Neurons and Nerve Impulse

The nervous system helps in controlling and responding to changes in the surroundings. It is made up of the brain, the spinal cord, and neurons. Among these, neurons carry signals from one part of the body to another. These signals are referred to as a nerve impulse or an action potential. Transmission occurs through electrical changes along the axon and chemical changes at synapses.

Structure of a neuron and a nerve impulse travelling through a synapse

Neuron: Unit of the Nervous System

A neuron is the structural and functional unit of the nervous system. Neurons help in detecting and receiving signals called stimuli. These signals are sent as electrical messages known as nerve impulses.

A nerve impulse occurs due to a change in the charge across the membrane. The impulse travels through the axon and is further transferred to the next neuron or muscle. Thus, the impulse travels through synapses, which can be chemical or electrical.

Parts of a Neuron

A neuron has the following parts:

Dendrite receives signals from other neurons.
Axon carries signals away from the cell body.
Axon terminal passes signals to the next cell.
Soma (Cell Body) contains the nucleus and cytoplasm.
Myelin Sheath covers the axon and speeds up signal flow.
Nodes of Ranvier are gaps in the myelin where impulses jump during saltatory conduction

Generation and Conduction of Nerve Impulse

Nerve impulses start when a stimulus reaches a neuron. It causes small changes in the neuron's membrane through ion channels that allow the movement of ions. Here is how the process occurs:

Resting Membrane Potential

In the resting state, i.e., before the stimulus arrives, the axon membrane allows potassium ions (K⁺) to move more easily than sodium ions (Na⁺).

Inside the axon, there are more K⁺ ions.
Outside the axon, there are more Na⁺ ions.

This creates a difference in ion levels across the membrane.

The sodium-potassium pump keeps the difference in charge steady. It moves 3 sodium (Na⁺) ions out of the cell. At the same time, it moves 2 potassium (K⁺) ions into the cell. This makes the inside of the membrane negative and the outside positive. This condition is called polarisation. The charge difference is known as the resting potential.

Action Potential (Impulse Generation)

Voltage-gated Na⁺ channels open in response to a stimulus that reaches threshold.

Na⁺ ions quickly enter the cell, changing the charge.
Now the inside becomes positive, and the outside becomes negative.

This change is called depolarisation. The electric signal produced is the action potential.

This signal moves along the axon. As it moves, voltage-gated K⁺ channels open to restore negative inside charge.

Impulse Conduction

The action potential keeps moving along the axon like a wave.

In myelinated neurons, the impulse jumps from one Node of Ranvier to another. This is called saltatory conduction and is fast.
In unmyelinated neurons, the impulse moves slowly along the whole axon. Thus, it is more energy-consuming and slower than conduction in myelinated neurons.

Synaptic Transmission

When the impulse reaches the end of the axon, it arrives at the axon terminal. This causes the release of neurotransmitters into the synaptic gap.

These chemicals pass the signal to the next cell by binding to receptors. Signals can be passed across either through a chemical synapse or an electrical synapse.

Summary

Neurons are the basic units of the nervous system. They carry signals as nerve impulses. These impulses start when a stimulus changes the charge across the neuron's membrane. At rest, the membrane is polarised. A strong stimulus causes depolarisation, forming an action potential.

The impulse travels along the axon and crosses synapses using neurotransmitters. In myelinated neurons, the impulse moves faster through saltatory conduction.

FAQs

Q1. What is an action potential?

A. The action potential is a sudden change in charge inside a neuron. It allows the nerve impulse to travel along the neuron quickly and efficiently.

Q2. What is the refractory period?

After a neuron generates and transmits an action potential, it cannot immediately fire another one. This brief interval, during which the neuron is unresponsive to a new stimulus, is called the refractory period.

Q3. What is a threshold stimulus?

A threshold stimulus is the minimum amount of stimulation required to initiate a nerve impulse in a neuron.