Theoretical bases of short-latency spike volleys in the peripheral vestibular system.

If a spike trigger zone exhibits the same sort of accommodation that has been found universally in peripheral axons and is an emergent property of the Hodgkin-Huxley model and other synthetic models of axonal membrane (1,2), then spike production will be favored by steep positive slopes of the waveform of the current into the trigger zone. Thus, large positive steps of axial current flowing nearly simultaneously into the trigger zones of many vestibular axons should produce a short-latency spike volley over those axons. Reasoning in terms of the small-signal (linear) transfer relationship for the cascade of components preceding the trigger zone, one can show that the ability to translate a stepwise change in head acceleration into a rapid increase in axial current into the trigger zone is more strongly dependent on the number of zeros at infinity from poles with long time constants (longer than those of the spike initiator) than it is on the values of the time constants themselves. Larger numbers of zeros at infinity make such translation increasingly difficult. Evidence from the work of Fernandez and Goldberg and others suggests (3-7) that the number is low. It may be lowest in jerk-sensitive units (8-9), in which case one would expect such units to make the greatest contributions to short-latency spike volleys.