The peripheral nerves of the arm communicate neural signals with sensitive end organs in the skin of the hand. These end organs tell the body what it "feels."
The neural signals make the muscles of the arm and hand operate the elbow, wrist and fingers. A lesion - or disruption of signals to the sensory organs in the skin and muscles - can be likened to what happens with the interruption of a telephone wire going to a telephone. The sensory end organs in the skin and the muscles may have a normal potential to operate - but with signal interruption in lines of communication they function irregularly or not at all.
Three major nerve branches supply sensory and motor (muscle) function to the hand and arm - the ulnar, the median and the radial nerve branches. These are listed in order of the frequency of their involvement in Hansen's disease (leprosy). Abnormality in any one of these peripheral nerve branches can result in a change from a normal sensory feedback (feeling) and muscle function of the hand. Hansen's disease patients are somewhat unique in that they can also develop temperature associated loss of sensory end organs superficially in skin in areas not typical to a specific peripheral nerve area of innervation. On the palm in early stages of the disease, a loss of feeling has most often been related to specific nerve branches, thus an ulnar, median, or radial nerve loss.
While Hansen's disease (HD) can affect the peripheral nerve branches or superficial areas of the skin, other diseases and injuries can affect the same peripheral nerve branches. Treatment developed to aid the rehabilitation of HD patients has been found useful in helping patients with insensitivity and muscle imbalance not HD related. Information learned from patients with multiple nerve involvement has increased our understanding of the normal hand and of the role insensitivity and muscle loss plays in deformity and disability. An understanding of the mechanics of deformity from sensory and motor loss provides insight into prevention and correction.