Innervation territories for touch and pain afferents of single fascicles of the human ulnar nerve. Mapping through intraneural microrecording and microstimulation.

The peripheral distribution of the fibre content of individual ulnar nerve fascicles supplying skin and muscles of the hand in human volunteers was indirectly mapped by tracing the fields of projected sensation evoked by intraneural electrical microstimulation (INMS) and by tracing receptive fields delineated through intraneural recording of afferent impulse activity elicited by natural stimulation of end organs. Moderate intensity suprathreshold INMS, delivered in cutaneous fascicles, induced nonpainful sensations projecting to stereotyped and coherent areas of skin, the fascicular projected fields (FPFs). Fascicular receptive fields (FRFs) were mapped during microneurographic recording by determining the area of skin which, when activated by light tactile stimuli, elicited afferent neural discharges recorded intraneurally. It was found that at a given electrode position in a skin nerve fascicle, moderate intensity INMS induced nonpainful sensations projecting to a cutaneous field (FPF) that coincides with the FRF, while high intensity INMS induced painful sensations projecting within the cutaneous field of nonpainful sensations. Pain induced by INMS in muscle nerve fascicles was projected to the muscles innervated by that fascicle and, in most instances, to areas beyond the muscular receptive field. The study demonstrates that individual ulnar nerve fascicles, at wrist levels, subserve well-defined cutaneous territories in the hand, and that the area of skin covered by the sum of all ulnar fascicular receptive or projected fields matches the maximal possible cutaneous distribution of the ulnar nerve. Insights of practical relevance regarding clinical expression of fascicular nerve injuries are also brought up by this study.