Touch and Vibrotactile Neurons in a Crotaline Snake's Trigeminal Ganglia

Abstract

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 excited by von Frey hair (5–10 mg) mechanical stimulation of the receptive field, and adapted slowly to a sustained stimulus. It was almost impossible to elicit 1:1 entrainment to sinusoidal movement. Vibration with touch was an adequate stimulus for the V + M neurons. The range of entrainment to sinusoidal movement was 5–300 Hz. Thresholds of V + M neurons to sustained mechanical stimulation could not be determined, but a response was obtained by stroking with a von Frey hair (5–10 mg). Receptive fields of both M and V + M neurons were found on the skin (scales) and the mucous membrane of the orofacial region. There was one receptive field of ∼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. Neurons 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 stem) axons of M and V + M neurons were 28.4 ± 5.7 m/sec (n = 11) and 30.8 ± 7.8 m/sec (n = 30), respectively.

Recorded neurons were labeled with intrasomal horseradish peroxidase electrophoresis. V + M neurons had larger somata than M neurons. All axons of M and V + M neurons were myelinated and similar in diameter. M and V + M neurons had similar central projection patterns. The projection of the thick central axon divided into a thinner ascending fiber and a thick descending fiber at the entry zone of the root to the brainstem. The former ran ipsilaterally 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 observed. Smaller myelinated and unmyelinated collaterals were given off at right angles from the descending fiber of the central axon into the TTD. They projected more densely to the rostral part than to the caudal part of the TTD.

All of these data were compared with data on warm-temperature neurons, previously obtained.

Key Words:

• trigeminal ganglion
• somal action potential
• touch neuron
• axon diameter
• central projection
• mechanoreceptors
• reptilian
• vibrotactile sense organs