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Physiological mechanisms of the receptor system

Antony W. Goodwin and Heather E. Wheat


Introduction

When we manipulate objects in our environment, a vast array of receptors in the skin, joints and muscles is activated. This information is relayed to the central nervous system and underlies two distinct but complementary aspects of hand function. Most obviously, these neural signals lead to haptic perception. We may sense how rough or smooth a surface is, or how curved an object is, whether it is soft or hard, whether the surface is slippery or sticky, how heavy it is and so on. Less obvious, but equally important, is the use the motor control system makes of these sensory signals in order to ensure appropriate hand movements resulting in stable grasps and effective complex manipulations. Some examples of common manipulations in our daily lives are: lifting a cup of coffee, opening a door, getting dressed, typing a manuscript, threading a needle.

Key features of the hand

The skeleton of the hand is made up of many bones (Fig. 1). Tendons inserting in each of the phalanges allow movements with many degrees of freedom, which is essential for the wide range of hand movements underlying the myriad complex manipulations we make. Some of the muscles from which the tendons originate are small muscles located in the hand, but most of the muscles are larger and are located some distance away in the forearm. Glabrous or hairless skin covers the front surface of the hand. This has a ridged appearance which is particularly striking on the fingertips giving rise to the fingerprint patterns which are unique to each individual. The back surface of the hand is covered in hairy skin.
During hand movements, many muscles are activated, angles change at many joints, contact with objects occurs at multiple points, and skin stretches or relaxes over the hand. Thus, sensory signals are relayed by mechanoreceptors in the muscles, in the joints, in the hairy skin, and in the glabrous skin. These mechanoreceptors are the terminal points of fibres, principally large myelinated axons that form the sensory components of the peripheral nerves innervating the hand, which are the median, ulnar and radial nerves. The cell bodies of these axons are located in the dorsal root ganglia and the central branches enter the spinal cord via the dorsal roots.
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