Reciprocal Reflexes

Two important principles should be recognized: (1) The myotactic and reverse myotactic reflexes

reflex measure the force between the muscle and its insertion. When the receptor of the Golgi tendon organ is stretched, either by passive stretching or muscle contraction, it activates an interneuron that inhibits the motor neurons going to the same muscle. Shown in blue are neurons that are excited when the Golgi tendon organ is stretched.

reflex measure the force between the muscle and its insertion. When the receptor of the Golgi tendon organ is stretched, either by passive stretching or muscle contraction, it activates an interneuron that inhibits the motor neurons going to the same muscle. Shown in blue are neurons that are excited when the Golgi tendon organ is stretched.

represent key ingredients in most movements ofthe body, from the most elementary to the most complex. These reflexes might be viewed as representing basic "units" of spinal motor programs. (2) These reflexes do not occur as isolated events; rather, they are ongoing and must be carried out in a coordinated fashion among related muscle groups. This is achieved by a third basic component of spinal motor programs, reciprocal reflexes (Fig. 4). For contraction of a selected muscle to succeed in accomplishing the intended movement of the body, synergistic muscles must also be activated to participate in the movement, and antagonistic muscles must be inhibited so they do not interfere. This is accomplished by interneuron networks in the spinal cord that provide excitatory

A. Muscles and neurons involved in a pain withdrawal reflex nociceptive stimulus dorsal nociceptive afferent nociceptive stimulus

excitatory interneuron a motor neurons to flexor a motor neurons to extensor dorsal nociceptive afferent excitatory interneuron a motor neurons to flexor a motor neurons to extensor

inhibitory interneurons

B. Crossed extensor pain reflex and central paths

B. Crossed extensor pain reflex and central paths

FIGURE 4 Circuitry of the polysynaptic flexor (withdrawal) and crossed extensor reflexes. Neurons that are excited by nociceptive input are shown in blue. (A) Pain withdrawal reflex; stimulated nociceptive fibers send excitatory information to activate interneurons (e) in the spinal cord. Interneurons then stimulate alpha motor neurons that activate flexors, and they inhibit alpha motor neurons that drive extensors. (B) A strong nociceptive stimulus can elicit a crossed extensor response through fibers that cross to the contralateral side to stimulate and inhibit appropriate alpha motor neuron groups. In the legs, this reflex keeps one standing. In the arms, it helps one push away from the irritant and maintain balance.

FIGURE 4 Circuitry of the polysynaptic flexor (withdrawal) and crossed extensor reflexes. Neurons that are excited by nociceptive input are shown in blue. (A) Pain withdrawal reflex; stimulated nociceptive fibers send excitatory information to activate interneurons (e) in the spinal cord. Interneurons then stimulate alpha motor neurons that activate flexors, and they inhibit alpha motor neurons that drive extensors. (B) A strong nociceptive stimulus can elicit a crossed extensor response through fibers that cross to the contralateral side to stimulate and inhibit appropriate alpha motor neuron groups. In the legs, this reflex keeps one standing. In the arms, it helps one push away from the irritant and maintain balance.

reciprocal connections among synergistic motor units and inhibitory reciprocal connections between pairs of antagonistic motor units. Cell bodies of these interneur-ons are dispersed throughout the length of the cord in the ventral horn and intermediate gray matter. Their processes ascend and descend in the anterolateral funiculi to reach appropriate levels where they decussate in the anterior white commissure to the contralateral side to link necessary components.

Other types of spinal motor programs provide protective reflexes that quickly remove the body or extremity from the dangerous object (the withdrawal or nociceptive reflex) or remove the potentially dangerous object from the body (the wipe or scratch reflex). Nociceptive fibers subserving the withdrawal reflex enter the spinal cord and activate interneurons that form excitatory connections, with flexor motor neurons supplying the entire ipsi-lateral limb and reciprocal inhibition of extensor muscles to that limb. The result is a rapid flexion of the limb. If the flexion reflex involves the leg, a compensatory extension of the opposite leg must occur to maintain balance. This is achieved automatically through a special type of reciprocal innervation to elicit a crossed-extensor reflex.

The scratch reflex is more complex in that it involves repetitive or rhythmic contraction and relaxation of muscles in the arm and fingers in response to a localized noxious stimulus. Even so it is achieved by interneuron circuits residing solely in the spinal cord and is indicative of the sophistication in motor programs that can be achieved without involvement from the brain.

Similar alternating movements are involved in walking. They are reflexive and are also programmed in the spinal cord. They do not require descending commands from upper motor neurons in motor centers of the brain. Though not completely understood, it has been demonstrated in experimental animals that complete transection of the cord at the midthoracic level leaves the hind limbs capable of generating coordinated walking movements. Groups of interneurons in the spinal cord act as central pattern generators, providing a series of alternating commands to control extension and contraction movements that provide the essential building blocks for locomotion. Additional interneuronal connections between the lumbar and cervical spinal segments are necessary to regulate swinging of the arms that accompanies walking.

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