in detail later. These tracts are of two general types.

(a) Ascending (sensory) - Conduct impulses toward the brain.

(b) Descending (motor) - Conduct impulses away from the brain.

e. There are two invaginations of the cord, the anterior median fissure and the posterior median sulcus.

    1. The hollow, CSF filled central canal is located in the center of the gray commissure. It communicates with

the fourth ventricle.

H. Peripheral nerves - The peripheral nerves all arise from the brain and cord. They subdivide many times and ultimately supply all of the body with both sensory and motor fibers. It is through the peripheral nerves that the CNS receives sensory information and sends motor information which ultimately controls all aspects of the body and behavior. There are two major groups of peripheral nerves.

1. Cranial nerves - These are the nerves that arise directly from the brain. There are 12 pair and they are classified as being motor, sensory, or mixed (having large numbers of both sensory and motor fibers). They are numbered (Roman numerals) and named as well. From anterior to posterior they are as follow.

I. Olfactory - Sensory - Olfaction (smell) - These terminate in the olfactory bulbs of the telencephalon. Olfactory tract then conduct impulses to olfactory center.

II. Optic - Sensory - These nerves carry visual information from the eyes. The two nerves meet to form the optic chiasma. At the chiasma the medial fibers cross over while the lateral fibers remain on the same side. These fibers then proceed through the optic tracts to the visual center.

III. Oculomotor - Motor - This nerve supplies four of the six extrinsic muscle that move the eye.

IV. Trochlear - Motor - This supplies the superior oblique (extrinsic muscle) of the eye.

V. Trigeminal - Mixed - This nerve receives sensory input from the various regions and sends motor fibers to the muscles of mastication and others. It consists of three major branches.

l. Ophthalmic - Supplies the upper facial region.

2. Maxillary - Supplies the upper jaw.

3. Mandibular - Supplies the lower jaw.

VI. Abducens - Motor - This nerve supplies the remaining extrinsic muscles of the eye.

VII. Facial - Mixed - This controls facial muscles, the salivary glands, and has sensory fibers for taste.

VIII. Vestibulocochlear) - Sensory - This nerve conducts hearing and equilibrium information from the inner ear.

IX. Glossopharyngeal - Mixed - It controls the muscles of swallowing, secretion of saliva, and transmits taste.

X. Vagus - Mixed - This controls most of the muscle movement in visceral organs and is sensory from these same organs.

XI. Accessory - Motor - This controls swallowing and the movement of the head.

XII. Hypoglossal - This muscle controls the movement of the muscles of the tongue.

The cranial nerves can be remembered in order by utilizing the first letter of each word in the following rhyme.

On Old Olympus Towering Top, A Friendly Vixen Grew Vines And Hops.

2. Spinal nerves - These nerves are given off from the spinal cord. There are 31 pairs and they are identified by the region of the vertebral column from which they exit.

a. Spinal nerves by region

(l) Cervical nerves - 8 pairs

(2) Thoracic nerves - 12 pairs

(3) Lumbar nerves - 5 pairs

(4) Sacral nerves - 5 pairs

(5) Coccygeal nerves - 1 pair

b. Organization of the spinal nerves - Each spinal nerve originates from a dorsal root (sensory) and a ventral root (motor). Spinal nerves are mixed.

(l) After exiting the intervertebral foramen each spinal nerve divides into a dorsal and ventral ramus. The dorsal rami supply the skin and musculature of the back. The ventral rami supply the skin and musculature of the front region as well as form plexuses which then supply the remainder of the body.

(2) Plexuses and peripheral nerves - The ventral ramus of many of the spinal nerves connect and interconnect together to form complex branching networks known as nervous plexuses. The peripheral nerves are derived from these plexuses. The distribution of spinal nerves to the major plexuses listed below.

(a) C1 - C4 - Cervical plexus - Supplies the diaphragm and the skin and muscles of the face and neck region.

(b) C5 - T1 - Brachial plexus - Supplies nerves to the arms.

(c) T2 - T12 - These do not contribute to any plexus. They serve the intercostal muscles and skin of the anterior thorax.

(d) L1 - C1 - Lumbosacral plexus - This supplies the nerves to the hip, pelvis, and legs.