Effects of spino-bulbo-spinal reflex volleys on flexor motoneurons of hindlimb in the cat

Summary o (1) Experiments were carried out on cats with chloralose anesthesia or decerebration to extend observations on the spino-bulbo-spinal (SBS) reflex and to analyze the effects of SBS reflex volleys on flexor motoneurons. (2) Stimulation of cutaneous afferents yielded late (SBS) reflex responses from the spinal ventral root 9 , as well as late responses in the primary afferents of flexor muscular and cutaneous origin. (3) A single volley in a cutaneous nerve yielded augmentation, early and late, and prolonged influences on the flexor mono- and polysynaptic reflexes (MSR and PSR). Early augmentation corresponded to facilitation at the spinal segment by direct volleys in cutaneous afferents. Late augmentation and other prolonged influences may be due to the SBS reflex system: (a) spinal transection at the C 1 level eliminated both; (b) they appeared under the same conditions, i.e. with similar spontaneous fluctuations, similarly blocked by pentobarbital anesthesia and asphyxia; and (c) the latency of late augmentation was the same as that of the SBS reflex response in the same ventral root. (4) Late augmentation seems to be due to the postsynaptic excitation, since there were EPSPs in flexor motoneurons at the same time as the SBS reflex. (5) The prolonged influences on the flexor reflexes included two different mechanisms: diminution of PSR and augmentation of MSR. The prolonged diminution of PSR and augmentation of MSR may be due to complex mechanisms underlying the supraspinal processes: presynaptic inhibition, postsynaptic inhibition, defacilitation and disinhibition of the motoneurons and spinal interneurons since (a) a late peak of the depolarization affecting the SBS reflex was obtained in afferent fibers of cutaneous and muscle origin following stimulation of the sural nerve, and (b) prolonged hyperpolarization and depolarization seen in flexor motoneurons involved two different processes, IPSP and defacilitation, and EPSP and disinhibition.