Conduction of Nerve Impulse — Revision Notes

⚡ 30-Second Revision

Resting Potential: — $-70,\text{mV}$, high $K^+$ inside, high $Na^+$ outside. Maintained by $Na^+/K^+$ pump (3 $Na^+$ out, 2 $K^+$ in) and $K^+$ leak channels.
Threshold Potential: — $approx -55,\text{mV}$. Required to trigger action potential.
Depolarization: — Rapid $Na^+$ influx via voltage-gated $Na^+$ channels. Membrane becomes positive.
Repolarization: — $Na^+$ channels inactivate, rapid $K^+$ efflux via voltage-gated $K^+$ channels. Membrane becomes negative.
Hyperpolarization: — Brief overshoot, membrane more negative than rest due to slow $K^+$ channel closure.
Absolute Refractory Period: — No new AP possible; $Na^+$ channels inactivated.
Relative Refractory Period: — Stronger stimulus can fire AP; membrane hyperpolarized.
Conduction Speed Factors: — Myelination (saltatory conduction), Axon Diameter (larger = faster).
Saltatory Conduction: — 'Jumping' of APs between Nodes of Ranvier in myelinated axons; faster, energy efficient.

2-Minute Revision

Nerve impulse conduction is the transmission of an action potential along a neuron. It starts from a resting membrane potential (around $-70,\text{mV}$), maintained by the $Na^+/K^+$ pump and $K^+$ leak channels, creating an electrochemical gradient.

A stimulus reaching the threshold potential (around $-55,\text{mV}$) triggers depolarization: voltage-gated $Na^+$ channels open, causing rapid $Na^+$ influx and making the inside positive. This is followed by repolarization: $Na^+$ channels inactivate, and voltage-gated $K^+$ channels open, leading to $K^+$ efflux and restoring the negative potential.

A brief hyperpolarization may occur before returning to rest. The action potential propagates unidirectionally due to the absolute refractory period, where $Na^+$ channels are inactivated. Conduction speed is enhanced by myelination, which allows for saltatory conduction (impulse 'jumps' between Nodes of Ranvier), and by a larger axon diameter.

This 'all-or-none' event ensures efficient and rapid communication throughout the nervous system.

5-Minute Revision

The conduction of a nerve impulse, or action potential, is the electrical signal that travels along a neuron. It's a dynamic process initiated from a stable resting membrane potential, typically $-70,\text{mV}$.

This resting state is maintained by the $Na^+/K^+$ pump, which actively transports 3 $Na^+$ ions out and 2 $K^+$ ions in, and by the differential permeability of the membrane to ions, primarily through $K^+$ leak channels.

This establishes high $Na^+$ outside and high $K^+$ inside the cell.

When a stimulus depolarizes the membrane to a critical threshold potential (around $-55,\text{mV}$), voltage-gated $Na^+$ channels rapidly open. This causes a massive influx of $Na^+$ ions into the cell, driven by both concentration and electrical gradients, leading to rapid depolarization (the rising phase) where the inside of the membrane becomes positive (e.g., $+30,\text{mV}$). This is an 'all-or-none' event.

Immediately after, $Na^+$ channels inactivate, and voltage-gated $K^+$ channels open. $K^+$ ions rush out of the cell, repolarizing the membrane (the falling phase) back towards its negative resting state. Due to the slow closure of $K^+$ channels, a brief hyperpolarization (undershoot) may occur, making the membrane even more negative than the resting potential, before the $Na^+/K^+$ pump and leak channels restore the precise resting potential.

The action potential propagates along the axon. In unmyelinated axons, it's continuous, regenerating at every point. In myelinated axons, it's saltatory: the myelin sheath insulates, and the impulse 'jumps' rapidly between unmyelinated Nodes of Ranvier, where voltage-gated channels are concentrated.

Saltatory conduction is significantly faster and more energy-efficient. Unidirectional propagation is ensured by the absolute refractory period, during which $Na^+$ channels are inactivated and cannot be reopened.

Conduction speed is directly proportional to axon diameter and the presence of myelination.

Prelims Revision Notes

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Resting Membrane Potential (RMP):

* Typically $-70,\text{mV}$ (inside negative relative to outside). * Maintained by: $Na^+/K^+$ pump (3 $Na^+$ out, 2 $K^+$ in, active transport), $K^+$ leak channels (more permeable to $K^+$ at rest), large intracellular anions. * High $Na^+$ and $Cl^-$ outside, high $K^+$ and organic anions inside.

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Action Potential (AP) Phases:

* Threshold Potential: Critical depolarization (e.g., $-55,\text{mV}$) required to trigger AP. * Depolarization (Rising Phase): Stimulus reaches threshold $\rightarrow$ voltage-gated $Na^+$ channels open rapidly $\rightarrow$ $Na^+$ influx $\rightarrow$ membrane potential becomes positive (e.

g., $+30,\text{mV}$). Positive feedback loop. * Repolarization (Falling Phase): Voltage-gated $Na^+$ channels inactivate $\rightarrow$ voltage-gated $K^+$ channels open slowly $\rightarrow$ $K^+$ efflux $\rightarrow$ membrane potential returns to negative.

* Hyperpolarization (Undershoot): $K^+$ channels close slowly $\rightarrow$ excessive $K^+$ efflux $\rightarrow$ membrane becomes more negative than RMP (e.g., $-80,\text{mV}$). $Na^+/K^+$ pump restores RMP.

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Refractory Periods:

* Absolute Refractory Period: During depolarization and early repolarization. No new AP possible, regardless of stimulus strength. Due to inactivation of voltage-gated $Na^+$ channels. Ensures unidirectional propagation. * Relative Refractory Period: During hyperpolarization. Stronger-than-normal stimulus required to fire new AP. Due to persistent $K^+$ efflux and hyperpolarization.

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Conduction Types:

* Continuous Conduction: In unmyelinated axons. AP regenerated at every point along the membrane. Slower. * Saltatory Conduction: In myelinated axons. AP 'jumps' between Nodes of Ranvier (unmyelinated gaps). Voltage-gated channels concentrated at nodes. Much faster and more energy-efficient.

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Factors Affecting Conduction Speed:

* Myelination: Myelinated $>$ Unmyelinated (due to saltatory conduction). * Axon Diameter: Larger diameter $>$ Smaller diameter (less resistance to current flow). * Temperature: Higher temperature (within physiological limits) $\rightarrow$ faster conduction.

Vyyuha Quick Recall

Nerve Impulse: 'NaK-DRH-SC'

NaK: — $Na^+/K^+$ pump maintains Resting Potential.
D: — Depolarization = $Na^+$ Influx.
R: — Repolarization = $K^+$ Efflux.
H: — Hyperpolarization = Extended $K^+$ Efflux.
S: — Saltatory Conduction = Speed (Myelin).
C: — Continuous Conduction = Slower (Unmyelinated).