Peripheral projections of nociceptive unmyelinated axons in the human peroneal nerve.

1. Previous knowledge of the anatomical course of unmyelinated (C) axons along a peripheral nerve has been scarce and has led to the concept of the axons in a constantly interchanging position. 2. Results obtained by microneurography in the peroneal nerve at knee or ankle levels in awake humans demonstrated that the receptive fields of neighbouring C units in the nerve cluster in close vicinity on the skin of the foot or the ankle. These findings indicate that C afferents run closely together throughout large portions of the peripheral nerve. 3. Intraneural microstimulation performed at neural sites where nociceptive C units were recorded induced painful sensations projected to the skin. When the stimulus intensity was increased, there was typically a concentric increase in the area of projected pain, rather than recruitment of several scattered pain projections. This finding further supports the hypothesis of a neighbouring relation of nociceptive C axons within nerve fascicles, implying spatial recruitment of adjacent axons in the nerve with adjacent peripheral projections. 4. A pain locognosia test performed during ischaemic block of impulse conduction in myelinated fibres demonstrated a fairly precise cerebral localization of noxious events on the foot from the input of C afferent fibres alone.