Ch. 15: Special Sense

What is considered the most dominant sense in humans?

The sense of sight.

What percentage of sensory receptors are in the eyes, and what portion of the brain is involved in visual processing?

About 70% of the body's sensory detectors are located here, and over half of the brain's outer layer is involved in some aspect of interpreting visual information.

Describe the basic structure of the eyeball.

A slightly irregular hollow sphere, about one inch across.

How much of the eyeball is typically visible?

Only the front one-sixth (1/6).

What protects the majority of the eyeball?

A cushion of fat within the bony socket.

What are the short, coarse hairs above the eyes called and what are their functions?

They help shield the eyes from sunlight and prevent sweat from entering.

What are the thin, skin-covered folds that protect the eye's surface?

Upper and lower protective coverings.

Where do these folds meet at the corners of the eye?

At the inner and outer angles.

What small structure is located at the inner angle and what does it produce?

A modified gland that secretes a whitish, oily substance.

How often does reflexive blinking typically occur and what is its purpose?

Every few seconds, to protect the eye and keep its surface moist.

What are the robust hairs projecting from the edges of the eyelids called and what is their function?

They are richly supplied with nerves and trigger a protective reflex when something approaches the eye.

What are the modified sebaceous glands in the eyelids called and what do they secrete?

They produce an oily substance that lubricates the eyelids and the eye's surface.

What are the modified sweat glands located near the eyelashes called?

Ciliary glands.

What is the transparent mucous membrane lining the eyelids and covering the front of the eyeball?

A lubricating membrane that prevents dryness.

What is the common name for inflammation of this membrane?

Pinkeye

What structures make up the tear-producing and draining system of the eye?

A gland that releases a saline solution with enzymes and antibodies, and a series of ducts and sacs.

What is the composition of tears?

A dilute salt solution containing antibacterial enzymes and antibodies.

What is the pathway of tears after being produced?

They flow across the eye, drain into small canals, then a sac, and finally a duct leading to the nasal cavity.

What common nasal symptoms are associated with excessive tearing?

Sniffles and watery nose.

Name the six muscles attached to the outside of the eyeball.

Lateral, medial, superior, and inferior straight muscles, and superior and inferior angled muscles.

Which cranial nerves control these muscles?

The abducens, trochlear, and oculomotor nerves.

What is the clinical term for "cross-eyed"?

Strabismus

What are the three main layers of the eyeball?

An outer protective layer, a middle vascular layer, and an inner sensory layer.

Describe the outer layer of the eyeball.

A dense, avascular connective tissue with two main parts.

What is the posterior portion of the outer layer called and what is its appearance and function?

The "white of the eye," a tough, protective layer that also anchors the eye muscles.

What is the anterior, transparent portion of the outer layer called and what is its function?

A clear window that bulges forward and is a major light-bending structure.

What happens when this anterior layer is irritated?

It triggers reflex blinking and tearing due to numerous pain receptors.

What is a common surgical procedure involving this anterior layer?

Corneal transplants.

Describe the middle layer of the eyeball.

A vascular layer with three main parts.

What is the posterior, highly vascular, and pigmented part of the middle layer called?

A dark brown membrane that nourishes the eye layers and absorbs stray light.

What structure in the middle layer controls the shape of the lens?

A ring of smooth muscle bundles connected to suspensory ligaments.

What is the function of the suspensory ligaments?

They hold the lens in place.

What is the visible, colored part of the eye and what does it control?

A pigmented diaphragm called the iris with smooth muscle that adjusts the size of the central opening.

What is the central opening in the colored part of the eye called?

The pupil.

Describe the inner layer of the eyeball.

A sensory layer containing photoreceptor cells.

What are the three main types of neurons in the sensory layer (from back to front)?

Photoreceptors, bipolar cells, and ganglion cells.

What are the photoreceptor cells responsible for vision in dim light and peripheral vision?

Rod cells.

Do these cells provide sharp images or color vision?

No, they are more sensitive to light but produce less detailed, black and white vision.

Where are these dim light receptors most concentrated in the sensory layer?

In the outer regions of the sensory layer.

What clinical condition is associated with a deficiency in the function of these dim light receptors?

Night blindness.

What are the photoreceptor cells responsible for color vision and sharp detail?

Cone cells.

Where are these color and detail receptors most concentrated?

At the central focal point of the sensory layer.

What clinical condition is associated with a deficiency in the function of these color receptors?

Colorblindness

What type of nerve is formed by the axons of the ganglion cells?

The optic nerve.

What happens to the optic nerves after they leave the eyes?

They cross over at a structure called the optic chiasma and continue as optic tracts to the visual cortex.

What is the area where the optic nerve exits the eye called and what does it create?

The optic disc, which forms a "blind spot" due to the absence of photoreceptors.

What blood vessels enter and exit the eye through the optic nerve?

The central artery and central vein.

What is the area of the sensory layer responsible for sharp central vision called, and what is its central pit?

The macula lutea with the fovea centralis.

What can happen when the sensory layer detaches from the underlying layer?

A detached retina.

What structure precisely focuses light onto the sensory layer?

The lens.

How does the lens adjust for viewing distant objects?

