Segmental responses of abdominal motoneurons in decerebrate cats

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As a first step towards elucidating the synaptic organization underlying segmental responses of abdominal muscles I recorded the responses of branches of the left cranial (L 1L) and caudal (L 2L) and right caudal (L 2R) lumbar (iliohypogastric) nerves to electrical shocks of different intensities to the caudal branch of L 2L in nine decerebrate paralyzed and ventilated cats. If such reflex responses subserve a respiratory function, then they should be bilaterally similar; if they do not, lateral asymmetry should be evident. At intensities activating only large diameter axons (i.e. spindle and tendon organ afferents), stimulation typically elicited in the rostral branch of L 2L a brief (approximately 1.6 ms) short-latency (approximately 1.8 ms) excitation followed by a suppression of activity (approximately 8–26 ms). Responses increased in amplitude as stimulus intensity increased, the suppression of activity being interrupted by an excitation (latency approximately 5.4 ms, duration approximately 3.6 ms) in four cats. L 1L responses were similar. Contralateral responses in the same segment (L 2R) in five cats consisted of a suppression of activity in four, a short-latency (approximately 3.3 ms) excitation being present in three; increases in stimulus intensity in two additional cats elicited these excitatory and inhibitory responses. I conclude: (1) the variable responses between cats reflect differences in nerve bundles and, therefore, target muscles, from which the recordings were made; and (2) because of the lateral asymmetry of responses, abdominal afferent activation elicited postural (rotational) rather than respiratory reflexes.