Generation and Conduction of Nerve Impulse — Definition

Definition

Imagine your nervous system as a vast, intricate network of electrical wires, but instead of metal, these wires are living cells called neurons. The 'electricity' that travels through these wires is what we call a nerve impulse or an action potential. It's not quite like the electricity in your home; it's a rapid, temporary change in the electrical charge across the neuron's outer membrane.

Think of the neuron's membrane as a fence separating two different environments: the inside of the cell and the outside. In its normal, 'resting' state, the inside of the neuron is slightly more negative than the outside.

This difference in charge is called the resting membrane potential, typically around $-70,\text{mV}$. This negative charge is maintained by a delicate balance of ion concentrations (like sodium ions, $ext{Na}^+$, and potassium ions, $ext{K}^+$) and the activity of a special pump called the sodium-potassium pump, which actively pushes three $ext{Na}^+$ ions out for every two $ext{K}^+$ ions it brings in, consuming energy in the process.

When a neuron receives a stimulus (like touching something hot or thinking about moving your hand), it causes some ion channels in the membrane to open. If enough positive ions (mainly $ext{Na}^+$) rush into the cell, the inside becomes less negative, moving towards zero.

This process is called depolarization. If this depolarization reaches a critical level, known as the threshold potential (usually around $-55,\text{mV}$), it triggers an 'all-or-none' event – meaning, once the threshold is hit, a full-blown nerve impulse will fire, regardless of the strength of the original stimulus beyond that point.

It's like flushing a toilet; once you push the handle past a certain point, the flush completes itself fully.

During the peak of the action potential, the inside of the cell briefly becomes positive (around $+30,\text{mV}$) due to a massive influx of $ext{Na}^+$. Immediately after this, the $ext{Na}^+$ channels close, and potassium channels open, allowing $ext{K}^+$ ions to rush out of the cell.

This outflow of positive charge makes the inside of the cell negative again, a process called repolarization. Sometimes, too many $ext{K}^+$ ions leave, causing the membrane potential to become even more negative than the resting potential for a brief moment; this is called hyperpolarization or the 'undershoot'.

Finally, the $ext{Na}^+/\text{K}^+$ pump and the natural leakage of ions restore the membrane to its original resting potential, ready for the next impulse. This entire sequence – depolarization, repolarization, and hyperpolarization – happens incredibly fast, in just a few milliseconds.

Once generated, this electrical signal doesn't just stay put; it travels along the length of the neuron's axon. In unmyelinated neurons, the impulse moves continuously, like a wave, by sequentially depolarizing adjacent segments of the membrane.

In myelinated neurons (which have a fatty insulating sheath called myelin), the impulse 'jumps' from one unmyelinated gap (called a Node of Ranvier) to the next. This 'jumping' is called saltatory conduction and makes the impulse travel much faster and more efficiently, saving energy.

This rapid and precise communication is what allows our brains to think, our muscles to move, and our senses to perceive the world.