HUMAN MEDICAL BIOLOGY, ANATOMY, PHYSIOLOGY AND PATHOLOGY - Ya.I. Fedonyuk 2010

ANATOMY, PHYSIOLOGY, PATHOLOGY

SECTION 3. ANATOMICAL AND PHYSIOLOGICAL ASPECTS OF BODY FUNCTION SELF-REGULATION

GENERAL ISSUES OF THE ANATOMY AND PHYSIOLOGY OF SENSORY SYSTEMS

SKIN (THE ORGAN OF TOUCH, TEMPERATURE, AND PAIN SENSATION)

3. FUNCTIONS OF THE SKIN

The Skin performs excretory, thermoregulatory, sensory, and protective Functions.

Excretory function. The excretion process is essential for Homeostasis. It rids the body of metabolic products, foreign and toxic substances, as well as excess Water, salts, and Organic compounds. The Excretory Function of the skin is carried out by sweat and Sebaceous Glands. Sweat removes water, salts, urea, ammonia, uric acid, and lactic acid. Consequently, the skin participates in The regulation of Water-Salt METABOLISM, maintenance of osmotic pressure, and Blood reaction. However, it cannot completely replace the Functions of the Kidneys.

Perspiration occurs reflexively, triggered by ambient Temperature. A healthy adult excretes about 500 ml of sweat per day, which evaporates immediately, keeping the skin from feeling damp. Increased sweating is observed during emotional stress, intense physical work, and severe pain.

Sebum (about 20 g is secreted per day) lubricates the Hair and skin, keeping it soft.

Thermoregulatory function. The intensity of heat loss depends on the ambient temperature. In the cold, Blood Vessels constrict, leading to reduced heat loss. When the air temperature rises, skin blood vessels dilate, increasing the volume of blood and promoting heat loss. An important mechanism of heat loss—and the only one when body and environmental temperatures are equal—is the evaporation of sweat from the skin surface.

Protective function. As the body's outer covering, the skin performs a protective function. This is ensured by its high mechanical strength, elasticity, electrical resistance, low permeability, and bactericidal properties. The skin also acts as a blood reservoir.

Sensory function. There are four types of cutaneous sensitivity: Touch (pressure), warmth, cold, and pain.

Tactile analyzer. In terms of factual indicators in humans, the tactile analyzer ranks first. Tactile receptors belong to mechanoreceptors and are represented by receptors for touch, pressure, and vibration. Touch receptors are located in the Superficial layer of the skin, while pressure receptors lie deeper. Surface receptors adapt easily, sparing us from the sensation of prolonged contact, such as clothing. Pressure receptors are slowly adapting, which also serves a purpose: we hold a cup in our hands as long as we feel it there; if the sensation disappears, we drop it.

The total number of receptors exceeds 600,000; however, on the scalp, it reaches 200–300 per 1 cm², whereas on the shin, there are only 10–12 touch receptors. The stimulus for tactile receptors is mechanical deformation of the skin. Uniform pressure on the skin is not felt due to receptor adaptation. The Nerve Impulse travels to the Spinal Cord and Brain, reaching the postcentral gyrus in the Cerebral Cortex.

Temperature analyzer. Temperature receptors are located in the skin and mucous membranes. There are also central thermoreceptors in the spinal cord and Hypothalamus. Temperature fluctuations are perceived by Two Types of receptors: some are excited by cold, others by warmth. Temperature receptors can also be excited by an inadequate stimulus. For example, cold receptors can be excited by heat, which explains the sensation of cold when immersing a limb in hot water. Temperature receptors adapt to the ambient temperature. The conducting pathway of the temperature analyzer consists of nerve fibers running to the spinal cord and brain as part of the spinothalamic tract. Temperature sensitivity is investigated using a thermoesthesiometer by touching the skin with a heated or cooled wire.

Pain analyzer. From a physiological standpoint, pain is an affective, emotional coloring of sensation caused by a blow, heat, cold, or pricking. "Pain is the watchdog of health," as they said in Ancient Greece. Pain is necessary up to certain limits. In many cases, it allows us to assess the degree and nature of disruptions to the body's integrity. At the same time, pain is a cruel enemy of man, draining strength and depressing the psyche.

As long as pain warns of danger or illness, it is necessary and useful. Once the information is heeded and pain turns into suffering, it must be eliminated.

Today, most researchers tend to recognize pain as an independent type of sensation with its own pain receptors, pathway system, and centers. This is confirmed by the fact that the transection or analgesic blockade of certain nerve pathways leads to the disappearance of pain while fully preserving the sensations of touch, warmth, and cold.

Pain receptors are called nociceptors. Their total number is 4,000,000. From the receptor, a fiber runs to the spinal cord. Next, the impulse is transmitted to the subcortical and cortical centers of the brain.

The body also possesses an anti-pain (antinociceptive) system. It includes physiologically active substances produced in the brain: enkephalins and endorphins. Endorphins have a pain-relieving effect 100 times stronger than morphine. Therapeutic measures such as acupuncture, hypnosis, and auto-suggestion stimulate the human antinociceptive system.