Slowly adapting type I (SAI) and type II (SAII) mechanoreceptors in the skin were studied in anaesthetized cats and rats employing mechanical stimuli every 30 s. Individual stimuli rose within 200 ms to a plateau force which was kept constant through a feedback control unit for 2000 ms. 2. In cats, close arterial infusion of neomycin (2.5 mg/min) as sulphate was given through a side branch into the femoral blood stream for 5, 10 or 20 min at a rate of 0.025 ml/min. At other times saline was infused at the same rate. 3. After 20 min of neomycin infusion (total 50 mg) nervous discharge of cat SAI receptors was suppressed to about 30% of the control responses before neomycin infusion. Nervous responses were reduced more profoundly during the plateau phase of stimulation than during the dynamic phase. The interspike interval histogram was severely distorted. 4. In contrast, cat SAII receptors maintained about 70% of their control response after 20 min of neomycin infusion. The interspike interval histogram showed an orderly shift towards longer intervals maintaining its normal shape. 5. In rats, intradermal microinfusion of neomycin (30 micrograms/min) through a glass micropipette into the immediate vicinity of the receptor under investigation resulted in severe transient suppression of SAI receptor responses to about 10% of the control level. Receptor responses recovered almost completely about 1 h after the end of neomycin application. 6. It is concluded that the observed differences between the two types of slowly adapting mechanoreceptors are consistent with the hypothesis that the SAI receptor functions as a secondary sensory receptor, with a synaptic link between the Merkel cell and the primary afferent neurone.
