HND 202 (Special Senses) PDF

**SPECIAL SENSES --** **THE EYE** **STRUCTURE:** The eyeball is roughly spherical and about 24mm in diameter. It has a tough **fibrous coat** with an **opaque** posterior part called the **sclera** which is continuous with the **strongly** curved transparent **cornea.** **ANATOMY OF THE EYES** L...

**SPECIAL SENSES --** **THE EYE** **STRUCTURE:** The eyeball is roughly spherical and about 24mm in diameter. It has a tough **fibrous coat** with an **opaque** posterior part called the **sclera** which is continuous with the **strongly** curved transparent **cornea.** **ANATOMY OF THE EYES** Light enters the eye through the **cornea**, passes through the **aqueous humor**, the pupillary aperture of the **iris**, the **lens** and **vitreous body** before striking the photosensitive **retina** which lines the posterior **2/3** of the eye ball. The retina is separated from the sclera by a **vascular** pigment called the **choroid** which continues into the ciliary body and the iris. The eye is usually directed in such a way that the image of the object focused on falls on the **fovea** which is a **depression** in the retina. Nerve fibres arising in the retina run across the retinal surface to pass out the eyeball through **perforations** in the sclera. **DIAGRAM OF THE EYE** C:\\Users\\NDFIED\\Desktop\\IMG-3987.PNG **EXTERNAL PROTECTION OF THE EYE** The human eye is protected by a strong **bony orbit** and by the **eye lids** which are lined by the **conjuctival** on the inner surfaces. A number of eye lashes project from the **margins** of the eyelid. The eyelids also contain **tarsal glands** which secrete an **oily fluid** that covers the edge of the lid and prevents tear fluid from flowing over. The eyelids are closed by a muscled called **orbicularis oculi** while the upper lid is raised by another muscle called the **levator palpebrae superioris** supplied by a branch of the **oculomotor** nerve. The eyelids close (blink) by reflex when the cornea, conjuctival, or eye lashes are touched, when a bright light shines into the eye or when an object suddenly approaches the eye. Blinking occurs spontaneously about **20 times** a minute and probably serves to renew the fluid film over the cornea. A blink last about **300 milliseconds** which implies a blackout for about **1/10** of our walking time. The eyelids can also be closed voluntarily. Apart from these, the lids normally close during **sneezing** and **sleep**. The fluid that moistens the conjuctival and cornea is secreted partly by the **lachrimal glands** in the upper and outer part of the orbit and partly by the **accessory lachrimal** glands on the inner surfaces of the lids. The rate of production of this tear fluid is usually adjusted to compensate exactly for the rate of loss by evaporation when foreign body or other irritants gets into the eye. The discharge of tear fluid, the rate of secretion of tear fluid from lachrimal glands into the upper conjunctival sac increases by **reflex** activation of the **parasympathetically innervated glands**. If the rate of secretion exceeds that of drainage into **Narso-Lachrimal** duct through small **orifices** called **puncta lachrimalia**, tears spill over to the cheek (It is referred to as Lachrimation). The tear fluid produced during lachrimation is an **isotonic solution** of **bicarbonate** and **Sodium chloride**. It has a ph of about **7.4** ad low protein content. The fluid which also contains a **bacteriosidal enzymes** (lysosymes) serves to wash foreign bodies or irritant materials out of the conjuctival sac. **PHYSIOLOGY** 1. **Cornea:** It is composed mainly of **collagen fibres anteriorly**. The cornea is **bounded** by a **condensed** stomal layer covered by a stratified epithelium of remarkable regularity. It contains some of the oxygen required for its metabolism directly from the atmosphere through its **anterior epithelium**. The **anterior** surface of the cornea forms the **major** optical focusing component of the eye. At the edge of the cornea the epithelium continues as the conjunctiva. Posteriorly the cornea is bounded by an elastic membrane and an endothelium. 