Significance of monosynaptic and polysynaptic reflexes in neonatal rat spinal cord in vitro.

Stimulation of a dorsal root in an isolated spinal cord, elicited a monosynaptic reflex (MSR) in the corresponding ventral root at a latency of 3-5 msec and a dorsal root reflex (DRR) in an adjacent dorsal root 2-3 times the latency of MSR. The magnitude of DRR was 10 times smaller than the MSR. These reflexes could be decreased by the agents which suppress synaptic transmission e.g. either by decreasing the [Ca2+]0 or by increasing [Mg2+]0. In the absence of magnesium ([Mg2+]0) in the perfusing medium, the MSR was 15-20% greater in its magnitude and the segmental polysynaptic reflex (PSR) was distinctly seen at 8-10 msec latency. Segmental PSR disappeared when [Mg2+]0 ions are added to perfusing medium probably indicating the involvement of N-methyl-D-aspartate receptors. Further, in this preparation homosynaptic inhibition and pre- or post-synaptic inhibitions could be elicited. This preparation, thus serves as useful model for the study of central synaptic transmission to study the pharmacological or neurotoxic agents.