Chapter 15: Special senses

I. The eyes and vision: eyes contain 70% of all sensory receptors in the body A. Accessory structures of the eye

1. Eyebrows 2. Eyelids (palpebrae)

a. Canthus: (canthi) - lateral and medial angles of the eye b. Lacrimal caruncle: the raised bump at the medial canthus

c. Eyelashes: are innervated at the root

d. Tarsal glands: produce an oily secretion that lubricates surface of eye 3. Conjunctiva: delicate mucous membrane

a. Well supplied with blood vessels

4. Lacrimal apparatus: lacrimal gland plus lacrimal duct

5. Extrinsic eye muscles: control eyeball position

a. Rectus muscles: Superior, inferior, lateral, medial

b. Oblique muscles: inferior and superior

B. Structure of the eye: the eye is a hollow sphere filled with fluids called

humors

1. Tunics (coats): form the wall of the eye a. Fibrous tunic: the outermost avascular dense fibrous C.T. coat

1) 2 main regions: a) Sclera: the white of the eye

b) Cornea: anterior clear part of the eyeball – contains many pain

receptors (1) Structure: it is a modified part of the fibrous tunic (2) Stratified squamous: outer protective layer

(3) Simple squamous: lining that contains sodium pumps

(4) Avascular: not accessible to immune system

b. Vascular tunic: middle coat of eye with 3 main regions 1) Choroid

2) Ciliary body: continuous with choroid anteriorly a) Mostly smooth muscle

3) Iris: visible colored part of eye shaped like a doughnut

a) Smooth muscle

(1) Radial: sympathetic (2) Circular: parasympathetic

c. The sensory tunic (retina): innermost tunic with 2 layers 1) Pigmented layer: absorbs light 2) Neural layer: nervous layer – an out pocket of the brain

a) Photoreceptors: rods and cones – 250 million per eye b) Bipolar cells: connect photoreceptors to the ganglion cell c) Ganglion cells: axons form optic nerve

3) Optic disc: the area where the optic nerve exits the eye

4) Macula lutea and fovea centralis: lateral to blind spot a) Fovea centralis: a pit in the center of the macula

(1) Contains only cones b) Macula lutea: small oval spot

(1) Contains mostly cones

2. Internal cavities and fluids

a. Posterior segment: (behind the lens) filled with vitreous humor 1) Vitreous humor: a clear gel within the posterior segment

b. Anterior segment: divided into 2 chambers 1) Anterior chamber: in front of the iris 2) Posterior chamber: behind the iris but in front of the lens 3) Aqueous humor: a watery fluid within the anterior segment

3. Lens: biconvex transparent disc composed of densely packed proteins called

crystallins

a. Epithelium: cuboidal on anterior face b. Lens fibers: layered like an onion

1) Lens fibers were formerly the cuboidal cells C. Physiology of vision

1. Overview of basics a. Visible light: the 400-700 nm part of the electromagnetic spectrum b. Refraction and lenses

1) Refraction: the bending of light, which is due to: a) Passing through different media densities

b) As it hits curved surfaces 2) Lenses: a transparent object curved on one or both sides

a) Convex: curved outward b) Concave: curved inward

c. Focusing for distant vision

d. Focusing for close vision

1) Accommodation of the lens (focusing)

a) Near point: the closest point of focus

b) Loss of elasticity with age: near point increases

2) Constriction of the pupils: enhances the effect of

accommodation

3) Convergence of eyeballs: controlled by occulomotor nerve 4) Vision and problems

a) Emmetropia ( a ): normal vision b) Myopia ( b ): nearsightedness

c) Hyperopia ( c ) : farsightedness

d) Astigmatism: unequal curvatures in different parts of the cornea or

lens

2. Photoreception a. Functional anatomy: modified neurons built like inverted columnar

cells with their tips buried in pigmented layer of retina 1) Rods and cones: possess an outer region connected to an inner

region by a thin neck

2) Retinal (absorbs light) plus opsin: forms the 4 light sensitive

photopigment

3) Photopigment in the dark: photoreceptors are constantly

depolarized

4) Photopigment in the light: photoreceptors are hyperpolarized

5) Generating an image a) Stimulate all photoreceptors: all we see is white b) Stimulate no photoreceptors: we see black

6) Rods: responsible for black/white vision a) Photopigment: rhodopsin

b) Low visual acuity: 100 or more rods feed into each ganglion cell 7) Cones: responsible for color vision

a) Photopigments: 3 kinds of photopsins

b) High visual acuity: 1 (or few) cones feed into a single ganglion

cell 8) Rod distribution in the retina

a) No rods in the fovea centralis b) Rods increase toward the periphery of the retina

3. Light and dark adaptation a. Light adaption

1) Photoreceptor bleaching: even a tiny amount of light can bleach

rods

b. Dark adaption

4. Binocular vision and depth perception requirements a. Each eye: sees slightly different image (from different angles) b. Optic chiasm: fibers from each eye cross over to opposite hemisphere c. Depth perception: also called 3D vision

1) Distance between eyes is constant 2) Object distance from eyes varies 3) Eyes turn medially during focus 4) Brain calculates distance from objects

