Changes in peripheral and central axonal projections of sensory fibers following muscle nerve transection

Experiments on regenerating muscle nerves have revealed that both muscle spindles and Golgi tendon organs can be reinnervated, although the former in greater proportion. The chapter discusses whether fibers that reinnervate these receptors after peripheral nerve transection are ones that previously innervated such receptors or whether reinnervation is random. The central projections of these fibers have been used to identify them as group Ia fibers, normally innervating muscle spindles, or group Ib fibers, normally innervating Golgi tendon organs. The central projections of these fibers can be distinguished quite accurately by recording in the homonymous motoneuron pool with an extracellular microelectrode. The stimulation of single group Ia fibers reliably evokes an electric field potential consisting of either or both a brief triphasic spike signalling the arrival of the afferent impulse, followed by a negative potential mirroring the intracellularly recorded synaptic potential that reflects the synaptic event. The stimulation of group Ib fibers reliably causes no potentials recorded by the same electrode. These differences were anticipated from a long history of studies on the physiology and anatomy of groups Ia and Ib fibers, but were established in a group of nine normal animals using techniques identical to those used in the experimental preparations.