chapter 15 study guide: special senses

What are the four main structures in the eye matching section?

The retina is the sensory layer; the lens focuses light; the aqueous humor helps maintain intraocular pressure; the fovea centralis provides the sharpest vision.

What is the retina?

The sensory layer at the back of the eye that contains rods and cones, which detect light.

What is the lens?

A transparent structure that focuses light onto the retina by changing shape during accommodation.

What is aqueous humor?

A watery fluid in the anterior part of the eye that nourishes nearby structures and helps maintain intraocular pressure.

What is the fovea centralis?

A small area in the retina with a high concentration of cones that provides the sharpest, most detailed vision.

What is the iris?

The colored part of the eye that controls the amount of light entering through the pupil.

What is the pupil?

The opening in the center of the iris through which light enters the eye.

What is the cornea?

The transparent front surface of the eye that provides much of the eye's light refraction.

What is the vitreous humor?

The gel-like substance filling the posterior segment of the eye that helps maintain its shape.

What is accommodation?

The process in which the lens changes shape to focus on objects at different distances.

What happens to lens elasticity as a person ages?

The lens becomes less flexible, making accommodation more difficult and causing difficulty focusing on nearby objects.

What is a cataract?

A clouding of the eye's lens that causes blurred or obscured vision.

What causes cataracts, and how are they treated?

Cataracts occur when proteins in the lens change and clump together, making it cloudy. They are treated with surgery to remove the cloudy lens and usually replace it with an artificial lens.

What is the macula lutea?

The central region of the retina responsible for detailed central vision; it contains the fovea centralis.

What is the optic disc?

The location where the optic nerve leaves the eye. It has no photoreceptors and creates the blind spot.

What is the blind spot?

The area of the visual field corresponding to the optic disc, where there are no rods or cones.

Why does the blind spot usually not create a noticeable gap in vision?

The two eyes have overlapping visual fields, so one eye can often provide information about an area that falls on the other eye's blind spot. The brain also fills in missing information.

What are rods?

Photoreceptors that are very sensitive to light and are especially important for vision in dim lighting. They do not provide color vision.

What are cones?

Photoreceptors responsible for color vision and sharp detail, especially in bright light.

What is the main function of rods?

Vision in dim light.

What is the main function of cones?

Color vision and detailed vision in brighter light.

Where are cones especially concentrated?

In the fovea centralis, the area of greatest visual acuity.

Why does the iris matter for vision?

It adjusts pupil size to regulate how much light enters the eye.

What is the correct pathway of light through the eye?

Light passes through the cornea, aqueous humor in the anterior chamber, pupil, lens, vitreous humor in the posterior segment, and then reaches the retina.

How does light travel from the eye to the brain?

Light reaches the retina and activates photoreceptors. Signals pass through bipolar cells and ganglion cells, travel along the optic nerve, partially cross at the optic chiasm, and continue through the optic tracts to the thalamus. They then travel through optic radiations to the primary visual cortex in the occipital lobe, where visual information is processed.

What is the optic chiasm?

The location where some fibers from each optic nerve cross to the opposite side of the brain.

Why do both pupils constrict when light is shined into one eye?

The pupillary light reflex sends signals through the optic nerve and brainstem, which activate both oculomotor nerves to constrict both pupils.

Which eye muscle moves the eye outward?

The lateral rectus muscle.

Which eye muscle moves the eye inward?

The medial rectus muscle.

Which eye muscle rotates the eye upward and laterally?

The inferior oblique muscle.

Which eye muscle would need to work for the right eye to look left?

The right medial rectus muscle. It moves the right eye inward toward the nose.

What is strabismus?

A condition in which the eyes are misaligned and do not point in the same direction.

What is exotropia?

A type of strabismus in which one or both eyes turn outward.

How can strabismus affect a child's vision, and how is it treated?

Misaligned eyes may not work together properly and can lead to amblyopia, or reduced vision in one eye. Treatment may include glasses, patching the stronger eye, vision therapy in selected cases, or surgery depending on the cause.

Why can prolonged reading cause eye strain?

The ciliary muscles must maintain accommodation to focus on nearby text, and the eyes must coordinate to keep the text clear. Prolonged near work can cause fatigue, discomfort, and temporary blurred vision.

What are the major structures of the external ear?

The pinna (auricle), external acoustic meatus, and tympanic membrane are associated with the external ear.

What is the pinna?

The visible part of the external ear that collects sound waves and directs them into the ear canal.

What is the external acoustic meatus?

The ear canal that carries sound waves toward the tympanic membrane.

What is the tympanic membrane?

The eardrum, which separates the external ear from the middle ear and vibrates in response to sound waves.

What are ceruminous glands?

Modified apocrine sweat glands in the external ear canal that produce cerumen, or earwax.

What is the pharyngotympanic tube?

Also called the auditory or Eustachian tube, it connects the middle ear with the nasopharynx.

What is the purpose of the pharyngotympanic tube?

It equalizes air pressure on both sides of the eardrum and helps drain fluid from the middle ear.

Why do your ears pop when you swallow or yawn?

The pharyngotympanic tubes open to help equalize pressure between the middle ear and the surrounding environment.

What are the three auditory ossicles?

The malleus, incus, and stapes. They transmit and amplify vibrations from the tympanic membrane to the oval window.

What is the cochlea?

The spiral-shaped inner-ear structure that contains the receptors for hearing.

Where are the receptors for hearing located?

In the organ of Corti within the cochlea.