2. **Anterior Chamber** **and** **Aqueous Humour:** The anterior chamber of the eye is filled with a fluid called aqueous humor whose composition is similar to that of the plasma **without** the protein. It also contains less glucose and urea. The less glucose contained in the aqueous humor is probably due to its utilization by the lens and other tissues in the eye. The aqueous and vitreous humor also contain high concentration of ascorbic acid (Vit. C). The aqueous humor is **continually** been produced by the cilliary glands. It drains into the **sinus venosus sclera** (veins located under the nose) and the **venus plexus** which lies within the sclera at the cornea scleral junction. The continuous production and foo of the aqueous humor causes a higher pressure in the inside of the eye ball than the outside. A maintenance of these intra ocular pressure which is normally **10 -- 20mmHg** is very important and crucial to a healthy sight. If this pressure rises unduly as is the case of the drainage channel become blocked. Blood flow in the retina and cheroidal circulation become impeded causing retinal damage (**glaucoma**). 3. **The Lens:** This is composed of ribbon-like fibres arranged in a concentric **lammae**. The cortex of the lens is **softer** than the central nucleus. The lens has a very high protein content (about 35%) and ascorbic acid content. It is enclosed by a strong elastic membranous capsule attached to the ciliary body by sensory ligaments. The thickness of the lens capsule is not uniformed. The part covering the posterior surface is uniformly thin, while that covering the anterior surface is thicker but with a thin central portion. The anterior surface of the lens is much less curved than the posterior surface. The increased convexity of the central portion of the anterior surface **increases** the **optical power** of the lens (accommodation). Although the lens has no blood supply, it is a metabolically active tissue which continues to grow throughout life. Only about **1/3** of the energy required by the lens is obtained by **oxidative metabolism**. As the lens ages, it becomes harder and experiences loss of accommodation (presbyopia). At birth, the lens is pale yellow in colour with a pigment that absorbs strongly in the near ultra-violent. However, in old age, the lens becomes more yellow and cloudy. In old age, the lens is very liable to become opaque (cataract). 4. **The iris:** This is a heavily pigmented screen containing muscle fibres which lies in front of the lens. The inner edge, forms the margin of the circular pupil. It has a sphincter muscle which can constrict the pupil to as little as **1mm** in diameter. It also has some readially arranged **myoepithelial** which helps to **dilate** the pupil although the sympathetically innervated dilator muscle does not play a significant role in controlling the diameter of the pupil. **Injuries** of the cervical cord which interrupts the sympathetic pathway give rise to a small pupil. A condition known as **miosis**. Such a condition which also paralyses the smooth muscle of levator palpebral superioris causes the eyelid to droop (ptosis) and the eyeball to retract (enophthalmos). These signs constitute the **horners' syndrome**. 5. **The Retina:** This is **formed** from the optic vesicle which grows out from the fore brain of the embryo. The retina is the light sensitive portion of the eye. It contains the **cones** which are responsible for **colour vision** and **the rods** which are mainly responsible for **black** and **white vision** and for vision in the dark. When either rods layers of neurons in the retina into **optic nerves fibres** and the cerebral cortex. The functional components of the retina are arranged in layers from the outside to the inside as follows: a. Pigmented layer b. Layer of rods and cones projecting to the pigments c. Outer nuclear layer containing the cell bodies of rods and cones d. Outer plexiform layer e. Inner nuclear layer f. Inner plexiform layer g. Ganglionic layer h. Layer of optic nerve fibres i. Inner limiting membrane. **A minute areas** in the center of the retina called the **fovea** is the part capable of acute and detailed vision. The central fovea which is only **0.3mm** in diameter is composed almost entirely of **cones**. These cones have a special structure that aids their detection of details in the visual image. The foveal cones have special long and slendar bodies in contrast to the much fatter cones located more peripherally in the retina. In the foveal region, the blood vessels, ganglion cells, inner nuclear layers of cells and plexiform layers are all displaced to **one side** rather than resting directly on top of the cones. This arrangement, allows