The spatial distribution and functional properties of type I mechanoreceptor units of the sural nerve of the rabbit.

The functional characteristics and spatial arrangement of Type I mechanosensitive units found in the sural nerve of the anaesthetized rabbit have been investigated. There are approximately 108 Type I units innervating a skin area of about 59.1 cm2 supplied by the sural nerve giving an innervation density of 1.83. This innervation density of 1.83 does not vary over the skin territory innervated by the sural. The dome/fibre ratio was 2.7. The domes were found to be distributed in a dispersed but fairly regular pattern. The average nearest neighbour distance was 5.3 mm. A stimulus of 5.5 threshold units showed the existence of a small receptive field of radius 2 mm surrounding each dome; at a 3 mm radius even the largest 2 mm indentation failed to evoke a response. The response of the Type I units exhibited 4 distinct phases: (1) a dynamic phase; (2) a transient adaptive phase; (3) a highly irregular static phase; (4) an off phase. The impulse frequency/static displacement relationship was best fitted by a power function. The impulse frequency-velocity characteristic became horizontal at an average value of 15 micrometers/ms. A power function provided the best fit for this relationship. The possible physiological significance of the differences between properties of Type I receptor populations in the rabbit and cat are discussed.