Explain resting potential, threshold, depolarization, repolarization, refractory periods and saltatory conduction.
Estimated time: 110 minutes
IB syllabus: C2.2 · SL and HL
An unstimulated neuron typically has a membrane potential near −70 mV: the cytoplasmic side is negative relative to the outside. The value varies among cell types. Unequal ion distributions, selective resting permeability and intracellular negative molecules all contribute. The sodium–potassium pump uses ATP to export three Na⁺ for every two K⁺ imported, maintaining the concentration gradients on which repeated signalling depends.
At rest, K⁺ leak channels make the membrane more permeable to K⁺ than to Na⁺. K⁺ tends to leave down its concentration gradient, leaving unbalanced negative charge behind, while the growing electrical attraction opposes further loss. Membrane voltage reflects this balance of chemical and electrical forces. The pump is essential over time, but the rapid phases of one action potential are produced mainly by ions moving through voltage-gated channels.
A stimulus that depolarizes the membrane to threshold—often around −55 mV—opens voltage-gated Na⁺ channels. Na⁺ enters down its electrochemical gradient, causing further depolarization and opening more channels. This positive feedback produces the steep rising phase and an overshoot above 0 mV. A sub-threshold change decays; a threshold-crossing change produces a stereotyped spike. Stronger stimuli increase firing frequency, not the amplitude of each all-or-none spike.
Na⁺ channels then inactivate and slower voltage-gated K⁺ channels open. K⁺ leaves, repolarizing the membrane. Because K⁺ permeability remains high briefly, the voltage becomes more negative than resting potential: hyperpolarization. As K⁺ channels close and resting permeabilities dominate again, the membrane returns toward rest. Only a tiny fraction of the total ions crosses during one spike, so the bulk gradients change very little.
Na⁺ entry in one axon region creates local current that depolarizes adjacent membrane to threshold. New voltage-gated channels open there, regenerating the action potential rather than letting one pulse of ions travel from the cell body to the terminal. Propagation is normally one-way because recently active Na⁺ channels are inactivated during the absolute refractory period. During the later relative refractory period, hyperpolarization means an unusually strong stimulus is required.
Myelinated axons conduct impulses much faster than non-myelinated fibres of similar diameter. Myelin reduces current loss between exposed nodes, so the signal can cover longer distances between regions where the action potential is regenerated. Increasing axon diameter also increases speed by reducing internal resistance.
Myelin increases membrane resistance and reduces capacitance across internodes, so local current spreads farther and charges the next node rapidly. Voltage-gated Na⁺ channels are concentrated at nodes of Ranvier, where action potentials are regenerated. The impulse appears to jump between nodes—saltatory conduction—although current still spreads continuously through the axoplasm beneath myelin.
Giant squid achieve relatively rapid conduction with exceptionally wide unmyelinated axons, but myelinated vertebrate axons achieve higher speeds with far less space. Conduction speed is positively correlated with axon diameter, and myelinated fibres are faster than non-myelinated fibres of comparable size. A correlation describes the relationship; it does not by itself identify every causal mechanism.
On a voltage–time graph, identify baseline resting potential, threshold, rising depolarization, peak, falling repolarization, undershoot and recovery. Do not infer propagation speed from one trace unless distance is also known. Conduction velocity requires the separation between two recording sites divided by the difference in arrival time; spike duration at a single site is a different measurement.
Move through the voltage trace, vary stimulus strength and compare continuous with saltatory conduction.
Detect · transduce · integrate · respond
An action potential reaches two electrodes 0.24 m apart at 1.8 ms and 4.8 ms. What is the conduction velocity?
Tetrodotoxin blocks voltage-gated Na⁺ channels but does not block the sodium–potassium pump. Which immediate result is expected in an exposed axon region?
Practice focused questions or see how IB combines this topic with ideas from elsewhere in the course.
Matching parts: 3(a), 3(b)
Matching part: 24