The surrounding muscles relax, tightening the ligaments and flattening the lens.

How does the lens adjust for viewing close objects?

The surrounding muscles contract, loosening the ligaments and making the lens more rounded.

What is the process of the lens changing shape to focus light called?

Accommodation

What is the clouding of the lens called?

Cataracts

What visual defect is caused by unequal curvatures in the cornea or lens?

Astigmatism

How does the lens change with age and what is a common consequence?

It becomes less flexible, leading to difficulty focusing on near objects and the need for reading glasses.

What is the condition where distant objects are blurry because light focuses in front of the sensory layer?

Nearsightedness (myopia).

What is the condition where nearby objects are blurry because light focuses behind the sensory layer?

Farsightedness (hyperopia).

What is the fluid-filled space in the front part of the eye called and what fluid does it contain?

The anterior segment, containing aqueous humor.

What two chambers is this segment partially divided into by the iris?

The anterior and posterior chambers.

Describe the aqueous humor.

A clear fluid similar to blood plasma.

Is aqueous humor continuously produced and drained?

Yes

What are the functions of the aqueous humor?

Maintaining pressure within the eye and providing nutrients to the lens, cornea, and parts of the sensory layer.

What condition results from increased pressure within this fluid-filled space?

Glaucoma

What are the consequences of glaucoma if left untreated?

Compression of the sensory layer and optic nerve, leading to blindness.

What is the fluid-filled space in the back part of the eye called and what fluid does it contain?

The posterior segment, containing vitreous humor.

What are the functions of the vitreous humor?

Transmitting light, supporting the lens, holding the sensory layer against the vascular layer, and contributing to intraocular pressure.

Summarize the pathway of light through the eye to the sensory layer.

Light passes through the cornea, anterior chamber, pupil, posterior chamber, lens, posterior segment, and finally reaches the retina.

What is the outer, visible part of the ear called and what is its function?

The auricle or pinna, which directs sound waves into the ear canal.

What is the canal leading from the outer ear to the eardrum called?

The external acoustic meatus or auditory canal.

What lines this canal and what glands are found there?

Skin containing hairs, sebaceous glands, and modified sweat glands.

What do the modified sweat glands in the ear canal secrete and what is its function?

Cerumen (earwax), which traps foreign particles and repels insects.

What is the membrane separating the outer and middle ear called?

The tympanic membrane or eardrum.

Describe the eardrum.

A thin, translucent membrane of connective tissue covered by skin externally and mucous membrane internally.

What happens to the eardrum when sound waves reach it?

It vibrates.

What are some clinical issues related to the eardrum and outer ear canal?

Ruptured eardrum and swimmer's ear (outer ear infection).

What is the small, air-filled cavity containing the middle ear bones called?

The tympanic cavity or middle ear.

What membrane forms the outer boundary of this cavity and what bony wall forms the inner boundary?

The eardrum laterally and a bony wall with two openings medially.

What are the two openings in the bony wall of the middle ear called?

The oval window and the round window.

What are the three small bones in the middle ear called?

Malleus (hammer), incus (anvil), and stapes (stirrup).

What tube connects the middle ear to the nasopharynx and what is its normal state?

The pharyngotympanic or auditory tube, which is usually flat and closed.

What is the function of this tube during swallowing or yawning?

To equalize pressure between the middle ear and the outside.

What is a common infection of the middle ear called?

Otitis media.

What is the bony labyrinth of the inner ear responsible for?

Hearing and balance.

What spiral-shaped structure in the inner ear contains the hearing receptors?

The cochlea.

What is the receptor organ for hearing located within the cochlea called?

The spiral organ or organ of Corti.

What type of receptors are found here and what do they respond to?

Hair cells that depolarize in response to sound vibrations in fluid.

What nerve transmits auditory information from the cochlea to the brain?

The cochlear nerve.

What structures in the inner ear are responsible for balance and equilibrium?

The vestibule and semicircular canals.

What type of receptors are found in these balance structures and what do they respond to?

Hair cells that are stimulated by fluid movement due to changes in body position.

What nerve transmits balance information from the inner ear to the brain?

The vestibular nerve.

What condition can be caused by overstimulation of the motion receptors in the inner ear?

Motion sickness.

How do medications for this condition typically work?

By suppressing the central nervous system's reception of sensory input related to motion.

What are some potential side effects of these medications?

Drowsiness

What is an infection of the inner ear called and what are some of its symptoms?

Otitis interna, with symptoms like dizziness and vertigo.

Summarize the pathway of sound waves through the ear to the auditory cortex.

Sound waves travel through the outer ear, vibrate the eardrum, move the middle ear bones, vibrate the oval window, travel through fluid in the cochlea, stimulate hair cells, generate nerve impulses in the cochlear nerve, and reach the auditory cortex.

What is the general term for loss of hearing?

Deafness

What is conduction deafness?

Hearing loss due to problems with sound wave transmission to the inner ear fluids.

What are some causes of conduction deafness?

Impacted earwax, perforated eardrum, middle ear infection, or fusion of the middle ear bones.

How can some forms of conduction deafness be treated?

With a hearing aid.