Conduction of Nerve Impulse — Definition

Definition

Imagine your nervous system as a vast network of electrical wires, and a nerve impulse is like an electrical signal traveling along one of these wires. In biology, these 'wires' are called neurons, and the 'signal' is a rapid, temporary change in the electrical charge across the neuron's outer membrane. This change is called an action potential.

Normally, a neuron is at a 'resting state,' meaning the inside of its membrane is slightly negatively charged compared to the outside. This is called the resting membrane potential, maintained by a delicate balance of ion concentrations (like sodium and potassium) and the activity of the sodium-potassium pump, which actively pushes ions against their concentration gradients.

When a neuron receives a strong enough stimulus, it triggers a series of events. First, specific channels in the membrane, called voltage-gated sodium channels, open up. This allows positively charged sodium ions to rush into the cell, making the inside of the membrane rapidly become positive. This rapid change from negative to positive is called depolarization, and it's the 'rising phase' of the action potential.

Once the inside becomes sufficiently positive, the sodium channels close, and another set of channels, the voltage-gated potassium channels, open. Positively charged potassium ions then rush out of the cell, making the inside of the membrane negative again.

This process of returning to a negative charge is called repolarization, the 'falling phase.' Sometimes, the potassium channels stay open a little too long, causing the membrane to become even more negative than the resting potential for a brief period; this is called hyperpolarization.

Crucially, once an action potential is generated at one point on the axon, it doesn't just stop there. The influx of sodium ions at one spot depolarizes the adjacent region of the membrane, triggering new voltage-gated sodium channels to open there.

This creates a domino effect, with the action potential 'jumping' or 'propagating' along the axon. It always moves in one direction because the region that just fired is in a 'refractory period,' meaning its sodium channels are temporarily inactivated and cannot open again immediately.

This ensures the signal travels efficiently from the neuron's cell body towards its axon terminals, never backward.

In many neurons, especially those involved in fast communication, the axon is covered by a fatty insulating layer called the myelin sheath. This sheath is not continuous but has gaps called Nodes of Ranvier.

In myelinated axons, the action potential doesn't have to be regenerated at every point; instead, it 'jumps' from one Node of Ranvier to the next. This process, called saltatory conduction, significantly speeds up the transmission of the nerve impulse, making our reflexes and thoughts incredibly fast.