Chapter 15: The Special Senses

Question 1

Eye:

Answer 1

o Only anterior one-sixth of eye’s surface is visible. The remainder is enclosed and protected by fat cushion and body orbit walls.
o Has accessory structures that protect it and aid in the eye’s function.

Question 2

Accessory Structures of the Eye:

Answer 2

o Eyelids (palpebrae): Seperated by palpebral fissure (“eyelid slit”)
o Palpebral Conjunctiva: Transparent mucous membranes that line eyelids and folds back over anterior eye surface.
o Bulbar Conjunctiva: Very thin and covers only whites of eyes, not cornea.
o Tarsal Plates: Internally supports eyelids; anchors eyelid muscles.
o Levator palpebral superioris: Gives upper eyelid mobility; raises eyelid. Used for pretection, and spreading secretions to moisten eye.
o Orbicularis Oculi: Contracts to close eye.

Question 3

Lacrimal Apparatus:

Answer 3

o Lacrimal gland and ducts that drain into nasal cavity.

o In orbit above lateral end of eye.

Question 4

Lacrimal Secretion (Tears):

Answer 4

o Dilute saline solution containing mucus, antibodies, and lysozyme; an enzyme that destroys bacteria to prevent infection
o Blinking spreads tears toward medial commissure.
o Tears enter paired lacrimal cnaliculi via lacrimal puncta.
o Tears drain into lacrimal sac and then into nasolacrimal duct then empties into nasal cavity at inferior nasal meatus.

Question 5

Extraocular Eye Muscles:

Answer 5

o Four rectus muscles elevates and depresses eye and moves eye laterally and medially.
o Superior Oblique rotates eye downward and somewhat laterally.
o Inferior oblique rotates eye up and laterally.
o Oculomotor Nerve: serves all extrinsic eye muscles. Exceptions: lateral rectus (abducens VI) and Superior oblique (trochlear IV).

Question 6

Three Layers of Eye Walls:

Fibrous Tunic

Answer 6

Outermost, Protective layer of the eye. Made of dense avascular connective tissue. Two regions are Sclera and Cornea.
• Sclera: Opaque posterior region. Protects, and shapes eyeball. Anchors extrinsic eye muscles. Continuous with dura mater of brain posteriorly.
• Cornea: Transparent anterior 1/6 of fibrous layer. Nourished by tears and aqueous humor. Bends light as it enters eye. Sodium pumps of corneal endothelium on inner face help maintain clarity of cornea. Numerous pain receptors contribute to blinking and tearing reflexes.

Question 7

Three Layers of Eye Walls:

Vascular Tunic

Answer 7

Where all the arteries and blood of the eye are stored. Middle pigmented layer that forms middle coat of eyeball. 3 regions: choroid, ciliary body, and iris.
• Choroid Region: Posterior portion of uvea. Supplies blood to all layers of eye. Brown pigment (melanocytes) absorbs light to prevent light scattering and visual confusion.
• Ciliary Region: Ring of tissue surrounding lens. Smooth muscle bundles (ciliary muscles) control lens shape. Capillaries of ciliary processes secrete fluid. Ciliary zonule (suspensory ligament) holds lens in potition.
• Iris: Visible colored part of eye. Lies between cornea and lens. Pupil is the central opening that regulates amount of light entering eye. Close vision and bright light; sphincter (constrictor) pupillae, distant vision and dim light; dilator pupillae (radial muscles).

Question 8

Three Layers of Eye Walls:

Inner (Nervous) Tunic

Answer 8

Location of the nerves, retina, rods and cones of the eye. Delicate two-layered membrane.
• Outer Pigmented layer: is single-cell thick lining, pigment cells absorb light and prevent its scattering. Phagocytize photoreceptor cell fragments. Stores vitamin A utilized by photoreceptor cells.
• Inner Neural Layer: Transparent layer extends to posterior ciliary body; junction of ora serrate: the saw-toothed margin posterior to ciliary body. Composed of 3 main types of receptors: Photoreceptors, bipolar cells, and ganglion cells. Signals spread from photoreceptors to bipolar cells to ganglion cells. Ganglion cell axons exit eye a the thick optic nerve.

