Touch is the sensory system concerned with surface sensation, found in all external body surfaces including the skin and some mucous membranes. Touch sensation is crucial in the detection and recognition of objects at the body surface, including those explored by the limbs, and also in the protection of these surfaces from injury. Functional categories of touch sensation include light touch (including movement of hairs), heat, cold, pressure, and pain sensation. These are to some degree physiologically distinct. Receptors for all the senses are particularly concentrated and developed over the face and hands. When the various types of skin receptor are stimulated, they activate nerve impulses in cutaneous nerves; these impulses pass via the spinal cord and brain stem to the brain, where coding and perception occur. With painful stimuli, reflex withdrawal movements may be induced at the segmental level.