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The Special Senses Chapter 15 The Special Senses • Chemical senses – Taste (gustation) – Smell (olfaction) • Vision • The ear – Hearing – Equilibrium • ***NOTE: Touch is not part of special senses, it is part of the NERVOUS SYSTEM 2 Sense of Taste • Taste buds are the sensory receptor organs located in the oral cavity. – Approximately 10,000 with most in the papillae of the tongue Taste Buds • Each consists of 50-100 epithelial cells Receptor cells – also called gustatory or taste cells -Gustatory Hairs Disolved molecules bind & induce receptor cells to generate impulses in sensory nerve fibers – Basal cells – act as stem cells which divide and differentiate into supporting cells (taste bud cells are replaced every 7-10 days) Five taste sensations • • • • • Sweet—front middle Sour—middle sides Salty—front side/tip Bitter —back “umami”—posterior pharynx M&M, Fig. 16.1 Taste Sensations • Cravings are often related to needs such as protein, sugar, salt or vitamin C • Bitterness is often related to poisons and spoiled food which is why we don’t find it pleasant Taste • In order to taste, the food must dissolve in saliva, diffuse into the taste pore and contact the gustatory hair • This then allows the taste to be transmitted to the brain • About 80% of taste is smell • Mouth also contains thermo-receptors and mechanoreceptors Cranial Nerves of Taste Anterior 2/3 tongue: VII (Facial) Posterior 1/3 tongue: IX Glossopharyngeal) Pharynx: X (Vagus) M&M, Fig. 16.2 Sense of Smell • Olfactory epithelium is located on the roof of the nasal cavity and similar to taste buds is composed of: – Olfactory receptor cells • Attached are olfactory cilia that increase surface area – Supporting cells – Basal cells Smell • Humans can distinguish between approximately 10,000 smells • Smell also relies on odor chemicals being in solution to be transmitted. Smells dissolve in mucus in the nose to be transmitted. Smell Disorder Anosmia: absence of the sense of smell – Trauma – Colds or allergies producing excessive mucus – Polyps causing blockage – 1/3 are from zinc deficiency Ear/Hearing M&M, fig. 16.17 • 3 Parts to the Ear • Outer Ear: auricle is elastic cartilage attached to dermis, gathers sound • Middle ear: ear ossicles transmit and modulate sound • Inner ear: cochlea, ampullae and semicircular canals sense sound and equilibrium Hearing • Ear is composed of 3 main structures: – Outer ear • consists of the auricle and external auditory canal. The auricle helps to direct sound while the external auditory canal extends the sound to the eardrum or tympanic membrane. • External auditory canal also aids in keeping the ear clean and free of foreign object through the secretion of cerumen (ear wax) • As sound enters, it causes the ear drum to vibrate which transfers the sound to the middle ear Middle Ear • External auditory canal ends at tympanic membrane which vibrates against malleus on other side • Inside middle ear chamber – malleusincus stapes which vibrates on oval window of inner ear • Muscles that inhibit vibration when sound is too loud – Tensor tympani m. (inserts on malleus) – Stapedius m. (inserts on stapes) M&M, fig. 16.19 Sound in external acoustic meatus hits tympanic membrane (eardrum) – it vibrates Pressure is equalized by the pharyngotympanic tube (AKA eustachian or auditory tube) 16 TM causes ossicles in air filled middle ear to move: – Malleus (hammer) – Incus (anvil) – Stapes (stirrup) These are 3 of the smallest bones of the body Ossicles articulate to form a lever system that amplifies and transmits the vibratory motion of the TM to fluids of inner ear cochlea via oval window 17 Auditory pathway 18 Inner Ear/Labyrinth • Static equilibrium, linear motion M&M, fig. 16.20 – Utricle, saccule are egg-shaped sacs in center (vestibule) of labyrinth • 3-D motion, angular acceleration – 3 semicircular canals for X,Y,Z planes • Sound vibrations – Cochlea (“snail”) Auditory Nerve (Acoustic) VIII receives stimulus from all to brain Vestibular n.—equilibrium Cochlear n.—hearing Hearing – The inner ear • Also known as the labyrinth because of the shape • Made up of 2 divisions: the bony labyrinth and the membranous labyrinth – Bony labyrinth is made up of the: » Vestibule: contains the saccule and utricle sacs that are related to equilibrium » Cochlea: contains the spiral organ of Corti which is related to hearing » semicircular canals: contains ampulla on semicircular ducts that respond to equilibrium – Membranous labyrinth is a continuous series of sacs and ducts within the bony labyrinth • The bony labyrinth is filled with perilymph while the membranous labyrinth floats in it Sound • Sound is a pressure disturbance. Alternations between high and low pressure creates vibrations which in turn create sound waves. • The sound waves are able to travel through the ear where they eventually excite hair cells in the organ of Corti. These send the transmission to the brain. Terminology, remember… • Optic – refers to the eye • Otic – refers to the ear • Getting eyedrops and ear drops mixed up is probably not a good idea 22 The Eye and Vision The Eye and Vision • Vision is the dominant sense in humans • 70% of sensory