Postsynaptic potentials in toad's spinal motoneurons due to muscle afferent volleys.

Although some investigators are dubious of a direct connection between the dorsal root fibers and the motoneurons in frog spinal cord21), evidences for the monosynaptic connection between them have recently been presented by ARAKI3), BROOKHART and FADIGA7)and FADIGA and BROOKHART16) with toad's and frog's spinal cord. In their experiments a single orthodromic stimulus delivered to a dorsal root set up a monosynaptic excitatory postsynaptic potential (EPSP) which, however, never reached by itself motoneuron threshold, , unless either the potential was augmented by summation of plurisynaptic EPSP's, or the threshold was lowered by cathodal polarization. As is widely known, the monosynaptic reflex is more pronounced in antigravity muscles which support the animal body resisting the action of gravity. In this sense, it might be expected that, in frogs or toads, the monosynaptic reflex would be more developed in the muscles of forelimbs than in those of hindlimbs. In the present study, therefore, an attempt was made to explore the monosynaptic reflex in the rostral segments of toad's spinal cord which innervate forelimb muscles. Along with EPSP, inhibitory postsynaptic potential (IPSP) elicited by volleys in muscle afferents was the subject of the present study, on which only little has so far been known.