Question 9

Accessories of Eye:

Answer 9

o Humors: Internal eye is filled with these protective and lubricating fluids.
o The lens: Separates the internal cavity into anterior and posterior segments(cavities).

Question 10

Optic Disc (Blind Spot):

Answer 10

o In retina.
o Site where optic nerve leaves eye.
o Lacks photoreceptors.

Question 11

Photoreceptors of the Retina:

Answer 11

o Rods: Dim-light and peripheral vision receptors. More numerous and sensitive to light than cones; don’t provide sharp images or color vision.
o Cones: Our vision receptors for bright light and provide high-resolution color vision.

Question 12

Macula Lutea of Retina:

Answer 12

o Small circular structure on retina.
o Located exactly at posterior pole; lateral to blind spot of each eye.
o Mostly cones located here.

Question 13

Fovea Centralis of Macula Lutea of Retina:

Answer 13

o Tiny pit in center of macula with all cones. Hence the best vision here.
o Retinal structures abutting vitreous humor are displaced to sides.
o Allows light to pass almost directly to photoreceptors than through several retinal layers.
o Greatly enhances visual acuity(detail).

Question 14

The 2 Segments of the Eye:

Answer 14

o Anterior Segment: Between Cornea and Iris.
• Contains aqueous humor.
• Plasma like fluid continuously formed by capillaries of ciliary processes.
• Drains via scleral venous sinus (canal of schlemm) at sclera-cornea junction.
• Supplies nutrients and oxygen mainly to lens and cornea but also to retina, and removes wastes.
o Posterior Segment: Between Iris and Lens. Filled with vitreous humor that binds water.
• Transmits light; supports posterior surface of lens.
• Holds neural layer of retina firmly against pigmented layer.
• Contributes to intraocular pressure.
• Floaters: Debris in vitreous humor of older people.

Question 15

Glaucoma:

Answer 15

• Blocked drainage of aqueous humor that increases pressure and causes compression of retina and optic nerve.
• Leads to blindness.

Question 16

The 3 Steps of Circulation of Aqueous Humor:

Answer 16

o 1) Aqueous humor forms by filtration from the capillaries in the ciliary processes.
o 2) Aqueous humor flows from the posterior chamber through the pupil into the anterior chamber. Some also flows through the vitrous humor.
o 3) Aqueous humor is reabsorbed into the venous blood by scleral venous sinus.

Question 17

Lens:

Answer 17

o Biconvex, transparent, flexible, and avascular.
o Changes shape to precisely focus light on retina.
o Enclosed in thin, elastic capsule and help in place by ciliary zonule just posterior to iris.
o 2 Regions: Lens epithelium anteriorly; lens fibers form bulk of lens.
o Lens fibers are filled with transparent protein crystalline.
o Lens becomes more dense, convex, less elastic with age.

Question 18

Cataracts:

Answer 18

Clouding of lens. Consequence of aging, diabetes, heavy smoking, frequent exposure to intense sunlight. Fixed with plastic lens replacement surgery (cataracts surgery).

Question 19

Optics and the Eye:

Answer 19

o Eyes respond to visible light
• Small portion of electromagnetic spectrum
• Wavelengths 400-700 nm
o Light
• Packets of energy that travel in wavelike fashion at high speeds.
• Color of light objects reflect determines color eye perceives.

Question 20

Refraction and Lenses:

Answer 20

o Light travels in straight lines and is blocked by any nontransparent object.
o When light travels in a given medium, its speed is constant.
• Speed of light changes when it passes from one transparent medium into another having a different density.
• Light speeds up as it passes into a less dense medium and slows as it passes into a denser medium.
o Due to change in speed of light, bending or refraction, of a light ray occurs when it meets the surface of a different medium at an oblique angle.
o Light passing through convex lens is bent so that rays converge at a single point called the focal point.
• Image formed by convex lens at focal point is upside-down and reversed right to left.
o Concave lenses diverge light preventing light from focusing.