light to pass unimpeded to the cones. In general, rods and cones are **composed** of: 1. An outer segment 2. Inner segment 3. The nucleous and 4. Synaptic body. A **light sensitive** photochemical is usually contained in the outer segment. In the rods, it is rhodopsin while in the cones, it is one of the three colour photochemicals called colour pigment. Both rhodopsin and the colour pigments are **conjugated** proteins. The protein portion of the rods is called scotopsin while that of the cones is called photopsin. A black pigment called melanin in the pigment layer of the retina prevents light reflection throughout the globe of the eyeball. This aids clear vision. The pigment area also stores vit. A. The nutrient blood supply for the internal layers of the retina is independent of the either structures of the eye and it's derived from the central retinal artery. Whereas the outer segment of the retina especially that of rods and cones depend on diffusion from the choroid blood vessels for their nutrition and oxygen. **PHYSIOLOGY OF THE EYE** For **effective** vision, rays of light reflected from an object must enter the eye and be focused on the retina. This implies that these rays must be refracted. To achieve this refraction, the eye uses its **refracting** media comprising of the; 1. Cornea 2. Aqueous humour 3. Lens and 4. Vitreous humour The lens has a refractive index of **1.413** while the rest have **1.33** as their refractive index. The **greatest** refraction of a ray entering the eye occurs between the lens and the cornea as well as when it is leaving the lens. **ACCOMMODATION** This refers to the focusing of the lens system of the eyes to achieve a high degree of visual acuity for both near and far object. This results from the contraction or relaxation of the ciliary muscles of the eye. Contraction causes an increased refractive power while relaxation causes a decrease refractive power. When the eyes suddenly changes distance of the fixation point, the lens changes its strength appropriately to achieve a new state of focus within a fraction of a second. **PATHOLOGY OF VISION** 1. **EMMETROPIA (Normal Vision):** The eye is considered normal or emmetropic if parallel rays of light from distant object are in sharp focus on the retina. The emmetropic eye can see all distant object clearly with its ciliary muscle in a relaxed state. However, to see objects at close range, the ciliary muscle must contract and produce appropriate accommodation.  2. **HYPEROPIA (Long sightedness):** This is usually either due to a too short eyeball or a lens system that is too weak. In this condition, parallel rays are not bent sufficiently by the relaxed lens so as to focus on the retina. Images, focus behind the retina. This can be correct by adding an extra refractive power through the use of a convex lens in front of the eye. 3. **MYOPIA (Short Sightedness):** This is a condition where light rays coming from distant object are focused in front of the retina. This is usually due to too long eyeball or too much refractive power in the lens system of the eyes. A myopic person has a definite limiting far point for clear vision. This is corrected by placing a concave spherical lens in front of the eyes. Short-Sightedness (Myopia) \| Causes and Treatment \| Patient 4. **ASTIGMATISM:** This is a refractive error of the eye which causes the virtual image in one plane to focus at a different distant from that of the plane at right angle. This can occur as a result of excessive curvature of the cornea in one plane of the eye. The accommodating power of the eye can never compensate for astigmatism because accommodation changes the curvature of the eye lens equally in both plans. This disorder is however corrected with the use of **cylindrical lens**. **ABNORMALITIES OF THE EYES** 1. **Cataract:** This occurs mainly in older people and refers to a cloudy or opaque area or areas in the lens. In the early stage of formation, the proteins in some lens fibres becomes denatured and later coagulate to form opaque areas in place of the normal transparent protein fibre. This condition is usually corrected by surgery. When this is done, the eye loses a large proportion of its refractive power which must be replaced by a powerful convex lens in front of the eye. 2. **GLAUCOMA:** The average normal intraocular pressure is about **15mmHg** with a range of 12 -- 20mmHg. In a normal eye, this pressure remains constant within [+] 