G. T. Coleman

Impulse propagation over tactile and kinaesthetic sensory axons to central target neurones of the cuneate nucleus in cat

Paired, simultaneous recordings were made in anaesthetized cats from the peripheral and central axons of individual tactile and kinaesthetic sensory fibres. The aim was to determine whether failure of spike propagation occurred at any of the three major axonal branch points in the path to their cuneate target neurones, and whether propagation failure may contribute, along with synaptic transmission failures, to limitations in transmission security observed for the cuneate synaptic relay. No evidence for propagation failure was found at the two major axonal branch points prior to the cuneate nucleus, namely, the T‐junction at the dorsal root ganglion, and the major branch point near the cord entry point, even for the highest impulse rates (∼400 impulses s−1) at which these fibres could be driven. However, at the highest impulse rates there was evidence at the central, intra‐cuneate recording site of switching between two states in the terminal axonal spike configuration. This appears to reflect a sporadic propagation failure into one of the terminal branches of the sensory axon. In conclusion, it appears that central impulse propagation over group II sensory axons occurs with complete security through branch points within the dorsal root ganglion and at the spinal cord entry zone. However, at high rates of afferent drive, terminal axonal propagation failure may contribute to the observed decline in transmission security within the cuneate synaptic relay.

Tactile sensory function in the forearm of the monotreme Tachyglossus aculeatus

Peripheral tactile neural mechanisms in the forepaw of the echidna (Tachyglossus aculeatus, from the order Monotremata) were investigated 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 in anesthetized echidnas from 29 single tactile sensory nerve fibers isolated in fine strands of the median or ulnar nerves of the forearm. Controlled tactile stimuli were applied to the forepaw glabrous skin to classify fibers, initially, into two broad divisions, according to their responses to static skin displacement. One displayed slowly adapting (SA) response properties, and the other showed a selective sensitivity to the dynamic components of the skin displacement. The SA class was made up of low‐threshold SA fibers and other less sensitive SA fibers, and the purely dynamically sensitive tactile fibers could be subdivided according to vibrotactile sensitivity and receptive field characteristics into a rapidly adapting (RA) class, sensitive to low‐frequency (≤50‐Hz) vibration, that resembled a corresponding RA class in placental species, and another class, sensitive to a broader range of vibrotactile frequencies (∼50–300 Hz), that may represent a monotreme equivalent of the Pacinian corpuscle (PC)‐related fiber class of placental mammals. The differential tactile sensitivity of the three principal fiber classes and their individual coding characteristics, determined by quantitative stimulus‐response analysis, indicate, first, that this triad of fiber 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. J. Comp. Neurol. 459:173–185, 2003.

An intact peripheral nerve preparation for examining the central actions of single kinaesthetic afferent fibres arising in the wrist joint of the cat

Intraneural microstimulation of single tactile or kinaesthetic afferent fibres arising in the hand of conscious human subjects has revealed marked differences among the different classes in their capacity to generate a perceptual response. In order to test the hypothesis that these different capacities might reflect a differential security in the transmission of singnals across synaptic junctions in the dorsal column nuclei or other levels of the somatosensory pathway we have previously developed a paired, simultaneous recording paradigm in the cat to analyze transmission characteristics within the dorsal column nuclei for single identified tactile and muscle sensory fibres of the forearm. These studies have depended upon the use of a fine peripheral nerve or nerve fascicle preparation in which it is possible to monitor the activity of single sensory fibres while the nerve remains in continuity with the central nervous system. Although we have previously described a preparation that allows the activity of single joint afferent fibres from the hindlimb to be monitored in the intact medial articular nerve, these hindlimb kinaesthetic afferents fail to project directly to the dorsal column nuclei. In the present study we report a forearm nerve preparation in the cat that permits the simultaneous recording of activity from individual wrist joint afferent fibres and their target neurones of the dorsal column nuclei. When this nerve is freed from nearby tissue over a distance of 2-4 cm and left in continuity, it is possible with a silver hook electrode to monitor the impulse activity of each group II joint afferent fibre with an excellent signal-to-noise ratio. The preparation should prove ideal for examining the central actions and security of transmission across the dorsal column nuclei for single, identified joint afferent fibres of the forearm.