Special Senses (Vision) PDF

SPECIAL SENSES Prepared by: Julius John Salamanes, RN, MSc.,LPT, USRN VISION The visual system includes the eyes, the accessory structures, and sensory neurons The eyes are housed within bony activities in the skull called orbits. Only the anterior 1/6 of the eye’s surfac...

SPECIAL SENSES Prepared by: Julius John Salamanes, RN, MSc.,LPT, USRN VISION The visual system includes the eyes, the accessory structures, and sensory neurons The eyes are housed within bony activities in the skull called orbits. Only the anterior 1/6 of the eye’s surface can normally be seen Accessory Structures of the Eye The accessory structures protect, lubricate, and move the eye. 1. Eyebrows ▪ protect the eyes by preventing perspiration from running down the forehead and into the eyes, causing irritation ▪ also help shade the eyes from direct sunlight 2. Eyelids ▪ With their associated lashes, protect the eyes from foreign objects ▪ Blinking, which normally occurs about 20 times per minute, also helps keep the eyes lubricated by spreading tears over the surface. 3. Conjunctiva ▪ a thin, transparent mucous membrane covering the inner surface of the eyelids and the anterior surface of the eye ▪ essential for protecting, lubricating, and maintaining the overall health of the eye. 4. Lacrimal apparatus ▪ consists of lacrimal gland situated in the superior lateral corner of the orbit and a nasolacrimal duct and associated structures in the inferior medial corner of the orbit. ▪ The lacrimal gland produces tears, which passes over the anterior surface of the eye. Most of the fluid produced by the lacrimal glands evaporates from the surface of the eye, but excess tears are collected in the medial angle of the eyes by small ducts called lacrimal canaliculi. These canaliculi open into a lacrimal sac, an enlargement of the nasolacrimal duct, which opens into the nasal cavity. o Tears are produced by the lacrimal gland and flow across the eye to enter the puncta, small openings in the eyelids. From there, the tears drain into the lacrimal sac, then through the nasolacrimal duct, and finally into the nasal cavity. This is why you often experience a runny nose when you cry. ▪ Tears lubricate and cleans the eye 5. Extrinsic eye muscles (Extraocular muscles) ▪ controls movement of the eyeball ACTION CRANIAL NERVE SUPPLY Lateral rectus Moves the eye laterally (outward) VI Medial rectus Moves the eye medially (inward) III Superior rectus Moves the eye upwards and medially (inward) III Inferior rectus Moves eye downwards and medially (inward) III Inferior oblique Moves eye upwards and laterally (outward) III Superior oblique Moves eye downwards and laterally (outward) IV COATS OF THE EYE A. Fibrous tunic- outermost tunic 1. Sclera ▪ the firm, white, outer connective tissue layer of the posterior 5/6 of the fibrous tunic ▪ a small portion can be seen as the “white of the eye” ▪ helps maintain the shape of the eye, protects the internal structures and provides attachment sites for the extrinsic eye muscles  2. Cornea ▪ the transparent anterior sixth of the eye, which permits light to enter ▪ As part of the focusing system of the fibrous tunic, it also bends, or refracts, the entering light ▪ the only tissue that can be transplanted from one person to another without the worry of rejection because it has no blood vessels thus it is beyond the reach of the immune system B. Vascular and Muscular tunic (UVEA)- middle tunic 1. Choroid ▪ the posterior portion of the vascular tunic, associated with sclera ▪ very thin structure consists of a vascular (blood-rich nutritive) network and many melanin- containing pigment cells, so that it appears black. The dark pigment prevents light from scattering inside the eye Anteriorly, the vascular tunic consists of ciliary body and iris. 2. Ciliary body ▪ continuous with the anterior margin of the choroid ▪ contains smooth muscles called ciliary muscles, which attach to the perimeter of the lens by suspensory ligaments ▪ with ciliary processes producing aqueous humor ▪ adjusts the lens for focusing, allowing the eye to see objects clearly at different distances. o The lens- is a flexible, biconvex, transparent disc. It is essential for focusing light onto the retina, allowing for clear vision at varying distances through accommodation, and maintaining clarity to ensure optimal light transmission. 3. Iris ▪ the colored part of the eye ▪ attached to the anterior margin of the ciliary body, anterior to the lens ▪ a contractile structure consisting mainly of smooth muscle surrounding an opening called the pupil. Light passes through the pupil, and the iris regulates the diameter of the pupil, which controls the amount of light entering the eye. o Parasympathetic stimulation of the oculomotor nerve (III) causes the circular smooth muscles of the iris to contract, constricting the pupil, whereas sympathetic stimulation causes the radial smooth muscles of the iris to contract, dilating the pupil. o Pupillary constriction occurs when the parasympathetic nervous system contracts the sphincter pupillae in response to bright light, while dilation happens when the sympathetic nervous system contracts the dilator pupillae in dim light or during stress.  