Tactile unit properties after human cervical spinal cord injury

Properties of tactile afferent units innervating the glabrous skin of the hand were examined by microneurography in 11 individuals who had no or impaired touch perception due to chronic (> 1 year post-injury) cervical spinal cord injury (SCI). The results were compared with published control data. The adaptation properties [fast adaptation unit (FA) or slow adaptation unit (SA)], the indentation force threshold, and the size of the receptive field of each unit were assessed using calibrated von Frey hairs. Units were classified as type I (FAI, SAI) or type II (FAII, SAII) if the receptive field was small and well defined or large and diffuse, respectively. Sensitivity to skin stretch was also used to distinguish between the two slowly adapting unit types. In SCI subjects, 66 tactile afferents (and 16 unclassified muscle receptors) were sampled from all the glabrous skin areas typically innervated by the median nerve. Each unit type was represented in similar proportions in the SCI and control data. Similarly, there was an increase in the innervation density from the palm to the finger tips in SCI subjects, as found in control subjects. Afferent axon conduction velocities were not different for the SCI and control data. Indentation force thresholds and receptive field sizes of each unit type were also similar with one exception. The receptive fields for FAII units were larger after SCI. However, when data from all unit types were pooled, receptive fields were larger and indentation force thresholds were higher in SCI subjects. This may be the consequence of the thin, smooth, soft and significantly more compliant glabrous skin of the SCI subjects, factors which may influence the transmission of force from the skin surface to the receptors. Twenty-four units showed sustained responses to passive bending of the fingers and usually an increase in firing rate during finger extension. Eight of these units were slowly adapting tactile units. The others were unclassified units located deep within the forearm. Morphological estimates indicated no loss of myelinated nerve fibres in the median nerve 6 months after SCI. Motor as well as sensory nerve fibres were therefore unaffected below the elbow. Thus, tactile units were largely intact after chronic human cervical SCI despite changes in the mechanical transmission in the skin, and perception deficit.