The Special Senses

1. The Special Senses Human Anatomy Chapter 16

2. I. The chemical senses: taste and smell Four of the five senses are localized in the face and head area. The receptors of touch are spread over the surface of the body.

3. A. Taste (gestation)- Taste is perceived by chemoreceptors. 1. taste buds- Taste buds are found in the tongue, some in the posterior region of palate, inner surface of cheeks, on the pharynx and epiglottis. Taste buds form papillae and large vallate papillae. Each taste bud has supporting, gustatory, and basal cells. The taste pore allows contact with the saliva containing dissolve chemicals that stimulate taste. Replacement of cells occurs every 7-10 days. If an entire taste is bud is destroyed it is replaced at the rate the entire epithelial layer is replaced (may take a month)

5. 2. taste sensations and gustatory pathway- there are four basic tastes: sweet, sour, salty, bitter. The facial nerve, glossopharyngeal nerve, and vagus nerve carry taste sensations. The parietal lobe has gustatory areas.

6. B. Smell (olfaction)- Smell is perceived by chemoreceptors found in the olfactory epithelium; they are olfactory receptor cells. This surface is covered with a layer of mucus that dissolves odor molecules. These cells are connected to filament of the olfactory nerve. Smell is sent to the limbic system (associate smell with emotions) and to areas of the cortex for identification and analysis.

7. C. Disorders of the chemical senses-Anosmia- results when one can’t smell due to damage to the olfactory nerve or zinc deficiency. Uncinate fits- detect imaginary smells caused by damage to olfactory nerve or epileptic aura.

9. II. The eye and vision 40% of the cerebral cortex is dedicated to processing images captured by the eye. The eye is protected by a cushion of fat found within the optical orbit. A. accessory structures of the eye- These are structures that protect the eye. 1. eyebrows- shade from sunlight, keep sweat away from the eyes. 2. eyelids- used for blinking, and have glands that produce “eye grit”. The eyelashes at the margin trap particles and produce reflexive blinking. Tarsal glands release oil that forms a film and slows water evaporation. 3. conjuctiva- palpebral conjuctive is a transparent mucous membrane that covers the inner sufaces of eyelids. Ocular conjuctiva covers the white of the eye except the area of the cornea. If the conjuctiva become inflamed due to bacteria or virus, it is known as pink eye. 4. lacrimal apparatus- consists of lacrimal gland and lacrimal duct, it secrets a fluid to keep eye moist. Blinking the eye spreads the fluid on the surface. Lacrimal fluid kills bacteria. 5. extrinsic eye muscles- Six muscles that move the eye- superior, inferior, lateral, and medial rectus; also superior and inferior oblique.

14. B. Anatomy of the eyeball- This organ is designed to protect the photoreceptors, capture and process and image. The eye has an anterior and posterior pole, it is protected by a three layer wall. 1. Fibrous tunic- most external, dense connective tissue, makes up the sclera and cornea. The cornea is transparent and allows light to enter the eye. It is transparent because it is avascular but contains pain receptors. It can easily be damaged but has good regeneration ability. 2. Vascular tunic- composed of the choroids, the ciliary body, and iris. a. The choroid is dark in color because it contains melanin to absorb light, highly vascularized. b. The ciliary body in an anterior continuation of the choroids, it encircles the lens an helps to adjust it. c. The iris is the colored part of the eye, it’s color comes from the amount of melanin in the iris body. The pupil is the opening of the iris. The iris has smooth muscles that control the size of the pupil. The papillary light reflex protects the eye from light damage.

17. 3. Sensory tunic is also called the retina. It contains melanocytes and a sheet of nervous tissue made up of photoreceptors. a. The photoreceptors consist of photoreceptor cells : rods= dark, grays and cones= bright, colors. There are blue, red and green light cones. They do not regenerate themselves. b. Rods and cones contact a bipolar neuron that synapses with ganglion cells. The axons of the ganglion cells enter the optic nerve and carry the stimulus to the occipital lobe. c. The retina has a “yellow spot” called the macula lutea at the posterior pole. At its center is a pit called the fovea centralis. It is an area that contains only cones thus providing visual acuity. d. The optic disc (blind spot) lies medial to fovea centralis. This is the site where the retina gathers to from the optic nerve. 4. Internal chambers and fluids- Fluids in these chambers nourish the eye tissues and maintain appropriate pressure. a. The posterior chamber is filled with vitreous humor that regulates eye pressure, maintains retina against the eye wall, and allows the image to travel across the eye to the retina. This is formed as an embryo and lasts the entire life time. b. The anterior chamber in filled with aqueous humor, it is more liquid than vitreous humor, periodically regenerated, and drains into scleral venous sinus.

