Chapter 8: Special Senses

What are the four traditional special senses?

Sight, hearing, taste, and smell.

What are the organs responsible for the special senses?

The eyes, ears, taste buds, and olfactory epithelium.

What are the accessory structures of the eye?

Extrinsic eye muscles, eyelids, conjunctiva, and lacrimal apparatus.

What is the palpebral fissure?

The space between the eyelids when the eye is open.

What are the medial and lateral canthi?

The medial and lateral corners where the eyelids meet.

What are tarsal glands?

Glands along the eyelid edges that secrete an oily substance to lubricate the eye.

What is the function of the conjunctiva?

It secretes mucus to keep the eye moist.

What is the lacrimal apparatus?

The lacrimal glands and ducts that produce and drain tears.

Where are the lacrimal glands located?

Superolateral to each eye.

What are the functions of tears?

To protect and moisten the eye.

What substances are found in lacrimal secretions?

Mucus, antibodies, and lysozyme.

What does lysozyme do?

Helps destroy bacteria.

Where do tears drain after entering the lacrimal canaliculi?

Into the nasolacrimal duct, which empties into the nasal cavity.

How many extrinsic eye muscles are there?

Six.

What is the function of the extrinsic eye muscles?

Produce eye movements and allow tracking of moving objects.

What are the three layers of the eyeball?

Fibrous, vascular, and sensory layers.

What structures make up the fibrous layer?

The sclera and cornea.

What is the sclera?

The tough, white connective tissue outer layer that protects the eye.

What is the cornea?

The transparent anterior part of the fibrous layer that allows light to enter.

Why is the cornea vulnerable to damage?

It is well supplied with nerve endings.

What are the three regions of the vascular layer?

Choroid, ciliary body, and iris.

What is the choroid?

The blood-rich, pigmented layer that nourishes the eye and prevents light scattering.

What is the ciliary body?

A smooth-muscle region that attaches to the lens through the ciliary zonule and helps with accommodation.

What is the iris?

The pigmented part of the eye that controls pupil size.

What is the pupil?

The rounded opening in the iris through which light enters.

What controls the opening and closing of the pupil?

Smooth muscle fibers in the iris.

What is the sensory layer of the eye?

The innermost layer, made primarily of the retina.

What are the two layers of the retina?

A pigmented layer and an inner transparent layer containing photoreceptors.

What is the function of the pigmented retinal layer?

Restricts light scattering, removes dead cells and photoreceptors, and stores vitamin A.

What are photoreceptors?

Light-sensitive cells called rods and cones.

What is the pathway of a visual signal through the retina?

Photoreceptors → bipolar cells → ganglion cells → optic nerve.

What is the optic nerve?

Carries visual impulses from the retina toward the brain.

Where does visual interpretation occur?

In the occipital lobe of the brain.

What is the optic disc?

The location where the optic nerve exits the eye.

Why is the optic disc called the blind spot?

It contains no rods or cones.

What happens when light hits the optic disc?

The object may disappear from view.

What are rods responsible for?

Seeing in dim light, gray tones, and peripheral vision.

Where are rods most densely located?

In the peripheral retina.

What are cones responsible for?

Color vision and seeing fine details in bright light.

Where are cones most densely located?

In the center of the retina.

What is the fovea centralis?

A small area with a high concentration of cones and the highest visual acuity.

What are the three types of cones?

Cones sensitive to blue, green, and red wavelengths.

What is the function of the lens?

To focus light onto the retina.

What holds the lens in place?

Suspensory ligaments, also called ciliary zonule fibers.

What is aqueous humor?

A clear, watery fluid in the anterior segment of the eye.

What is vitreous humor?

A gel-like substance in the posterior segment of the eye.

What are the two segments of the eye?

The anterior (aqueous) segment and posterior (vitreous) segment.

What do the aqueous and vitreous humors help maintain?

Eye structure and intraocular pressure.

What is an ophthalmoscope used for?

Illuminating and examining the fundus, including the retina, optic disc, and blood vessels.

What is refraction?

