Bidirectional communication of sensory afferent information in a peripheral nerve of the cat forelimb

Kinaesthetic information is derived from both muscle and joint nerves. However, the segregation, at peripheral levels, of inputs from these sources is by no means clear cut. In the present report, we demonstrate the complexity of peripheral innervation of joint and muscle structures in the cat's forearm, in particular, with evidence for bidirectional signalling for different classes of kinaesthetic afferents within a peripheral nerve segment. Three-way simultaneous recordings were carried out in the anaesthetized cat from single kinaesthetic afferents in three nerves that were freed from nearby tissue in the distal forearm, but remained in continuity. These were the wrist-joint nerve and two components of the indicis proprius nerve, one that projects proximally from the muscle to join the deep radial nerve, the other a distal extension of this nerve that runs through and beyond its own muscle to the region of the wrist-joint capsule where it forms an anastomosis with the wrist-joint nerve. Single-unit recording from the intact nerves demonstrated that some spindle afferent fibres from the indicis proprius muscle may take an "ectopic" path to the central nervous system, conveying their signals over an initial centrifugal path via the distal extension of the indicis proprius nerve, before looping back to project centripetally via the "classic" wrist-joint nerve. As some wrist-joint afferents themselves may project "ectopically" via the distal and then proximal segment of the indicis proprius nerve (rather than via the wrist-joint nerve), the recordings demonstrate that, within the distal segment of the indicis proprius nerve, there is bidirectional traffic of kinaesthetic afferent signals, with wrist-joint impulses travelling centripetally and muscle afferent signals travelling centrifugally. The findings emphasize the complexity of signalling that may be present in sensory nerves, on account of the "ectopic" paths taken by some afferents, and the need to activate deep inputs of joint or muscle origin by natural stimulation of the appropriate receptors in order to examine selectively the central actions and processing of either source of input.