Porth's Essentials of Pathophysiology, 4e

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Nervous System

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cause any purposeful muscle movement. Located in each segment of the anterior horn of the spinal cord are sev- eral thousand motor neurons called lower motor neurons (LMNs). Upper motor neurons (UMNs) control LMNs. They project from the motor strip in the cerebral cortex to the anterior horn of spinal cord and are fully contained in the central nervous system (CNS) (Fig. 36-3). Motor neurons segregate into two major categories: alpha and gamma. The alpha motor neurons are large nerve fibers that innervate the extrafusal muscle fibers responsible for muscle contraction and force generation. The much-smaller gamma motor neurons innervate the intrafusal muscle fibers of the muscle spindles, which help control basic muscle tone (to be discussed under spinal reflexes). Each alpha motor neuron undergoes multiple branchings, making it possible for a single alpha motor neuron to innervate a few to thousands of muscle fibers. In general, large muscles—those contain- ing hundreds or thousands of muscle fibers and provid- ing gross motor movement—have large motor units. This contrasts sharply with those that control the hand, tongue, and eye movements, for which the motor units are small and permit very precise control. The alpha motor neuron and the group of muscle fibers it innervates is called a motor unit. When an alpha motor neuron develops an action potential, all of the muscle fibers in the motor unit it innervates develop action potentials, causing them to contract simultane- ously. Thus, a motor neuron and the muscle fibers it innervates function as a single unit—the basic unit of motor control. Spinal Reflexes The spinal cord contains neural reflexes or coordinated, involuntary motor responses that are initiated by a stim- ulus applied to peripheral receptors. 3,5 Some reflexes, such as the flexor-withdrawal reflex, initiate movements to avoid hazardous situations, whereas others, such as the stretch reflex or crossed-extensor reflex, serve to integrate motor movements so they function in a coor- dinated manner. The anatomic basis of a reflex consists of an afferent neuron that synapses either directly with an effector neuron that innervates a muscle or with an interneuron that synapses with an effector neuron. Reflexes are essentially “wired into” the CNS so that they are always ready to function. With training, most reflexes can be modified to become part of more com- plicated movements. A reflex may involve neurons in a single cord segment (i.e., segmental reflexes), several or many segments (i.e., intersegmental reflexes), or struc- tures in the brain (i.e., suprasegmental reflexes). In most cases, reflex activities go on without our con- scious awareness. There is a significant amount of reflex circuitry in the spinal cord for the coordinated control of movements, particularly stereotyped movements con- cerned with locomotion. Many of these reflexes work equally well in decerebrate animals (those in which the brain has been destroyed) as long as the spinal cord is intact. Other spinal reflexes require the activity of the brain for their successful completion.

Trunk Shoulder Elbow

Hip

Knee

Ankle

Wrist

Hand

Toes

Little

Ring

Middle

Index Thumb

Neck

Brow

Eyelid and eyeball Face Lips

V o c a l i z a t i o n

Jaw

Tongue Swallowing M a s t i c a t i o n

S a l i v a t i o n

medial surface of the hemisphere in the premotor region (areas 6 and 8). It is intimately involved in the perfor- mance of complex, skillful movements that involve both sides of the body. The Cerebellum and Basal Ganglia In addition to the brain stem and cerebral cortex, the cerebellum and basal ganglia assist in the planning and execution of motor movements. They provide feedback circuits that regulate cortical and brain stem motor areas, and they receive indirect input from various areas of the motor cortex. 2 The cerebellum and basal ganglia do not send significant output directly to the spinal cord, but act on motor neurons in the brain stem. Although the precise contributions of the cerebellum and basal ganglia are still unclear, both are necessary for smooth movement and posture. The basal ganglia pro- vide the gracefulness of performance as well as the sup- portive posture for highly skilled movements. Cerebellar circuits are involved with the timing and coordination of movements that are in progress and with learning of motor skills. Damage to the cerebellum by vascular lesions of certain familial degenerative disorders pro- duces cerebellar ataxia, a characteristic loss of coordi- nation and accuracy of limb movement. The Motor Unit The motor neurons in the spinal cord represent the final pathway for integration of motor function. 2,3 Without the special neuronal circuits in the spinal cord, even the most complex motor control systems in the brain could not FIGURE 36-2. Representation of the relative extent of motor cortical area 4 devoted to muscles of the various body regions. Medial surface is at the left, lateral fissure is at the right, with pharyngeal and laryngeal muscle representation extending toward the insula. (From Penfield E, RasmussenT. The Cerebral Cortex in Man: A Clinical Study of Localization of Function. NewYork: Macmillan; 1968.)