Porth's Essentials of Pathophysiology, 4e

837

Organization and Control of Neural Function

C h a p t e r 3 4

Spinal Cord and Brain The central nervous system, which consists of the spinal cord and brain, gathers information about the environ- ment from the peripheral nervous system, processes this information, perceives part of it, and organizes reflexes and other behavioral responses. The Spinal Cord In the adult, the spinal cord is found in the upper two thirds of the spinal canal of the vertebral column (Fig. 34-11A). It extends from the foramen magnum at the base of the skull to a cone-shaped termination, the conus medullaris, usually located at the level of the first or second lumbar vertebra (L1 or L2) in the adult. The dorsal and ventral roots of the more caudal portions of the cord elongate during development and angle down- ward from the cord, forming what is called the cauda equina (from the Latin for “horse’s tail”). The filum ter- minale, which is composed of nonneural tissues and the pia mater, continues caudally and attaches to the second sacral vertebra (S2). Cross-Sectional Anatomy of the Spinal Cord The spinal cord is somewhat oval on transverse section, with the gray matter that forms the dorsal and ventral horns having the appearance of a butterfly or the let- ter “H” (see Fig. 34-11B). The central portion of the cord, which connects the dorsal and ventral horns, is called the intermediate gray matter . The intermediate gray matter surrounds the central canal. In the thoracic area, the small, slender projections that emerge from the intermediate gray matter are called the intermediolateral columns of the horns. These columns contain the vis- ceral output association neurons and the efferent neu- rons of the sympathetic nervous system. The gray matter is proportional to the amount of tissue innervated by a given segment of the cord (see Fig. 34-11B). Larger amounts of gray matter are present in the lower lumbar and upper sacral segments, which supply the lower extremities, and in the fifth cervical segment to the first thoracic segment, which supply the upper limbs. The white matter in the spinal cord also increases progressively toward the brain because ever more ascending fibers are added and the number of descending axons is greater. The spinal cord and the dorsal and ventral roots are covered by a connective tissue sheath, the pia mater, which also contains the blood vessels that supply the white and gray matter of the cord (Fig. 34-12). On the lateral sides of the spinal cord, extensions of the pia mater, the denticulate ligaments, attach the sides of the spinal cord to the bony walls of the spinal canal. Thus, the cord is suspended by both the denticulate ligaments and the segmental nerves. A fat- and vessel-filled epi- dural space intervenes between the spinal dura mater and the inner wall of the spinal canal.

■■ In terms of organization, the nervous system retains many early patterns of segmental development that were established during early embryonic life, with a longitudinal series of segments, each repeating the same fundamental pattern of a central cavity surrounded by an inner core of gray matter made up of nerve cells and a superficial layer of white matter containing axons of the longitudinal tract systems. Each of the body segments are connected to their corresponding central nervous system (CNS) segments by afferent sensory and efferent motor neurons of the peripheral nervous system (PNS). ■■ There are four types of afferent neurons that carry sensory information to the CNS through the dorsal root ganglia: general somatic afferents that carry sensory information from the skin and other somatic structures, special somatic afferents that are concerned with internal sensory information such as joint and tendon sensation, general visceral afferents that innervate visceral structures such as the gastrointestinal and genitourinary systems, and special visceral afferents that innervate gut-related visceral receptors such as taste buds and olfactory receptors. ■■ There are three types of efferent neurons in the ventral horn that synapse with lower motor neurons that exit the CNS in the ventral roots: general somatic efferent neurons that innervate skeletal muscles, general visceral efferent neurons that supply visceral structures innervated by the autonomic nervous system, and the pharyngeal efferent neurons that innervate pharyngeal muscles. arranged in three layers: an inner, a middle, and an outer layer. The inner layer of white matter contains the axons of neurons that connect neighboring segments of the nervous system. It contains a mixture of nerve cells and axons called the reticular formation and is the site of many important spinal cord and brain stem reflex circuits. The middle layer provides for longitudinal communication between the more distant segments of the nervous system; it contains most of the major fiber tract systems required for sensation and movement. The outer layer contains large-diameter axons that travel the entire length of the nervous system; it includes tracts needed for fine manipulative skills. ■■ Communication between longitudinal segments is provided by tracts that are