Tactile neural mechanisms in monotremes.
Monotremes, perhaps more than any other order of mammals, display an enormous behavioural reliance upon the tactile senses. In the platypus, Ornithorhynchus anatinus, this is manifest most strikingly in the special importance of the bill as a peripheral sensory organ, an importance confirmed by electrophysiological mapping that reveals a vast area of the cerebral cortex allocated to the processing of tactile inputs from the bill. Although behavioural evidence in the echidna, Tachyglossus aculeatus, suggests a similar prominence for tactile inputs from the snout, there is also a great reliance upon the distal limbs for digging and burrowing activity, pointing to the importance of tactile information from these regions for the echidna. In recent studies, we have investigated the peripheral tactile neural mechanisms in the forepaw of the echidna to establish the extent of correspondence or divergence that has emerged over the widely different evolutionary paths taken by monotreme and placental mammals. Electrophysiological recordings were made from single tactile sensory nerve fibres isolated in fine strands of the median or ulnar nerves of the forearm. Controlled tactile stimuli applied to the forepaw glabrous skin permitted an initial classification of tactile sensory fibres into two broad divisions, according to their responses to static skin displacement. One displayed slowly adapting (SA) response properties, while the other showed a selective sensitivity to the dynamic components of the skin displacement. These purely dynamically-sensitive tactile fibres could be subdivided according to vibrotactile sensitivity and receptive field characteristics into a rapidly adapting (RA) class, sensitive to low frequency (</=50 Hz) vibration and another class, sensitive to a broader range of vibrotactile frequencies (approx. 50-300 Hz). The differential tactile sensitivity of the three principal fibre classes and their individual coding characteristics, determined by quantitative stimulus-response analysis, indicate first, that this triad of fibre classes can subserve high-acuity tactile signalling from the echidna footpad and second, that peripheral tactile sensory mechanisms are highly conserved across evolutionarily-divergent mammalian orders.
Rowe
M J
MJ
School of Medical Sciences, The University of New South Wales, School of Physiology and Pharmacology, Sydney, N.S.W. 2052, Australia. M.Rowe@unsw.edu.au
Mahns
D A
DA
Bohringer
R C
RC
Ashwell
K W S
KW
Sahai
V
V
D000818:Animals; D005075:Biological Evolution; D002540:Cerebral Cortex / Q000473:pathology; D004594:Electrophysiology; D008465:Mechanoreceptors / Q000502:physiology; D008954:Models, Biological; D009007:Monotremata / Q000033:anatomy & histology / Q000502:physiology; D009474:Neurons / Q000378:metabolism*; D010987:Platypus / Q000033:anatomy & histology* / Q000502:physiology; D004442:Tachyglossidae / Q000033:anatomy & histology / Q000502:physiology; D013997:Time Factors