Porth's Essentials of Pathophysiology, 4e

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Nervous System

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branches end in the segment of entry; others ascend to adjacent segments, influencing intersegmental reflex function; and still others ascend in the dorsal column of the cord to the brain stem. Segmental branches establish monosynaptic contact with each of the LMNs that have motor units in the muscle containing the spindle receptor. This produces an opposing muscle contraction. Another segmental branch of the same afferent neuron innervates an internuncial neuron that is inhibitory to motor units of antagonistic muscle groups. This disynaptic inhibitory pathway is the basis for the reciprocal activity of agonist and antagonist muscles (i.e., when an agonist muscle is stretched, the antagonists relax). Reciprocal innervation is useful not only for the stretch reflex, but also for vol- untary movements. Relaxation of the antagonist muscle during movements enhances the speed and efficiency because the muscles that act as prime movers are not working against contraction of the opposing muscle. 1 Another function of the stretch reflex is to inform the CNS of the status of muscle length. Ascending impulses from stretch receptors in the contracting muscle fibers ultimately provide information about muscle length to higher centers in the cerebellum and cerebral cortex. When a skeletal muscle lengthens or shortens against tension, a feedback mechanism needs to be available for readjustment such that the spindle apparatus remains sensitive to moment-to-moment changes in muscle stretch, even while changes in muscle length are occur- ring. This is accomplished by the gamma motor neurons that adjust spindle fiber length to match the length of the extrafusal muscle fiber. Descending fibers of motor pathways synapse with and simultaneously activate both the alpha motor neurons of the contracting muscle and gamma motor neurons so that the sensitivity of the spindle fibers is coordinated with muscle movement. Golgi Tendon Reflex. The Golgi tendon organ helps control muscle tension. The major difference between the Golgi tendon organ versus the muscle spindle is that the muscle spindle monitors muscle length whereas the Golgi tendon organ monitors muscle tension. 3 When the Golgi tendon organs of a muscle tendon are stimulated by increased tension in the contracting muscle, signals are sent to the spinal cord to cause reflexive inhibitory effects in the respective muscle. Thus, this reflex pro- vides a negative feedback mechanism that prevents the development of too much tension in the muscle. Another possible function of the Golgi tendon reflex is to equalize contractile forces of separate muscle fibers by inhibiting those fibers that exert excessive ten- sion and permitting those that exert too little tension to become more excited by withdrawing the inhibition. This spreads the muscle load over all the muscle fibers and prevents damage to isolated areas of a muscle where small numbers of fibers might be overloaded. Central Control of the Spinal Reflexes. Normal muscle tone depends on stretch reflexes initiated by the muscle spindles, which monitor changes in muscle length; and

the Golgi tendon organs, which monitor muscle tension. The neural circuits responsible for stretch reflexes provide higher centers of the nervous system with a mechanism for adjusting muscle tone. Disorders of muscle tone are frequently associated with lesions of the motor system, especially those that interfere with descending pathways. Stretch reflexes are hyperactive when lesions of the cor- ticospinal tract (e.g., stroke or spinal cord injury) reduce or disrupt the inhibitory effect of the brain on the spi- nal cord, and they are hypoactive or absent in cases of peripheral nerve damage or anterior spinal cord injury. Central control over the gamma motor neurons also permits increases or decreases in muscle tone in antici- pation of changes in the muscle force. Through its coor- dinated control of the muscle’s alpha and the spindle’s gamma motor neurons, the CNS can suppress the stretch reflex. This occurs during centrally programmed move- ments, such as pitching a baseball, that require a muscle to produce a full range of unopposed motion. Without this programmed adjustability of the stretch reflex, any movement is immediately opposed and prevented. Motor Pathways The primary motor cortex (Brodman area 4) is struc- tured into six well-defined layers. Those in layers I through IV project to the premotor and somatosensory areas on the same side of the brain, the opposite side of the brain, or to subcortical structures such as the thala- mus and basal ganglia. Efferent motor neurons in layers V and VI descend to the brain stem and spinal cord. The axons of these UMNs project through the subcortical white matter and internal capsule to the deep surface of the brain stem, through the ventral bulge of the pons, and on to the ventral surface of the medulla, where they form a ridge or pyramid (see Fig. 36-3). The majority of corticospinal UMNs cross the midline in the pyramidal ridge to form the lateral corticospinal tract on the oppo- site side of the spinal cord. The remaining UMNs do not cross to the opposite side, but pass down the ventral column of the cord, mainly to cervical levels, where they cross and innervate contralateral LMNs. Traditionally, motor tracts have been classified as belonging to one of two motor systems: the pyramidal and extrapyramidal systems. According to this clas- sification system, the pyramidal system consists of the motor neurons that cross the midline in the pyramidal ridge. All other descending UMNs emanating from the motor cortex and basal ganglia are generally grouped together as the extrapyramidal system. Disorders of the pyramidal tracts (e.g., stroke) are characterized by spasticity and paralysis, whereas those affecting the extrapyramidal tracts (e.g., Parkinson disease) result in involuntary movements, muscle rigidity, and immobil- ity without paralysis. As increased knowledge regard- ing motor pathways has emerged, it has become evident that the extrapyramidal and pyramidal systems are extensively interconnected and cooperate in the control of movement. 1