The characteristics of slowly adapting mechanical sensory neurons in the trigeminal ganglia of crotaline snakes

Mechanical pressure (P) and touch (M) neurons, two types of slowly adapting neurons, can be distinguished in the trigeminal ganglion of crotaline snakes, Trimeresurus flavoviridis, by their electrophysiological membrane properties. Intracellular stimulation and the recording of the responses from P and M neurons in vivo was performed with microelectrodes. The characteristics of the receptive field (RF) of both types of neurons were examined. P neurons had a much larger RF size and a higher mechanical threshold than those of M neurons. Both of them responded with discharges to mechanical stimulation. The active and passive electrophysiological membrane properties were measured from nine P and 18 M neurons. The active membrane properties of P neurons showed a larger amplitude and longer duration of action potential, a larger after-hyperpolarization with a longer duration to half-decay, and a higher electric threshold in response to intracellularly injected depolarizing current than M neurons. The passive membrane properties of P neurons showed a higher input resistance, much longer time constant, and larger capacitance than M neurons. The rebound spike which responded to injecting sufficient hyperpolarizing current was sometimes observed, and had a longer latency in P neurons than M neurons. These results indicated that some electrophysiological membrane properties of primary sensory neurons are dependent on their sensory modalities.