II. The chemical senses: taste and smell A. The olfactory epithelium and the sensation of smell

1. Location and structure of olfactory epithelium

a. Epithelium is in roof of nasal cavity: poor location for smelling

b. Cells of olfactory epithelium 1) Supporting cells: the bulk of the epithelium like in taste buds

2) Olfactory cells: chemoreceptor cells that are bipolar neurons with

olfactory cilia (non-motile)

3) Basal cells: at base of the epithelium give rise to new olfactory cells

2. Specificity of olfactory epithelium: humans can detect 10,000

different odors a. Olfactory cells: respond to combinations of chemical stimuli b. Smell genes: there are at least 400 smell genes

3. Activation of olfactory cells a. Chemical: must be volatile b. Must: dissolve in water c. Must: bind to hairs d. Receptor potential then generator potential, then action potential

B. Taste buds and the sensation of taste 1. Papillae and taste buds

a. Papilla (papillae): visible bumps on the tongue 1) Types of papillae that contain taste buds

a) Fungiform: mushroom shaped

b) Foliate: ridges on the posterior sides of the tongue

c) Circumvallate: largest and least numerous

b. Taste bud: several cell types 1) Support cells: the bulk of the taste bud 2) Gustatory cells: chemoreceptor cells

3) Basal cells: Give rise to new taste bud cells

2. Basic taste sensations a. Sweet: anterior tongue

b. Sour: lateral tongue

c. Salty: tip of tongue

d. Bitter: posterior tongue

e. Umami: pharynx 3. Physiology of taste

a. Chemical: must dissolve in saliva b. Must: contact gustatory hair c. Concentration: of the chemical determines the intensity of perception d. Sensitivity: bitter is the highest, the others are less e. Adaptation: is rapid partial in 3-5 seconds; complete in 1-5 min f. Starts as receptor potential:

4. Influence of other sensations on taste a. 80% of “taste” is actually smell b. Receptors add to the “richness” of the experience

III. The ear - hearing and balance A. Structure of ear

1. External (outer) Ear a. Auricle (pinna): elastic cartilage covered by skin b. External auditory canal: contains ceruminous glands

c. Tympanic membrane: shaped like a flattened cone protruding into

the middle ear

2. Middle ear: air-filled mucosa-lined cavity in the temporal bone

a. Openings associated with the middle ear 1) External auditory canal: sealed by the tympanic membrane 2) Oval and round windows: membrane-covered openings into the

inner ear 3) Auditory tube: a pressure-equalization tube leading to the

nasopharynx

b. Auditory ossicles: tiny bones connecting the tympanic membrane with

the membrane of oval window

1) Malleus – incus - stapes

2) Tiny synovial joints: connect the ossicles c. Muscles: 2 tiny skeletal muscles

1) Tensor tympani

2) Stapedius

3. Inner ear a. 2 main divisions

1) Bony labyrinth: bone lined with endosteum and filled with perilymph

2) Membranous labyrinth: membrane sacs and ducts within the

bony labyrinth

b. Bony labyrinth: the 3 parts described in detail 1) Vestibule: the central egg-shaped part

a) Utricle and saccule: 2 membranous labyrinth sacs contained

within the vestibule

2) Semicircular canals: extend off the posterior part of the vestibule

a) Semicircular ducts: membranous labyrinth sacs within the

canals

3) Cochlea: "snail shell" in Latin, contains membranous labyrinth tubes a) Scala vestibule: contains perilymph

b) Scala media: also called cochlear duct, contains endolymph

c) Scala tympani: continuous with scala vestibule, contains

perilymph

B. The physiology of hearing 1. Sound: a pressure disturbance originating from a vibrating object

a. Frequency: how many waves per unit time b. Amplitude: the height of the sound wave c. Wavelength: the distance between 2 crests in a sound wave

2. Transmission of sound to the inner ear a. Sound: enters external ear and strikes the tympanic membrane which

vibrates with same frequency

b. Ear drum is 20x larger than oval window

c. Resonance of basilar membrane

1) Schematic diagram of the membrane resonance

2) Summary of steps in sound production a) Sound moves fluids within cochlea b) High frequency sound: activates stiff hair cells close to the

oval window c) Low frequency sound: travels further into the cochlea to

activate floppy hairs distally d) Vibration of basilar membrane: triggers hair cells e) Humans can detect: 20 hz to 20,000 hz

C. Equilibrium and orientation: 2 forms, static and dynamic 1. Static (stationary) equilibrium

a. Macula

1) Support cells

2) Hair cells

3) Otolithic membrane

a) Otoliths

b. Maculae respond to changes in body position

1) Diagram:

c. Static equilibrium mechanism:

2. Dynamic (moving) equilibrium

a. Respond: to rotational and angular movements b. Crista ampullaris contains

1) Support cells 2) Hair cells: contain stereocilia plus 1 kinocilium 3) Cupula: is a gel-like mass 4) Diagram of a crista

c. Activating cristae

1) Endolymph moves opposite direction: bends hairs to

depolarize

2) Hyperpolarizes: when moving in the opposite direction