Question 21

Pathway of Light Entering Eye:

Answer 21

o	Cornea (refracted when entering here)
o	Aqueous humor
o	Lens (refracted when entering and leaving here)
o	Vitreous humor
o	Entire neural layer of retina
o	Photoreceptors

Question 22

The Majority of Refractory Power is Where?

Answer 22

The Cornea

Question 23

Change in Lens Curvature Allows for What?

Answer 23

Fine Focusing

Question 24

Focusing on Distant Vision:

Answer 24

o Eyes best adapted for distant vision.
o Far point of vision:
• Distance beyond which no change in lens shape needed for focusing. 20 feet for emmetropic (normal) eye. Cornea and lens focus light precisely on retina.
o Ciliary muscles relaxed.
o Lens stretched flat by tension in ciliary zonule.

Question 25

Focusing on Close Vision:

Answer 25

o Light from close object diverges as it approaches the eye.
o Requires eye to make active adjustments using three simultaneous processes:
• Accommodation of lenses: Changing lens shape to increase refraction. Near point vision. Closest point on which the eye can focus.
• Constriction of pupils: Accommodation pupillary reflex constricts pupils to prevent most divergent light rays from entering eye.
• Convergence of eyeballs: Medial rotation of eyeballs toward object being viewed.

Question 26

Presbyopia:

Answer 26

Loss of accommodation over age 50.

Question 27

The 3 Major Areas of the Ear:

Answer 27

o External (Outer) Ear: Hearing Only.
o Middle Ear (Tympanic Cavity): Hearing Only.
o Internal (Inner) Ear: Hearing and Equilibrium.
o Receptors for hearing and balance respond to separate stimuli, and are thus activated independently.

Question 28

Structures of the External Ear:

Answer 28

o Auricle (Pinna):
• Composed of Helix (Rim) and Lobule (Earlobe).
• Funnels Sound Waves into external acoustic meatus.
o External Acoustic Meatus:
• Short, curved tube that extends from auricle to eardrum lined with skin bearing hairs, sebaceous glands, and ceruminous glands.
• Transmits sound waves to eardrum.

Question 29

Structures of the Middle Ear:

Answer 29

o Tympanic Membrane:
• Boundary between external and middle ears.
• Connective tissue membrane that vibrates in response to sound.
• Transfers sound energy to bones of middle ear.
o Tympanic Cavity: Small, air-filled cavity in temporal bone.
• Flanked laterally by eardrum.
• Flanked medially by bony wall containing oval (vestibular) and round (cochlear) windows.
o Pharyngotympanic (auditory) tube:
• Otherwise known as Eustacian tube.
• Connects middle ear to nasopharynx.
• Equalizes pressure in middle ear cavity with external air pressure when swallowing or yawning.
• Eardrum vibrates freely only if pressure on both of its surfaces is the same; otherwise sounds are distorted.
o Auditory Ossicles:
• Three small bones in tympanic cavity: the malleus, incus, and stapes.
• “handle” of malleus secured to eardrum (tympanic membrane_ and base of staped fits into oval window.
• Transmits and amplifies sound vibrations from tempanic membrane to oval window. This sets fluids of internal ear into motion eventually exciting hearing receptors.
• Tensor tympani and stapedius muscles contract reflexively in response to loud sounds to prevent damage to hearing receptors.

Question 30

Otitis Media:

Answer 30

o Common middle ear inflammation; especially in children due to shorter, more horizontal pharyngotympanic tubes. Most frequent cause of hearing loss in children.
o Acute infectious forms, eardrum bulges and becomes inflamed and red.
o Mostly treated with antibiotics.
o Myringotomy: to relieve pressure if severe.

