Generation and Conduction of Nerve Impulse Explained

A nerve impulse is a brief electrical signal called an action potential that travels along a neuron. Generation and conduction depend on ion channels and unequal distribution of sodium and potassium ions across the membrane.

This process matters because it is the language of the nervous system. Sensation, reflexes and voluntary movement all begin with these signals, and many medicines and toxins work by altering ion channels.

Key Concepts

• Resting membrane potential: About -70 millivolts inside relative to outside, maintained by the sodium-potassium pump and selective permeability.
• Threshold: Critical voltage, around -55 millivolts, at which an action potential is triggered.
• Depolarisation: Rapid rise in voltage when voltage-gated sodium channels open and sodium rushes in.
• Repolarisation: Fall in voltage when sodium channels close and potassium channels open, letting potassium leave.
• Refractory period: Brief time after an impulse when another impulse is difficult or impossible, ensuring one-way travel.
• Saltatory conduction: Jumping of the impulse between nodes of Ranvier in myelinated axons.

How It Works

1. At rest, the pump moves 3 sodium ions out and 2 potassium ions in for each ATP, keeping inside negative.
2. A stimulus opens some sodium channels. If depolarisation reaches threshold, many sodium channels open suddenly.
3. Sodium inflow makes the inside briefly positive, up to about +30 millivolts: this is the spike.
4. Sodium channels inactivate and potassium channels open; potassium outflow restores negative voltage, sometimes briefly overshooting (hyperpolarisation).
5. The local current depolarises the next membrane segment, so the impulse propagates without weakening. In myelinated fibres it jumps node to node.

Worked Example

Voltage change from rest to peak = +30 - (-70) = 100 millivolts in roughly 1 millisecond. If a demyelinated segment slows conduction from 50 metres per second to 5 metres per second over a 0.5-metre pathway, normal time = 0.5 ÷ 50 = 0.01 second; slowed time = 0.5 ÷ 5 = 0.1 second. The tenfold delay can disrupt timing in vision and movement, as seen in multiple sclerosis.

Common Misconceptions

• Impulses travel like electrons in a wire: Wrong. The signal is a moving wave of ion flow across the membrane, not electron flow along the axon.
• Stronger stimulus makes a bigger action potential: Wrong. Action potentials are all-or-none; stronger stimuli increase firing frequency.
• The pump creates each spike directly: Wrong. The pump maintains gradients over time; channels create the spike rapidly.

Real-World Applications

ECG and EEG record electrical activity patterns clinically. Local anaesthetics block sodium channels to stop pain signals. Anti-epileptic drugs reduce excessive firing by modifying channel behaviour.

To conclude, a nerve impulse is a controlled, self-propagating voltage reversal driven by sodium and potassium channels. Its speed and frequency encode information throughout the body.