1. Overview of the Nervous System
Part 4 of 4

Nachum Dafny, Ph.D.
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The Peripheral Nervous System (PNS)

The PNS includes 31 pairs of spinal nerves, 12 pairs of cranial nerves, the autonomic nervous system and the ganglia (groups of nerve cells outside the CNS) associated with them. Also included in the PNS are the sensory receptor organs. The receptor organs are scattered in all parts of the body, sense and perceive changes from external and internal organs, then transform this information to electrical signals, which are carried via an extensive nervous network to the CNS (Figure 1.15). The cranial and spinal nerves contain nerve fibers that conduct information to-afferent-(Latin for carry toward) and from-efferent (Latin for carry away) the CNS. Afferent fibers convey sensory information from sensory receptors in the skin, mucous membranes, and internal organs and from the eye, ear, nose and mouth to the CNS; the efferent fibers convey signals from cortical and subcortical centers to the spinal cord and from there to the muscle or autonomic ganglia that innervate the visceral organs. The afferent (sensory) fibers enter the spinal cord via the dorsal (posterior) root, and the efferent (motor) fibers exit the spinal cord via the ventral (anterior) root. The spinal nerve is formed by the joining of the dorsal and the ventral roots. The cranial nerves leave the skull and the spinal cord nerves leave the vertebrae through openings in the bone called foramina (Latin for opening).

The PNS is divided into two systems: the visceral system and the somatic system. The visceral system is also known as the autonomic system. The autonomic nervous system (ANS) is often considered a separate entity; although composed partially in the PNS and partially in the CNS, it interfaces between the PNS and the CNS. The primary function of the ANS is to regulate and control unconsciousness functions including visceral, smooth muscle, cardiac muscle, vessels, and glandular function (Figure 1.16). The ANS can be divided into three subdivisions:

1. The sympathetic (or the thoracolumbar) subdivision associated with neurons located in the spinal gray between the thoracic and the upper lumbar levels;
2. The parasympathetic (or craniosacral) subdivision is associated with the 3rd, 7th, 9th and the 10th cranial nerves as well as with the 2nd, 3rd, and 4th sacral nerves;
3. The enteric subdivision is a complex neuronal network within the walls of the gastrointestinal system and contains more neurons than the spinal cord. The visceral (autonomic) system regulates the internal organs outside the realm of conscious control. The PNS component of the somatic system includes the sensory receptors and the neurons innervating them and their nerve fibers entering the spinal cord. The visceral and the somatic nervous system are primarily concerned with their own functions, but also work in harmony with other aspects of the nervous system.

Orientation to the Central Nervous System

This section illustrates representative sections through the CNS in order to acquaint the reader with prominent structures and help in the recognition of the level and orientation of the section. It also provides landmarks for locating nuclei and tracts involved in sensory and motor functions. Directional terms are used in describing the locations of structures in the CNS.

Keep in mind that certain terms were developed to describe the nervous system of quadrupeds and may have a slightly different meaning when applied to bipeds. For example, the ventral surface of the quadruped spinal cord is comparable to the anterior surface of the biped (Figure 1.18). In the following descriptions, the terms are applied to a standing human. The terms rostral and anterior refer to a direction towards the face/nose. The terms caudal and posterior refer to a direction towards the buttocks/tail. The terms inferior and superior generally refer to spatial relationships in a vertical direction (Figure 1.18). A coronal section is parallel to the vertical plane and a midcoronal section would divide the head into anterior and posterior halves (Figure 1.19). The sagittal section is also parallel to the vertical plane, but a midsagittal section would divide the head into right and left halves. The horizontal (axial) section is parallel to the horizontal plane and a mid-horizontal section would divide the head into superior and inferior halves. Transverse or cross sections of the spinal cord of humans are taken in a plane perpendicular to the vertical (i.e., in the horizontal plane of the head). Most electromagnetic imaging techniques produce images of the brain in the coronal, horizontal (axial) and sagittal planes. The representative sections are transverse sections through the spinal cord and brain stem and coronal sections through the telencephalon and diencephalon (Figure 1.17).

Transverse Section through the Spinal Cord. Figure 1.17A illustrates a section taken at the level of the thoracic spinal cord. The spinal cord neuron (gray matter) form a central core taking a butterfly configuration that is surrounded by nerve fibers (white matter). In the left and right halves of the spinal cord, the gray matter is organized into a dorsal horn and ventral horn with the intermediate gray located between them. In the thoracic spinal cord, which is illustrated in this figure, a lateral horn extends laterally from the intermediate gray (Figure 1.17A). The spinal cord white matter is subdivided into the posterior white column, the anterior white column and the lateral white column. The anterior white commissure joins the two halves of the spinal cord and is located ventral to the intermediate gray. The dorsal root fibers enter the spinal cord at the dorsolateral sulcus and the fibers of the ventral root fibers exit the spinal cord in numerous fine bundles through the ventral funiculus (see Figure 1.5).

Transverse Section through the Medulla. Figure 1.17B is a section taken at the level of the upper medulla. Landmark structures include the fourth ventricle, hypoglossal nucleus, inferior cerebellar peduncle, inferior olivary complex and the pyramids. As in the spinal cord section, the fiber tracts, the inferior cerebellar peduncle and pyramids, appear light in this section whereas the nuclei in the inferior olivary complex appear dark.

Transverse Section through the Pons. Figure 1.17C is a section taken at the level of the mid pons. Landmark structures include the fourth ventricle, the pons tegmentum, which includes the abducens nuclei; the pons base, which includes the corticofugal fibers and pontine nuclei; and the middle cerebellar peduncles.

Coronal Section through the Rostral Telencephalon. Figure 1.17D is a section taken at the level of the decussation of the anterior commissure. Landmark structures include the head of the caudate nucleus, the anterior limb of the internal capsule, the globus pallidus and putamen (important for controlling motor functions). The anterior commissure, a fiber bundle connecting the right and left frontal lobes, can be seen decussating (crossing the midline). The corpus callosum forms a thick band of decussating nerve fibers located above the lateral ventricles. Below the telencephalon afferent nerve fibers from each eye decussate in the optic chiasm and join uncrossed fibers to form the optic tract.

Coronal Section through the Midbrain-Diencephalon Junction. Figure 1.17E is a section taken at the level of the junction of the midbrain with the diencephalon. Notice that the plane of section differs from those of the previous sections. At this level, a landmark structure of the diencephalon is the thalamus, which surrounds the third ventricle. The posterior limb of the internal capsule separates the thalamus from the surrounding telencephalic structures (i.e., the globus pallidus and putamen). Lateral to the putamen is the insula while more dorsomedially the corpus callosum overlies the cavities of the lateral ventricles. Below the third ventricle are the red nucleus, substantia nigra and crus cerebri of the midbrain, which are the continuation of the internal capsule.

Section through the Midbrain. Figure 1.17F is a section that shows the main midbrain nuclei which include the tectum (superior colliculi) the periaqueductal gray, the red nuclei, substantia nigra and the cerebral peduncles.

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