Spinal Reflex Pathways

Muscle Stretch Reflex. Parts A and B show some of the connections made by groups Ia and II (not shown) afferent fibers from muscle spindle receptors. These afferent fibers are responsible for the muscle stretch reflex, in which the stretching of a muscle elicits a contraction of that muscle and its close synergists and a relaxation of its antagonists (see Plate 2-14). When the muscle is stretched, its spindle receptors are activated, thus causing increased firing of the spindle afferent fibers. The direct monosynaptic excitation of the motor neurons by these spindle afferent fibers contributes to the contraction of the stretched muscle and its synergists (B). Relaxation of the antagonist muscles is produced by a disynaptic inhibitory pathway involving an interneuron. The stretching of an extensor muscle leads to a reflex contraction of the extensors acting at that particular joint and to a simultaneous relaxation of the antagonistic flexor muscles.

Excitation and inhibition of motor neurons (B) are mediated by axosomatic or axodendritic synapses. Muscle spindle afferents, as well as other afferents, may also activate the circuits that modulate the action of afferent fibers by means of presynaptic afferent inhibition (A). Here, la fibers from either the flexors or the extensors activate an inhibitory neuron, which forms an axoaxonic synapse with a muscle spindle afferent fiber that terminates on an extensor motor neuron. The action of these synapses blocks or decreases the excitation of a motor neuron by muscle spindle afferents.

Recurrent inhibition is another type of neural interaction that controls the activity of motor neurons (C). It is produced by the collaterals of motor neurons that excite inhibitory interneurons known as Renshaw cells. When the motor neurons discharge, the Renshaw cells are activated by the motor neuron collaterals and fire a train of action potentials. The firing of the Renshaw cells causes inhibition of motor neurons of the same muscle and of other related, synergistic muscles. In addition to limiting the firing rate of motor neurons, this inhibition is also thought to restrict motor activity to the most intensely excited motor neurons.

Tendon Organ Reflex. Reflex actions evoked by Ib afferent fibers from Golgi tendon organs are shown in D. These fibers are activated by active (strong) tension in a muscle. When thus activated, the Ib fibers excite spinal interneurons, which inhibit the motor neurons that supply the particular muscle from which the Ib fibers originate and simultaneously excite the motor neurons that supply antagonist muscles. Thus the tendon organ reflex action is opposite to that produced by muscle spindle afferent fibers. The tendon organ reflex was once thought to play a role in protecting the muscles from excessive tension, but the excitatory influence of types Ia and II afferents during brief muscle contractions exceeds any inhibitory effects from the tendon organs. The tendon organ discharge may provide a “force feedback” signal whose inhibitory action opposes the excitatory “length feedback” signal provided by muscle spindle afferents during periods when the muscle is actively generating tension.

Flexor Withdrawal Reflex. Complex pathways are involved in the familiar flexor withdrawal reflex evoked by a painful stimulus (E). Such a stimulus activates nociceptive afferent fibers, which produce the firing of chains of neurons in the posterior horn of the spinal cord. These neurons, in turn, activate the interneurons in the anterior horn that excite flexor motor neurons and inhibit extensor motor neurons on the side of the painful stimulus. At the same time, commissural neurons activate circuits that excite extensor motor neurons and inhibit flexor motor neurons on the opposite side. The resulting reflex response is flexion or withdrawal of the stimulated limb and extension of the opposite limb.

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