Myelinated afferent fibres responding specifically to noxious stimulation of the skin.

1. The characteristics of receptors from the hairy skin of the hind limb of cat were studied by recording from single primary afferent fibres with fine micropipettes. The distinctive features of 513 fibres conducting under 51 m/sec are described.2. Seventy-four fibres conducting between 6 and 37 m/sec were classified as nociceptors because they responded only to damaging mechanical stimulation of the skin. These fibres responded maximally to pinching the skin with a serrated forceps or to cutting the skin. Noxious heat, noxious cold, acid applied to the receptive field and bradykinin injected into skin cuts did not evoke discharges from such receptors. Typically their receptive fields were 2-5 cm long by 1-2.5 cm wide and consisted of responsive spots (under 1 mm diameter) separated by unresponsive areas. There was a tendency for the most slowly conducting fibres so classified to be the least sensitive.3. Other afferent fibres had receptive fields similar to the nociceptors; however, they were excited by substantial but not noxious mechanical deformation. Their conduction velocities overlapped those of the nociceptors and extended upwards to 51 m/sec; the most rapidly conducting fibres tended to be the most sensitive to mechanical stimuli. These insensitive mechanoreceptors or moderate pressure receptors adapted more slowly than the nociceptors.4. The majority of fine myelinated axons originated from hair receptors and had conduction velocities concentrated between 14 and 22 m/sec.5. The possible relation of these observations to pain and reactions typical of pain is considered.