MCAT content library / Biology & biochemistry / Action potentials: distinguish firing, recovery and conduction speed
Separate a spike’s amplitude, timing, refractory behavior, and travel speed using original traces and paired recordings.
About 16 minutes, including the worked examples and checks.
From a membrane to a neural signal · Lesson 5 of 6 · Continue in this path
Useful foundations
On this page
- Concept: a spike is a regenerative conductance change
- Pattern: separate spike height from its time course
- Worked application: probe recovery with a second stimulus
- Rule: count spikes over time to assess firing rate
- Application: infer speed from the difference in arrival times
- Check your reasoning
- Continue learning
- Sources
Concept: a spike is a regenerative conductance change
An action potential is a rapid, self-propagating change in membrane voltage. In the familiar neuronal model, depolarization increases the opening of voltage-gated sodium channels; inward sodium current produces further depolarization. Once the regenerative response is recruited under the stated conditions, increasing the triggering stimulus does not simply scale the spike like a louder sound wave. This is the basis of all-or-none firing.2
Threshold is the point at which the relevant inward and outward current dynamics permit that regenerative response. Its numerical value depends on the cell and its recent state. A passage may supply a threshold for a simplified comparison; do not turn that value into a universal constant for every neuron.
Sodium-channel inactivation and the delayed rise of potassium conductance end the rising phase and promote repolarization. Continued potassium conductance can produce an afterhyperpolarization. These rapid events use existing ion gradients. The pump maintains gradients over time; it is not the channel mechanism that directly produces each millisecond rise and fall.3
Pattern: separate spike height from its time course
The traces, measurements, and perturbations here are original instructional examples. The altered condition selectively reduces delayed potassium conductance while the initial sodium response remains adequate to produce a spike. The following values define two schematic traces; straight lines between them are for visualization, not a fitted membrane model.
| Time, ms | Control Vm, mV | Altered Vm, mV |
|---|---|---|
| 0 | -70 | -70 |
| 1 | -70 | -70 |
| 1.5 | -55 | -55 |
| 2 | 30 | 30 |
| 2.5 | 10 | 20 |
| 3 | -65 | -10 |
| 4 | -80 | -60 |
| 5 | -70 | -70 |
Both traces begin at −70 mV and reach +30 mV, so the baseline-to-peak change is 100 mV in both. At 3 ms, however, control has returned to −65 mV while the altered trace is still at −10 mV. The supplied reduction in outward potassium conductance is consistent with that slower falling phase. The altered condition also lacks the control trace’s −80 mV undershoot.
Without the stipulated selectivity of the perturbation, the waveform alone would not prove a particular channel defect. Several current changes can broaden a spike. The useful conclusion is that amplitude and duration are distinct measurements, and a known current perturbation predicts a particular phase change.
Worked application: probe recovery with a second stimulus
After a spike, sodium channels temporarily remain unavailable because of inactivation, and elevated potassium conductance can continue opposing depolarization. These changing conditions create refractory behavior and limit firing frequency.4
| Interval after first upstroke | Ordinary test pulse | Stronger test pulse |
|---|---|---|
| 0.6 ms | No second spike | No second spike |
| 2.0 ms | No second spike | Second spike |
| 5.0 ms | Second spike | Second spike |
The 0.6 ms result is consistent with an absolute refractory interval: the membrane cannot generate a new propagated spike through the usual sodium-channel mechanism at that point. The 2.0 ms result is consistent with relative refractoriness, because a stronger pulse succeeds when the ordinary pulse does not. By 5.0 ms, the ordinary test pulse is again effective in this preparation.
These observations bound recovery for the tested conditions; they do not establish identical refractory durations in every neuron. “No second spike” with only one weak pulse would also be insufficient to distinguish absolute refractoriness from a raised excitation requirement. The stronger-pulse comparison supplies the missing distinction.
Rule: count spikes over time to assess firing rate
In a separate 0.10 s observation window, one stimulus condition produces three spikes and another produces six. Their mean rates over that window are 30 and 60 spikes/s. If both conditions retain a 100 mV spike amplitude, the increased stimulus is represented by more spikes per unit time in this example, not taller individual spikes.
All-or-none describes recruitment under specified conditions. It does not guarantee that spike amplitude survives every change in extracellular sodium, channel availability, temperature, or recording position. Compare like with like, and distinguish a single-fiber intracellular spike from a compound extracellular response involving many axons.
Application: infer speed from the difference in arrival times
A local current from an active region depolarizes neighboring excitable membrane, where voltage-gated channels regenerate the signal. Myelin reduces current leakage across internodes and permits rapid spread between nodes. In saltatory conduction, the regenerative events occur at nodes while current still spreads through the intervening continuous axon; the signal does not teleport across empty space.5
Researchers record the same axon 2 cm and 8 cm from a stimulation site. Spikes arrive at 1.4 ms and 2.4 ms. The extra 6 cm takes 1.0 ms, so v = 0.060 m/0.0010 s = 60 m/s. Using the difference cancels a shared delay between the stimulus and actual spike initiation.
After a localized insulation change between the electrodes, arrivals are 1.4 ms and 3.4 ms, with spikes still reaching both sites. The extra segment now takes 2.0 ms, giving 30 m/s. Conduction speed has halved even if the successfully propagated spikes retain their amplitude. More severe disruption could prevent a distal spike, in which case one cannot assign its arrival time from a missing response.
