Neural signalling practice questions.

Saltatory conduction increases the speed of nerve impulses because the action potential:

• Travels continuously along the whole membrane
• Jumps between nodes of Ranvier
• Moves in both directions at once
• Is generated only at the synapse

Answer: Jumps between nodes of Ranvier

Myelin insulates the axon so ion exchange occurs only at gaps called nodes of Ranvier. The impulse effectively jumps from node to node, greatly increasing conduction velocity.

Which neurotransmitter is released at neuromuscular junctions to stimulate skeletal muscle?

• Dopamine
• Acetylcholine
• Serotonin (5-HT)
• GABA

Answer: Acetylcholine

Acetylcholine is released from motor neuron endings at the neuromuscular junction, binding receptors on the muscle fibre and triggering depolarisation and contraction. It is then broken down by acetylcholinesterase.

The myelin sheath around a neuron

• slows the nerve impulse
• speeds up the impulse
• produces neurotransmitters
• detects stimuli

Answer: speeds up the impulse

Myelin insulates the axon, allowing saltatory conduction that speeds transmission of the impulse.

During the depolarization phase of an action potential, which ion movement is primarily responsible for the rapid change in membrane potential?

• Na⁺ diffusing into the axon through voltage-gated channels
• K⁺ diffusing out of the axon through voltage-gated channels
• Cl⁻ diffusing into the axon through leak channels
• Ca²⁺ being pumped out of the axon by active transport

Answer: Na⁺ diffusing into the axon through voltage-gated channels

Depolarization occurs when voltage-gated sodium channels open and Na⁺ flows into the axon, making the inside more positive; potassium efflux causes the later repolarization phase.

A neuron has a threshold potential of −55 mV. A stimulus depolarizes the membrane from −70 mV to only −60 mV. Determine the most likely outcome.

• A full action potential of normal amplitude is triggered
• No action potential is triggered because threshold is not reached
• An action potential of reduced amplitude is triggered
• The membrane potential becomes permanently fixed at −60 mV

Answer: No action potential is triggered because threshold is not reached

Action potentials are all-or-nothing; because the depolarization to −60 mV does not reach the −55 mV threshold, no action potential is generated.

State the role of the sodium–potassium pump in maintaining the resting potential of a neuron.

• It moves 3 Na⁺ out and 2 K⁺ in per cycle using ATP
• It moves 3 Na⁺ in and 2 K⁺ out per cycle using ATP
• It allows Na⁺ and K⁺ to diffuse freely down their gradients
• It moves 2 Na⁺ out and 3 K⁺ in per cycle without ATP

Answer: It moves 3 Na⁺ out and 2 K⁺ in per cycle using ATP

The pump actively transports 3 Na⁺ out of and 2 K⁺ into the cell per ATP hydrolysed, establishing the concentration gradients underlying the resting potential.