Voltage-gated Na⁺ channels open rapidly, allowing Na⁺ to enter and depolarize the neuron. Delayed opening of voltage-gated K⁺ channels allows K⁺ to leave, repolarizing the membrane.

An afferent sensory neuron carries input to the central nervous system. An interneuron processes the input and signals an efferent motor neuron, which activates an effector to produce a response.

Acetylcholinesterase in the synaptic cleft, largely anchored to the basal lamina, rapidly hydrolyzes acetylcholine into choline and acetate. Choline is transported back into the presynaptic terminal and reused to synthesize acetylcholine.

Sympathetic activity prepares the body for stress by increasing heart rate, promoting glycogen breakdown, stimulating the adrenal medulla, and dilating the pupils. Gallbladder contraction that aids digestion is promoted by parasympathetic activity.

Most sympathetic postganglionic neurons release norepinephrine, which acts on adrenergic receptors. Sympathetic postganglionic neurons supplying sweat glands instead release acetylcholine (ACh). Preganglionic autonomic neurons, parasympathetic postganglionic neurons, and somatic motor neurons at neuromuscular junctions also release ACh.

Statements I, II, and III are correct. Nicotinic receptors conduct Na⁺ inward and K⁺ outward, producing a net inward current and a graded depolarization. An action potential occurs only if threshold is reached. Acetylcholine binding to postsynaptic receptors does not directly create presynaptic vesicles.

Axons, dendrites, and cell bodies are neuronal structures. Myelin is produced by glial cells and surrounds many, but not all, axons. An action potential is an electrical signal, not an anatomical structure.

Local anesthetics reversibly block voltage-gated Na⁺ channels. This suppresses the initiation and propagation of action potentials in sensory axons, preventing pain signals from reaching the central nervous system.

The spinal cord has central, butterfly-shaped gray matter surrounded by white matter. The cerebrum has a superficial gray-matter cortex overlying white matter; it also contains deep gray-matter nuclei. The corpus callosum is a white-matter tract connecting the cerebral hemispheres.

All listed structures and divisions are associated with the peripheral nervous system. The PNS includes sensory pathways and somatic and autonomic motor pathways. The sympathetic division belongs to the autonomic nervous system. The vagus nerve carries sensory and motor fibers, including parasympathetic fibers supplying many thoracic and abdominal organs.

Statements A–D are true, so none is an exception. Most mature neurons have limited capacity for cell division. This does not mean every injured axon is irreparable: some peripheral axons can regenerate if the cell body survives. Myelinating Schwann cells form myelin in the PNS, whereas oligodendrocytes form myelin in the CNS.

The rising phase is driven primarily by Na⁺ influx through voltage-gated Na⁺ channels. Delayed opening of voltage-gated K⁺ channels allows K⁺ efflux during repolarization. Elevated K⁺ conductance can persist during the undershoot as Na⁺ channels recover from inactivation.

Forebrain parts are cerebrum and diencephalon (thalamus and hypothalamus) and hindbrain parts are pons, medulla oblongata, and cerebellum.

Myelin electrically insulates the axon and speeds conduction. Local current spreads passively through internodes, and action potentials are regenerated at the nodes of Ranvier. Thus, reduced conduction speed is not a feature of normal saltatory conduction.

The cerebellum coordinates movement and balance. The hypothalamus helps regulate sexual drive, hunger, thirst, and other homeostatic functions. The pons and medulla contain respiratory control centers, while the cerebrum supports sensory processing and voluntary motor control.

During normal signaling, the refractory membrane behind an action potential helps maintain propagation toward the axon terminals. At a typical chemical synapse, neurotransmitter is released presynaptically and acts on postsynaptic receptors, so transmission is one-way. Artificial stimulation in the middle of an axon can produce propagation in both directions, and many electrical synapses are bidirectional.