Jump Gaps, Set the Pace

Science · The Human Body · ages 17-18

Name ______________________ Date ____________

Two axons have the same myelination and temperature. Why does the wider one usually conduct faster?
- Its sodium channels remain open between impulses
- Its internal resistance to current is lower
- Its voltage-gated sodium channels need no stimulus
Why does myelination usually speed conduction compared with an unmyelinated axon of the same diameter?
- Current spreads between nodes, where spikes regenerate
- Myelin grows new sodium channels during every impulse
- Myelin carries each spike without any current in the axon
What defines the absolute refractory period in an axon membrane region?
- A stronger stimulus can fire another spike while potassium channels remain open
- Inactivated sodium channels prevent another spike, even with a strong stimulus
- Myelin must grow back around the membrane before another spike can occur
Assume the minimum interval between the starts of two spikes is 2 milliseconds. There are 1000 milliseconds in one second. What is the theoretical maximum number of spikes per second?
Answer: ______________
During normal propagation, why does a spike not immediately re-excite the membrane just behind it?
- Myelin allows current to flow only toward the axon terminals
- All sodium ions behind the spike have been used up completely
- Recently active membrane is refractory and cannot fire again yet
A healthy axon is warmed moderately within its physiological temperature range. What usually happens to conduction?
- Channel gating becomes slower, reducing conduction speed
- Sodium channels stop recovering, preventing every later spike
- Channel gating becomes faster, speeding conduction
During the relative refractory period, no stimulus can trigger another action potential in that membrane region. This claim is true.
Circle one: True False
An isolated axon is resting throughout. A researcher triggers an action potential in its middle. How can it initially spread?
- Only toward the terminals, because myelin sets direction
- In both directions into excitable membrane
- Only toward the cell body, because channels face that way

Answer key

For grown-ups. Fold this page away before handing over the rest.

Jump Gaps, Set the Pace W1-mt_x3M0sv6oD6-s1

Its internal resistance to current is lower · Greater diameter lowers internal resistance, allowing local current to spread more readily along the axon.
Current spreads between nodes, where spikes regenerate · Myelin reduces current leak and capacitance. Current spreads between nodes, where concentrated sodium channels regenerate the action potential.
Inactivated sodium channels prevent another spike, even with a strong stimulus · Sodium-channel inactivation prevents another action potential during the absolute phase. A stronger stimulus can work during the later relative phase.
500 · Divide the time available by the minimum start-to-start interval: 1000 / 2 = 500 spikes per second. This is a theoretical upper limit.
Recently active membrane is refractory and cannot fire again yet · Current spreads both ways, but membrane just behind the spike is refractory. Resting membrane ahead can regenerate the signal.
Channel gating becomes faster, speeding conduction · Moderate warming within the stated range usually speeds channel gating and conduction. This does not predict the effects of excessive heating.
False · False. A stronger stimulus can trigger another spike during the relative phase. No stimulus can do so during the absolute phase.
In both directions into excitable membrane · Both neighbouring regions are initially excitable, so a middle stimulus can launch spikes in both directions. Each leaves refractory membrane behind it.