22. 5. Lens- a transparent disc that changes shape to adjust to focus light. The lens sends images that are upside down and reverse from left to right. The cerebral cortex reverses the image. With age the lens thickens and becomes less flexible reducing its ability to focus light. C. Eye as an optical device- eyes are design to see best at far distances, when looking at objects close up, the pupil constricts. The cornea is powerful at bending light so it converges on the retina at a single focal point. The lens is powerful at focusing objects. D. Visual pathways= please study the figure on page 484. Image travels from retina to optic nerve to optic chiasma to optic tract. This tract travels to midbrain some tract branch out to control eye reflexes, other branch to the hypothalamus. From the hypothalamus to the thalamus to cerbral cortex (visual cortex in occipital lobe). Images are sent at different angles from both eyes, the brain process that information to create just one image. E. Disorders of the eye- this book covers many eye disorders, including focusing disorders on pg 482-483). Please read these for your own knowledge.

28. III. The Ear: Hearing and equilibrium The ear is divided into three regions: outer, middle, and inner ear. The first two are involved in hearing, the third is involved in equilibrium. A. Outer ear- Consists of the auricle (pinna) and the auditory canal (acoustic meatus). 1. Auricles consist of elastic cartilage and skin. Its function is to gather sound waves and funnel the to the auditory canal. The way the sound bounces off the auricles allows the brain to detect the direction of sound. 2. External auditory canals run from the auricle to the eardrum. The skin contains modified sebaceous glands called ceruminous glands that produce cerumen (ear wax). Sound travels through the canal until it meets the eardrum, this causes it to vibrate, thus the bones in the middle ear vibrate. B. Middle ear is also known as the tympanic cavity that lies within the temporal bone. It has four openings: the epitympanic recess that leads to t he mastoid process, the oval window connecting to the inner ear, the round window leads to the temporal bone and the pharyngotympanic tube (auditory tube) leads to the pharynx. C. The inner ear is also called the labyrinth that lies within the temporal bone. The membranous labyrinth are sacs and ducts within the bony labyrinth. Both types of labyrinths are filled with fluid.

31. The three ossicles are smallest bones in the body. The connect to each other in a linear fashion, the malleus (hammer) attaches to the eardrum and to the incus (anvil) which attaches to the stapes (stirrup). The stapes attaches to the oval window and send vibrations into the inner ear. There are two smooth muscles that connect to these ossicles and reduce vibration in very noisy environments.*read about middle ear infections for your own knowledge-its interesting.

32. 1. Vestibule- part of the bony labyrinth, lies medial to middle ear connecting to the oval window. It contains sensory epithelium called macula that detects the position of the head, changes in linear acceleration. The macula have receptors called hair cells that synapse with the vestibular nerve. Macula can run in horizontal and vertical positions to detected tiling and elevation or depression of the head. 2. Semicircular canals-part of the bony labyrinth, lie posterior of vestibule. There at three canals and each lies along a different plane. The crista ampullaris detects rotational acceleration via the hair cells. 3. Cochlea-part of the bony labyrinth, forms a spiraling chamber, it contains receptors for hearing arranged in rows of hair receptors that detect the vibrations traveling through the fluid in the chamber (endolymph). These hearing receptors are called the spiral organ of Corti. As the hair cells detect vibration they send a signal to the cochlear nerve that joins the vestibular nerve, the impulse travels to the brain. D. Equilibrium and auditory pathways Equilibrium information travels to the brain through the vestibular nerve to the brainstem and then to the cerebellum or to the reflex centers. Auditory information travels from cochlear nerve to brain stem to the thalamus and then to auditory cortex or from brainstem to auditory reflex center, E. Disorders of equilibrium and hearing- Once again there are many disorders discussed in this chapter see page 495.

41. IV. The special senses throughout life. 1- Smell and taste are sharp in newborns and children have more taste buds than adults. Taste and smell declines around 40. 2- Before birth photoreceptors are fully formed, after birth visual experience fine tunes the the neurological connections in the visual pathway. The following also occurs: a- birth to 3 months- see only gray tones, uncoordinated eye movements, images are blurry, can’t follow objects. b. 3-6- images can be focus on fovea centralis, can follow moving object c. by 6 month- depth perception is present and color vision is well developed. 3. As we age the lens loses its clarity, scattering light and the pupil does not adjust as efficiently. Thus by age 70 is visual acuity is dramatically lowered. 4. Newborns have reflexive responses to sound. Ability to hear fine sounds develops after birth. By 3rd/4th months sounds can be localized. By 12 months they know all the sounds in their language. 5. It is uncommon to have congenital defects in the eye but more common for the ear. 6. We are born with 20,000 hair cells in each ear and they are progressively destroyed, by the time you reach puberty you have lost ability to hear high pitch sounds. Deterioration continues and its rate depends on the degree of noise you are exposed throughout life.

42. THE END