Peripheral projections of nerve fibres in the human median nerve

Elementary sensations of definable quality were evoked by intraneural electrical stimulation of sensory units through tungsten microelectrodes inserted in the median nerve of awake human subjects. The most commonly reported sensations were tapping, pressure, dull pain and sharp pain, which correlated with recording from RA units, SA I units, C nociceptors and A delta nociceptors, respectively. All sensations were projected more often to the fingertips than to the palm. The multifocal projections of sensations recruited by excitation of small groups of neighbouring nerve fibres were significantly more scattered in space than anticipated if these fibres had preserved an orderly intraneural map, implying lack of somatotopy in the nerve. Mean projected field sizes were 3.4 mm2 for sharp pain, 9.2 mm2 for tapping/vibration, 29.3 mm2 for pressure and 35.4 mm2 for dull pain, and in addition the latter two showed a rising size gradient from fingertip to palm. Consideration of projected field size as a variable in discriminative touch suggests that input from RA units and A delta nociceptors may be of importance in stimulus localization, while SA I units may play an additional role in two point discrimination.