Vision

The oculomotor nerve supplies four of the six eye muscles and the muscle that raises the upper lid, and carries parasympathetic fibers that narrow the pupil. The optic nerve is sensory, and the abducens supplies only the muscle that turns the eye outward.

• Optic nerve (II):
• Correct: Oculomotor nerve (III):
• Abducens nerve (VI):

The primary visual cortex lies in the occipital lobe at the back of the brain. The postcentral gyrus holds the primary somatosensory cortex, and the temporal lobe holds the primary auditory cortex.

• Correct: In the occipital lobe:
• In the postcentral gyrus:
• In the temporal lobe:

An activated G protein–coupled receptor protein switches on a G protein inside the cell, which switches an enzyme on or off and so changes the level of a second messenger.

• It opens as an ion channel:
• Correct: It switches on a G protein, which changes the activity of an enzyme:
• It moves into the nucleus:

The optic disc is where the ganglion cell axons leave the eye as the optic nerve and the central retinal vessels enter. It has no photoreceptors, so it makes the blind spot.

• Fovea centralis: The fovea is the small cone-only pit of sharpest vision, beside the optic disc rather than where the nerve leaves.
• Ciliary body: The ciliary body is the ring of muscle and folds around the lens at the front of the eye.
• Correct: Optic disc: Correct. The spot where the optic nerve leaves the eye is the optic disc, the blind spot.
• Choroid: The choroid is the dark, vessel-rich middle layer of the wall, not a spot on the retina.

Near vision needs more bending. Parasympathetic fibers in the oculomotor nerve contract the ring-shaped ciliary muscle, which shrinks toward the lens. The zonule fibers go slack, and the elastic lens rounds up and refracts more.

• The ciliary muscle relaxes, the zonule fibers tighten and the lens flattens: That is what happens when you look from near to far, not far to near.
• The ciliary muscle contracts, the zonule fibers tighten and pull the lens rounder: Taut zonule fibers pull the lens flat, not round. When the ciliary muscle contracts, the fibers slacken.
• The cornea bulges forward and bends light more strongly: The cornea does most of the bending, but its shape is fixed. Only the lens changes for near vision.
• Correct: The ciliary muscle contracts, the zonule fibers go slack and the lens rounds up: Correct. Slack zonules let the elastic lens round up.

In a long eye (myopia), the cornea and lens bring distant light to a focus before it reaches the retina. A concave lens spreads the rays slightly first, which moves the focus back onto the retina.

• Behind the retina; a convex lens: A focus behind the retina is hyperopia, caused by an eye that is too short.
• In front of the retina; a convex (converging) lens: The focus is in front of the retina, but a convex lens would add bending and move it even farther forward.
• Behind the retina; a concave lens: A long eye puts the focus in front of the retina, not behind it.
• Correct: In front of the retina; a concave lens: Correct. Myopia: focus in front of the retina, corrected with a diverging (concave) lens.

The fovea holds only tightly packed cones. Each foveal cone connects through its own bipolar cell to its own ganglion cell, so its signal reports one tiny spot, and light reaches the cones without passing through the other layers.

• Rods there feed many ganglion cells each, which multiplies their signal: The fovea has no rods, and rods converge many-to-one onto ganglion cells, which lowers acuity rather than raising it.
• It lies closest to the lens, so the image there is least blurred: The fovea is at the back of the eye, not especially close to the lens; its sharpness comes from its wiring.
• Correct: Each cone has its own ganglion cell, and the layers above are pushed aside: Correct. Low convergence and an unobstructed path give the fovea its acuity.
• It has the most blood vessels, so its photoreceptors receive the best oxygen supply: Blood vessels are kept away from the fovea; vessels in front of it would block light.

In photoreceptors, light turns a signal off. It lowers cGMP, closes the cation channels that carried the dark current, hyperpolarizes the rod and reduces its glutamate release.

• cGMP in the outer segment: down. Active rhodopsin switches on transducin, which switches on the enzyme that breaks down cGMP.
• Number of open cation channels: down. These channels are held open by cGMP, so they close as cGMP falls.
• Membrane potential (how positive the inside is): down. With the Na+ inflow cut off while K+ still leaks out, the rod hyperpolarizes from about −40 toward −70 mV.
• Glutamate release onto bipolar cells: down. A hyperpolarized rod releases less neurotransmitter; the fall in glutamate is the light signal.

The outer half of each eye's field is seen by the nasal half of each retina. Those are exactly the axons that cross in the middle of the optic chiasm. A pituitary tumor growing up from below typically presses there, causing bitemporal hemianopia.

• Correct: The middle of the optic chiasm: Correct. Crossing fibers from both nasal retinas run through the middle of the chiasm.
• The left optic tract: A left optic tract lesion removes the right half of the field in both eyes, not the outer half of each.
• The right optic nerve: A right optic nerve lesion blinds the right eye only.
• The occipital lobe on both sides: Damage to both occipital lobes would cause far wider loss; a single mass pressing on one structure fits the chiasm.

When the right eye is lit, both pupils narrow, so both efferent paths (both oculomotor nerves and both iris sphincters) work. When the left eye is lit, nothing happens, so the signal never gets in: the afferent path from the left eye, the left optic nerve, is damaged.

• Left oculomotor nerve: A left oculomotor lesion would leave the left pupil wide whichever eye is lit, but here it narrows when the right eye is lit.
• Right oculomotor nerve: A right oculomotor lesion would stop the right pupil narrowing, but it narrows when the right eye is lit.
• Correct: Left optic nerve: Correct. Light in the damaged eye cannot start the reflex; light in the healthy eye drives both pupils.
• Right optic chiasm: The chiasm has no separate "right" half for one eye's pupil fibers, and damage there would not silence one eye's input completely.

The ventral stream, running from the occipital lobe forward into the temporal lobe, identifies what things are, including faces. His ability to locate and grasp the cup shows that the dorsal stream still works.

• The dorsal stream in the parietal lobe: The dorsal stream guides reaching and locating, which he does well.
• The lateral geniculate nucleus: Damage to the lateral geniculate nucleus would cause loss of part of the visual field, not a loss of recognition with normal reaching.
• The optic chiasm: Chiasm damage causes field loss, typically the outer halves, not a loss of object recognition.
• Correct: The ventral stream in the temporal lobe: Correct. Recognizing objects and faces is the job of the ventral "what" stream.