The functional properties and innervation density of type II mechanoreceptor units of the sural nerve of the rabbit.

The functional properties of Type II mechanoreceptor units of the sural nerve of the rabbit have been examined, using controlled mechanical stimuli applied to the skin. The response of Type II units exhibited 5 phases: (1) a resting spontaneous phase (2) an open phase, (3) a transient adaptive phase, (4) a regular static phase, and (5) an off phase. The impulse frequency/indentation depth relationship for the static phase was best fitted by a linear function with a slope of 0.015 and an intercept of 2.47. An almost as good fit could be obtained with a power function (exponent of 0.57 and a multiplying constant of 17.2). The impulse frequency/velocity characteristics was best fitted by a power function; it became horizontal at an average value of 5 micrometer/s. The mean threshold if Type II units was 17.2 micrometer; there was no change in response frequency once the displacement reached 1340 micrometer. There are approximately 206 Type II units innervating a skin area of 59.1 cm2, giving an innervation density of 3.42. The significance of the similarities and differences between Type II units in the rabbit compared with those in the cat are discussed.