Questions: Neuron Structure and Function

Question 1

At which location on a neuron is the action potential typically initiated?

• A. At the tips of the dendrites, where signals are received
• B. At the soma, where the nucleus integrates information
• C. At the axon hillock, where the axon emerges from the soma
• D. At the axon terminal, just before the synapse

Answer: (C)

The axon hillock has the highest density of voltage-gated sodium channels and the lowest threshold for firing. Incoming signals from dendrites and the soma summate at the axon hillock; if the combined depolarization reaches threshold, an action potential is triggered there and propagates down the axon.

Question 2

Myelin sheaths wrap the axon in a continuous, unbroken insulating layer along its entire length.

• T. True
• F. False

Answer: False

Myelin wraps the axon in segments, with small gaps called nodes of Ranvier left exposed between each segment. These gaps are essential: voltage-gated sodium channels are concentrated at the nodes, and the action potential 'jumps' from node to node (saltatory conduction). A completely unbroken myelin sheath would actually prevent propagation.

Question 3

How does myelination increase the speed of action potential propagation along an axon?

Model answer: Myelin electrically insulates the axon between nodes of Ranvier, preventing ion leakage and forcing the action potential to 'jump' from one node to the next (saltatory conduction) rather than propagating continuously along the entire membrane. This is much faster because the electrical signal spreads passively and rapidly through the insulated segments, and regeneration only needs to occur at the widely spaced nodes.

Unmyelinated axons depolarize every patch of membrane in sequence, which is slow and metabolically expensive. Saltatory conduction reduces both the time and energy cost of propagation. This is why demyelinating diseases like multiple sclerosis — where myelin is damaged — cause profound deficits in motor and sensory function.