Free MCAT-style practice, 3A
MCAT Sensory Systems, Vision and Hearing Practice Questions
Ten free MCAT-style practice questions on sensory receptors and adaptation, phototransduction, the visual pathway, cochlear hair cells, the vestibular system and smell, with full explanations.
Quick concept primer
Every sense converts a stimulus into a graded receptor potential, which triggers action potentials if it reaches threshold. Receptors are classed by stimulus (mechanoreceptors, thermoreceptors, nociceptors, chemoreceptors, photoreceptors) and by adaptation: phasic receptors fire at a change and then fall silent, while tonic receptors keep firing for as long as the stimulus lasts. Nearly all sensory pathways relay through the thalamus; smell is the exception.
Photoreceptors work backward from most neurons. In the dark, high cGMP holds cation channels open, so rods and cones are depolarized and release glutamate. Light converts 11-cis retinal to all-trans retinal, activating transducin and a phosphodiesterase that breaks down cGMP, so the channels close and the cell hyperpolarizes. Cones, packed in the fovea, give color and sharp vision; rods dominate the periphery and dim light. Nasal retinal fibers cross at the optic chiasm, so each hemisphere sees the opposite visual field.
In the cochlea, sound moves the basilar membrane and bends hair cell stereocilia; potassium from the high K+ endolymph flows in and depolarizes the cell. The base responds to high pitches and the apex to low pitches. Semicircular canals sense rotation, and the utricle and saccule sense linear acceleration and gravity.
Traps: light hyperpolarizes photoreceptors, and K+ entry, not exit, depolarizes hair cells.
10 practice questions
Question 1
Which sequence correctly describes what happens in a rod cell when light strikes it?
- cGMP rises, Na+ channels open, the cell depolarizes, and glutamate release increases
- Retinal converts from all-trans to 11-cis, K+ channels close, and the cell fires an action potential
- cGMP falls, cGMP-gated Na+ channels close, the cell hyperpolarizes, and glutamate release decreases
- Rhodopsin is degraded, Ca2+ channels open, and acetylcholine is released
Show answer and explanation
Answer: C. cGMP falls, cGMP-gated Na+ channels close, the cell hyperpolarizes, and glutamate release decreases
Light isomerizes 11-cis retinal to all-trans retinal in rhodopsin, which activates transducin and phosphodiesterase. Phosphodiesterase lowers cGMP, so the cGMP-gated cation channels close, the rod hyperpolarizes, and it releases less glutamate onto bipolar cells. Choice A describes the dark state, rods signal with graded potentials rather than action potentials, and their transmitter is glutamate.
Question 2
A subject can just detect an increase from a 200 g weight to a 206 g weight. According to Weber's law, what is the smallest increase the subject should detect when holding a 500 g weight?
- 6 g
- 9 g
- 30 g
- 15 g
Show answer and explanation
Answer: D. 15 g
Weber's law says the just noticeable difference is a constant fraction of the baseline stimulus. Here the fraction is 6 / 200 = 0.03, or 3 percent. Three percent of 500 g is 15 g. Answering 6 g assumes a constant absolute difference, which is exactly what Weber's law rejects, and 30 g doubles the fraction.
Question 3
Patients with lesions at different sites in the visual pathway were tested.
| Patient | Visual field finding |
|---|---|
| 1 | Total loss of vision in the right eye only |
| 2 | Loss of the outer (temporal) half of the visual field in both eyes |
| 3 | Loss of the left half of the visual field in both eyes |
Which lesion best explains the finding in Patient 3?
- Right optic tract
- Left optic tract
- Right optic nerve
- Midline of the optic chiasm
Show answer and explanation
Answer: A. Right optic tract
The left visual field of both eyes is processed by the right hemisphere: it falls on the temporal retina of the right eye and on the nasal retina of the left eye, whose fibers cross at the chiasm. Both sets travel together in the right optic tract, so a lesion there removes the left field in both eyes. A right optic nerve lesion blinds only the right eye (Patient 1), and a midline chiasm lesion cuts crossing nasal fibers, causing the outer field loss seen in Patient 2.
Question 4
A worker exposed to loud noise for years has lost hair cells near the base of the cochlea but not near the apex. Which hearing change is most likely?
- Loss of low-frequency hearing only
- Loss of high-frequency hearing
- Loss of all hearing in that ear
- Loss of balance without hearing loss
Show answer and explanation
Answer: B. Loss of high-frequency hearing
The basilar membrane is tonotopic. Near the base it is narrow and stiff and vibrates most for high frequencies, while near the apex it is wide and floppy and responds to low frequencies. Losing basal hair cells therefore removes high-pitched hearing while low pitches are spared. Balance depends on the vestibular hair cells, not the cochlea.