Question 31

Structures of the Internal Ear:

Answer 31

o Also called the labyrinth because of it’s complicated shape.
o Complex structures carved out of temporal bone behind eye socket.
o Inner Ear has two major divisions:
• Bony labyrinth: Filled with perilymph (CSF). Forms semicircular canals and outer channels of cochlear and vestibule.
• Saccule and Utricle: Two membranous sacs suspended in vestibule; contain maculae that respond to pull of gravity and changes in position of head.
• Ampulla: Enlarged swelling at end of each semicircular canal, houses equilibrium receptor region, crista ampullaris.
• Membranous labyrinth: Forms semicircular ducts and inner spaces of vestibule and cochlea in bony labyrinth filled with endolymph.
o Contains membranous cochlear duct, which houses spiral receptor organ of hearing (spiral organ of corti).
o Cochlear branch of nerve VIII runs from spiral organ to brain
o Spiral orgain composed of supporting cells and hearing receptor cells (Cochlear hair cells)
• Inner hair cells (1 row)
• Outer hair cells (3 rows)

Question 32

Structures of the Cochlea:

Answer 32

o Scala Vestibuli: Ends at oval window, contains perilymph.
• Vestibular Membrane: seperates scala vestibule from cochlear duct.
o Scala Media: (cochlear duct) Contains endolymph
o Scala Tympani: Terminates at round window; contains perilymph.
• Basilar Membrane: Seperates scala tympani from cochlear duct.

Question 33

Properties of Sound:

Answer 33

o Sound is a disturbance of air pressure, with alternating areas of high and low pressure produced by a vibrating object and propagated by molecules of the medium.
o Sound travels more slowly than light and depends on an elastic medium for its transmission.
o Sound wave: Alternating patterns of compressions and rarefactions.
o Frequency: The number of waves that pass a given point in a given time. (Pitch) Depends on size of wavelength. Highest energy in smallest wavelengths.
o Amplitude: Height of sine wave crests. (Loudness)

Question 34

The 4 Steps of Sound Transmission to Internal Ear:

Answer 34

o 1) Sound waves vibrate the tympanic membrane.
o 2) Auditory ossicles vibrate. Pressure is amplified.
o 3) Pressure waves created by the stapes pushing on the oval window move through fluid in the scala tympani.
o 4) Sounds with frequencies below hearing travel through the helicotrema and don’t excite hair cells. Sounds in the hearing range go through the cochlear duct, vibrating basilar membrane and deflecting hairs on inner hair cells.

Question 35

Sound Transmission to Internal Ear:

Answer 35

o High Frequency: Sounds cause basilar membrane to vibrate near the base (narrow and stiff).
o Low Frequency: Sounds cause basilar membrane to vibrate near apex (wide and flexible).
o Fluid vibrations cause basilar membrane to vibrate, which causes hair cells in spiral organ to brush against tectorial membrane.
o The middle ear amplifies air waves by bone conduction.
o The inner ear transforms air waves into fluid waves.
o The inner ear also transduces physical energy into nerve energy.

Question 36

Equilibrium and Orientation:

Answer 36

o Utricle and Saccule contain Maculae: a flat epithelial patch containing hair cells that function as sensory receptor organs.
• Monitors position of head in space with respect to gravity.
• Essential for maintaining appropriate posture and balance.
o Equilibrium sense depends on inputs from internal ear and on vision and information from stretch receptors of muscles and tendons.
o Vestibular apparatus:
• Equilibrium receptors in semicircular canals and vestibule.
• Vestibular receptors monitor static equilibrium (gravity).
• Semicircular canal receptors monitor changes in head rotation (dynamic equilibrium).
o Maculae:
• Utricle: horizontal orientation of macula.
• Saccule: vertical orientation of macula.
• Otholithic membrane: thick, gelatinous, glycoprotein layer that contains tiny stones (otoliths) composed of calcium carbonate.
• Macular hair cells have long stereocilia which are embedded in otolithic membrane.

Question 37

Static Equilibrium:

Answer 37

o When your head starts/stops moving in a linear direction, inertia causes the otolith membrane to slide backward or forward like a greased plate over the hair cells, bending the hairs.
• Going forward, otolith membranes of utricle maculae lag behind, bending hairs backward.
• Sudden stop, otolith membrane slides abruptly forward, bending hair cells forward.
o Hair cells release NT continuously but movement of hairs modifies amount of NT’s released.