2mmHg. Glaucoma is a disease of the eye in which intraocular pressure become pathologically high often reaching 60mmHg -- 70mmHg. Pressures about 25 -- 30mmHg can cause loss of vision when maintained for long period. Extremely high pressures can cause blindness within hours. High pressures compress the optic nerve thereby blocking appropriate nutrition and causing the death of the fibre which leads to the brain. This can be caused by any condition that blocks the trabecular spaces. It is treated with eye drops that reduces the eye secretion or increases the absorption of aqueous humour. When this fails, surgery is employed to open the trabecular spaces. **THE EAR** The ear is the organ concerned with hearing and balancing. It has three division; the external, middle and inner ear. **DIAGRAM OF THE EAR**  **THE EXTERNAL EAR:** This is composed of the trumpet-like, cartilaginous external ear (auricle) which may be helpful in localizing the source of a sound. From the auricle, the external auditory meatus which is about **2.5cm** long in the adult connects the external to the **tympanic** membrane of the middle ear. The skin of the outer half of the meatus possesses numerous ceruminous or wax secreting glands which secretes a substance called **cerumen** which is waxy in texture. It also helps to trap particles entering the meatus. The auditory meatus also contains hair and sweat glands on its wall. **THE MIDDLE EAR:** This starts from the tympanic membrane or ear drum which is about 1cm in diameter. It is composed of radial and circular collagenous fibres covered externally by stratified **squamous** epithelium which is continuous with the skin. Internally, it is covered by a mucus membrane comprising of ciliated columnar epithelium. The outer surface of the membrane is concave while the inner surface is convex. The middle ear also contains a pharyngotympanic tube called Eustachian tube which connects it to the nasal pharynx. The pharyngeal end of the tube is normally closed but opened during swallowing to allow the air pressure in the ear to be equalized with that of the atmosphere. Reduced acuity in hearing occurs if the tube is blocked by a swelling of the mucus membrane which usually occur during cold condition. The tube therefore serves to equalize the air pressure within the ear cavity with that of the exterior. This equalization is facilitated by swallowing. The middle ear also contains three small bones called ossicles. They are malleus, (hammer), incus (anvil) and stapes (stirrup). The malleus articulates with the incus which further attaches to the stapes. The **base** of the stapes is attached to the **margins** of the fenestra vestibuli (oval window) by fibrous tissues. The ear drum and the ossicles serves as a type of **transformer** that conveys vibrations of the light medium (air) to the denser medium (water) of the internal ear. The area of the eardrum is about 90mm^2^ while that of the stapes is only about 3.2mm^2^. This therefore creates more pressure in the fluid under the stape which therefore causes the ear drum to move. This mechanism is known as **impedance matching function.** In conditions of a familiar and progressive deafness, in young people called **otosclerosis**, the base of the stapes become joined by bones to the margins of the fenestra vestibuli. This condition causes severe deafness. **THE INNER EAR:** The inner ear contains receptors for hearing and equilibrium. It comprises the **cochlea** which is primarily concerned with **hearing** as well as the semi-circular canals and **otolith** organs which are concerned with the maintenance of balance. The cochlea is a system of curled tubes, and consists of three (3) tubes curled side by side. These are; 1. The scala vestibuli 2. The scala media 3. The scala tympani The scala vestibuli and scala media are separated from each other by the **Reissner's membrane** (vestibular membrane) while the scala tympani and the scala media are separated from each other by the **basilar membrane**. On the surface of the Basilar membrane, there is a structure called the **organ of cort**; which contains a series of **electromechanically sensitive** cell called the hair cells. They are the receptive end organs that generate nerve impulses in response to sound vibration. The Reissner's membrane maintains a special kind of fluid in the scala media which is required for the normal function of the sound receptive hair cell. The inside of