The Senses. Special Senses. Chapter 8. Accessory Structures of the Eye. The Eye and Vision. Accessory Structures of the Eye

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1 The Senses Chapter 8 Special Senses General senses of touch Temperature Pressure Pain Special senses Smell Taste Sight Hearing Equilibrium The Eye and Vision 70% of sensory receptors are in eyes Each eye has over a million nerve fibers Protection for the eye - bony orbit - surrounding fat Accessory Structures of the Eye Eyelids Eyelashes Meibomian glands - modified sebaceous glands - oily secretion to lubricate Ciliary glands Modified sweat glands between eyelashes Conjunctiva - Membrane lining eyelids; connects to eye surface; secretes mucus to lubricate Accessory Structures of the Eye Lacrimal apparatus Lacrimal gland produces lacrimal fluid Lacrimal canals drains lacrimal fluid from eyes Lacrimal sac provides passage of lacrimal fluid towards nasal cavity Nasolacrimal duct empties lacrimal fluid into the nasal cavity Function of the Lacrimal Apparatus Properties of lacrimal fluid - Dilute salt solution (tears) - Contains antibodies and lysozymes Protects, moistens, & lubricates the eye Empties into the nasal cavity 1

2 Extrinsic Eye Muscles Muscles attach to the outer surface of the eye Produce eye movements Structure of the Eye The wall is composed of three tunics Fibrous tunic outside layer Choroid middle layer Sensory tunic inside layer Figure 8.2 Figure 8.3a Sclera The Fibrous Tunic White connective tissue layer Seen anteriorly as the white of the eye Cornea Transparent, central anterior portion Allows for light to pass through Repairs itself easily The only human tissue that can be transplanted without fear of rejection Choroid Layer Blood-rich nutritive tunic Pigment prevents light from scattering Modified interiorly into two structures Cilliary body smooth muscle Iris Pigmented layer that gives eye color Pupil rounded opening in the iris Sensory Tunic (Retina) Contains receptor cells (photoreceptors) Rods Cones Signals pass from photoreceptors via a two-neuron chain Bipolar neurons Ganglion cells Signals leave the retina toward the brain through the optic nerve Retina 2

3 Neurons of the Retina Figure 8.4 Neurons of the Retina and Vision Rods Most are found towards the edges of the retina Allow dim light vision and peripheral vision Perception is all in gray tones Cones Allow for detailed color vision Densest in the center of the retina Fovea centralis area of the retina with only cones Optic disk (Blind spot) - No photoreceptor cells There are three types of cones Different cones are sensitive to different wavelengths Color blindness is the result of lack of one cone type Cone Sensitivity Lens Biconvex crystal-like structure Held in place by a suspensory ligament attached to the ciliary body Figure 8.6 Internal Eye Chamber Fluids Aqueous humor - Watery fluid in chamber between lens & cornea Similar to blood plasma Helps maintain intraocular pressure Provides nutrients for the lens and cornea Reabsorbed into blood by the canal of Schlemm Vitreous humor - Gel-like substance behind lens Keeps the eye from collapsing Lasts a lifetime and is not replaced Lens Accommodation Light must be focused to a point on the retina for optimal vision eye is set for distance vision (over 20 ft away) lens must change shape to focus for closer objects 3

4 Images Formed on the Retina Photoreceptors of retina Optic nerve Visual Pathway Optic nerve crosses at the optic chiasma Optic tracts Thalamus (axons form optic radiation) Visula cortex of the occipital lobe Figure 8.10 Eye Reflexes Internal muscles controlled by autonomic nervous system - Bright light causes pupils to constrict (radial and ciliary muscles) Viewing close objects causes accommodation External muscles control eye movement to follow objects Viewing close objects causes convergence (eyes moving medially) Cataracts lens which has become opaque or clouded Causes? Diabetes, old age, pollution? STRABISMUS visual defect in which the eyes are misaligned and point in different directions misalignment of the eyes GLAUCOMA disease of the eye in which damage occurs to the optic nerve, typically as a result of an elevated pressure within the eye.. Damage to the optic nerve causes progressive loss in peripheral vision and can eventually lead to blindness. 4

