Human Anatomy and Physiology (with Age-Related Features of the Child's Body) - Sapin M.R., Sivoglazov V.I. 2002

Nervous system
Sensory organs
Organs of taste and smell

The sensations of taste and smell result from the action of chemical substances on specialized sensory Cells within the gustatory and olfactory Organs. The perception of taste and various odors plays a crucial role in human life. Taste and smell provide valuable information about food quality and the environment, influencing a person's emotional state and behavior.

The ORGAN OF TASTE

In humans, The Organ of taste is represented by numerous (about 2000) taste buds located in the Stratified Epithelium of the mucous membrane of the Tongue, soft palate, fauces, Pharynx, and epiglottis. Taste buds are particularly abundant in the epithelium of the filiform, fungiform, foliate, and vallate papillae. Taste buds are ellipsoidal in shape and consist of closely packed receptor (gustatory) and supporting cells. At the apex of each taste bud, There is a taste pore that leads into a small taste pit formed by the tips of the gustatory cells. Microvilli are present On the surface of each gustatory Cell facing the taste pit. Sensory gustatory cells perceive sweet, bitter, salty, sour, or combinations of these four types of taste stimuli. To affect the gustatory cells, these substances must be dissolved in liquid. The solvent in the Oral Cavity is saliva. The dissolved substance penetrates the taste bud through the taste pore at its apex, exciting the gustatory cells. This excitation is transmitted to adjacent nerve endings, where a Nerve Impulse is generated. The nerve impulse travels to the Brain along nerve fibers that enter the taste bud. The nerve impulse from the anterior ²/₃ of the tongue is transmitted via the nerve fibers of the lingual nerve, and then the chorda tympani of the Facial Nerve. From the vallate papillae, soft palate, and palatoglossal arches, it passes along the fibers of the Glossopharyngeal nerve, and from the epiglottis, via the Vagus nerve. The Cell bodies of the first Neurons of the gustatory pathway lie in the corresponding ganglia of Cranial Nerves VII, IX, and X. The axons of these cells travel within these nerves to the solitary tract Nucleus located in the Medulla Oblongata, where they synapse with its neurons. The central processes of these Nerve Cells project to the thalamus. The axons of the thalamic neurons project to the cortical end of the gustatory analyzer, located in the cortex of the parahippocampal gyrus, uncus, and hippocampus (Ammon's horn).

Class="center">The Organ of Smell

The organ of smell is located in the olfactory region of the nasal mucosa. This includes the superior nasal concha and the corresponding area of the nasal septum, where the mucous membrane is covered with olfactory epithelium. Olfactory receptor cells have long central and short peripheral processes. The number of olfactory cells in humans is about 40 million. The peripheral process (dendrite) ends in a thickening called the dendritic bulb (olfactory vesicle), at the apex of which there are 10—12 motile olfactory cilia that come into contact with odorants. Odorant molecules, first dissolving in the secretion of the mucous glands, interact with the receptor Proteins of the cilia, triggering a nerve impulse. A single molecule of an odorant is sufficient to excite a single olfactory (sensory) cell. The central processes (axons) of the olfactory cells pass between supporting cells and gather into olfactory filaments, which, numbering 20—40, enter the cranial cavity through the cribriform plate of the Ethmoid bone and proceed to the olfactory bulb. The olfactory bulb contains the next (second) neurons of the olfactory pathway. The axons of these cells form the olfactory tract, the fibers of which then pass through the anterior perforated substance and the olfactory triangle. They then follow a complex path within the fornix, as well as around the corpus callosum, to reach the mammillary bodies and the cortical end of the olfactory analyzer located in the uncus and parahippocampal gyrus. Olfactory impulses are also directed to the Hypothalamus, hippocampus, and amygdala, which are part of the limbic system involved in The formation of emotional reactions.

The Cytology/cytology/66.html">Skin and its Derivatives

The skin performs diverse Functions: protective, thermoregulatory, respiratory, and metabolic. Skin glands produce sweat and sebum. Under normal conditions, about 500 ml of Water, dissolved salts, and End products of Nitrogen METABOLISM are excreted daily with sweat. The skin is actively involved in vitamin metabolism. The synthesis of vitamin D under The Influence of ultraviolet rays is particularly important. The surface area of an adult's skin reaches 1,5—2 m². This surface serves as an extensive receptor field for tactile, pain, Temperature, and cutaneous sensitivity. Various stimuli are perceived by thermoreceptors, mechanoreceptors, and nociceptors located in the skin. The first perceive temperature changes, the second detect Touch, and the third perceive painful stimuli. The cell bodies of sensory neurons, whose dendrites transmit impulses from these receptors, reside in the spinal ganglia and sensory ganglia of the cranial nerves.