Normally, the region just activated is refractory, helping the spike advance into excitable membrane ahead. Artificial stimulation in the middle of a resting axon can produce propagation in both directions. The usual physiological direction of signal flow is therefore not evidence that an isolated axonal segment is structurally capable of conducting in only one direction.4
The AAMC includes threshold, firing frequency, myelin, nodes, and propagation. For a new trace, identify the baseline and axes, compare the affected phase, then separate three questions: was a spike initiated, how long did it last, and how quickly did it reach the next recording site?1
ACTIVE PRACTICE · EVIDENCE TO INTERPRETATION
Separate Refractoriness From Conduction Speed
The original paired-stimulus experiments test recovery at 0.6, 2.0, and 5.0 ms. The separate axon experiment records responses 2 cm and 8 cm from the stimulus.
Review the reasoning boundary
Interpret only the stipulated experiment and its controls. The exercise provides teaching feedback, not a calibrated assessment score.
Check your reasoning
Choose an answer before opening its feedback. Each option explains the reasoning behind that choice.
Check 1
In a stipulated neuronal model, a perturbation reduces delayed outward potassium current after the spike peak while leaving the initial inward sodium current intact. Which immediate waveform change is most directly predicted?
A. A slower return toward the resting voltage after the peak.
Correct. Reducing the delayed outward current makes repolarization slower under the supplied conditions.
B. A loss of the initial inward current that produces the upstroke.
Review this choice. The stem explicitly preserves the initial inward current.
C. An instantaneous equalization of sodium concentrations across the membrane.
Review this choice. A channel-current change does not instantly eliminate the bulk concentration gradient.
D. A proportional doubling of spike amplitude whenever stimulus strength doubles.
Review this choice. All-or-none recruitment does not imply a linear stimulus-to-amplitude relationship.
The principle to carry forward
Connect the stated current change to the phase during which that current contributes.
Check 2
For one preparation, the supplied absolute refractory interval lasts from the first upstroke until 1.5 ms afterward. A second ordinary electrical test pulse is delivered 1.0 ms after that upstroke. Which result follows from this stipulated interval?
A. A stronger second pulse elicits another spike because partial recovery is sufficient.
Review this choice. That pattern describes relative refractoriness. The supplied time lies within the absolute interval, before the ordinary regenerative response is available.
B. The ordinary pulse elicits another spike because the bulk ion gradients remain.
Review this choice. Maintained gradients do not eliminate channel inactivation. Channel availability, not instant depletion of the gradient, limits firing during this interval.
C. No second propagated spike through the usual sodium-channel mechanism during that interval.
Correct. The second pulse occurs within the specified absolute interval, when the usual regenerative response is unavailable.
D. The second pulse elicits a spike whose amplitude scales directly with pulse strength.
Review this choice. This confuses graded responses with action-potential recruitment and also ignores the stipulated absolute interval.
The principle to carry forward
Use the explicitly supplied recovery interval rather than importing a universal refractory duration.
Check 3
Two recording sites on the same axon are 4.0 cm apart. Their arrival-time difference increases from 0.80 ms to 1.60 ms after a perturbation. Successful spikes retain the same baseline-to-peak amplitude. What change is established by these measurements?
A. Speed falls from 50 m/s to 25 m/s, while measured spike amplitude is unchanged.
Correct. 0.040 m/0.00080 s = 50 m/s and 0.040 m/0.00160 s = 25 m/s. The longer delay halves speed at a fixed distance.
B. Speed rises from 25 m/s to 50 m/s, while measured spike amplitude is unchanged.
Review this choice. A larger travel time at fixed distance means slower, not faster, propagation.
C. Speed is unchanged, while spike amplitude must fall by half.
Review this choice. The stem directly reports unchanged amplitude and a changed arrival-time difference.
D. Speed falls by fourfold because both time values are in milliseconds.
Review this choice. Converting both times consistently preserves their twofold ratio; it does not create a fourfold change.
The principle to carry forward
Calculate distance over the difference in arrival times, and keep amplitude as a separate observation.
Continue learning
From a membrane to a neural signal
Lesson 5 of 6.
- Previous in From a membrane to a neural signal: Resting membrane potential: balance forces, currents and gradients
- Next in From a membrane to a neural signal: Synaptic transmission: locate the change and predict the response
- Return to this path’s overview
See every step in From a membrane to a neural signal
From a membrane to a neural signal
Connect membrane properties and gradients to signal propagation and synaptic integration.
- Lipids and membrane properties
- Membrane transport and osmosis
- Neuron structure: locate a failure in input, conduction or output
- Resting membrane potential: balance forces, currents and gradients
- Action potentials: distinguish firing, recovery and conduction speed · You are here
- Synaptic transmission: locate the change and predict the response
Related lessons
Browse biology & biochemistry · MCAT content library
MCAT preparation and roadmap · Aim High, Score Higher
Sources
1. AAMC: B/B content category 3A
2. OpenStax Anatomy and Physiology 2e: The Action Potential
3. OpenStax Biology 2e: How Neurons Communicate
4. Purves et al.: The Refractory Period
5. Purves et al.: Increased Conduction Velocity as a Result of Myelination