Contributions by Individual Guard Hairs and Their Interactions in Response of Forelimb Guard Hair Afferent Unit

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1. In anesthetized, immobilized cats, single primary afferent axonal responses to movement of forelimb G hairs were recorded in the ipsilateral cuneate fasciculus with a glass-insulated tungsten microelectrode.

2. One or more innervated receptive-field G hairs were isolated with a thin shield, which prevented all other receptive-field hairs from being driven by the test mechanical stimulus.

3. Spike trains were elicited by mechanical stimulation of one or more innervated hairs either by continuous air jet or by trains of fixed displacement transients and were recorded either in the steady state or over an isochronal period beginning with the onset of the maintained mechanical stimulation.

4. Maximal intensity air-jet stimulation of one hair evoked a mean discharge frequency that was significantly lower than that evoked by applying the same stimulus to the entire receptive field.

5. Steady-state discharge evoked by air- jet stimulation of a single hair had a) an inter- spike interval distribution with a dead time, a monotonic rising limb to a single mode, and a monotonic falling limb; and b) serial interspike-interval independence except for the shortest intervals.

6. The isochronal mean frequency elicited by maximal intensity air-jet stimulation of pairs of innervated hairs was less than that elicited by whole-field stimulation. While the dual-hair response frequency was greater than that elicited by either hair alone, it was either less than, greater than, or close to the sum of the individually elicited frequencies.

7. Comparison of the dual-hair-evoked spike train with that which resulted from the superposition of the two individually evoked spike trains revealed that during dual-hair stimulation, there is: a) a deficit of the shortest predicted intervals; and b) either less than, greater than, or approximately the same distribution of other classes of predicted intervals. The relative amounts of a and b determine the degree of non-arithmetic summation of constituent frequencies (item 6, above).

8. Electrical stimulation of the parent afferent axon elicited a long-duration cycle of elevated mechanical threshold as tested from a single innervated hair. However, electrical stimulation of the parent axon during continuous air-jet stimulation did not significantly change the pattern of air-jet-evoked spike generation in the poststimulus histogram except for the absence of an early greater-than-steady-state elevation of conditional firing probability for short mean interval units.

9. Using brief triangular displacement of innervated hairs at sufficiently high periodic repetition rates, the output spike frequency evoked by driving N hairs was found to be less than that evoked by driving N + 1 hairs. The interspike-interval distribution (IID) of the spike train evoked by such stimulation has a mode at the interstimulus interval or multiples thereof.

10. The marked difference between IIDs of responses to air-jet and periodic repetitive stimuli most likely reflects aperiodic components of hair movement under the air jet.

11. Processing of information by hair follicle and touch afferent units is contrasted.

12. Summation of generator potentials or of blocked impulses at sites of probabilistic transmission is proposed as the explanation of the observed facilitation; the branch points in the arborization of the parent axon are proposed as critical sites of probabilistic transmission.

13. The output discharge frequency in the parent axon is modeled by a set of equations that separately describe the effects of facilitation and refractoriness.


Copyright © 1980 The American Physiological Society