The skin is divided into the epidermis and the skin proper, or dermis (Fig. 100).

The epidermis is formed by Stratified squamous keratinized epithelium, the thickness of which (0,03—1,5 mm) depends on the body region. Thus, in areas subjected to constant mechanical pressure (palms, soles), it is thicker than on the chest, abdomen, and other PARTS OF THE body. Among the basal Cells of the epithelium, there are pigment-producing cells rich in melanin pigment granules (melanocytes), The amount of which determines skin color. Melanin protects the skin from ultraviolet rays. The cells of the basal and spinous layers, located deep within the integumentary epithelium, are functionally combined into the germinative layer. Above it lies the granular layer, consisting of several layers of flattened cells containing large keratohyalin granules. As the cells move into the upper layers, keratohyalin is converted into keratin.

Fig. 100. Structure OF THE Skin:

1 — stratum corneum, 2 — epidermis, 3 — basal layer, 4 — Connective Tissue fibers (Collagen, elastic, and reticular) and cells, 5 — papillary layer, 6 — bundles of myocytes, 7 — Hair bulb, 8 — reticular layer, 9 — hair ROOT, 10 — sweat gland, 11 — sebaceous gland, 12 — lobules of adipose tissue

Above the granular layer lies the clear layer, formed by 3—4 layers of flat, non-nucleated, highly refractive cells. The superficial layer (stratum corneum) consists of numerous layers of cornified scales containing the protein keratin and air bubbles. This layer is waterproof, dense, and elastic, and most importantly, it prevents the penetration of microorganisms. Cornified scales are constantly shed and replaced by new ones that migrate to the surface from deeper cell layers. During their migration to the surface, these cells gradually undergo keratinization. A complete turnover of cells in the epidermis of the human sole occurs within 10—30 days.

The dermis, or skin proper, is 1—2,5 mm thick and is formed by connective tissue. It is divided into the papillary and reticular layers. The papillary layer lies beneath the epidermis. This layer consists of loose, irregular connective tissue that forms papillae projecting into the epidermis. Due to these papillae, ridges separated by skin furrows are visible on the skin surface. The ridges, corresponding to the elevations of the dermal papillae, and the furrows between them form a highly individual, complex pattern on the skin surface, especially on the palms and soles, which persists throughout a person's life. The structure of this skin relief is widely used in medicine and for personal identification in forensics. The papillary layer contains myocytes associated with hair follicles. In the dermis of the face, Scrotum, nipple of the breast, and the dorsum of the limbs, there are independent bundles of myocytes not associated with hair follicles. Their contraction produces the well-known phenomenon of 'goosebumps'.

Beneath the papillary layer lies the reticular layer, which consists of dense irregular connective tissue containing large bundles of collagen fibers that form a network. On the sole, elbows, and terminal Phalanges of the fingers, which are subjected to constant pressure, the mesh of the network is wide-looped. Along with collagen fibers, the reticular layer contains a network of elastic fibers and a small number of reticular fibers. The roots of hairs, Sweat Glands, and Sebaceous Glands are embedded in the reticular layer.

The bundles of collagen fibers of the reticular layer merge into the subcutaneous tissue (hypodermis), which contains adipose tissue. This layer plays an important role in thermoregulation and serves as a fat depot for the body. Adipose tissue reaches its greatest development in the buttocks and soles, where it performs a mechanical function. In the skin of the eyelids and scrotum, the adipose layer is absent. As a rule, the adipose layer is more developed in women.

Skin derivatives

Hair. Almost the entire skin is covered with hair, with the exception of the palms, soles, the transitional zone of the Lips, the glans Penis, and the Labia minora. The greatest density of hair is typically on the HEAD. The pattern of hair growth depends on sex and age and is classified as a secondary sexual characteristic. During Puberty, increased hair growth begins in the armpits, pubic area, and, in men, on the face, limbs, chest, and abdomen. There are Three types of hair: terminal hair (hair of the head, pubis, beard, mustache), bristly hair, and vellus hair on the rest of the body surface.