5 PTERYGIUM wedge-shaped fibrovascular growth of conjunctiva that extends onto the cornea benign lesions that can be found on either side of the cornea. Astigmatism irregularity in the shape of the cornea or the lens. Instead of being shaped round, the cornea is shaped oval, causing a blurred image at all distances. Patients may notice blurred or ghost images close up or far away. present in various degrees Hyperopia (Farsightedness) -unable to see near objects without extreme focusing. images are formed behind the retina eye too short, or the refractive powers of cornea & lens are too weak Myopia (Nearsightedness)-Distant objects are unclear in cases of myopia. condition of the eye in which images are formed in front of the retina the eye is relatively too long or refractive powers of the cornea & lens are too strong. LASIK Surgery The process: After your eye has been numbed with "eye drop" anesthesia, an instrument known as an eyelid speculum will be positioned to hold your eyelids open. You will remain awake and comfortable throughout the procedure. A small suction ring will be placed around the cornea and serves as a platform for the microkeratome. The microkeratome separates the surface layers of the cornea, and the corneal flap is folded back. You will be asked to look at a target light while the Excimer laser reshapes the corneal tissue. A clicking sound can be heard as each microscopic layer of tissue is vaporized. This process will last from seconds to minutes, depending on the amount of correction necessary. The corneal flap is then placed back into its original position and allowed to dry for a few minutes. Keratoconus Corneal degenerative disorder Cornea becomes progressively thin and steep The front of the eye bulges. 5

6 Conjunctivitis Pink eye Infection of conjuctiva Caused by bacteria or virus Highly contagious Color Blindness Complimentary Colors Stare at the flag for 30 seconds. Then look at a white surface. What happens & why? Brain interprets Fooling your brain 6

7 Fooling your brain 7

8 Human Hearing sound wave vibrates ear drum amplified by bones converted to nerve impulses in cochlea Organs of Hearing Organ of Corti Located within the cochlea Receptors = hair cells on the basilar membrane Gel-like tectorial membrane is capable of bending hair cells Cochlear nerve attached to hair cells transmits nerve impulses to auditory cortex on temporal lobe Organs of Hearing Figure 8.13 Slide 8.27a Slide 8.27b 8

9 Mechanisms of Hearing Mechanisms of Hearing Vibrations from sound waves move tectorial membrane Hair cells are bent by the membrane An action potential starts in the cochlear nerve Continued stimulation can lead to adaptation Figure 8.14 Slide 8.28 Slide 8.29 Organs of Equilibrium Organs of Equilibrium Receptor cells are in two structures Equilibrium has two functional parts Vestibule Static equilibrium Semicircular canals Dynamic equilibrium Figure 8.16a, b Figure 8.16a, b Slide 8.30a Slide 8.30b Static Equilibrium Maculae receptors in the vestibule Report on the position of the head Send information via the vestibular nerve Anatomy of the maculae Hair cells are embedded in the otolithic membrane Otoliths (tiny stones) float in a gel around the hair cells Movements cause otoliths to bend the hair cells Slide 8.31 Function of Maculae Figure 8.15 Slide

