Segmental synaptic responses in the canine spinal cord for stimulation of the nerves of trunk muscles

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Responses of alpha-motoneurons of trunk muscles (multifidus, longissimus, iliocostalis, obliquus externus abdominis and rectus abdominis) were investigayted intracellularly by stimulation of these trunk nerves or cutaneous nerves in decerebrate spinal cats.Motoneurons of the back muscles (multifidus, longissimus and iliocostalis) and abdominal muscles (obliquus externus abdominis and rectus abdominis) were excited bilaterally by stimulation of the nerves of the back or abdominal muscles and the cutaneous nerves. The dominant excitations were polysynaptic responses equal to or over 3 synapses. There were no crossed Ia IPSP responses between the left and right back muscles or abdominal muscles. Only a few bisynaptic IPSP responses existed in motoneurons of the back muscles after stimulation of the abdominal muscle nerves, and in those of the abdominal muscles after stimulation of the back muscles. These results indicated that ipsilateral back and abdominal muscles, left and right back or abdominal muscles and diagonal back and abdominal muscles act not as reciprocal muscles but as coordinate muscles. Tere were strong connections between nieghboring muscle because monosynaptic EPSPs were found in motoneurons of the multifidus after stimulationof the longissimus muscle nerve and in those of the longissimus after stimulation of the iliocostalis muscle nerve.Segmental synaptic responses in cats under alphs-chloralose anesthesia was complex and fifferent from those in spinal cats. Although polysynaptic EPSP responses were dominant in most of examined motoneurons in chloralose anesthtized cats, polysynaptic IPSP responses existed much more in chloralose anesthetized cats than in spinal cats. Dominant polysynaptic IPSP responses were found in the longissimus motoneurons after stimulation of the ipsilateral multifidus nerve. These results indicate that descending inputs from the brain may give inhibitory effects on the trunk motoneurons.