receptors in humans are in the eyes • 40% of the cerebral cortex is involved in processing visual information • The eye (or eyeball) is the visual organ – Diameter 2.5 cm (1 inch) – Only anterior 1/6 visible – Lies in bony orbit – Surrounded by a protective cushion of fat 24 Sense of Sight • Accessory structures include – – – – – Eyebrows Eyelids Conjunctiva Lacrimal Apparatus Extrinsic Eye Muscles Sense of Sight • Eyebrows – Short, coarse hairs that shade the eye and prevent sweat from trickling into the eye • Eyelids (palpebral fissure— “eyelid slit”) – Main function is to keep the eye from drying and to keep out foreign objects • This is largely accomplished via blinking; • Eyelashes are related to this as well; any pressure will trigger blinking • Lacrimal caruncle – Contains sudoriferous and sebaceous gland that produces “sleep”, “sandman’s sand” or “eye-boogers”. • Orbicularis muscles encircle the eye; when they contract the eye closes • Upper eyelid is larger and more motile • Eyelid muscles contract to blink every 3-7 seconds – Clears eyes of debris – Prevents drying out Accessory Structures eyelids cont…. • Eyelashes: rich in nerve endings; anything that touches them triggers blinking • Tarsal (Meibomian) glands: posterior to the eyelashes (on the eyelid) – Produce oily secretion that lubricates eye – Prevents lids from sticking together • Sebaceous glands called ciliary glands lie between the hair follicles. • Inflammation of these glands results in a sty Sense of Sight • Conjunctiva – A transparent mucous membrane that lines the eyelid and the white area of the eye – Prevents the eye from drying out by producing a lubricating mucus – Pink Eye (Conjunctivitis) Sense of Sight • Conjunctiva – A transparent mucous membrane that lines the eyelid and the white area of the eye – Prevents the eye from drying out by producing a lubricating mucus – Pink Eye (Conjunctivitis) • Lacrimal Apparatus – This is made up of the lacrimal gland and ducts. • Glands produce secretions (tears) • Tears are spread across the eyeball and drain into the lacrimal sac and eventually nasolacrimal duct • Tears (lacrimal fluid) contains mucus, antibodies, and lysozyme Accessory Structures eyelids cont…. • Lacrimal Apparatus: lacrimal glands and ducts • Lacrimal gland; lies in the orbit above the eyeball • Continually releases a saline solution: tears – Contains mucus, antibodies and lysosomes • Spread across eyeball and drain into the lacrimal punctum: little red dots in the corner of the eye • Drain into lacrimal sac and then into nasal cavity – stuffiness Lacrimal Gland Infection • http://bjo.bmj.com/content/suppl/2005/02 /22/89.3.DC1/893report.full M&M, fig. 16.4 Movement of eye Sense of Sight • Extrinsic Eye Muscles – There are 6 eye muscles that work together to keep the eye in orbit and allow movement of the eyeball. • • • • • • Lateral Rectus Medial Rectus Superior Rectus Inferior Rectus Inferior Oblique Superior Oblique Eye Muscle Disorders Diplopia: double vision (seeing one object as two) Strabismus: cross-eyed • Amblyopia Lazy eye Sense of Sight • The eyeball is a slightly irregular hollow sphere with anterior and posterior poles • The inside of the eyeball is filled with liquids called humors and the wall of the eyeball is made up of 3 tunics – Fibrous tunic – Vascular tunic – Sensory tunic Fibrous Tunic • The outermost coat of the eye composing the sclera and the cornea – Sclera is opaque white and acts as a support for the eyeball and anchor for the extrinsic eye muscles – Cornea is a transparent bulge in the anterior region of the sclera that allows light to enter IRIS • Behind the cornea are the iris and the pupil. The iris is the colorful part of the eye. When we say a person has blue eyes, we really mean the person has blue irises! PUPIL • The iris controls how much light goes through the pupil. The pupil is the black circle in the center of the iris, and it lets light enter the eye. The pupils will get smaller when a light shines near them and they'll open wider when the light is gone. Eye as lens/optical device M&M, fig. 16.7 Light path: Cornea Pupil Lens Retina Cataract (opaque lens) Nearsightedness Myopia • An eye that is too long or a cornea that is too steep causes myopia (or nearsightedness). In nearsighted eyes, the image isn't focused precisely inside the eye, causing blurring in the distance. The more nearsighted you are, the more blurred the distant object appears, and the thicker your glasses need to be. Most nearsighted people feel that their condition is severe, due to their dependence on glasses and contact lenses. In fact, only one in ten nearsighted individuals are actually in the "severe" or "extreme" categories. Notice geese are clear but the city is blurry. Far-Sighted • Hyperopia • An eye that is too short, or a cornea that is not steep enough causes hyperopia (or Farsightedness). People with hyperopia see blurry when looking at close objects. Young people can slightly overcome hyperopia by using their focusing muscles to make the image clear. This gets harder as they get older. Currently, there are restricted options to correct hyperopia. Most operations are still under development. What is blurry this time? Cow Eye Dissection