C. Nervous tunic- innermost tunic 1. Retina ▪ covers the posterior 5/6 of the of the eye and is composed of two layers: a. outer pigmented retina- with the choroid, keeps light from reflecting back into the eye b. inner sensory retina- contains photoreceptor cells, called rods and cones RODS- stimulated by low intensity light; for night vision (scotopic vision); responsible for peripheral vision CONES- stimulated by high intensity light; for day/color vision (photopic vision) Note: There are three types of cones, each sensitive to a different color: blue, green, or red CHAMBERS OF THE EYE 1. Anterior chamber-located between the cornea and the iris 2. Posterior chamber- located between the iris and the lens Important: The anterior and posterior chambers are filled with aqueous humor (watery fluid), which helps maintain pressure within the eye, refracts light, and provides nutrients to the inner surface of the eye. The aqueous humor is produced by the ciliary body. 3. Vitreous chamber- located posterior to the lens; filled with vitreous humor. The vitreous humor helps maintain pressure within the eye and holds the lens and retina in place. Unlike the aqueus humor, the vitreous humor does not circulate. Two major features of the posterior region of retina when examined within ophthalmoscope a. Macula ▪ a small spot near the center of the posterior retina ▪ In the center of the macula is a small pit, the fovea centralis. o The fovea centralis is the part of the retina where light is most focused when the eye is looking directly at an object. Also, the region with the greatest ability to discriminate fine images, which explains why objects are best seen straight ahead. b. Optic disc ▪ a white spot just medial to the macula, through which a number of blood vessels enter the eye and spread over the surface of the retina ▪ contains no photoreceptor cells and do not respond to light; it is therefore called the “blind spot of the eye”  ▪ point where the optic nerve fibers exit the eye REFRACTIVE MEDIA OF THE EYES 1. Cornea 2. Aqueous humor (in the anterior and posterior chamber) 3. Lens 4. Vitreous humor When light passes from one substance to another substance that ahs a different density, its speed changes and its rays are bent, or refracted. Light rays are bent in the eye as they encounter the cornea, aqueous humor, lens and vitreous humor. The refractive or the bending power of the cornea and humors is constant. However, that of the lens can be changed by changing its shape- that is by making it more or less convex, so that light can be properly focused on the retina. The greater the lens convexity, or bulge, the more it bends the light. On the other hand, the flatter the lens, the less it bends the light. The resting is “set” for distant vision. In general, light from a distant source (over 20 feet) approaches the eye as parallel rays, and no change in lens shape is necessary for it to be focused properly on the retina. However, light from a close object tends to scatter and diverge, or spread out, and the lens must bulge to make close vision possible. To achieve this, the ciliary body contracts, allowing lens to become more convex. The ability of the eye to focus specifically close objects (those less than 20 ft away) is called ACCOMODATION. VISUAL PATHWAY Electrical signals pass from the photoreceptors (rods and cones) to the bipolar cells and then to the ganglion cells Axons of ganglion cells join together and pierce back of the retina (optic disc) to form the optic nerve Optic nerves have nasal fibers (from the medial side) and temporal fibers (from the lateral side). Nasal fibers receive impulse from the temporal visual field and the temporal fibers receive impulse from the nasal visual field. Nasal fibers decussate (cross) at the level of the optic chiasm while temporal fibers leave the optic chiasm without decussating. Fibers from the optic chiasm going to the lateral geniculate bodies are called optic tract. Lateral geniculate bodies (in the thalamus) are the last relay station of the visual pathway. o Most of the optic tract axons terminate in the thalamus. Some axons do not terminate in the thalamus but separate from the optic tracts to terminate in the superior colliculi, the center for visual reflexes. Cells from the lateral geniculate bodies in the thalamus project fibers to the primary visual area and are called optic radiation or geniculocalcarine tract. The optic radiations project to the primary visual area (visual cortex) in the occipital lobe of the brain. The visual cortex is the area of the brain where vision is perceived.  