1. Thermal stimulation of frog skin produces a discharge in afferents in the dorsocutaneous nerve. The characteristics of this response have been examined with regard to static and dynamic sensitivity to thermal stimuli and to mechanical sensitivity. Frog cutaneous receptors respond only to cooling, with no response to warming through the same thermal range. 2. The static temperature at which these receptors are maximally active is about 24° C for Rana pipiens and about 27° C for R. catesbiana. 3. The dynamic sensitivity of frog cutaneous receptors is linearly related to both stimulus slope and magnitude. Maximum dynamic sensitivity was between −90 and −120 impulses/° C.sec. 4. Antidromic occlusion experiments demonstrate relative insensitivity of these receptors to tonic mechanical stimulation. At high stimulus intensities, however, larger fibres are recruited into the response; this recruitment of action potentials of larger amplitude is a linear function of both stimulus slope and magnitude. 5. Spike heights are linearly related to conduction velocities in the dorsocutaneous nerve; tonic mechanoreceptors have a mean spike height of 28·4±0·6 μV and conduction velocities about 6–8 m/sec, whereas these temperature sensitive receptors have spike heights 15·8±0·4 μV and conduction velocities about 3–4 m/sec. 6. Maximum dynamic sensitivity skin is increased following stimulation of the first or second sympathetic ganglion. This increase is both marked and progressive, reaching a maximal enhancement of about 150–160% control at a stimulus rate of 5 stimuli/train, each train delivered once every 5 sec. 7. Static sensitivity of the cold receptors is also increased following sympathetic stimulation. This increased sensitivity is shown by both increased discharge rate within the same thermal range and by decreased temperature of maximum static sensitivity. 8. Sympathetic modulation of dynamic thermal sensitivity is mimicked by epinephrine and norepinephrine in doses of 10−6‐10−7 g/ml. Ephedrine, another adrenergic agonist, also mimics the enhancement of cold receptors by sympathetic stimulation. 9. Larger fibres are recruited to account for the increased sensitivity of thermoreceptors following sympathetic stimulation and epinephrine application. 10. Propranolol and phentolamine both block the enhancement of the response by sympathetic stimulation, but propranolol blocks the response of the receptor to thermal stimulation as well. Reserpine pre‐treatment blocks the effect of sympathetic stimulation on the cold response.
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