10 Dynamic Equilibrium Dynamic Equilibrium Crista ampullaris receptors in the semicircular canals Tuft of hair cells Cupula (gelatinous cap) covers the hair cells Action of angular head movements The cupula stimulates the hair cells An impulse is sent via the vestibular nerve to the cerebellum Figure 8.16c Figure 8.16c Slide 8.33a Slide 8.33b Basic Types of Hearing Loss Causes of Hearing Loss Conductive Hearing Loss sound is not conducted efficiently through the outer ear canal to the eardrum and the ossicles of the middle ear. Sensorineural Hearing Loss damage to the inner ear (cochlea), or to the nerve pathways from the inner ear to the brain. Conductive: External Ear congenital malformation where pinna and ear canal fail to form blockage in ear canal - foreign body or accumulated cerumen (ear wax) Middle ear perforation in tympanic membrane (ear drum) from trauma or disease otitis media (ear infection) broken ossicular chain due to head trauma or trauma to the ear Causes of Hearing Loss Sensorineural: genetic disorders hearing loss from aging ototoxic drugs such as some antibiotics cancer treatments - chemotherapy and radiation therapy head trauma - fractured temporal bone excessive noise exposure diseases of the vascular system such as sickle cell anemia kidney disease Ménière's disease acquired infections such as influenza, meningitis, mumps, and syphilis Tumors of or near the nerve of hearing and balance Prevention Beware of loud or dangerous recreational activities Limit exposure to things such as: Firecrackers Loud Concerts Shooting a gun Avoid situations where you have to shout to someone less than an arm s length away Keep volume of tvs, radios, stereos, and ipods low Avoid medications that are dangerous to your hearing Try and keep noise levels reasonable 10

11 Hearing Aids: Behind The Ear (BTE) In The Ear (ITE) Treatment In the Canal (ITC) Completely in The Canal (CIC) Tinnitus Causes ringing, humming, buzzing, and cricket-like sounds Can be in one ear, both ears, or the head No actual cure for it Other Hearing Disorders Sudden Sensorineural Hearing Loss (SSHL), or sudden deafness Rapid loss of hearing Ménière s disease Causes dizziness, tinnitus, and hearing loss Usher Syndrome affects both hearing and sight Is inherited Test Your Hearing ng_loss_simulator/index.cfm Chemical Senses Taste and Smell Both senses use chemoreceptors Stimulated by chemicals in solution Taste has four types of receptors Smell can differentiate a large range of chemicals Both senses complement each other and respond to many of the same stimuli Slide

12 Olfaction The Sense of Smell Olfactory Epithelium Olfactory receptors are in the roof of the nasal cavity Neurons with long cilia Chemicals must be dissolved in mucus for detection Impulses are transmitted via the olfactory nerve Interpretation of smells is made in the cortex Slide 8.35 Figure 8.17 Slide 8.36 The Sense of Taste Taste buds house the receptor organs Location of taste buds Most are on the tongue Soft palate Cheeks Figure 8.18a, b The Tongue and Taste The tongue is covered with projections called papillae Filiform papillae sharp with no taste buds Fungifiorm papillae rounded with taste buds Circumvallate papillae large papillae with taste buds Taste buds are found on the sides of papillae Slide 8.37 Slide 8.38 Structure of Taste Buds Structure of Taste Buds Gustatory cells are the receptors Have gustatory hairs (long microvilli) Hairs are stimulated by chemicals dissolved in saliva Impulses are carried to the gustatory complex by several cranial nerves because taste buds are found in different areas Facial nerve Glossopharyngeal nerve Vagus nerve Slide 8.39a Slide 8.39b 12

13 Anatomy of Taste Buds Taste Sensations Sweet receptors Sugars Saccharine Some amino acids Sour receptors Acids Bitter receptors Alkaloids Salty receptors Metal ions Umami beef taste MSG Discovered by the Japanese Figure 8.18 Slide 8.40 Slide 8.41 Taste Sensations Historically, the tip of the tongue was believed to be most sensitive to sweet and salty substances, its sides to sour, the back of the tongue to bitter and the pharynx to umami. Actually there are only slight differences in the locations of the taste receptors in different regions of the tongue. Most taste buds respond to two, three, four or even all five taste modalities. Taste Sensations Taste likes and dislikes have a homeostatic value. Sugar & Salt satisfies the body s need for carbohydrates and minerals Sour (acidic) typically sources of Vitamin C Umami guides the intake of proteins Bitter many natural poisons and spoiled foods Developmental Aspects of the Special Senses Formed early in embryonic development Eyes are outgrowths of the brain All special senses are functional at birth Slide