HEARING (AUDITORY) AND BALANCE The organs of hearing and balance are proportioned into three areas: the external, middle and inner ears. Anatomy and Function of the Ear A. EXTERNAL (OUTER) EAR 1. Pinna or auricle ▪ the fleshy part of the external ear on the outside of the head ▪ collects sound waves and direct it into the external auditory canal 2. External auditory canal ▪ a passageway that leads to the eardrum ▪ lined with hairs and ceruminous (wax) glands, which produce cerumen, a modified sebum commonly called earwax 3. Tympanic membrane (eardrum) ▪ a thin membrane that separates the external ear from the middle ear Note: The auricle collects sound waves and directs them toward the external auditory canal, which transmits them to the tympanic membrane. Sound waves reaching the tympanic membrane cause it to vibrate. B. MIDDLE EAR-air-filled cavity located medial to the tympanic membrane 1. Auditory ossicles- receives vibrations from the tympanic membrane and transmits to the oval window o Malleus (hammer)- attached to the medial surface of the tympanic membrane o Incus (anvil)- connects malleus to the stapes o Stapes (stirrups)- presses on the oval window 2. Two covered openings on the medial side and connect the middle ear to the inner ear o Oval window- receives vibrations from the three auditory ossicles; plays a crucial role in transmitting and amplifying sound waves from the middle ear to the inner ear, initiating fluid motion within the cochlea, and facilitating the auditory process. o Round window- essential for pressure equalization, sound transmission, protection of inner ear structures, and facilitating the movement of the basilar membrane, all of which contribute to the auditory process.  3. Eustachian tube (Auditory tube) o opens into the pharynx and enables air pressure to be equalized between the outside air and middle ear cavity o crucial for maintaining proper tympanic membrane (eardrum) function and ensuring efficient sound transmission. o The Eustachian tube connects the middle ear to the nasopharynx, allowing air to flow in or out to equalize pressure, especially during activities like swallowing or yawning. C. INNER (INTERNAL) EAR contains bony chambers called bony labyrinth enclosing the membranous labyrinth 1. Bony labyrinth- filled with a clear fluid PERILYMPH a. Cochlea b. Vestibule c. Semicircular canals 2. Membranous labyrinth- filled with a clear fluid ENDOLYMPH. a. Saccule b. Utricle c. Cochlear duct d. Semicircular ducts The space between the bony and membranous labyrinths is filled with a fluid called PERILYMPH. HEARING: THE COCHLEA Cochlea ▪ is shaped like a snail shell and contains a bony core shaped like a screw ▪ involved in HEARING ▪ divided into three channels 1. scala vestibuli situated above the scala media (cochlear duct) and extends from the oval window at the base of the cochlea to the helicotrema at the apex. The primary function of the scala vestibuli is to convey sound vibrations from the oval window (which is connected to the stapes bone of the middle ear) through the cochlea. When sound waves enter the oval window, they create pressure waves in the perilymph of the scala vestibuli. The pressure waves created in the scala vestibuli stimulate the basilar membrane within the cochlear duct. This, in turn, activates the hair cells in the organ of Corti, leading to the transduction of sound waves into electrical signals that are sent to the brain for auditory processing. 2. scala tympani located in the lower part of the cochlea, beneath the scala media (the cochlear duct) and above the scala vestibuli. It extends in parallel with the scala vetibuli from the apex back to the round window. The scala tympani, along with the scala vestibuli, is involved in the process of sound transduction. When sound waves enter the cochlea, they create pressure waves in the perilymph of the scala vestibuli, which then travel through the cochlea and eventually reach the scala tympani, allowing for the movement of hair cells in the organ of Corti.  The movement of fluid in the scala tympani, in response to sound waves, helps stimulate the hair cells in the cochlear duct, leading to the generation of nerve impulses that are sent to the brain for auditory perception. Note: These two channels (scala vestibuli and scala tympani) are perilymph-filled spaces between the walls of the bony and membranous labyrinths. The wall of the membranous labyrinth that lines the scala vestibuli is called the vestibular membrane. The wall of the membranous labyrinth that lines the scala tympani is called the basilar membrane. 3. cochlear duct (scala media) formed by the space between the vestibular membrane and the basilar membrane and is filled with endolymph situated between the scala vestibuli and the scala tympani. It extends from the base of the cochlea at the oval window to the apex of the cochlea. The cochlear duct plays a crucial role in the process of sound transduction. When sound waves enter the cochlea through the oval window, they create pressure waves in the perilymph of the scala vestibuli. This movement causes the basilar membrane to vibrate, which in turn displaces the hair cells in the Organ of Corti against the tectorial membrane. o ORGAN OF CORTI (spiral organ)- specialized structure found inside the cochlear duct and is the receptor for hearing. ▪ The organ of Corti contains specialized sensory cells called hair cells, which have hairlike