Nerve Impulse Conduction: Resting and Action Potential

NCERT grounding

NCERT Class 11 Biology, Chapter 18 — Neural Control and Coordination, Section 18.3.1 — anchors this subtopic with three load-bearing statements. First: "Neurons are excitable cells because their membranes are in a polarised state." Second: the resting membrane is far more permeable to K+ than to Na+, and the sodium-potassium pump moves three Na+ out for every two K+ in. Third: when a stimulus opens Na+ channels at a site, the membrane there is depolarised, an action potential is generated, and the wave is then carried forward by local currents along the axon. NIOS Biology Chapter 17 (Section 17.6) reinforces the same mechanism and explicitly states the all-or-none and threshold rules that NEET has tested repeatedly.

"The electrical potential difference across the resting plasma membrane is called as the resting potential. The electrical potential difference across the plasma membrane at the site A is called the action potential, which is in fact termed as a nerve impulse."

NCERT Class 11 Biology · Chapter 18 · Section 18.3.1

Resting potential — the polarised axon

A neuron is not a passive wire. Even when it is not firing, the axonal membrane carries a stored electrical charge — the inside of the axon is roughly seventy millivolts negative relative to the outside, and that voltage difference is the fuel that every nerve impulse spends. NCERT calls this voltage the resting potential and locates its origin in the differential permeability of the axonal membrane to ions and in the active transport carried out by the sodium-potassium pump.

Two compartments separated by the axolemma set the stage. The axoplasm, the fluid inside the axon, contains a high concentration of potassium ions (K+) and large, negatively charged proteins that cannot cross the membrane. The extracellular fluid (ECF) outside the axon contains a high concentration of sodium ions (Na+) and a relatively low K+ concentration. So the two ions are stockpiled on opposite sides — K+ inside, Na+ outside — and each one is poised to flow down its own concentration gradient if the membrane lets it.

The membrane is selective. In the resting state it is far more permeable to K+ than to Na+ (it has a few open K+ "leak" channels), and is effectively impermeable to the trapped axoplasmic proteins. K+ therefore leaks slowly outward down its gradient, leaving an excess of unpaired negative charges (the proteins) behind. The inner face of the membrane becomes net negative; the outer face, where K+ accumulates, becomes net positive. That charge separation is the resting potential.

Left to itself, the slow K+ leak and the slow Na+ inward seep would eventually erase these gradients. The Na+/K+ ATPase pump — an integral membrane protein that hydrolyses one molecule of ATP per cycle — restores them. NCERT states the exact stoichiometry: the pump moves three Na+ ions out of the axon for every two K+ ions it brings in. Because three positive charges leave and only two return per cycle, the pump itself is electrogenic and contributes a small additional negativity to the inside. More importantly, it permanently maintains the steep Na+ gradient (Na+ high outside) and K+ gradient (K+ high inside) on which every impulse depends.

This stored voltage is what makes a neuron excitable. The membrane is sitting on a gradient that, if the right channels open, can collapse violently and rapidly in either direction. The job of the resting potential is to hold the gun cocked; the action potential is the trigger pull.

The action potential, step by step

When a stimulus — mechanical, chemical, thermal or electrical — reaches the axonal membrane at a site, the local voltage-gated Na+ channels at that site sense the small initial depolarisation and snap open. The membrane there becomes briefly, freely permeable to Na+. Sodium ions, which were held outside at high concentration, rush inward down both their concentration gradient and the prevailing electrical gradient.

The result is dramatic. Within roughly a millisecond, enough positive charge has poured into the axon at that site to reverse the polarity of the membrane: the inside swings from about minus seventy millivolts up to roughly plus thirty millivolts, and the outside becomes briefly negative relative to the inside. NCERT names this reversed potential difference the action potential and explicitly identifies it as the nerve impulse. The membrane at that site is said to be depolarised.

The reversal is short-lived. NCERT writes that the rise in Na+ permeability is "extremely short-lived" and is "quickly followed by a rise in permeability to K+." Two events happen almost simultaneously: the voltage-gated Na+ channels inactivate (they close and become temporarily unresponsive), and a slower set of voltage-gated K+ channels open. K+, now held inside the axon at high concentration and facing an inside that has briefly become positive, rushes outward.

That K+ efflux drains positive charge out of the axon. The membrane potential falls back from plus thirty millivolts toward minus seventy — the process is repolarisation. Because K+ channels close a bit sluggishly, K+ keeps leaving briefly beyond the resting value and the membrane dips slightly below minus seventy millivolts. This undershoot is hyperpolarisation. Finally the K+ channels close, the Na+/K+ pump quietly tidies up the ion accounting, and the membrane is restored to the resting state, ready to fire again.

