Hearing and equilibrium

Mechanically gated channels open when the membrane or a structure attached to the channel is stretched or pushed. Voltage-gated channels respond to voltage, and ligand-gated channels to a bound chemical.

• A change in membrane voltage:
• Correct: A physical force that stretches or deforms the membrane:
• A chemical messenger binding to it:

The middle and inner ear are hollowed out of the temporal bone, whose external acoustic meatus is the bony part of the ear canal.

• Correct: Temporal bone:
• Occipital bone:
• Sphenoid bone:

A stimulus opens or closes ion channels in a sensory receptor cell, producing a graded change in its membrane potential. Its size depends on the strength of the stimulus.

• An action potential in a sensory neuron's axon:
• The resting potential of a sensory cell:
• Correct: A graded change in a sensory cell's membrane potential caused by a stimulus:

The incus is the middle ossicle. It links the malleus, attached to the eardrum, to the stapes in the oval window.

• Malleus: The malleus is the ossicle attached to the inside of the eardrum, the first in the chain.
• Stapes: The stapes is the last ossicle, whose footplate sits in the oval window.
• Correct: Incus: Correct. The middle bone of the chain is the incus.
• Tympanic membrane: The tympanic membrane is the sheet stretched across the end of the ear canal, not a bone.

The eardrum vibrates, the ossicles pass the vibration to the oval window, the stapes sets up waves in the perilymph, the basilar membrane vibrates, and the tectorial membrane slides across the hair cells and bends their stereocilia.

• Correct order: 1. Sound waves vibrate the tympanic membrane 2. The malleus, incus and stapes carry the vibration across the middle ear 3. The stapes rocks in the oval window 4. Pressure waves travel through the perilymph of the scala vestibuli 5. The basilar membrane vibrates 6. The hair cells' stereocilia bend

High frequencies vibrate the narrow, stiff base of the basilar membrane most, close to the oval window. Low frequencies travel farther and peak near the wide, floppy apex. Which hair cells are stimulated tells the brain the pitch.

• Near the apex, where the membrane is wide and floppy: The wide, floppy apex vibrates most for low frequencies, not high ones.
• Evenly along the whole length of the membrane: Each frequency makes the traveling wave peak at one place; the membrane does not vibrate equally along its length.
• Correct: Near the base, where the membrane is narrow and stiff: Correct. High pitch is coded near the base.
• At the round window, where the membrane ends: The round window is where the scala tympani ends at the middle ear; the basilar membrane lies along the cochlear duct.

Bending toward the tallest stereocilium stretches the tip links, which pull mechanically gated channels open. K+ from the endolymph flows in and depolarizes the cell, which then releases more glutamate onto its sensory neuron.

• Tip links slacken, channels close and the cell hyperpolarizes: This is what happens when the bundle bends away from the tallest stereocilium.
• Correct: Tip links stretch, channels open, K+ flows in and the cell depolarizes: Correct. Toward the tallest: channels open and the cell depolarizes.
• The hair cell fires an action potential along its axon to the brainstem: Hair cells have no axon. They release neurotransmitter onto sensory neurons, which fire the action potentials.
• Tip links stretch, channels open, K+ flows out and the cell hyperpolarizes: K+ flows in, not out, because the endolymph around the stereocilia is rich in K+ and positively charged.

Bone conduction beating air conduction on the right means sound is blocked on its way through the right external or middle ear: a conductive loss. The Weber test fits, because a conductive loss makes the fork louder in the affected ear.

• Sensorineural hearing loss in the right ear: A right sensorineural loss would send the Weber sound to the left ear, and air conduction would still beat bone on the right.
• Correct: Conductive hearing loss in the right ear: Correct. Weber to the right plus bone better than air on the right means a right conductive loss.
• Sensorineural hearing loss in the left ear: A left sensorineural loss would also send the Weber sound right, but it would not make bone beat air in the right Rinne test.
• Conductive hearing loss in the left ear: A left conductive loss would send the Weber sound to the left ear.

Auditory fibers cross at several levels, beginning at the superior olivary nuclei, so each ear reaches both auditory cortices. Losing one cortex does not deafen either ear, although locating sounds and understanding speech can suffer.

• He is deaf in the right ear: Unlike touch, hearing from each ear reaches both hemispheres, so one-sided cortical damage does not deafen the opposite ear.
• He is deaf in the left ear: The left ear also sends signals to the intact right cortex.
• Correct: Neither ear goes deaf, since each ear reaches both hemispheres: Correct. Bilateral pathways protect against deafness from one-sided cortical damage.
• He loses the ability to hear high-pitched sounds in both of his ears: Frequency is mapped across each auditory cortex; losing one side does not remove high pitches from both ears.

During steady spinning, the endolymph catches up with the canal and the cupula returns upright. When she stops, the fluid's inertia keeps it flowing, which bends the cupula the opposite way and signals a turn in the other direction.

• Otoliths have come loose from the maculae: Loose otoliths cause brief positional dizziness, not the normal after-spin sensation everyone feels.
• Correct: The endolymph keeps flowing and bends the cupula the opposite way: Correct. The lagging fluid now overshoots and bends the cupula the other way.
• The cupula is heavy and is pulled downhill by gravity when she stops: The cupula has the same density as endolymph and no crystals, so gravity does not pull it.
• The vestibulocochlear nerve keeps firing from fatigue: Nerve fatigue would reduce firing, not create a clear sense of turning in a particular direction.

The semicircular canals signal the head turn to the vestibular nuclei, which drive the eye muscles to turn both eyes the same amount in the opposite direction. The image stays still on the retinas and the sign stays sharp.

• Both eyes turn right, ahead of the head: Turning the eyes with the head would make the image slide across the retinas, the opposite of what the reflex does.
• Only the right eye turns left: The reflex moves both eyes together, keeping them aligned.
• Both eyes stay fixed in their sockets and the image slides: Without the reflex the image would slide and blur; the reflex prevents this.
• Correct: Both eyes turn left by the same amount: Correct. The eyes counter-rotate to hold the image steady.