6.2.3 Nerve Impulses

Key Features of Nerve Impulses

1. Action Potential:

• A nerve impulse is initiated when the neurone membrane becomes depolarised, creating an action potential.
• An action potential occurs if the stimulus exceeds the threshold, typically -55mV.

2. Phases of an Action Potential:

• Depolarisation:
• When stimulated, voltage-gated sodium ion channels open, allowing Na⁺ ions to diffuse into the neurone.
• The inside becomes less negative and can reach a value of approximately +40mV.
• Repolarisation:
• Sodium ion channels close, and voltage-gated potassium ion channels open, allowing $K⁺$ ions to diffuse out.
• This restores the inside of the neurone to a negative charge.
• Hyperpolarisation:
• Potassium ion channels remain open longer than necessary, causing the potential to drop below the resting potential (more negative than -70mV).
• Resting Potential Re-established:
• The sodium-potassium pump restores the resting potential by actively transporting 3 $Na⁺$out and 2 $K⁺$ in.

3. Refractory Period:

• After an action potential, there is a short period where the neurone cannot generate another impulse.
• This ensures impulses are unidirectional and prevents overlap.

4. Propagation of Nerve Impulses:

• The action potential causes localised changes in the membrane potential, triggering adjacent sections of the membrane to depolarise.
• This results in the impulse moving along the neurone.

5. Saltatory Conduction:

• In myelinated neurones, the action potential jumps between the nodes of Ranvier, significantly increasing the speed of transmission.
• In non-myelinated neurones, the impulse travels continuously along the membrane, which is slower.

Significance of Nerve Impulses:

• They enable rapid communication between sensory receptors, the central nervous system, and effectors.
• They are essential for responses to stimuli, such as reflex actions and voluntary movement.

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