The whole event at a single patch of membrane takes roughly one to two milliseconds. Note carefully what the Na+/K+ pump does not do during the action potential: it does not move charge fast enough to depolarise the membrane, and it does not actively pump anything during repolarisation. Depolarisation is driven entirely by passive Na+ inflow through voltage-gated channels, and repolarisation by passive K+ outflow through a different set of voltage-gated channels. The pump is the slow background bookkeeper, not the trigger.

Propagation along the axon

A nerve impulse must travel. The patch of axon at site A has just reversed polarity — outside negative, inside positive — but the patch immediately ahead at site B is still at resting state with outside positive and inside negative. Adjacent regions with opposite polarity cannot coexist. A local current begins to flow: on the inner surface of the membrane, positive charge flows from A toward B; on the outer surface, current flows from B back to A to complete the circuit. NCERT describes this loop explicitly in Section 18.3.1.

That local current is enough to depolarise site B past its own threshold. Site B's voltage-gated Na+ channels open, Na+ floods in, and a fresh action potential is generated there. Meanwhile site A has already entered repolarisation and is becoming refractory. The wave moves one patch forward; the patch behind it cools off and resets. This is the self-propagating nature of the nerve impulse — every patch regenerates the signal in full as it passes, so the impulse does not decay with distance the way a passive electrical signal would in a copper wire.

Saltatory conduction in myelinated fibres

Conduction in an unmyelinated axon — one patch at a time, every patch — is reliable but slow. The vertebrate nervous system speeds it up dramatically by wrapping most peripheral and central long-distance axons in a fatty insulating sheath called myelin. In the PNS, myelin is laid down by Schwann cells; in the CNS, by oligodendrocytes (NEET 2017 Q.114). Myelin is interrupted at regular intervals by exposed gaps called nodes of Ranvier, where the axolemma is in direct contact with the extracellular fluid.

Myelin is an electrical insulator. Beneath the myelin sheath the axonal membrane cannot exchange ions with the ECF, so the action potential cannot regenerate on the internodes. Voltage-gated Na+ and K+ channels, instead of being spread evenly along the axon, are clustered at the nodes of Ranvier. The action potential, once fired at one node, cannot fire on the internode that follows; the local current spreads passively beneath the myelin to the next node and triggers a fresh action potential there. The impulse therefore appears to jump from node to node — saltatory conduction (from Latin saltare, to leap).

For NEET, two related facts are worth memorising. Saltatory conduction is faster and more energy-efficient — faster because only the nodes need to spend time on the full action-potential cycle, and more efficient because the Na+/K+ pump has fewer Na+ ions to pump back out per metre of axon. Diseases that strip away myelin (such as multiple sclerosis in humans) slow or block conduction even though the axon itself remains anatomically intact — proof that the myelin sheath is functionally essential, not just a passive covering.

All-or-none law, threshold and refractory period

A nerve impulse does not come in shades. NCERT and NIOS both state the law in the same form: a stimulus must reach a threshold strength to open enough voltage-gated Na+ channels to ignite the regenerative cycle. Below threshold, the small leak of Na+ is mopped up by ongoing K+ outflow and pump activity, and no impulse is generated. At or above threshold, the action potential fires at its full, characteristic amplitude — and increasing the stimulus further does not make the impulse bigger, faster or stronger. This is the all-or-none law.

How then does the nervous system encode a stronger stimulus? Two ways. First, by frequency coding: a stronger stimulus makes the same axon fire more action potentials per second. Second, by population coding: a stronger stimulus recruits additional sensory axons with higher thresholds, so more fibres in the same nerve fire together. Neither mechanism changes the amplitude of any single impulse.

The refractory period is the time after an action potential during which the axon either cannot, or cannot easily, fire again. In the absolute refractory period, voltage-gated Na+ channels are still inactivated; they will not open no matter how strong the stimulus. In the slightly longer relative refractory period, the K+ channels are still open and the membrane is hyperpolarised, so a larger-than-usual stimulus is needed to push the membrane back up to threshold. The refractory period has two important consequences. It places a ceiling on the maximum firing rate of an axon (typically a few hundred impulses per second). And it ensures one-way propagation: the patch behind a moving impulse is refractory, so the impulse cannot travel backward along the axon.

Worked examples

Common confusion & NEET traps