Cutaneous primary afferent properties in the hind limb of the neonatal rat.

1. Single cutaneous primary afferent units innervating the hind limb were recorded from the L4 dorsal root ganglion in neonatal rats aged 0-14 days. 2. Conduction velocities were less than 1.0 m s-1 on day 0 but by day 1 had clearly divided into a faster conducting group whose velocities markedly increased with age, reaching 15 m s-1 on day 14, and a slower conducting group that increased only slightly to 1.5 m s-1 on day 14. 3. All major cutaneous receptor types could be identified from day 0. Slowly adapting and rapidly adapting touch and hair mechanoreceptors had the same response patterns and thresholds described for such receptors in the adult, but peak firing frequency and ability to follow high frequency electrical stimulation was low. At birth, these afferents could only follow 20 Hz stimuli but by day 14 this had increased to 200 Hz. 4. Nociceptors were also present from birth. Polymodal nociceptors responding to intense mechanical, thermal and chemical skin stimulation were completely mature in their firing frequencies and response patterns from day 0. High threshold mechanoreceptors responded at somewhat lower frequencies than the adult but otherwise were comparable. 5. A prominent group of receptors responding to firm pressure on the skin were observed in the first post-natal week but not on day 14. It is proposed that these may represent maturing touch or hair receptors or alternatively a group of temporary receptors that are lost during development. 6. On the first post-natal days, there was no relation between conduction velocity and receptor type. After this time, however, afferents which have A beta fibres in the adult tended to conduct faster in the neonate than those with A delta and C fibres. There were exceptions however demonstrating that receptor properties and afferent axons do not always mature in parallel. 7. These results show that considerable maturation of cutaneous receptors must occur before birth. They suggest that central rather than peripheral maturation is responsible for the post-natal changes in dorsal horn physiology and cutaneous sensory reflexes. Furthermore since C polymodal nociceptors are fully functional at birth, the delayed development of neurogenic oedema and of certain specific C-evoked reflexes could be due to slow maturation of critical chemical factors in C fibres.