Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Sense Organs. Analyzers
The Skin and Its Derivatives
Cutaneous Analyzers

The Significance of cutaneous sensitivity in human life is immense. The Skin perceives Touch, pressure (tactile sensation), warmth, cold, and pain. Each of these stimuli is detected by specialized receptors—sensory endings located at various depths within the skin—which vary in both shape and Structure. These receptors are distributed unevenly across the body; they are particularly abundant in the skin of the fingertips, palms, soles, Lips, and external genitalia, while being far less numerous in the skin of the back.

Touch and pressure (tactile sensitivity) are perceived by approximately 500,000 receptors. These are mechanoreceptors, which include free nerve endings penetrating the epidermis to detect pressure, as well as encapsulated nerve endings (those possessing a distinct capsule). Encapsulated sensory endings include the large lamellar corpuscles (Vater-Pacini corpuscles) and tactile corpuscles (Meissner's corpuscles), both located in the dermis proper. The Senses of touch and pressure not only enable us to recognize objects, but also to determine their shape, size, and the texture of the material from which they are made.

Temperature Sensation (the sense of cold and warmth) is mediated by different receptors. Some are excited by The Effect of cold on terminal nerve corpuscles (Krause end bulbs), while others respond to the effect of warmth on bulboid corpuscles (Golgi-Mazzoni corpuscles). Cold receptors, which extend between epidermal Cells, are located more superficially than warm receptors. There are significantly more cold receptors (about 250,000) than warm ones (about 30,000). The skin of the extremities (especially exposed areas) is less sensitive to temperature changes than the skin of the trunk (covered areas). It is well established that thermal receptors adapt to changes in ambient temperature.

The sensation of pain is perceived by specialized free nerve endings. The number of pain receptors in human skin is very high, approximately 100–200 per 1 cm² of skin surface, with the total number reaching 2 to 4 million. The site of pain origin is localized by a person quite precisely.

Pain sensations are also perceived by nerve endings in the mucous and serous membranes, as well as in Internal Organs. Often, pain is felt not only in the damaged organ itself, but also in other PARTS OF THE body, such as specific skin areas. Such pain is referred to as referred or radiating pain. For example, during a spasm of the coronary Arteries of the Heart (CORONARY HEART DISEASE), pain manifests not only in the chest (behind the Sternum), but also in the area of the left scapula and down the arm.

Pain sensations are of paramount importance as danger signals. They trigger protective and defensive mechanisms: Muscle tone increases, heart rate and Respiration accelerate, and there is an increased secretion of Hormones Involved in mobilizing the body's defenses (such as Adrenal hormones like adrenaline).

Nerve impulses generated in cutaneous receptors travel to the Spinal Cord, specifically to its sensory and motor centers, which participate in forming automatic, subconscious, protective, and defensive Reflexes at the spinal segment level (such as withdrawing a hand from a burn or a prick). Simultaneously, sensory impulses from Skin Receptors pass via the sensory nuclei of the dorsal horns of the spinal cord or Cranial Nerves along pathways leading to the cortical end of the general sensitivity analyzer, specifically the Neurons of the postcentral gyrus. Within the Cerebral Cortex, in the postcentral gyrus, higher-level analysis and conscious perception of all modalities—tactile, thermal, and nociceptive—perceived by the respective skin receptors take place.

The Pathways of the cutaneous analyzer include the lateral spinothalamic tract, which conducts pain and temperature sensitivity from the somatic skin receptors, as well as the anterior spinothalamic tract and partially the medial lemniscus, which convey the sense of touch and pressure from the somatic skin to the cerebral cortex.

Review and Self-Assessment Questions:

1. Indicate the locations of taste buds. To which areas of the cerebral cortex are gustatory impulses directed?

2. Describe the pathways followed by nerve impulses from olfactory receptors to the cerebral cortex.

3. Name the layers of the skin. What skin structures form the pattern on its surface (ridges and furrows)?

4. List the Skin Appendages (derivatives) and characterize each of them.

5. Describe The structure of the mammary (breast) glands, along with their age-related and functional features.

6. Discuss the cutaneous analyzers and their significance in human life.