Nerve impulse conduction is the process by which electrical signals travel along neurons, enabling our body to think, feel, and act. Where biology provides the cells and ion channels, physics explains the electrical signals and currents. In this blog, we’ll explore how our body conducts nerve impulses.

Structure of Neuron

Before diving into the magic of signal transmission, let’s meet the star of the show: the neuron.

The Neuron is the structural and functional unit of the neural system. Truly, the star of the show. A neuron consists of the main cell body and the cytoplasmic projections arising from it.

A typical cell body consists of cytoplasm, nucleus, and cell membrane. It has branched projections known as dendrites, which receive the nerve signals. This is where it all begins.

Moving on, the axon is a single, long, uniformly thick projection. You can think of it as a hollow pipe, and the content flows inside like water. It’s the main part that we’ll be dealing with. In myelinated nerve fibers, Schwann cells form myelin sheaths around the axon. The gaps between the myelin sheaths are known as nodes of Ranvier. The axon terminals are branched, and at the end of these branches, you’ll find little knob-like structures known as synaptic knobs through which transmission of nerve impulses occurs.

Fun fact #1: Neurons are the longest cells in our body, motor neurons can run from the spinal cord to the toes!

Membrane potential

Okay, so now we know what a neuron looks like. But how does this cell transmit signals at such great speed? To understand this, we first need to look into the membrane potential.

In technical terms, a nerve impulse is defined as a wave of depolarization of the membrane of the nerve cell. In layman’s words, a nerve impulse is when the charge (polarity) of the nerve cell membrane is reversed. That means the negative side becomes positive and vice versa.

Wait, now positive and negative sides?

To clear this confusion, let’s take a closer look at the charges present on the membrane- in this case, the axonal membrane or the axolemma.

When there is no signal transmission, the membrane is said to be in a polarized state. Here, the inner side of the membrane towards the axoplasm, the cytoplasm of the axon, is negatively charged, and the outer side is positively charged.

The presence of polarity is due to the distribution of Na+ and K+ ions across the membrane. In the resting state, there are more Na+ ions on the outside than on the inside and more K+ ions on the inside than on the outside.

But, both of these ions are positive, so how is the inner membrane negative?

The reason is that the inner membrane isn’t absolutely negative. In fact, both the inner and outer membranes are positive. In reality, the inner membrane is less positive compared to the outer membrane. That’s why we call it ‘negative’ relative to the outer surface.

Think of two friends, one with 10 million dollars and another with 1 million. From a general perspective, both of them are rich, but when assessed side by side, one can be considered poor as compared to the other. The same can be said for the membranes; both of them are positive, but the inner membrane is relatively less positive (negative) when compared to the outer membrane. At rest, the inside of the neuron is about -70mV compared to the outside. This is what we call the resting membrane potential.

The polarization and depolarization of the membrane is controlled by the channels prsent on it.

Voltage-gated channels (VGC): only open when the membrane potential reaches a certain value

Ligand-gated channels (LGC): open when a specific molecule (like a neurotransmitter) binds to them.

Mechanical gated channels (MGC): open in response to physical forces such as stretch or pressure.

These channels are the real players, because their opening and closing control the game of electrical signals.

Fun fact #2: The number of neurons in our brain is roughly equal to the number of stars in the Milky Way Galaxy.

so, we do have more than one braincell!

Nerve impulse conduction

Finally, we get to the part where biology and physics join hands.

The process is explained by the “Ionic Theory of Nerve Impulse” proposed by A.L. Hodgkin and A.F. Huxley (1952). The theory states that electrical events in the nerve fiber are governed by the differential permeability (selective permeability) of its membrane to sodium and potassium ions, and that these permeabilities are regulated by the electric field across the membrane.

That is a very fancy sentence! Let’s understand it in simple terms.

Basically, when the membrane is in a polarized state, there is a -70mV relative potential on the inner side of the membrane which means that the outer surface of the membrane is more positive than the inner membrane surface. When an electrical signal is received at the axonal membrane, the Na+ stimulus-gated channels are stimulated, which causes the Na+ ions rush to the inner surface from the outer surface of the membrane. This surge in Na+ inside the neuron causes depolarization of the membrane, i.e., membrane potential increases to — 55 mV, which is the threshold potential. At this potential, the action potential is carried down the axonal membrane, and the Na+ VGC’s open, which leads to a further increase in the membrane potential to +30mV known as repolarization. This stimulates the K+ VGC’s to open, allowing K+ to move to the outer surface.

At this point, the membrane now becomes hyperpolarized, where the potential now decreases to -85mV. To balance this, the (Na+)- (K+) pump gets to work and restores the potential back to -70mV. Thus, the signal is transmitted throughout the neuron.

Fun fact #3: Some nerve impulses can travel up to a speed of 275 mph, so, against Formula 1 cars, I’d say action potential takes the win.

Conclusion

The human body is a remarkable integration of physics, chemistry, and biology, resulting in one of the most finely tuned systems in the world. All the functions of our organs are perfectly coordinated by the brain and the neural system. Neurons receive stimuli and transmit them to the brain through a series of nerve impulse transmissions, and the resulting responses are transmitted back to the organs within seconds. This entire operation requires precision and efficiency. Thus, these minute cells and transmissions play a vital role in our day-to-day life; without them, normal functioning would be impossible.