Question 38

Semicircular Ducts:

Answer 38

o Located inside the semicircular canals.
o Oriented in all 3 dimensions: Anterior, posterior, and lateral (horizontal).
o Ampullae=enlarged ends which contain crista ampullaris.
o Crista ampullaris similar to macula, with stereocilia embedded in gel-like cupula.

Question 39

Crisa Ampullaris:

Answer 39

o Sensory receptor for rotational acceleration. Major stimuli are rotational movements.
o Cristae respond to changes in velocity of rotational movements of the head.
o Bending of hairs in cristae causes depolarizations and rapid impulses reach brain at faster rate.
o Rotation causes endolymph to bend cupula in opposite direction.
o Bending causes hair cells to change the rate of impulse generation.

Question 40

The Chemical Senses:

Answer 40

o Sense of smell (olfaction) and taste (gustation).
o Receptors for smell(olfaction) and taste (gustation) are chemoreceptors. They respond to chemicals (odorants) in aqueous solution.
• Smell receptors are excited by airborne chemicals that dissolve in fluids coating nasal membranes.
• Taste receptors are excited by food chemicals dissolved in saliva.

Question 41

Olfaction:

Answer 41

o Low threshold-only few molecules needed.
o Odorants bind to receptors.
o Depolarization occurs.
o Nerve impulses are triggered.
o Anosmia=inability to smell
o Hyposmia=decreasing ability to smell
o Adaptation=decreasing sensitivity to odors.
o Olfactory adaptation is rapid
• 50% in 1 second.
• Complete in 1 minute
o Olfactory Epithelium: In roof of nasal cavity, organ of smell.
• Covers superior nasal conchae
• Contains olfactory sensory neurons: bipolar neurons with radiating olfactory cilia.
• Olfactory stem cells lie at base of epithelium
o Bundles of nonmyelinated axons of olfactory receptor cells form olfactory nerve.
• Project superiorly through openings in cribiform plate of ethmoid bone.

Question 42

Olfactory Pathway:

Answer 42

o Axons of olfactory cells synapse on olfactory bulbs.
o Axons of 2nd order sensory neurons travel in olfactory tracts to primary olfactory cortex, don’t have to pass through thalamus first.
o Some then relay to thalamus and to oribitofrontal cortex. Sometimes limbic structures as well.

Question 43

Gustation:

Answer 43

o Special sense of taste= chemical sense
o Involves specialized receptor cells called taste buds.
o Taste buds are located in the tongue, cheeks, soft palate, pharynx, and epiglottis.
o Lingual Papillae: Protrusions of the tongue.
• Filiform Papillae: No taste buds! Texture.
• Foliate Papillae: Taste buds present only up to age 2-3
• Fungiform Papillae: Contain taste buds scattered over entire tongue surface.
• Vallate Papillae: Surrounded by deep trench. Contain half of all taste buds.
o Taste requires dissolving of substances in saliva.
o Ageusia= defect in taste
o Five Primary Taste Sensations: Salty, sweet, sour, bitter, umami.

Question 44

Gustation Pathways:

Answer 44

o 1) Dissolved in substance contacts gustatory hairs of taste bud.
o 2) Receptor potentials developed in gustatory hairs cause the release of NT that gives rise to nerve impulses.
o 3) Nerve Impulse formed in 1st-order neuron of cranial sensory neuron: VII: serves anterior 2/3 of tongue. IX: Serves posterior 1/3 of tongue and part of palate. X: serves palate, pharynx, and epiglottis.
o 4) All taste fibers synapse in medulla oblongata on 2nd order sensory neuron.
o 5) 2nd order sensory neuron projects to: hypothalamus and amygdala: gagging, vomiting, and other autonomic reflexes. And thalamus.
o 6) 3rd order sensory neuron from thalamus projects to cortex of insula and post-central gyrus. (conscious identification of taste)