Receptors 2022-2023 PDF

THE SENSE ORGANS Receptors - The sense organs are formed of sensory units called receptors. Receptors convey informations about the external world to the C.N.S. - They are classified according to functions into: 1. Somatic and visc...

THE SENSE ORGANS Receptors - The sense organs are formed of sensory units called receptors. Receptors convey informations about the external world to the C.N.S. - They are classified according to functions into: 1. Somatic and visceral receptor system (superficial and deep sensation). 2. Proprioceptor system (detection of body position in space). 3. Chemoreceptor system (taste and smell). 4. Photoreceptor system (vision). 5. Audioreceptor system (hearing). - on the basis of their distribution in the body, they are also classified into: 1- Exteroceptors: They respond to external stimuli and are at or close to the surface. ▪ General (cutaneous) sense organs: as free and encapsulated nerve terminals in the skin. ▪ Special sense organs: as olfactory, visual, auditory & gustatory (taste). 2- Proprioceptors: They respond to stimulation of deeper tissue and are concerned with detection of movements and position e.g. muscle spindle, tendon spindle, Pacinian corpuscle and vestibular receptors of the ear. 3- Interoceptors: They are present in the walls of viscera, glands and blood vessels. They are responding to tension, vascular chemoreceptors and baroreceptors. I- Somatic and visceral receptor system: They are responsible for the following senses: (A) Touch and pressure: 1- Pacinian corpuscle: Sites: ▪ dermis of the skin especially that of palms, soles and tips of fingers. ▪ the mesenteries and periosteum. Structure (Figure 9-1): ▪ an encapsulated nerve terminal. ▪ oval in shape. ▪ In histological sections, it resembles sliced onion consisting of 20-70 layers of fibroblasts separated by thin collagen fibers and tissue fluid. ▪ The myelinated nerve fiber enters one pole of the corpuscle, loses its myelin sheath, and then loses its neurolemmal sheath. The naked fiber runs in the longitudinal axis of the corpuscle and terminates by an expanded end. ▪ mainly associated with the detection of vibration and posture of the body. 2- Meissner's corpuscle: Sites: ▪ dermal papillae of hairless skin of palms, soles, lips, glans penis, clitoris and nipples. Structure (Figure 9-2): ▪ an encapsulated nerve terminal. ▪ oval in shape, lying with its long axis perpendicular to the surface. ▪ consists of fibroblasts and thick collagen fibers surrounding an inner core of Schwann cells and nerve terminals. ▪ The myelinated nerve fiber enters the base of the corpuscle and gives 2-9 branches that make a spiral course. They are enfolded by horizontally arranged Schwann cells. ▪ responds to light touch.  3-Ruffini's corpuscle: Sites: ▪ joints and dermis of both hairy and hairless skin. Structure (Figure 9-3): ▪ It is partly encapsulated. ▪ It is spindle–shaped surrounded by thin collagen capsule ▪ The myelinated nerve fiber penetrates the side of the corpuscle, and breaks up into numerous non-myelinated terminal branches located between layers of collagen fibers which run parallel to the longitudinal axis of the corpuscle and intermingle with collagen bundles in the surrounding C.T. ▪ mechanoreceptor responding to tensional forces. 3- Krause end bulb: Sites: ▪ C.T. of conjunctiva, tongue and dermis of external genitalia. Structure (Figure 9-4):  ▪ It is an encapsulated spherical nerve terminal. ▪ The myelinated fiber penetrates the corpuscle and breaks up into numerous non- myelinated terminals which end by bulbous expansion. ▪ It is thought to be responsible for cold sensation. 