microvilli on their surfaces. It is situated on the basilar membrane within the cochlear duct. ▪ The hair tips are embedded within an acellular gelatinous shelf called the tectorial membrane. The tectorial membrane is integral to the auditory system, enabling the conversion of sound waves into electrical signals that the brain interprets as sound. ▪ When sound vibrations cause the basilar membrane to move, the Organ of Corti detects these movements. The displacement of the hair cells against the tectorial membrane generates electrical signals that are transmitted to the auditory nerve fibers, leading to the perception of sound. ▪ The Organ of Corti contains sensory hair cells that detect vibrations caused by sound waves and convert them into electrical signals, which are then sent to the brain for auditory processing. The perilymph is located in the spaces surrounding the cochlea and the semicircular canals, serving as a cushion and pressure equalizer, while endolymph is found within the cochlear duct and the membranous labyrinth, playing a crucial role in sound transduction and balance. Their different ionic compositions and locations are essential for the proper functioning of the auditory and vestibular systems. o The composition of perilymph is similar to that of extracellular fluid, containing a higher concentration of sodium (Na+) and a lower concentration of potassium (K+) ions.  o Endolymph has a unique ionic composition, characterized by a high concentration of potassium (K+) and a low concentration of sodium (Na+) ions. Neuronal Pathways for Hearing The senses for hearing and balance are both transmitted by the vestibulocochlear nerve (CN VIII). o Cochlear nerve is the portion of the vestibulocochlear nerve involved in hearing o Vestibular nerve is involved in balance Hair cells → Cochlear nerve → Cochlear nucleus → other areas of the brainstem and to inferior colliculus in the midbrain → thalamus→ Auditory cortex (temporal lobe) of the cerebrum Important: Conductive hearing loss is primarily due to problems in the outer or middle ear affecting sound transmission, while sensorineural hearing loss involves damage to the inner ear or auditory pathways, impacting the processing of sound. The treatment approaches and implications for each type of hearing loss can differ significantly, emphasizing the importance of accurate diagnosis and management. BALANCE: THE VESTIBULE AND SEMICIRCULAR CANALS Two components of sense of balance or equilibrium 1. Static equilibrium- associated with the VESTIBULE and is involved in evaluating the position of the head relative to gravity. Vestibule- divided into two chambers namely: a. utricle b. saccule Each chamber contains specialized patches of epithelium called the maculae, which are surrounded by endolymph. The maculae, like the spiral organ, contain hair cells. The tips of the microvilli of these cells are embedded in a gelatinous mass weighted by otoliths, particles composed of protein and calcium carbonate. Function of the vestibule is essential for detecting linear acceleration and changes in head position, contributing to the overall balance and spatial orientation of the body. It plays a vital role in coordinating movements and maintaining stability. 2. Dynamic equilibrium- associated with the SEMICIRCULAR CANALS and is involved in evaluating changes in the direction and rate of head movements. Semicircular canals ▪ involved in dynamic equilibrium ▪ canals are placed at nearly right angles to one another, enabling a person to detect movement s in essentially any direction ▪ The base of each semicircular canal is expanded into an ampulla. Within each ampulla, the epithelium is specialized to form a crista ampullaris. ✓ Each crista consists of a ridge of epithelium with a curved, gelatinous mass, the cupula, which is structurally and functionally very similar to the maculae, except that it has no otoliths. The  cupula functions as a float that is displaced by the endolymph movement within the semicircular canals. ▪ Function of semicircular canals: essential for detecting rotational movements of the head, contributing to balance, spatial orientation, and coordination of movements in relation to the body’s position in space. Neuronal Pathways for Balance Axons forming the vestibular portion of the vestibulocochlear nerve (CN VIII) project to the vestibular nucleus in the brainstem. Axons run from this nucleus to numerous areas of the CNS , such as the cerebellum and cerebral cortex (parietal lobe). TASTE (GUSTATORY) SENSATION TASTE BUDS ▪ sensory structures that detect taste stimuli ▪ are oval structures located on the surface of certain papillae, which are enlargements on the surface of the tongue ▪ also distributed throughout other areas of the mouth and pharynx, such as on the palate, the root of the tongue, and the epiglottis ▪ Each taste bud is composed of two types of cells: a. gustatory (taste) receptor cells ▪ the primary sensory cells within taste buds that detect taste stimuli (such as sweet, sour, salty, bitter, and umami) and send signals to the brain, allowing for the perception of taste. ▪ the interior part of the taste buds responsible which contains hairlike processes called taste hairs, which extend into a tiny opening in the surrounding stratified epithelium,  called a taste pore. b. supporting cells- specialized epithelial cells that form the exterior supporting capsule of each taste bud and encapsulate the taste cells Types of papillae 1. circumvallate papillae- with taste buds Located at the back of the tongue, forming a V-shaped row just in front of the sulcus terminalis (a groove that separates the tongue into anterior and posterior sections). These are large, dome-shaped papillae surrounded by a trench. They contain many taste buds and are particularly sensitive to bitter tastes. 