Primary afferent depolarization in frog spinal cord is associated with an increase in membrane conductance.

The mechanism of primary afferent depolarization (PAD) was studied in the isolated frog spinal cord using intrafibre recording (microelectrodes filled with 0.6 M potassium sulfate) from large myelinated axons of dorsal roots. Standard current–clamp technique was used to obtain voltage–current (V–I) relationship. It was found that: (i) PAD is voltage dependent: its amplitude and rate of rise are increased with hyperpolarization; (ii) the slope of the linear part of the V–I curve obtained during PAD is decreased compared with the V–I curve at rest; (iii) the PAD equilibrium potential, estimated by extrapolation, ranged from −66 to −40 mV. These results suggest that PAD is associated with an increase in conductance of primary afferent terminals and thus seem to provide the first experimental evidence for the hypothesis that shunting of primary afferent membrane is the mechanism of presynaptic inhibition in the vertebrate nervous system.