the cochlea contains a fluid known as perilymph while the **cochlea duct** contains another fluid known as endolymph. The fluid in the scala vestibule and scala tympani (Perilymph) is similar in chemical composition to the cerebrospinal fluid except that it contains a higher percentage of protein. The endolymph in the scala media also has high potassium and low sodium composition like the intracellular fluid. C:\\Users\\NDFIED\\Desktop\\IMG-3989.JPG **PATHOLOGY OF THE EAR** Although ear disorders are rarely fatal, they impair hearing activity. They include 1. **Inflammatory lesions:** Inflammations of the ear which includes otitis media usually occur in children. They produce a serous exudate (if viral) but may become suppurative if complicated by bacteria infection, with organism streptococcus pneumonia, Herpes influenza and murazela calaliz. Repeated cases of acute otitis media leads to chronic diseases which may perforate the ear drum, encroaching on the ossicles or labirinte spreading into the mastoid cases and even penetrating into the cranial vault to produce a temporal cerebritis or abscess. 2. **Otosclerosis:** This is an abnormal bone deposition at the rim of the oval window into which the foot plate of the stapes fits. It usually affect both ears. It begins in childhood and is familial following autosomal dominant transmission with veritable penetrance. It progresses slowly over time and results in a remarkable hearing loss. 3. **Tumours:** Epithelial and mesenchyme tumours are rare in the ear except basal cell or squirmous cell casinomas of the pinna. These casinomas tend to occur in the elderly and may be associated with actinic radiation. The basal cell or squirmous cell region of the pinna commonly invade locally, like squirmous cell casinomas arising in the external canal may invade the cranial cavity or metastasize to regional nodes which are often fatal. **HEARING ABNORMALITIES** **Deafness:** This is usually divided into two types (a) those caused by an impairment of the cochlea or the auditory nerve (b) those caused by an impairment of the physical structure of the ear which conduct sound wave to the cochlea. This is classified as conduction deafness. **Nerve deafness** usually result in permanent deafness while in **conduction deafness,** sound waves dcan still be conducted into the cochlea by means of bone conduction by a sound generator applied to the skull over the ear. **THE CHEMICAL SENSES** **Taste and Smell** The senses of taste and smell allow us to separate undesirable or even lethal food from those that are pleasant or nutritious. Both senses are strongly tied to primitive, emotional and behavioural functions of our nervous system. **SENSE OF TASTE (GUSTATION)**: The sense of taste is confined to the mouth region. The **structures** containing the receptors responsible for the sensation of taste are the **taste bud**. They are about a thousand in adult and are placed mainly on the peripheral part of the **dorsum** of the tongue. They are mostly found in the groove surrounding the valtale papillae. Though some occur on the soft palate while a few occur on the epiglottis. In children, they are more numerous and are more widely distributed over the tongue and on the inside of the cheeks. The oval taste bud embedded in the stratified epithelium covering the tongue contain thin **fusiform** taste cells surrounded by supporting cells just like the staves of a barrel. The taste cells have micro villi on their free surfaces. The cells are elongated and cylindrical forming an opening at the surface of the tongue. These opening are known as taste pores. The taste buds are grouped into structures called papilla which are distributed on different parts of the tongue. These papilla gives the tongue a rough surface. All the nerve fibres that mediates the sense of taste pass centrally to the medulla oblongata. **PRIMARY SENSATIONS OF TASTE** Psychophysiologic and neurophysiologic studies has identified at least 13 possible chemical receptor in the taste cells. They are: 1. 2 sodium receptor 2. 2 potassium 3. 1 chloride 4. 1 Adenosine 5. 1 inosine receptor 6. 2 sweet 7. 2 bitter 8. 1 glutamate 9. 