Subdivision of primary afferents from passive cat muscle spindles based on a single slow-adaptation parameter

A universal model of single-unit sensory receptor action

We present a model governing the operation of all "isolated" sensory receptors with their primary afferent neurons. The input to the system is the intensity of a sensory signal, be it chemical, optical etc., and the output is the rate of neural impulse transmission in the afferent neuron. The model is based on the premise that information is conserved when transmitted by an efficient sensory channel. Earlier studies on this informational or entropic model of receptor function were constrained to the cases where the sensory stimulus assumed the form of an infinite step function. The theory was quite successful in treating mathematically all responses to such elementary sensory stimuli--both neural, and by extension, psychophysical. For example, the theory unified the logarithmic and power laws of sensation. However, the earlier model was incapable of predicting responses to time-varying stimuli. The generalized model, which we present here, accounts for both steady and time-varying signals. We show that more intense stimuli are remembered by sensory receptors for longer periods of time than are less intense stimuli.

Repetitive activity of a branched Hodgkin-Huxley axon with multiple encoding sites

Afferent activity in a receptor afferent fiber with several encoding sites is generally believed to represent the activity of the fastest pacemaker that resets all more slowly encoding sites. Alternatively, some impulse mixing as well as some nonlinear summation of receptor current to a single encoder have been considered. In this article the repetitive firing activity of a Hodgkin-Huxley axon consisting of two branches that join into a single stem axon was investigated. The model axon was stimulated by constant-current injection into either the right or the left or both branches. It was found that the model axon generated an (infinite) train of action potentials if the input current was large enough. The discharge frequency found was constant, and on combined stimulation of both branches with different current, the site of impulse initiation was always in the branch receiving the higher input current, excluding a simple impulse mixing. On the other hand, the combined stimulation of both branches evoked repetitive firing with a higher frequency than expected by the pacemaker-resetting hypothesis. Moreover, a stimulus that is subthreshold for repetitive firing if injected into one branch yields repetitive firing when it is injected into both branches, a behavior inconsistent with impulse mixing and pacemaker resetting. On the other hand, current injection into one branch allowed repetitive activity only within a rather limited range of firing frequencies. Using distributed current injection into both branches, however, allowed many more different firing frequencies. Such behavior is inconsistent with both pacemaker resetting and (nonlinear) input current summation. Consequently, the repetitive firing behavior of a branched Hodgkin-Huxley axon with multiple encoding sites appears to be more complex than postulated in the simple hypotheses.