A hair has a shaft projecting above the skin surface and a root embedded within the skin. The hair root is located in a hair follicle, which is formed by an epithelial (root) sheath and a connective tissue Sheath of the hair. The arrector pili Muscle is attached to the hair sheath. A sebaceous gland opens into the follicle. Upon contraction, the muscle elevates the hair and compresses the sebaceous gland, causing its secretion to be released.

The hair root expands into the hair bulb, which is responsible for hair growth. The hair shaft consists of a medulla and a cortex. The cornified scales of the hair contain pigment granules and air bubbles.

With age, the number of air bubbles increases, and pigment synthesis gradually ceases, causing the hair to turn gray. Hair is shed and replaced at intervals ranging from 2—3 months to 2—3 years.

Nails. A nail is a cornified plate lying on a connective tissue nail bed, bounded at the base and sides by nail folds. The nail grows due to the division of the germinative layer of the nail bed epithelium in the root region. The dividing cells, like the epithelial cells of the epidermis, undergo keratinization as they move forward.

Skin glands. These include sweat, sebaceous, and Mammary Glands.

Sweat glands, numbering about 2—2,5 million, are simple tubular glands. Their secretory portions are coiled, forming glomeruli. A long excretory duct pierces the skin and opens onto its surface. The secretion of sweat glands—sweat—consists of 98% water and 2% organic and inorganic substances (mineral salts, urea, uric acid).

Sebaceous glands are simple alveolar glands located at the boundary between the papillary and reticular layers of the dermis. Sebaceous glands are absent only on the palms and soles, with the greatest number found on the scalp, forehead, Cheeks, and chin. The total mass of sebum secreted by the glands per day can reach 20 g. The gland consists of an alveolar secretory portion and a short excretory duct that opens into a hair follicle. Sebum, being bactericidal, not only lubricates the hair and epidermis but also protects the skin from microbes. During puberty in boys, the function of the sebaceous glands is activated, which is associated with the influence of Male Sex Hormones.

The mammary gland (breast) is located on the anterior surface of the pectoralis major muscle. In virgins, its mass is about 150–200 g, and in lactating women, 300–400 g. In the center of the anterior surface of the gland, there is a pigmented nipple surrounded by a pigmented areola. On The surface of the nipple, 10–15 lactiferous pores open. The skin of the nipple and areola contains numerous myocytes, whose contraction causes the nipple to erect.

The mammary gland is a modified sweat gland. In men, the gland is undeveloped. In an adult woman, it consists of 15–20 lobes, interspersed with adipose and loose Fibrous connective tissue. Each lobe is a compound alveolar gland, the excretory duct of which runs radially toward the nipple. Before reaching the nipple, the duct dilates to form a lactiferous sinus. In non-lactating women, the secretory portions of the gland consist only of lactiferous alveolar ducts. Under the influence of estrogen and progesterone, from the end of the 5th month until the end of Pregnancy, alveoli form at their ends. Glandular cells (lactocytes) are surrounded by basket-like myoepithelial cells located on the basement membrane. Their contraction forces milk into the ducts. Milk secretion is stimulated by prolactin (lactotropic hormone of the Pituitary Gland). After the Lactation period ends, the mammary gland gradually undergoes involution. Only a few alveoli persist.

In a newborn girl, the secretory portions of the mammary glands are undeveloped, with only a rudimentary duct system present. During the prepubertal period, adipose tissue grows rapidly. By puberty, the gland becomes rounded, but its enlargement is mainly due to adipose tissue.

The mammary glands begin to secrete immediately after childbirth. During the first 2–3 days, the glands secrete colostrum, which differs in composition from mature milk. Colostrum contains less casein protein and has a yellowish color. Milk secretion begins on the 3rd day; it contains water, organic, and inorganic substances. The white color of milk is due to tiny suspended fat droplets (up to 2 billion per 1 ml of milk). Milk contains 2–4% fat and about 4% casein protein. Milk also contains lactalbumin and lactoglobulin. CARBOHYDRATES in the form of lactose account for 3–6%, and salts (0.75%) are represented by phosphates, sulfates, and chlorides of potassium, calcium, sodium, and other elements. Milk contains Vitamins A, B, C, and D, as well as Antibodies. Lactating women produce up to 0.5–1.0 L of milk per day.