4- Merkel's tactile disc: Sites: ▪ deep layers of the epidermis of soles and palms. Structure (Figure 9-5): ▪ It is un-encapsulated nerve terminal. ▪ The naked fiber penetrates the basement membrane of the epidermis and ends as a terminal disc around the base of modified epidermal cell called Merkel's cell.  5- Sensory endings around hair follicles: - They are also called pretrichial nerve endings. Sites: ▪ around hair follicles. Structure: ▪ Unmyelinated fibers (free nerve endings) are arranged circularly and longitudinally around most of the length of the hair follicle. ▪ The sensation of touch is elicited when a hair is bent. (B) Heat, Cold and Pain: Free nerve endings: Sites: ▪ epidermis of the skin, and epithelium of the cornea and oral cavity. Structure (Figure 9-6): ▪ unmyelinated sensory nerve fibers that branch in the C.T (dermis). ▪ They penetrate the basement membrane and extend between the lower cell layers of the stratified epithelium (epidermis). II. Proprioceptor system (Neuromuscular spindles): 1- Muscle spindle: ▪ This type of receptors is sensitive to muscle stretch and reflexly controls the muscle tone, movement and posture. ▪ They are present between skeletal muscle fibers. ▪ The muscle spindle is a fusiform structure that consists of stretchable connective tissue capsule containing tissue fluid and few specialized muscle fibers (intrafusal muscle fibers) of two types (Figure 9-7): 1- Nuclear bag fibers: which are; - Thicker and longer - Have expanded central part containing many nuclei. - Supplied by primary (annulospiral) nerve endings that are coiled around the expanded central part. 2- Nuclear chain fibers: which are - Thinner and shorter. - Have regular diameter & the nuclei are arranged in row. - They are also supplied by primary (annulospiral) endings around the central part of the fiber. - In addition, they are supplied by two secondary (flower spray) endings, one on each side of the primary ending.  ▪ Muscle fibers outside the capsule are called extrafusal muscle fibers. 2- Tendon spindles (Figure 9-8): ▪ These are similar to muscle spindles, but the intrafusal fibers are intracapsular collagen fibers. ▪ They are found near the insertion sites of muscle fibers in tendons & joints. III. Chemoreceptor system (Taste and smell). 1-Taste buds ▪ Site: present mainly in the tongue, embedded within the stratified epithelium of the  circumvallate, foliate and fungiform papillae. They are also present on the soft palate, glossopharyngeal arch, epiglottis and posterior wall of the pharynx. ▪ Structure: 1-The taste bud has a small opening at the epithelial surface called taste pore through which chemicals enter the bud. 2-EM shows that taste buds are composed of 4 types of cells (Figure 2-6): - Type I and type II are supporting (sustentacular) cells that are tall with microvilli on their surface. - Type III (sensory cells) are tall, with microvilli on the surface and pale stained euchromatic nuclei. Each cell is characterized by the presence of numerous vesicles, which are in close proximity to dentritic processes of sensory nerves at its basal surface. - Type IV is a relatively undifferentiated and short basal cell.  2- Olfactory mucosa ▪ The olfactory chemoreceptors are present in the olfactory epithelium covering the olfactory area. ▪ This is a pseudostratified columnar epithelium composed of 3 types of cells. 1- Supporting cells: - tall cells with apically located nuclei. - The apex of the cell is broad and the base is narrow.  - The cell has microvilli on its apical surface. - It contains a yellow pigment similar to lipofuschin pigment that gives the epithelium its yellow color. 2- Basal cells - They are short and rounded, located at the basal region of the epithelium. - They may act as undifferentiated stem cells or supporting cells. 3- Olfactory cells - bipolar nerve cells. - The nuclei lie in a level below the nuclei of the supporting cells. - The apical portion is a modified dendrite that ends at the surface of the epithelium in a bulb (the olfactory vesicle). - This vesicle gives rise to several long, almost immotile cilia (6-8). These cilia lie flat along the surface, and act as receptors. - The afferent axons of these bipolar neurons unite in small bundles which pass through the cribriform plate of the ethmoid bone forming olfactory nerve that enters the brain. The cilia of olfactory cells and the microvilli of supporting cells are submerged in a fluid layer consisting mainly of serous secretion of glands in the lamina propria beneath the olfactory epithelium called Bowman's glands. Serous secretion of Bowman’s glands: 1- Serves as a solvent for odoriferous substances. 2- Clear the surface of the olfactory epithelium.