The nature and location of certain phasic mechanoreceptors in the cat's foot.

1. The locations of the receptor endings of twenty-two phasic, primary afferent units which could be excited by small (less than 40 mum) point displacements of cats' large foot pads have been determined by dissection.2. Eleven of these units had endings located within or immediately below the skin on the plantar surface of the pad; eight had endings located deep within the pad and three had endings on the digits.3. All the deep and digital endings were of the PC type of Jänig, Schmidt & Zimmermann (1968) whilst five of the superficial endings were definitely of the RA type of Jänig et al. (1968), and another five could have been. One superficial ending had properties intermediate between those of RA and PC receptors.4. It was shown by dissection that the endings of four of the PC type units were Pacinian corpuscles and such corpuscles were seen within 1 mm of the apparent location of six other PC receptors. Pacinian corpuscles were found close to only two of the eleven superficial receptor locations.5. It is therefore concluded that of the two functionally distinct types of phasic receptors studied, the PC receptors are Pacinian corpuscles located deep in the pad or on the digits, whilst the RA receptors are located within or immediately below the skin and are not Pacinian corpuscles.