MCAT - Organ Systems:: Nervous System

Which part of the neuron is primarily responsible for receiving synaptic input from other neurons?

- **A)** Dendrites
- **B)** Axon terminal
- **C)** Myelin sheath
- **D)** Node of Ranvier

The resting membrane potential of a typical neuron (around -70 mV) is established primarily by:

- **A)** Active transport of glucose across the membrane
- **B)** Equal permeability to all ions
- **C)** The Na+/K+-ATPase pump maintaining ion gradients, combined with the membrane's much higher resting permeability to K+ than Na+ (via leak channels)
- **D)** Complete impermeability of the membrane to all ions

An action potential's rapid depolarizing upstroke is caused by:

- **A)** Efflux of K+ through voltage-gated K+ channels
- **B)** A rapid influx of Na+ through voltage-gated Na+ channels that open once threshold is reached
- **C)** Influx of Cl- through ligand-gated channels
- **D)** Active pumping of Na+ into the cell by the Na+/K+-ATPase

The absolute refractory period, during which a neuron cannot fire another action potential no matter how strong the stimulus, is due to:

- **A)** Complete closure of all ion channels
- **B)** Hyperpolarization caused by Cl- influx
- **C)** Depletion of ATP within the axon
- **D)** Voltage-gated Na+ channels being inactivated (in a distinct inactivated state, not simply closed) and unable to reopen until the membrane repolarizes

Saltatory conduction, in which action potentials appear to 'jump' between nodes of Ranvier, occurs because:

- **A)** Myelin insulates the internodal membrane, so voltage-gated Na+ channels are concentrated at the nodes, and the action potential regenerates only at these nodes, greatly increasing conduction velocity
- **B)** Myelin actively pumps the action potential along the axon
- **C)** Unmyelinated regions conduct faster than myelinated ones
- **D)** Saltatory conduction only occurs in unmyelinated C fibers

At a typical chemical synapse, arrival of an action potential at the presynaptic terminal triggers neurotransmitter release via which sequence?

- **A)** Direct electrical current flow into the postsynaptic cell without any chemical intermediate
- **B)** Neurotransmitter diffusing continuously regardless of electrical activity
- **C)** Voltage-gated Ca2+ channels open, Ca2+ influx triggers synaptic vesicle fusion with the presynaptic membrane, releasing neurotransmitter into the synaptic cleft by exocytosis
- **D)** Sodium channels on the postsynaptic membrane open directly in response to presynaptic depolarization

An excitatory postsynaptic potential (EPSP) is typically produced by neurotransmitter binding that causes:

- **A)** Opening of ligand-gated cation channels (e.g., permeable to Na+), depolarizing the postsynaptic membrane toward threshold
- **B)** Opening of Cl- channels, hyperpolarizing the membrane
- **C)** Closure of all postsynaptic ion channels
- **D)** Opening of K+ channels exclusively, hyperpolarizing the membrane

An inhibitory postsynaptic potential (IPSP) is typically produced by opening of which type of channel?

- **A)** Voltage-gated Na+ channels
- **B)** Ligand-gated Cl- channels (e.g., GABA-A receptors) or K+ channels, which hyperpolarize or stabilize the membrane away from threshold
- **C)** Voltage-gated Ca2+ channels in the postsynaptic membrane
- **D)** Aquaporin water channels