2. fungiform papillae- with taste buds Scattered across the tongue, especially concentrated at the tip and sides. mushroom-shaped and contain a few taste buds. They play a role in taste sensation, particularly for sweet and salty tastes. 3. foliate papillae- with taste buds Found on the sides of the tongue, particularly near the back These papillae are leaf-shaped and contain taste buds that are more active in children but tend to decline with age. They help in detecting sour tastes. 4. filiform papillae-no taste buds; only gustatory cells the most numerous papillae and are cone-shaped spread across the entire surface of the tongue, especially in the central area main function is to provide a rough surface for moving food around the mouth and sensing texture. FIVE BASIC TYPES OF TASTE SENSATIONS (sensed acutely in particular regions of the tongue) 1. Sweet- tip of the tongue 2. Bitter- back of tongue 3. Salty- over most of tongue 4. Sour- sides of tongue 5. Umami (savory) Although all taste buds are able to detect all five basic taste sensations, each taste bud is usually most sensitive to one class of stimuli. Most taste sensations are strongly influenced by olfactory sensation. This influence can be demonstrated by comparing taste of some food before and after pinching your nose. It is easy to detect that the sense of taste is reduced while the nose is pinched. Neuronal Pathways for Taste Taste sensations are carried by three cranial nerves: o facial nerve (CN VII) transmits sensations from the anterior 2/3 of the tongue o glossopharyngeal nerve (CN IX) carries taste sensations from the posterior 1/3 of tongue o In addition,vagus nerve (CN X) carries some taste sensations from the root of the tongue. Three cranial nerves (CN VII, IX and X)→ gustatory (taste) portion of brainstem nuclei→ thalamus→ taste area in the parietal lobe of the cerebral cortex OLFACTION (SMELL) The sense of smell, called olfaction, occurs in response to airborne molecules, called odorants, which enter the nasal cavity. Olfactory neurons are bipolar neurons within the olfactory epithelium, which lines the superior part of the nasal cavity. The mucus keeps the nasal epithelium moist, traps and dissolves airborne molecules, and facilitates the removal of molecules and particles from the nasal epithelium. Airborne odorants become dissolved in the mucus on the surface of the epithelium and bind to receptor molecules on the membranes of the specialized cilia. There are at least 400 functional olfactory receptors in humans Neuronal Pathways for Olfaction Axons from olfactory neurons form the olfactory nerves (CN I)→ pass through the foramina of the cribriform plate and enter the olfactory bulb→ olfactory tracts→ Amygdala and hippocampus → olfactory cortex (located within the temporal)  Olfaction is the only major sensation that is relayed directly to the cerebral cortex without first passing through the thalamus. Within the olfactory bulb and olfactory cortex are feedback loops that tend to inhibit transmission of action potentials resulting from prolonged exposure to a given deodorant. This feedback, plus the temporary decreased sensitivity at the level of the receptors, results in adaptation to a given odor. Relations between Taste and Smell ▪ Taste and smell are separate senses with their own receptor organs, yet they are intimately entwined. ▪ Tastants, chemicals in foods, are detected by taste buds, which consist of special sensory cells. When stimulated, these cells send signals to specific areas of the brain, which make us conscious of the perception of taste. ▪ Similarly, specialized cells in the nose pick up odorants, airborne odor molecules. Odorants stimulate receptor proteins found on hairlike cilia at the tips of the sensory cells, a process that initiates a neural response. ▪ Ultimately, messages about taste and smell converge, allowing us to detect the flavors of food. POINTS TO REMEMBER The sensations of smell and taste are closely related, both structurally and functionally, and both are initiated by the interaction of chemicals. The sense of vision is initiated by interaction of light with sensory receptors Both hearing and balance function in response to the interaction of mechanical stimuli with sensory receptors. Hearing occurs in response to sound waves, and balance occurs in response to gravity or motion.