What is the primary reason for the negative resting potential inside a neuron?

The negative resting potential is primarily due to three factors: the greater permeability of the neuronal membrane to potassium ions (K$^+$) at rest through K$^+$ leak channels, allowing K$^+$ to diffuse out of the cell; the active transport of ions by the sodium-potassium pump, which expels three Na$^+$ ions for every two K$^+$ ions it brings in, creating an electrogenic effect; and the presence of large, negatively charged proteins and organic phosphates inside the cell that cannot cross the membrane.

What is the 'threshold potential' and why is it important for an action potential?

The threshold potential is the critical level of depolarization (typically around $-55, ext{mV}$) that must be reached for an action potential to be initiated. It's important because it's the point at which a sufficient number of voltage-gated sodium channels open rapidly, leading to a massive influx of Na$^+$ ions and the explosive depolarization characteristic of an action potential. If the stimulus is sub-threshold, these channels won't open sufficiently, and no action potential will fire.

Explain the 'all-or-none' principle in the context of action potentials.

The 'all-or-none' principle states that once a stimulus reaches the threshold potential, an action potential will fire with its full, consistent amplitude and duration, regardless of the strength of the stimulus beyond that threshold. Conversely, if the stimulus is below the threshold, no action potential will be generated. It's like flipping a light switch: it's either fully on or fully off; there's no 'half-on' state for an action potential.

What is the role of the sodium-potassium pump during an action potential?

The sodium-potassium pump (Na$^+$/K$^+$ ATPase) does not directly participate in the rapid depolarization and repolarization phases of a single action potential. Its crucial role is to maintain the steep concentration gradients of Na$^+$ and K$^+$ across the membrane. By actively pumping Na$^+$ out and K$^+$ in, it restores the ion balance that was slightly disturbed by the passive ion movements during an action potential, ensuring the neuron can fire repeatedly and maintain its resting potential.

Why are refractory periods important for nerve impulse transmission?

Refractory periods are vital for two main reasons: Firstly, they ensure the unidirectional propagation of the nerve impulse, preventing it from traveling backward along the axon. Secondly, they limit the frequency at which a neuron can fire action potentials, allowing for proper signal encoding and preventing overstimulation. The absolute refractory period ensures distinct, separate action potentials, while the relative refractory period allows for modulation of firing rate based on stimulus intensity.

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Conduction of Nerve Impulse

Resting and Action Potential

Biology

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Version 1Updated 22 Mar 2026

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The resting potential is the electrical potential difference across the plasma membrane of an excitable cell (like a neuron or muscle cell) when it is not actively transmitting a signal. It is typically negative inside relative to the outside, primarily established by the differential permeability of the membrane to potassium ions, the activity of the sodium-potassium pump, and the presence of neg…

Quick Summary

The electrical activity of neurons is governed by the membrane potential, which is the voltage difference across the cell membrane. The resting potential is the stable, negative charge (around $-70,\text{mV}$ ) maintained by a neuron when inactive.

This is established by the differential permeability of the membrane to ions, primarily potassium (K $^+$ ) through leak channels, and the active transport of ions by the sodium-potassium pump (Na $^+$ /K $^+$ ATPase), which pumps 3 Na $^+$ out and 2 K $^+$ in, maintaining concentration gradients.

When a neuron receives a sufficient stimulus, it reaches a threshold potential (around $-55,\text{mV}$ ), triggering an action potential. This involves a rapid sequence of events: depolarization (inside becomes positive, due to rapid influx of Na $^+$ through voltage-gated Na $^+$ channels), followed by repolarization (inside returns to negative, due to efflux of K $^+$ through voltage-gated K $^+$ channels), and sometimes a brief hyperpolarization (undershoot).

After an action potential, the neuron enters a refractory period (absolute and relative), preventing immediate re-firing and ensuring unidirectional signal propagation. This 'all-or-none' electrical signal is the basis of nerve impulse transmission.

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Key Concepts

Establishing Resting Potential: Role of K

^+

Leak Channels

At rest, the neuronal membrane is significantly more permeable to potassium ions (K $^+$ ) than to sodium ions…

Depolarization: The Rapid Influx of Na

^+

When a stimulus causes the membrane potential to reach the threshold (e.g., $-55,\text{mV}$ ), voltage-gated…

Repolarization: The Efflux of K

^+

and Na

^+

Channel Inactivation

The depolarization phase is quickly terminated by two events. First, the voltage-gated Na $^+$ channels…

Resting Potential: —

approx -70,\text{mV}

. Maintained by Na

^+

^+

pump (

3,\text{Na}^+_{\text{out}} / 2,\text{K}^+_{\text{in}}

) and K

^+

leak channels (

P_{\text{K}^+} gg P_{\text{Na}^+}

Threshold Potential: —

approx -55,\text{mV}

. Required to trigger AP.

Action Potential Phases:

- Depolarization (Rising): Voltage-gated Na $^+$ channels open $ightarrow$ Rapid Na $^+$ influx $ightarrow$ Inside becomes positive ( $approx +30,\text{mV}$ ). - Repolarization (Falling): Voltage-gated Na $^+$ channels inactivate + Voltage-gated K $^+$ channels open $ightarrow$ Rapid K $^+$ efflux $ightarrow$ Inside becomes negative. - Hyperpolarization (Undershoot): Prolonged K $^+$ efflux $ightarrow$ Membrane more negative than rest ( $approx -80,\text{mV}$ ).

Refractory Periods:

- Absolute: No AP possible (Na $^+$ channels inactivated). - Relative: AP possible with stronger stimulus (some Na $^+$ channels reset, K $^+$ channels still open).