MCQs

The threshold potential (usually around -55 mV) is the tipping point where voltage-gated Na⁺ channels open en masse, triggering the explosive positive feedback loop of an action potential.

Following peak depolarization, voltage-gated K⁺ channels open and K⁺ rushes out of the cell down its electrochemical gradient, restoring the negative internal charge during repolarization.

Because a neuron cannot fire another action potential during the absolute refractory period regardless of stimulus strength, this period sets a strict upper limit on how frequently the neuron can fire.

Electrical synapses do not use neurotransmitters; instead, they utilize gap junctions (connexons) that physically connect the cytoplasm of adjacent cells, allowing instantaneous, bidirectional current flow.

Neuromuscular junctions (motor end plates) are the highly specialized synapses where motor neurons release acetylcholine to trigger skeletal muscle contraction.

Norepinephrine (and epinephrine from the adrenal medulla) acts on sympathetic target organs to mediate the 'fight or flight' stress responses, such as increased heart rate and bronchodilation.

Depolarization refers to any shift in membrane potential toward a less negative (or more positive) value, generally caused by the influx of positive ions like sodium.

Synaptic fatigue occurs when the rate of neurotransmitter release exceeds the rate of its synthesis and recycling, causing a temporary failure of synaptic transmission despite incoming action potentials.

Conduction velocity is positively correlated with both axon diameter (reducing internal electrical resistance) and the presence of myelination (allowing fast saltatory conduction).

The Na⁺/K⁺ pump is an active transport ATPase that hydrolyzes ATP to move sodium and potassium against their concentration gradients, consuming a massive portion of the neuron's metabolic energy.

The patellar reflex is a monosynaptic spinal stretch reflex mediated specifically by the femoral nerve and the lumbar spinal cord segments L2, L3, and L4.

The efferent (motor) pathway carries signals away from the CNS to the target effectors (muscles or glands). The afferent (sensory) pathway carries signals toward the CNS.

Reciprocal inhibition ensures coordinated movement by using inhibitory interneurons to relax antagonistic muscles while excitatory pathways stimulate the agonist muscles.

Contralateral reflex arcs (e.g., the crossed extensor reflex) involve interneurons that cross the spinal cord midline to produce a motor response on the side of the body opposite to the stimulus.

GABA is the major inhibitory neurotransmitter in the brain, functioning primarily by opening chloride channels on postsynaptic neurons to induce hyperpolarization. Glutamate is the major excitatory neurotransmitter.

Temporal summation occurs when a single presynaptic neuron fires multiple times in rapid succession, allowing the resulting graded potentials to add together before the previous ones dissipate.

Multiple sclerosis is an autoimmune disorder characterized by the progressive demyelination of axons in the CNS, which disrupts saltatory conduction and leads to severe neurological deficits.

Because the membrane segment immediately behind the advancing action potential is in its refractory period (Na⁺ channels are inactivated), the impulse cannot spread backward, ensuring unidirectional propagation.

Nodes of Ranvier are the unmyelinated gaps between adjacent myelin segments (internodes) where action potentials are regenerated during saltatory conduction.