Monotreme tactile mechanisms: From sensory nerves to cerebral cortex

Electrophysiological recordings from single tactile sensory nerve fibres supplying the limb extremities in the echidna (Tachyglossus aculeatus) reveal a remarkable resemblance between monotreme peripheral tactile mechanisms and those of placental mammals. The similarities apply to a concatenation of attributes, including the classification of sensory fibre types and aspects of functional properties and tactile coding capacities. The analysis demonstrates that high-acuity tactile signalling from the distal forelimb in the monotreme is based upon a triad of major tactile fibre classes as is the case for placental mammals. Furthermore, the functional similarity between corresponding classes in monotreme and placental species suggests that peripheral tactile sensory mechanisms are highly conserved across evolutionarily-divergent mammalian orders. Evidence for a unique and striking dependence upon tactile sensory mechanisms in monotremes comes from both behavioural observations on the animals and from the exceptional prominence given to the representation of tactile inputs in the cerebral cortex of these species. In the platypus, for example, almost half of its lissencephalic cortex is allocated to the processing of inputs from the bill. Furthermore, within the specialized area of bill representation in the platypus cortex, the receptive fields of individual neurones are amongst the smallest ever recorded within the somatosensory areas of cortex (often <1mm in diameter), presumably conferring great fidelity and precision on the tasks of tactile localization and discrimination involving the bill. However, in both the platypus and the echidna there is a complete and highly ordered somatotopic representation of tactile inputs from the contralateral body surface, conforming with the so-called primary somatosensory cortex (SI) of other mammals. Controversy applies to the issue of whether additional, multiple body representations are present in the monotreme cortex, as neither Lende (1964) nor Bohringer and Rowe (1977) found evidence of this, in contrast to Krubitzer et al. (1995a), who have argued for four body representations in the cortex of both the echidna and the platypus.