Neuroanatomy and Physiology

Neuroanatomy

by Jessie C. Huang, M.D.

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.

Skull

The skull is formed by the bones of calvaria and the skull base. (Fig. 1). 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). 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. It 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.

Spinal Column

The bony spinal column (Fig. 2) 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). 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). 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.

Brain

The brain is divided into five parts on an embryological basis:

  1. Telencephalon

    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.
    • The cerebrum (Fig 3) 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). 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. The fissures and sulci divide each hemisphere into four main areas called frontal, parietal, temporal and occipital lobes (Fig. 4). Seen on a horizontal, cross-sectional view of the brain (Fig. 5), (Fig. 6), (Fig. 7) is the outer layer of the cerebral hemisphere, the gray matter or the cortex, which is composed of neuron (nerve)cell bodies. 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. 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.
    • 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 (Fig 5).

  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 (Fig. 8). 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.

  3. Mesencephalon

    The mesencephalon along with the pons and the medulla oblongata together form a wedge-shaped structure, the brainstem(Fig. 8), (Fig. 9). 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. 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. 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.

  4. Pons and Cerebellum

    The pons and cerebellum (Fig. 7), (Fig. 8), (Fig. 9) 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. 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.

  5. Medulla Oblongata

    The medulla oblongata(Fig. 8), (Fig. 9) is the last division of the brain. It becomes continuous with the spinal cord at the foramen magnum (Fig. 10). 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.

Spinal Cord

The spinal cord (Fig. 10) 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. 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. A cross section of the spinal cord (Fig. 11) 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.

Cranial Nerves

There are 12 pairs of cranial nerves (Fig. 9).

  1. The Olfactory Nerve (Cranial Nerve I) arises in the nasal mucosa and conveys smell to the brain
  2. The Optic Nerve (Cranial Nerve II) deliveres images of sight from the eye's retina to the brain.
  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
  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.
  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.
  6. The Abducens Nerve (Cranial Nerve VI) This nerve arises in the Pons and goes to eye muscles which rotate the eye outward.
  7. The FacialNerve (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.
  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.
  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.
  10. 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.
  11. The Accessory Nerve (Cranial Nerve XI) arises in the medulla supplies the muscles which elevate the shoulder as occurs with a "scrug".
  12. The Hypoglossal Nerve (Cranial Nerve XII) This motor nerve comes from the medulla and goes to the muscles of the tongue.

Cerebral Cortex:

The cerebral cortex is highly developed in man and certain areas are associated with specific neurological functions (Fig. 12).

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. Most of CSF is provided by the choroid plexuses that reside in lateral, third and fourth ventricles(Fig. 13), (Fig. 14). 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. 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. 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. The CSF is then absorbed primarily through arachnoid villi into the superior sagittal sinus and joins the blood circulation (Fig. 14).

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.

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. 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.

Further Reading

  1. Anderson, JE. Grant&ae;lig;s Atlas of Anatomy. Eighth Edition. Williams and Wilkins. Baltimore-London, 1983.
  2. Clemente, Camine D. Anatomy. A Regional Atlas of the Human Body. Second Edition. Urban & Schwarzenberg. Baltimore - Munich 1981.
  3. DeArmond SJ and et al. Structure of the Human Brain. Photographic Atlas. Third Edition. Oxford Press. New York-Oxford, 1989.
  4. Gilman,S. and Newman, SW. Essentials of Clinical Neuroanatomy and Neurophysiology. Seventh Edition. F.A. Davis Company. Philadelphia, 1987.
  5. Wilkins, RH and Rengachary, SS. Neurosurgery. McGraw-Hill,

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