Question 5
When stereocilia of a cochlear hair cell bend toward the tallest stereocilium, mechanically gated channels open and K+ flows into the cell, depolarizing it. Why does K+ move inward here, when K+ usually leaves neurons?
- The hair cell has no Na+/K+ ATPase
- K+ is pumped in by the channels using ATP
- The perilymph surrounding the cell body is rich in K+
- The endolymph bathing the stereocilia has an unusually high K+ concentration
Show answer and explanation
Answer: D. The endolymph bathing the stereocilia has an unusually high K+ concentration
The tops of hair cells face endolymph, a fluid with a very high K+ concentration and a positive potential, so opening channels there lets K+ flow down its electrochemical gradient into the cell and depolarize it. The cell body sits in perilymph, which resembles ordinary extracellular fluid with low K+. Ion channels do not use ATP to move ions.
Question 6
A person standing in an elevator feels the elevator start to accelerate upward. Which structures detect this movement?
- The semicircular canals
- The utricle and saccule
- The organ of Corti
- The tympanic membrane
Show answer and explanation
Answer: B. The utricle and saccule
The utricle and saccule are otolith organs: dense crystals on a gel layer lag behind during linear acceleration and bend hair cells, and they also signal head tilt relative to gravity. The saccule is especially sensitive to vertical acceleration. The semicircular canals detect rotational movement of the head, and the organ of Corti and tympanic membrane serve hearing.
Question 7
Signals from each of the following senses relay through the thalamus before reaching the cerebral cortex EXCEPT:
- Vision
- Hearing
- Smell
- Touch
Show answer and explanation
Answer: C. Smell
Olfactory neurons send axons through the cribriform plate to the olfactory bulb, which projects directly to olfactory cortex and limbic areas without a thalamic relay. Vision relays in the lateral geniculate nucleus, hearing in the medial geniculate nucleus, and touch in the ventral posterior nuclei of the thalamus.
Question 8
A receptor in the skin fires a burst of action potentials when steady pressure is first applied, stops firing while the pressure is held constant, and fires again briefly when the pressure is removed. How is this receptor best classified?
- A rapidly adapting (phasic) mechanoreceptor
- A slowly adapting (tonic) mechanoreceptor
- A thermoreceptor
- A nociceptor that sensitizes over time
Show answer and explanation
Answer: A. A rapidly adapting (phasic) mechanoreceptor
Firing only at the onset and offset of a stimulus is the signature of a rapidly adapting, or phasic, receptor, which signals change rather than a steady state; Pacinian corpuscles behave this way and suit vibration. A tonic receptor would keep firing throughout the held pressure. The stimulus is mechanical, so thermoreceptors and nociceptors do not fit.
Question 9
When a person shifts focus from a distant mountain to a book held close to the face, which change occurs in the eye?
- The ciliary muscle relaxes and the lens flattens
- The pupil dilates through sympathetic activity
- The cornea changes curvature to increase its refractive power
- The ciliary muscle contracts, the suspensory fibers slacken, and the lens becomes rounder
Show answer and explanation
Answer: D. The ciliary muscle contracts, the suspensory fibers slacken, and the lens becomes rounder
For near vision, parasympathetic signals contract the ring-shaped ciliary muscle, which reduces tension on the suspensory fibers (zonules). The elastic lens then rounds up and bends light more strongly. Relaxation of the ciliary muscle flattens the lens for distant vision, near focus is usually paired with pupil constriction, and the cornea's curvature is fixed.
Question 10
On a dark night, a faint star is easier to see when a person looks slightly to one side of it rather than straight at it. What best explains this?
- The fovea contains only rods, which are less sensitive than cones
- The optic disc lies at the center of the visual field
- Rods, which are far more sensitive in dim light, are concentrated outside the fovea
- Cones in the periphery are more sensitive to dim light than rods
Show answer and explanation
Answer: C. Rods, which are far more sensitive in dim light, are concentrated outside the fovea
Looking straight at an object focuses it on the fovea, which is packed with cones that need relatively bright light. Rods are much more sensitive and dominate the peripheral retina, so looking slightly away puts the faint star on rod-rich retina. The fovea lacks rods, and the optic disc, the blind spot, sits on the nasal side of the retina rather than at the center of gaze.