Which structure predominates in the white matter of the brain?

Correct Answer: A

Rationale: White matter in the brain is primarily composed of myelinated axons, which are nerve fibers coated with myelin to speed up signal transmission. Neuronal cell bodies dominate gray matter, not white matter. Ganglia and dendrites from the enteric nervous system are not primary components of brain white matter.

Correct Answer: B

Rationale: Voltage-gated Na⁺ channels open when the membrane potential reaches the threshold (around -55 mV), triggering an action potential. They don’t open at resting potential, during repolarization, or at overshoot.

Correct Answer: D

Rationale: Acetylcholine (ACh) is released at the neuromuscular junction to stimulate skeletal muscle contraction. Norepinephrine acts at sympathetic synapses, not the neuromuscular junction.

Correct Answer: C

Rationale: The trochlear nerve (CN IV) is the smallest cranial nerve by axon count and diameter, innervating only the superior oblique muscle. The olfactory nerve (I) has many fibers, and oculomotor (III) and abducent (VI) are larger.

Correct Answer: D

Rationale: The facial nerve (CN VII) motor nucleus is in the pons, controlling facial expression muscles. The hypothalamus and midbrain are incorrect, and the third ventricle floor is unrelated.