The effect of transection and cold block of the spinal cord on synaptic transmission between Ia afferents and motoneurones.

Composite excitatory postsynaptic potentials were elicited in lumbar motoneurones by Ia afferents from muscles of the triceps surae group. These excitatory postsynaptic potentials were examined in the same cell before, during and after interruption of descending spinal pathways. After transection or cold block of the spinal cord at T12-T13, the amplitude of composite excitatory postsynaptic potentials showed no significant change for a period of up to seven hours after transection. However, there was a reduction in amplitude of the monosynaptic reflex in the extensor motoneurones which may be due to an observed hyperpolarization and reduction in membrane time constant in these neurones. The reduction in amplitude of the monosynaptic reflex observed in spinal shock can be attributed to the effects of these changes, rather than to a decrease in the size of the monosynaptic excitatory postsynaptic potential.