Generation and Conduction of Nerve Impulse — Revision Notes

⚡ 30-Second Revision

Resting Potential: — $\approx -70,\text{mV}$. Inside negative. Maintained by $\text{Na}^+/\text{K}^+$ pump ($3\text{Na}^+_{\text{out}} / 2\text{K}^+_{\text{in}}$) and $\text{K}^+$ leak channels.
Threshold Potential: — $\approx -55,\text{mV}$. Minimum depolarization to trigger AP.
Depolarization (Rising Phase): — Stimulus reaches threshold $\rightarrow$ Rapid opening of voltage-gated $\text{Na}^+$ channels $\rightarrow$ $\text{Na}^+$ influx $\rightarrow$ Membrane potential becomes positive (up to $+30,\text{mV}$). All-or-none principle.
Repolarization (Falling Phase): — $\text{Na}^+$ channels inactivate $\rightarrow$ Slow opening of voltage-gated $\text{K}^+$ channels $\rightarrow$ $\text{K}^+$ efflux $\rightarrow$ Membrane potential returns to negative.
Hyperpolarization (Undershoot): — Slow closing of $\text{K}^+$ channels $\rightarrow$ Excessive $\text{K}^+$ efflux $\rightarrow$ Membrane potential more negative than RMP.
Absolute Refractory Period: — During depolarization/early repolarization. $\text{Na}^+$ channels open/inactivated. No new AP possible. Ensures unidirectional flow.
Relative Refractory Period: — During late repolarization/hyperpolarization. Stronger stimulus can trigger AP.
Conduction:

- Continuous: Unmyelinated axons. Slower. Sequential depolarization. - Saltatory: Myelinated axons. Faster, energy-efficient. AP 'jumps' between Nodes of Ranvier.

2-Minute Revision

Nerve impulse, or action potential, is an electrochemical signal. A neuron at rest maintains a negative resting membrane potential (RMP) of about $-70,\text{mV}$, due to the $\text{Na}^+/\text{K}^+$ pump and $\text{K}^+$ leak channels.

When a stimulus depolarizes the membrane to a threshold (around $-55,\text{mV}$), voltage-gated $\text{Na}^+$ channels rapidly open, causing $\text{Na}^+$ influx and a rapid rise in potential to $+30,\text{mV}$ (depolarization).

This is an 'all-or-none' event. Immediately, $\text{Na}^+$ channels inactivate, and voltage-gated $\text{K}^+$ channels open, leading to $\text{K}^+$ efflux and a fall in potential back to negative (repolarization).

A brief hyperpolarization may occur due to slow $\text{K}^+$ channel closure. During the absolute refractory period, no new impulse can be generated, ensuring unidirectional propagation. In unmyelinated axons, conduction is continuous.

In myelinated axons, the impulse 'jumps' between Nodes of Ranvier (saltatory conduction), which is significantly faster and more energy-efficient.

5-Minute Revision

The nerve impulse, or action potential, is the neuron's electrical signal. It starts from a resting membrane potential (RMP), typically $-70,\text{mV}$, where the inside of the neuron is negative relative to the outside. This RMP is established by the **$\text{Na}^+/\text{K}^+$ pump** (pumping $3,\text{Na}^+$ out and $2,\text{K}^+$ in) and the higher permeability of the membrane to $\text{K}^+$ ions through leak channels.

When a neuron receives a stimulus, if it's strong enough to depolarize the membrane to a threshold potential (around $-55,\text{mV}$), an action potential is triggered following the all-or-none principle. This initiates the depolarization phase, where voltage-gated $\text{Na}^+$ channels rapidly open, causing a massive influx of $\text{Na}^+$ ions. The membrane potential quickly reverses, becoming positive (e.g., $+30,\text{mV}$).

Next is the repolarization phase. At the peak of depolarization, $\text{Na}^+$ channels inactivate, and voltage-gated $\text{K}^+$ channels slowly open. This leads to a rapid efflux of $\text{K}^+$ ions, restoring the negative charge inside the cell. The membrane potential then briefly dips below the RMP, a phase called hyperpolarization or undershoot, due to the slow closing of $\text{K}^+$ channels. Finally, the $\text{Na}^+/\text{K}^+$ pump and leak channels restore the RMP.

During and immediately after an action potential, the neuron enters a refractory period. The absolute refractory period (during depolarization and early repolarization) prevents any new action potential, ensuring unidirectional propagation. The relative refractory period (during late repolarization/hyperpolarization) allows a new action potential only with a stronger-than-normal stimulus.

Conduction along the axon differs. In unmyelinated axons, it's continuous conduction, a slower, sequential depolarization. In myelinated axons, the impulse undergoes saltatory conduction, 'jumping' between unmyelinated Nodes of Ranvier. This is significantly faster and more energy-efficient, as action potentials are only regenerated at the nodes. Factors like axon diameter and myelination directly influence conduction velocity.

Prelims Revision Notes

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Resting Membrane Potential (RMP):

* Value: Typically $-70,\text{mV}$ (inside negative). * Maintenance: Primarily by $\text{Na}^+/\text{K}^+$ pump (active transport: $3\text{Na}^+_{\text{out}} / 2\text{K}^+_{\text{in}}$) and differential permeability due to $\text{K}^+$ leak channels (more $\text{K}^+$ efflux than $\text{Na}^+$ influx at rest). * Ions: High $\text{Na}^+$ outside, high $\text{K}^+$ inside.

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Action Potential (AP) Phases:

* Stimulus: Causes local depolarization. * Threshold Potential: Critical level ($\approx -55,\text{mV}$) to trigger AP. All-or-none principle applies. * Depolarization (Rising Phase): * Event: Rapid opening of voltage-gated $\text{Na}^+$ channels.

* Ion Movement: Massive $\text{Na}^+$ influx. * Potential Change: Inside becomes positive (up to $+30,\text{mV}$). * Repolarization (Falling Phase): * Event: Inactivation of voltage-gated $\text{Na}^+$ channels; slow opening of voltage-gated $\text{K}^+$ channels.

* Ion Movement: Rapid $\text{K}^+$ efflux. * Potential Change: Inside returns to negative. * Hyperpolarization (Undershoot): * Event: $\text{K}^+$ channels close slowly. * Ion Movement: Excessive $\text{K}^+$ efflux.

* Potential Change: Membrane potential becomes more negative than RMP (e.g., $-80,\text{mV}$).

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Refractory Periods:

* Absolute Refractory Period: During depolarization and early repolarization. $\text{Na}^+$ channels are open or inactivated. No new AP possible. Ensures unidirectional propagation. * Relative Refractory Period: During late repolarization and hyperpolarization. Some $\text{Na}^+$ channels reset. Stronger-than-normal stimulus can trigger AP.

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Conduction of Nerve Impulse:

* Continuous Conduction: In unmyelinated axons. Slower. Sequential depolarization of adjacent membrane segments. * Saltatory Conduction: In myelinated axons. Faster (up to 50x), energy-efficient. AP 'jumps' from one Node of Ranvier (unmyelinated gaps rich in channels) to the next.

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Factors Affecting Conduction Velocity:

* Axon Diameter: Larger diameter $\rightarrow$ Faster conduction. * Myelination: Presence of myelin $\rightarrow$ Faster (saltatory conduction). * Temperature: Higher temperature $\rightarrow$ Faster conduction (within physiological limits).

Vyyuha Quick Recall

NaK+ pump sets the Rest, then Depolarization is Na+ in a Rush. Repolarization is K+ out, then Hyperpolarization's a K+ Slow-down. Saltatory Jumps are Fast!