1 hydrogen These receptor capabilities have been grouped into five categories called the primary sensations of taste namely; 1. Sour 2. Salty 3. Sweet 4. Bitter and 5. Umami taste. **SOUR TASTE** This is caused by acid (H^+^ (hydrogen ion)) and its intensity is approximately proportional to the logarithm of the H^+^ conc. i.e. the more acidic the food, the stronger the sour sensation. E.g. include vinegar, which contains acetic acid, citrus fruit which contains citric acid and sour milk which contains lactic acid. **SALTY TASTE** This is elicited by ionized salt mainly by the Na^+^ concentration of a food. The quality of the taste varies from one salt to the other. The cations of the salts especially sodium cations are mainly responsible for the salty taste. Though the typical salt substance is NaCl, saltiness is not a specific property of the chloride ion since sodium bromate, sodium iodide, sodium sulphate and sodium nitrate all have salty taste. **SWEET TASTE** The sweet taste is not caused by any single class of chemical. A number of organic substances such as sucrose, glycerin, sodium cyclamate, saccharine and a few other inorganic substance such as salts of lead and beryllium all have sweet taste. The amino acid are sweet while polypeptide are often bitter. The sweetest sugar is fructose followed by sucrose followed by glucose, etc. Other substances with the sweet taste include glygos, sugar, alcohols, aldehydes, ketones, amides, esters, some amino acid and some small proteins. **BITTER TASTE** The bitter taste just like sweet taste is not caused by a single chemical agent. The substances that give bitter taste are also almost entirely organic. Two particular classes of substances are especially likely to cause the bitter taste sensations. They are 1. Long chain organic substances that contain Nitrogen. 2. Long chain organic substances that contain alkaloids. The alkaloids include many of the drugs sued in medicine e.g. quinine, caffeine, strychnine, and nicotine. **UMAMI TASTE** Umami is a Japanese word meaning delicious which implies pleasant taste sensation qualitatively different from sour, salty, sweet or bitter taste. It is the dominant taste of foods containing L-glutamate such as meat extract and ageing cheese. **DIAGRAM OF TASTE BUD**  **THE SENSE OF SMELL (OLFACTION)** This is the least understood of all, this is because it is poorly developed in human being when compared with lower animal. The **receptor** cell for the smell sensation olfactory cells which are **bi-polar** nerve cells derived from the central nerve cells. The mucosa end of the olfactory cells form a knob from which 4 -- 25 olfactory hairs (olfactory cilia) project into the mucus that coats the inner surface of the nasal cavity. These projecting olfactory cilia forms a dense mat in the mucus which reacts to odours in the air and stimulate the olfactory cell. In between the olfactory cells in the olfactory membrane, there are many small bow man's gland that secrete mucus on the surface of the membrane. صدق الله العظيم الاسراء اية 58. By Dr. Abdel Aziz M. Hussein Lecturer of Physiology Member of American Society of Physiology. - ppt download **PHYSIOLOGY OF SMELL** The portion of each olfactory cell that responds to the olfactory stimuli is the **olfactory cilia.** The deodorant substance on coming in contact with the olfactory membrane surface diffuses into the mucus which covers the cilia. It subsequently binds with receptor proteins on the surface of the membrane of each cilium. On excitation of the receptor protein, an α-subunit breaks away from the G-protein on the inside of the folding protein and immediately activates Adenyl cyclase which is attached to the inside of the ciliary membrane near the receptor cell body. The activated cyclase in turn converts many molecules of intracellular ATP into cyclic AMP's. Finally, Cyclic Amp activates another membrane protein which is a greater Na^+^ channel that opens its gate for large amount of Na^+^ to pour through into the receptor cell cyntoplasm. The Na^+^ increases the electrical potential in the positive direction inside the cell membrane. Thus, exciting the olfactory neutron and transmitting action potentials into the CNS through olfactory nerves. In addition to basic chemical mechanism by which olfactory cells are stimulated. Several factors affect the degree of stimulations. For instance, 1. Only volatile substances that can be sniffed into the nostrils can be smelled. 2. The stimulating substances must be at least water soluble so that it can pass through the mucus to reach the olfactory cilia. 3. The substance must be at least slightly lipid soluble because lipid constituents of the cellium itself are a weak barrier to non-lipid soluble odourant.