Cutaneous sensation. Nerve endings located at various depths in the skin perceive touch, temperature, and pain. Each stimulus is detected by specialized receptors that differ from one another in shape and structure. Receptors are distributed unevenly; they are abundant in the skin of the fingertips, palms, soles, lips, and external genitalia. There are far fewer receptors in the skin of the back. The Significance of cutaneous sensation in human life is immense.

Touch and pressure (tactile sensation) are perceived by approximately 500,000 receptors located in the skin. These are mechanoreceptors, which include free nerve endings that penetrate the epidermis and perceive pressure, as well as encapsulated endings. Encapsulated sensory nerve endings include large lamellar corpuscles (Pacinian corpuscles) and tactile corpuscles (Meissner's corpuscles) located in the dermis.

The sensations of touch and pressure allow us not only to recognize objects but also to determine their shape, size, and The Nature of the material from which they are made.

Temperature Sensation (the feeling of cold and warmth) is perceived by different receptors. Some are excited by the action of cold on nerve corpuscles (Krause end-bulbs), while others are excited by the action of heat on Golgi-Mazzoni corpuscles. Cold receptors, which penetrate between epidermal cells, are located more superficially than warmth receptors. There are far more cold receptors (about 250,000) than warmth receptors (about 39,000). The skin of the limbs (arms and legs), especially exposed areas, is less sensitive than the skin of the trunk (covered areas). Receptors that perceive temperature stimuli adapt to changes in environmental temperature (air, water), as if 'getting used' to them. For example, water that initially feels very hot is gradually perceived as less hot, or even just warm. A hand or FOOT also 'gets used' to cold water.

The sensation of pain is perceived by specialized free nerve endings. The number of pain receptors (nociceptors) in human skin is very high, approximately 100–200 per 1 cm² of skin surface. The total number of such receptors reaches 2–4 million. A person can pinpoint the Location of pain quite accurately. Nerve endings perceive pain not only in the skin but also in mucous and serous membranes, and in Internal Organs. Often, pain is felt not only in the damaged organ but also in other parts of the body, such as specific areas of the skin. Such pain is called referred or radiating pain. For example, during a spasm of the coronary Arteries of the Heart (coronary artery disease), pain is felt not only in The Heart (substernally) but also in the left scapula and arm.

Pain sensations are of great importance because they arise during tissue damage as danger signals that trigger protective and defensive mechanisms (such as increased muscle tone, elevated heart rate, and rapid breathing). The secretion of Hormones Involved in mobilizing the body's defenses (Adrenal hormones—adrenaline and corticosteroids) is also enhanced.

Nerve impulses originating in Skin Receptors travel not only to the Spinal Cord, into its sensory and motor centers, which participate in forming automatic, subconscious, protective, and defensive Reflexes at the level of spinal cord segments. This refers, for example, to withdrawing a hand when burned or pricked. Here, pain or temperature impulses are transmitted to the sensory nuclei of the posterior horns of the spinal cord, and from there to the motor nuclei of the anterior horns. The corresponding motor impulses travel along the axons of the anterior horn motor neurons to the Muscles. Simultaneously, sensory impulses from skin receptors travel through the sensory nuclei of the posterior horns of the spinal cord or cranial nerves along pathways through the thalamus to the cortical end of the general sensory analyzer, specifically to the neurons of the postcentral gyrus. In the Cerebral Cortex, within the postcentral gyrus, higher analysis and conscious perception of all these sensations (tactile, thermal, and pain) perceived by the corresponding skin receptors take place. For conscious actions in response to the nerve impulses arriving at the postcentral gyrus, signals are transmitted from this gyrus via association fibers to the effector (motor, secretory) centers of the cerebral cortex (in the precentral gyrus) or to other subcortical centers.

Questions for REVIEW AND SELF-assessment:

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

2. Name the parts of the Nasal cavity that belong to the olfactory region. Describe the pathways of nerve impulses from olfactory receptors to the cerebral cortex.

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

4. List the appendages (derivatives) of the skin and characterize each of them.

5. Describe the structure of the mammary (breast) glands, their age-related and functional characteristics.