Myelinated afferent fibres innervating the primate skin and their response to noxious stimuli

1,079

693

SUMMARY1. Single unit impulses were recorded with percutaneously inserted tungsten needle electrodes from the median nerve in conscious human subjects.2. A sample of 334 low threshold mechanoreceptive units innervating the glabrous skin area of the hand were studied. In accordance with earlier investigations, the units were separated into four groups on the basis of their adaptation and receptive field properties: RA, PC, SA I and SA II units. 3. The locations of the receptive fields of individual units were determined and the relative unit densities within various skin regions were calculated. The over-all density was found to increase in the proximo-distal direction. There was a slight increase from the palm to the main part of the finger and an abrupt increase from the main part of the finger to the finger tip. The relative densities in these three regions were 1, 1P6, 4-2.4. The differences in over-all density were essentially accounted for by the two types of units characterized by small and well defined receptive fields, the RA and SA I units, whereas the PC and SA II units were almost evenly distributed over the whole glabrous skin area.5. The spatial distribution of densities supports the idea that the RA and SA I units account for spatial acuity in psychophysical tests. This capacity is known to increase in distal direction along the hand.6. On the basis of histological data regarding the number of myelinated fibres in the median nerve, a model of the absolute unit density was proposed. It was estimated that the density of low threshold mechanoreceptive units at the finger tip is as high as 241 u./cm2, whereas in the palm it is only 58 u./cm2.

Section: Resultsmentioning

confidence: 94%

Tactile sensibility in the human hand: relative and absolute densities of four types of mechanoreceptive units in glabrous skin.

Johansson¹,

Vallbo²

1979

1,079

693

“…It is likely that the majority of HTMs reported in previous studies in cat, rabbit and monkey (e.g. Perl, 1968;Fitzgerald & Lynn, 1977) were in fact what we term here type I AMHs. The response latency distribution for hairy skin AMHs had its peak near the end of the stimulus.…”

Section: Discussionmentioning

confidence: 99%

Evidence for two different heat transduction mechanisms in nociceptive primary afferents innervating monkey skin.

Treede

Meyer

Raja

et al. 1995

398

286

1. Mechano-and heat-sensitive A fibre nociceptors (AMHs) and C fibre nociceptors (CMHs) in hairy skin (forty-six AMHs and twenty-one CMHs) and in glabrous skin (fifty-nine AMHs and ten CMHs) of anaesthetized monkeys were tested with a 30 s, 53°C heat stimulus, delivered by a laser thermal stimulator (0a1 s rise time, 7-5 mm diameter). 2. Two types of heat response were observed in hairy skin AMHs. Type I AMHs had a peak discharge towards the end of the stimulus, response latencies to heat of up to several seconds, a median heat threshold greater than 53°C, and a mean conduction velocity of 25 m s-' (n = 33). Type II AMHs had a peak discharge within 1-3 s, a mean response latency of 120 ms, a median heat threshold of 46°C, and a mean conduction velocity of 15 m s-' (n = 13). Type I AMH fibres were sensitized to heat, whereas heat responses of type II AMHs were suppressed following the intense heat stimulus. 3. In glabrous skin, only type I AMHs were found. The absence of type II AMHs is consistent with the absence of first pain to heat in glabrous skin. 4. C fibre nociceptors in hairy skin had a peak discharge near stimulus onset, a mean response latency of 100 ms and a median heat threshold of 41°C. Heat responses of CMHs in glabrous skin were not significantly different from those in hairy skin. 5. Only type II AMHs had response latencies that were short enough to explain first pain to heat. Heat thresholds of type II AMHs were significantly higher than those of CMHs. 6. These results suggest two different heat transduction mechanisms in nociceptive afferents.For one, heat energy is quickly transduced into action potentials, and the peak discharge is reached soon after stimulus onset. For the other, the transduction of heat is distinctly slower, and the peak discharge occurs near the end of the stimulus. Chemically mediated sensitization may be involved in the second transduction mechanism.

“…Conduction distances, estimated from measurements along the surface of the leg, were generally 9-14 cm, so the figures obtained for conduction velocity are probably accurate to within 20 % for the faster fibres and 10% for the slower fibres (Perl, 1968).…”

Section: Methodsmentioning

confidence: 92%

Sensory receptors in the sheep's foot

Evans¹

1973

4. The hair follicle units fell into two categories: 'diffuse' units which innervated many hairs in an area of about 1-3 cm2 anywhere on the hairy skin above the hoof, and 'localized' units which innervated a few hairs within small areas of the skin adjacent to the hoof. The 'diffuse' units generally resembled the ordinary hair follicle units of other animals except that in the sheep the afferents had higher conduction velocities. The 'localized' units also had large myelinated axons, but their physiological properties differed in several aspects from the guard hair, tylotrich and carpal hair units which have been studied in other animals.