What is the organ of Corti?

The sensory structure inside the cochlea containing hair cells that convert sound-related vibrations into nerve signals.

What is the vestibule of the inner ear?

The region containing the utricle and saccule, which detect linear acceleration and head position relative to gravity.

What are otoliths?

Tiny calcium carbonate crystals in the utricle and saccule that move with gravity and linear acceleration, stimulating sensory hair cells.

What do the utricle and saccule detect?

Linear acceleration and the position of the head relative to gravity.

What do the semicircular canals detect?

Rotational or angular acceleration of the head.

What is the role of endolymph in balance?

Endolymph is fluid within the membranous labyrinth. Its movement during head motion bends sensory hair cells and helps the brain detect rotational movement.

What is the vestibular apparatus?

The inner-ear balance system, including the semicircular canals, utricle, and saccule, which detects head movement and position.

How does sound travel from the external ear to the brain?

Sound waves enter the pinna and external acoustic meatus, vibrate the tympanic membrane, and move the malleus, incus, and stapes. The stapes transmits vibrations through the oval window into the cochlea. Fluid movement bends hair cells in the organ of Corti, generating signals carried by the cochlear branch of the vestibulocochlear nerve (VIII) to the brainstem, then through auditory pathways to the thalamus and auditory cortex in the temporal lobe.

What is the vestibulocochlear nerve (VIII)?

A purely sensory cranial nerve with a vestibular branch for balance and a cochlear branch for hearing.

What is labyrinthitis?

Inflammation of the inner-ear labyrinth that can cause dizziness, vertigo, and balance problems, sometimes with hearing symptoms.

What is Ménière's disease or syndrome?

An inner-ear disorder associated with episodes of vertigo, ringing in the ears (tinnitus), fluctuating hearing loss, and sometimes nausea and vomiting.

What is the suspected cause of Ménière's disease?

It is commonly associated with abnormal accumulation or regulation of endolymphatic fluid in the inner ear, although the exact cause is not always known.

What is vertigo?

The sensation that you or your surroundings are spinning or moving when they are not.

What is tinnitus?

The perception of ringing, buzzing, roaring, or other sounds without an external sound source.

Why can a roller coaster cause wild sensations, dizziness, or nausea?

Rapid changes in head movement and acceleration stimulate the vestibular apparatus in the inner ear. The brain combines these signals with visual information and body-position signals; conflicting information can contribute to dizziness and nausea.

Why do dancers use spotting when spinning?

They keep their gaze fixed on a target for as long as possible, then quickly turn their head to reacquire it. This helps coordinate visual and vestibular information and can reduce dizziness.

Why does spotting help reduce dizziness?

It helps coordinate what the eyes see with movement detected by the vestibular system. It can reduce the sensation of continuous spinning, although it does not completely prevent dizziness in everyone.

What is the olfactory epithelium?

Specialized tissue in the upper nasal cavity containing olfactory receptor neurons that detect odor molecules.

What are olfactory receptors?

Chemoreceptors that respond to odor molecules and send signals through the olfactory nerve to the brain.

How can humans distinguish thousands of odors with fewer types of olfactory receptors?

Different odor molecules activate different combinations of olfactory receptors. The brain interprets these patterns as distinct smells.

What is olfactory adaptation?

A decrease in the perceived intensity of an odor during continued exposure to it.

What is the relationship between smell and taste?

Smell and taste work together to create flavor. When smell is reduced, food often seems less flavorful even when taste receptors still function.

Why does a bad cold make food taste different?

Nasal congestion prevents odor molecules from reaching olfactory receptors effectively. Because smell contributes greatly to flavor, food may seem bland or different.

Why does your nose run during and after crying?

Tears drain through the nasolacrimal ducts into the nasal cavity, adding fluid to nasal secretions and causing a runny nose.

What are taste buds?

Sensory structures containing taste receptor cells that detect dissolved chemicals in food.

What types of receptors are found in the mouth?

Chemoreceptors detect taste chemicals; mechanoreceptors detect touch and texture; thermoreceptors detect temperature; and nociceptors detect painful or irritating stimuli.

Which cranial nerve carries taste from the tip and front two-thirds of the tongue?

The facial nerve (VII), through its chorda tympani branch.

Which cranial nerve carries taste from the back one-third of the tongue?

The glossopharyngeal nerve (IX).

What is the only special sense that is not fully functional at birth according to the study guide?

Vision.

Why can a cornea be transplanted with a relatively low risk of rejection?

The cornea has no blood vessels, which limits direct access by many immune cells and reduces the risk of rejection compared with highly vascularized tissues.

What is the main function of the iris?

To regulate the amount of light entering the eye by changing the size of the pupil.

Which type of sensory receptor is continually replaced throughout adult life according to the study guide?

Olfactory receptor cells.

What percentage of the body's sensory receptors are located in the eye according to the study guide?

Approximately 70 percent.

Which structure is NOT part of the external ear: pinna, external acoustic meatus, tympanic membrane, or pharyngotympanic tube?

The pharyngotympanic tube, which connects the middle ear with the nasopharynx.

What is the difference between the vestibule and semicircular canals?

The vestibule contains the utricle and saccule, which detect linear acceleration and head position relative to gravity. The semicircular canals detect rotational acceleration.

How do the special senses help the body understand its surroundings?

Vision detects light; hearing detects sound; olfaction detects odors; taste detects chemicals in food; and equilibrium detects head movement and position.