Segmental responses of abdominal motoneurons in decerebrate cats

As a first step towards elucidating the synaptic organization underlying se gmental responses of abdominal muscles I recorded the responses of branches of the left cranial (L1L) and caudal (L2L) and right caudal (L2R) lumbar ( iliohypogastric) nerves to electrical shocks of different intensities to th e caudal branch of L2L in nine decerebrate paralyzed and ventilated cats. I f such reflex responses subserve a respiratory function, then they should b e bilaterally similar; if they do not, lateral asymmetry should be evident. At intensities activating only large diameter axons (i.e. spindle and tend on organ afferents), stimulation typically elicited in the rostral branch o f L2L a brief (approximately 1.6 ms) short-latency (approximately 1.8 ms) e xcitation followed by a suppression of activity (approximately 8-26 ms). Re sponses increased in amplitude as stimulus intensity increased, the suppres sion of activity being interrupted by an excitation (latency approximately 5.4 ms, duration approximately 3.6 ms) in four cats. L1L responses were sim ilar. Contralateral responses in the same segment (L2R) in five cats consis ted 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 ne rve bundles and, therefore, target muscles, from which the recordings were made; and (2) because of the lateral asymmetry of responses, abdominal affe rent activation elicited postural (rotational) rather than respiratory refl exes. (C) 2000 Elsevier Science B.V. All rights reserved.