The neurotransmitter systems mediating spinal locomotion in response to epidural spinal cord stimulation (ES) have not been identified. Here, we examine the role of the serotonergic system in regulating locomotor behavior of decerebrated cats during ES at L4 -L5. ES elicited coordinated, weight-bearing, hindlimb stepping with plantar foot placement on a moving treadmill belt. Ketanserin [a 5-hydroxytryptamine (serotonin) (5-HT) 2/7 receptor antagonist] depressed this locomotor activity: only weak rhythmic movements without plantar foot placement and depressed EMG activity were observed. Cyproheptadine, a nonselective 5-HT blocker, prevented facilitation of stepping by epidural stimulation. These data demonstrate an important role of the serotonergic system in facilitating locomotion in the presence of epidural stimulation. In the presence of ketanserin, passive movements of one forelimb in a step-like manner immediately induced stepping of both hindlimbs with EMG patterns similar to those observed with ES without ketanserin. Thus, a non-5-HT-dependent spinal circuitry projecting from the cervical to the lumbar region of the spinal cord can facilitate stepping. The specific neurotransmitters responsible for this forelimb-facilitated stepping of the hindlimbs are unknown. These data suggest that a 5-HT 2/7 receptor-dependent pathway that processes hindlimb locomotor-like proprioception to facilitate hindlimb stepping can be complemented with proprioceptive afferents from the forelimbs via a non-5-HT 2/7 receptor neurotransmitter system. Thus, different neurotransmitter receptor systems can be used to mediate the same type of sensory event, i.e., locomotor-like proprioception to facilitate the same motor task, i.e., hindlimb stepping.
Acute experiments on decerebrate cats were performed to study the mechanism of formation of the locomotor pattern in conditions of epidural stimulation of the spinal cord. These studies showed that only segments L3-L5 contributed to generating the stepping pattern in the hindlimbs. At the optimum frequency (5-10 Hz) of stimulation of these segments, formation of electromyographic burst activity in the flexor muscles was mainly due to polysynaptic reflex responses with latencies of 80-110 msec. In the extensor muscles, this process involved the interaction of a monosynaptic reflex and polysynaptic activity. In epidural stimulation, the stepping pattern was specified by spinal structures, while peripheral feedback had modulatory influences.
Acute experiments on decerebrate cats were performed to study the mechanism of formation of the locomotor pattern in conditions of epidural stimulation of the spinal cord. These studies showed that only segments L3-L5 contributed to generating the stepping pattern in the hindlimbs. At the optimum frequency (5-10 Hz) of stimulation of these segments, formation of electromyographic burst activity in the flexor muscles was mainly due to polysynaptic reflex responses with latencies of 80-110 msec. In the extensor muscles, this process involved the interaction of a monosynaptic reflex and polysynaptic activity. In epidural stimulation, the stepping pattern was specified by spinal structures, while peripheral feedback had modulatory influences.
Acute experiments on decerebrate and spinal cats were performed to study the role of the peripheral afferent input from hindlimb receptors in forming the locomotor pattern during epidural stimulation of the spinal cord. Evoked electromyographic activity in the muscles of the hindlimbs was analyzed, along with the kinematic parameters of stepping movements. Epidural stimulation (20-100 microA, 5 Hz) of segments L4-5 of the spinal cord was found to elicit well coordinated walking in the hindlimbs on a moving treadmill band. When the support conditions were changed (non-moving treadmill, unsupported position), epidural stimulation initiated walking with an unstable rhythm. This was associated with a change in the overall nature of the locomotor pattern and the internal structure of the stepping cycle. Alteration of the direction of movement of the treadmill band led to the appearance of backward walking. An increase in the speed of movement of the treadmill band increased the stepping frequency, mainly due to decreases in the extensor phase. Epidural stimulation applied 2-4 h after complete transection of the spinal cord at the T8-T9 level could elicit stepping movements, but only when the treadmill was moving. The role of peripheral feedback in generating the locomotor pattern in conditions of complete disconnection from supraspinal control increased significantly. These data show that peripheral feedback during epidural stimulation of the spinal cord can define the properties of the motor output.
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