TOUCH AND VIBROTACTILE NEURONS IN A CROTALINE SNAKES TRIGEMINAL GANGLIA

Thirty-five touch (M) neurons and 59 vibrotactile (V + M) neurons were recorded intrasomally in the trigeminal ganglion of a crotaline snake (the pit viper, Trimeresurus flavoviridis). The M neurons were excite d by von Frey hair (5-10 mg) mechanical stimulation of the receptive f ield, and adapted slowly to a sustained stimulus. It was almost imposs ible to elicit 1:1 entrainment to sinusoidal movement. Vibration with touch was an adequate stimulus for the V + M neurons. The range of ent rainment to sinusoidal movement was 5-300 Hz. Thresholds of V + M neur ons to sustained mechanical stimulation could not be determined, but a response was obtained by stroking with a von Frey hair (5-10 mg). Rec eptive fields of both M and V + M neurons were found on the skin (scal es) and the mucous membrane of the orofacial region. There was one rec eptive held of similar to 2 mm in diameter for each M or V + M neuron. The mean resting potentials (+/- SD) of M and V + M neurons were -57. 0 +/- 5.1 mV (n = 26) and -63.7 +/- 8.2 mV (n = 49), respectively. Neu rons of both modalities displayed no background discharge. The action potential of V + M neurons had a shorter mean duration than that of M neurons. The mean conduction velocities (+/- SD) of peripheral (and st em) axons of M and V + M neurons were 28.4 +/- 5.7 m/sec (n = Il)and 3 0.8 +/- 7.8 m/sec (n = 30), respectively. Recorded neurons were labele d with intrasomal horseradish peroxidase electrophoresis. V + M neuron s had larger somata than M neurons. All axons of M and V + M neurons w ere myelinated and similar in diameter. M and V + M neurons had simila r central projection patterns. The projection of the thick central axo n divided into a thinner ascending fiber and a thick descending fiber at the entry zone of the root to the brainstem. The former ran ipsilat erally to the principal sensory nucleus of the trigeminal nerve (TPR), and the latter ran to the descending nucleus of the trigeminal nerve (TTD) and beyond, where terminal arbors and bouton swellings were obse rved. Smaller myelinated and unmyelinated collaterals were given off a t right angles from the descending fiber of the central axon into the TTD. They projected more densely to the rostral part than to the cauda l part of the TTD. All of these data were compared with data on warm-t emperature neurons, previously obtained.