Sensory transduction in dorsal cutaneous mechanoreceptors of the frog,Rana pipiens

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Abstract
Sensory transduction was studied in dorsal skin mechanoreceptors of the frog,Rana pipiens. The skin was clamped and stretched before being stimulated with the tip of a glass rod mounted on a servo-controlled loudspeaker. Afferent activity was recorded extracellularly from a dorsal cutaneous nerve. Three groups of sensory units could be identified by the size of their recorded action potentials and their response to mechanical stimuli. Action potential amplitudes for the three groups were: 300 μV (group III). Group II were selected for further study because of their amplitude and their resistance to fatigue. Three types of mechanical stimuli were used to examine the dynamic properties of group II receptors, steps, sinusoids, and band-limited random displacement. In each case the responses could be well fitted by a power-law model with a fractional exponent of time or frequency. Random stimulation of a large number of group II receptors showed considerable variability in their sensitivity and in their dynamic behavior, as measured by the fractional exponent of frequency. However, the distributions of these two parameters were both unimodal and strongly clustered around the modes, suggesting that the recordings were from a single class of receptors. Varying the temperature of the receptors had little effect on their sensitivity or dynamic properties. This is in contrast to findings on other mechanoreceptors. Doubling the potassium concentration in the bathing solution affected the dynamic properties of the receptors within 5 min but several distinct patterns of change in dynamic behavior were seen. The mean conduction velocity in the afferent nerves was 2.84 m/s. On this evidence we suggest that free nerve endings are the most likely morphological class of receptors to be responsible for group II responses.