Skin as an organ of sense

The skin is richly supplied with nerves and various types of specialized sensory end-organs, which provide information regarding environmental changes; the body can then adjust its activities accordingly.

Four types of skin sensitivity are distinguished: pain, tactile, heat, and cold. The last two types are embraced under the common name of temperature, or thermal sense. The sense of touch, pressure, and vibration are related to tactile sense.

Temperature and tactile sensitivity are aroused by definite specific stimuli acting on the skin. The pain effect may be induced by various factors which, on reaching a definite stimulation threshold, are perceived as the sense of pain. The sensation of itching may also be considered a form of sensitivity.

Pain is a specific sensation with a pronounced emotional colouring. It is perceived by free nerve endings in the epidermis and dermis. Because of the different emotional colouring of pain sensitivity, acute and dull, stabbing, cutting and aching, dragging, and other types of pain are distinguished. Pain sensitivity may be greatly disturbed. Increased pain sensitivity is called hyperalgesia and hyperpathia; in hyperalgesia the sensation of pain is aroused even by a weak stimulus because the excitation threshold in such patients is decreased, whereas in hyperpathy the patient's sensitivity is increased and super-threshold stimuli are perceived as long-term very sharp pain. Diminished sensitivity to pain is called hypalgesia, while its loss is known as analgesia. Hyperalgesia of various skin areas is encountered very often in diseases of the viscera. In such cases increased skin sensitivity is encountered in only definite metameres which receive afferent fibres from the same spinal segment that contains the sensory fibres of the diseased organ (e.g. hyperalgesia of the skin on the neck, chest, upper part of the abdomen and back in diseases of the heart and lungs, hyperalgesia of the skin of the lower abdomen and in the region of the lower vertebrae in intestinal disease). These skin areas are called Zakharyin-Head's zones.

Patients with skin diseases, like some patients suffering from neuroses, often complain of a sensation of itching which at times is so unbearable that the desire to scratch becomes irresistible. Both exogenous and endogenous factors induce the itch sensation; the itch is quite often of psychogenic origin. A sensation of itching on skin areas previously subjected to analgesia gives reason to assume that it is conducted by nerve fibres other than those concerned with the sense of pain.

Dissociation (or splitting) of sensitivity is encountered in clinical practice, when one type of skin sensitivity is lost, while the others are preserved.

Skin thermoregulating function

Participation of the skin in the process of body thermoregulation is one of its most important physiological functions. The body temperature in healthy man is usually maintained at a constant level irrespective of the environmental temperature, whether high or low.

Heat emission is regulated by reflex (on stimulation of the skin temperature receptors) and by direct stimulation of the thermoregulation centres in the tuber cinereum and lateral wall of the third ventricle (hypothalamic region). The skin vascular reactions and the secretion of sweat, which are constituents of the process of heat emission, are controlled by sympathetic nerves and fibres arising from the sympathetic ganglia.