Porth's Essentials of Pathophysiology, 4e

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Nervous System

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cranial nerves. Consequently, the cranial nerves, which are arbitrarily numbered 1 through 12, retain the early pattern of development, with more than one cranial nerve branching from a single segment. The truly seg- mental nerve pattern of the cranial nerves is altered because all branches from segment 2 and most of the branches from segment 1 are missing. Cranial nerve 2, also called the optic nerve , is not a segmental nerve, but a brain tract connecting the retina (modified brain) with the first forebrain segment from which it developed. Cell Columns Anatomically, the body is organized into soma (e.g., skin, muscles, and skeletal structures of the body wall) and viscera (internal organs). The organizational struc- ture of the nervous system can be best explained and simplified as a pattern in which functionally specific PNS and CNS afferent and efferent somatic and visceral neurons are repeated as parallel cell columns running lengthwise along the nervous system. In this organiza- tional pattern, afferent neurons, dorsal horn cells, and ventral horn cells are organized as a bilateral series of 11 cell columns. The cell columns on each side can be further grouped according to their location in the PNS: four in the dorsal root ganglia that contain sensory neurons, four in the dorsal horn that contain sensory input association (IA) interneurons, and three in the ventral horn that contain motor neurons (Fig. 34-8). The processes of sensory neurons that enter the dorsal horn communicate with IA neurons, which then distributes the afferent information to local reflex circuitry and to more rostral segments of the CNS. The ventral horns contain output association (OA) interneurons and lower motor neurons, which project to the effector muscles. The afferent and efferent cell columns of the PNS and CNS, their projections, and the type of information they transmit are summarized in Table 34-1. Between the IA neurons and the OA neurons are networks of small interconnecting neurons arranged in complex circuits that provide the discreteness, appropri- ateness, and intelligence of responses to stimuli. Most of the billions of CNS cells in the spinal cord and brain gray matter are interconnecting neurons. Dorsal Horn Cell Columns. Four columns of affer- ent (sensory) neurons in the dorsal root ganglia directly innervate four corresponding columns of IA neurons in the dorsal horn (see Fig. 34-8). These are the special somatic and special visceral afferents and the general somatic and general visceral afferents. The special somatic afferent (SSA) neurons are con- cerned with internal sensory information such as joint and tendon sensation. They relay information to local reflexes concerned with posture and movement, as well as to the cerebellum, contributing to coordination of movement, and to the forebrain, contributing to expe- rience. Afferents innervating the vestibular system of the inner ear also belong to the special somatic afferent category.

Segments

Dorsal root

Dorsal root ganglion neuron

Spinal nerve

IA neuron

Ventral root

Dorsal root ganglion

33 of which form the spinal cord and spinal nerves, and 10 of which form the brain and its cranial nerves. The basic pattern of the CNS is that seen in the spi- nal cord—a central cavity surrounded by an inner core of gray matter and a superficial layer of white matter (Fig. 34-7). The brain retains this organization, but it also contains additional regions of gray matter that are not evident in the spinal cord. The gray matter is func- tionally divided into longitudinal columns of nerve cell bodies called the cell columns . The superficial white matter region contains the longitudinal tract systems of the CNS. The dorsal half or dorsal horn of the gray mat- ter contains afferent neurons. The ventral portion, or ventral horn , contains efferent neurons that communi- cate by way of the ventral roots with effector cells of the body segment. Many CNS neurons develop axons that grow longitudinally as tract systems that communicate between neighboring and distal segments of the neural tube. Each segment of the CNS is accompanied by two pairs of bundled nerve roots—a dorsal and ventral pair. The paired dorsal roots connect a pair of dorsal root ganglia and their corresponding CNS segment. The dor- sal root ganglia contain many afferent nerve cell bodies, each having two axonlike processes—one that ends in a peripheral receptor and another that enters the dorsal horn of its respective CNS segment. The paired ventral roots of each segment contain fibers supplying skeletal muscles and visceral structures of the body segment. All brain segments, except segment 2, retain some portion of the basic segmental organization of the ner- vous system in which each segment has multiple paired branches containing a grouping of component axons. The classic pattern of spinal nerve organization, which consists of a pair of dorsal and a pair of ventral roots, is a later development that has not occurred in the­ FIGURE 34-7. Three segments of the spinal cord showing dorsal roots entering the dorsal lateral surface of the cord and ventral roots exiting the ventral surface of the cord.The dorsal root ganglion contains dorsal root ganglion cells, whose axons bifurcate: one process enters the spinal cord in the dorsal root, and the other extends peripherally to supply the skin and muscle of the body.The ventral root is formed by axons from motor neurons in the spinal cord. IA, input association.