The bending of light as it passes through different structures of the eye.

What structures does light pass through as it enters the eye?

Cornea → aqueous humor → lens → vitreous humor → retina.

What is accommodation?

The adjustment of lens shape to focus light onto the retina.

What happens to the lens when viewing a close object?

The lens becomes more convex to focus diverging light.

What happens to the lens when viewing a distant object?

The lens is flatter for focusing parallel light rays.

How does the retinal image differ from the actual object?

It is inverted, reversed from left to right, and smaller.

What is the optic chiasma?

The location where fibers from the medial sides of the eyes cross to the opposite side of the brain.

What are optic tracts?

Fiber pathways carrying visual information after the optic chiasma.

Where do optic tracts send visual information?

To the thalamus, then through optic radiations to the occipital lobe.

What controls the internal muscles of the eye?

The autonomic nervous system.

What do the internal eye muscles control?

Pupil size and lens configuration.

Which cranial nerves control the extrinsic eye muscles?

Cranial nerves III, IV, and VI.

What is the photopupillary reflex?

Automatic pupil constriction in response to bright light.

Why do pupils constrict when viewing close objects?

To help produce more acute vision.

What are the three anatomical divisions of the ear?

External ear, middle ear, and inner ear.

Which parts of the ear are used only for hearing?

The external and middle ear.

Which part of the ear functions in hearing and balance?

The inner ear.

What structures make up the external ear?

The auricle (pinna) and external auditory meatus.

What is the auricle (pinna)?

The visible external part of the ear.

What is the external auditory meatus?

The auditory canal that carries sound waves toward the eardrum.

What is cerumen?

Earwax that traps foreign bodies and repels insects.

What is the tympanic membrane?

The eardrum; it vibrates when struck by sound waves.

What is the middle ear also called?

The tympanic cavity.

What are the oval window and round window?

Two openings in the bony wall separating the middle and inner ear.

What is the pharyngotympanic tube?

The auditory tube connecting the middle ear to the throat.

What is the function of the pharyngotympanic tube?

Equalizes pressure on both sides of the eardrum.

What actions can open the pharyngotympanic tube?

Swallowing and yawning.

What are the three auditory ossicles?

Malleus, incus, and stapes.

What are the common names of the auditory ossicles?

Malleus = hammer; incus = anvil; stapes = stirrup.

What is the function of the auditory ossicles?

Transmit eardrum vibrations to the fluid of the inner ear.

What does the stapes contact?

The oval window.

What are the three parts of the bony labyrinth?

Cochlea, vestibule, and semicircular canals.

Where is the vestibule located?

Between the cochlea and semicircular canals.

What fluid fills the bony labyrinth?

Perilymph.

What is the membranous labyrinth?

A system of membrane sacs suspended within the bony labyrinth.

What fluid fills the membranous labyrinth?

Endolymph.

What is the vestibular apparatus?

The inner-ear receptors responsible for equilibrium.

What are the two types of equilibrium?

Static equilibrium and dynamic equilibrium.

What is static equilibrium?

Detects head position relative to gravity when the body is not moving.

Where are the receptors for static equilibrium located?

In the vestibule, in structures called maculae.

What are maculae?

Receptor regions in the vestibule that detect head position relative to gravity.

What are otoliths?

Calcium-salt structures in the otolithic membrane.

How does static equilibrium detect head position?

Otoliths shift, pulling the otolithic membrane and bending hair cells.

Which nerve carries equilibrium information to the brain?

The vestibular branch of cranial nerve VIII.

What is dynamic equilibrium?

Detects angular or rotational movements of the head.

Where are the receptors for dynamic equilibrium located?

In the semicircular canals.

What is the ampulla?

The swollen region at the base of each semicircular canal containing the receptor region.

What is the crista ampullaris?

The receptor region in the ampulla containing hair cells.

What is the cupula?

The gelatinous cap covering the hair cells of the crista ampullaris.

How does dynamic equilibrium detect head rotation?

Endolymph movement bends the cupula and stimulates hair cells.

Where is the cochlea located?

In the inner ear.