1.2. Explain the generation and conduction of nerve impulses

This guide will help you answer 1.2. Explain the generation and conduction of nerve impulses.

The human nervous system enables communication between the brain, spinal cord, and body. Nerve impulses make this communication possible. These electrical signals allow us to move, sense, and respond to the environment. Understanding how nerve impulses are generated and conducted is key to appreciating how the body functions.

What is a Nerve Impulse?

A nerve impulse is an electrical signal that travels along a neurone (nerve cell). This signal allows communication between the brain, spinal cord, and other parts of the body. It is essential for processes like muscle contraction, sensory perception, and reflex actions.

Neurones consist of three main parts:

• Cell body – Contains the nucleus and vital organelles of the neurone.
• Dendrites – Extensions that receive signals from other neurones.
• Axon – A long fibre that transmits the nerve impulse to other neurones or target cells such as muscles or glands.

The movement of a nerve impulse involves a rapid change in electrical charge across the neurone’s membrane.

Resting Potential

In a resting state, a neurone is not transmitting an impulse. During this time, the neurone’s membrane maintains a difference in electrical charge. This is known as the resting potential, which is typically around -70 millivolts (mV).

The resting potential exists because of the unequal distribution of ions (charged particles) on either side of the neurone’s membrane:

• Sodium ions (Na⁺) are more concentrated outside the neurone.
• Potassium ions (K⁺) are more concentrated inside the neurone.

The neurone’s membrane is selectively permeable. This means some ions can pass through more easily than others. Special proteins called sodium-potassium pumps actively move sodium out of the neurone and potassium into the neurone. For every three sodium ions moved out, two potassium ions are moved in. This imbalance maintains the resting potential.

Action Potential

When a neurone is activated by a stimulus, the resting potential changes. If the stimulus is strong enough, it triggers the neurone to generate an action potential. This is a rapid and temporary reversal of the electrical charge across the membrane.

The stages of an action potential include:

1. Depolarisation – When a stimulus reaches the neurone, sodium channels in the membrane open. Sodium ions (Na⁺) rush into the cell because they are more concentrated outside. This sudden influx of positive charges causes the membrane’s electrical charge to become less negative. If the charge reaches a threshold of about -55mV, it triggers a full action potential. The inside of the neurone briefly becomes positive (+30mV).
2. Repolarisation – After the peak of depolarisation, sodium channels close, and potassium channels open. Potassium ions (K⁺) flow out of the neurone, restoring the inside to a negative charge.
3. Hyperpolarisation – The outflow of potassium can cause the membrane potential to dip below the resting potential. This stage is known as hyperpolarisation. Sodium-potassium pumps then restore the resting potential by moving sodium out and potassium back in.

The entire process of an action potential occurs in just a few milliseconds. Once an action potential occurs at one part of the axon, it triggers neighbouring sections, creating a chain reaction.

The All-or-Nothing Principle

A key feature of action potentials is the all-or-nothing principle. If the stimulus is strong enough to reach the threshold, a full action potential occurs. If it does not reach the threshold, no action potential is generated. There are no partial signals – the neurone either fires completely or not at all.

Conduction of Nerve Impulses

Once an action potential is generated, it must be transmitted along the axon to reach the neurone’s target. The way this conduction occurs depends on whether the neurone’s axon is myelinated or non-myelinated.

Non-Myelinated Axons

In neurones without myelin, the action potential moves in a wave-like manner down the axon. Each segment of the axon undergoes depolarisation and repolarisation in turn. This process is relatively slow compared to myelinated conduction.

Myelinated Axons and Saltatory Conduction

Some neurones have an insulating layer called the myelin sheath. Myelin is made of fatty material and is produced by specialised cells called Schwann cells. It wraps around the axon, leaving small gaps called nodes of Ranvier.

In myelinated neurones, the action potential jumps from one node of Ranvier to the next. This process is called saltatory conduction. It is much faster than conduction in non-myelinated axons. The myelin prevents ion flow across the membrane in covered areas, so the electrical signal must “leap” to the uncovered nodes.

The speed of conduction is important for rapid responses, such as withdrawing your hand from a hot surface.

Synaptic Transmission

The conduction of a nerve impulse does not always end with the same neurone. Most neurones are connected to others or to target cells like muscles or glands. The junction between two neurones is called a synapse.

When the impulse reaches the end of the axon, it cannot jump directly to the next neurone. Instead, chemical messengers called neurotransmitters are released.

Here’s how synaptic transmission works:

1. The nerve impulse arrives at the end of the axon, called the axon terminal.
2. Small sacs called vesicles release neurotransmitters into the synaptic cleft (the small gap between neurones).
3. The neurotransmitters bind to receptors on the next neurone’s membrane, causing ion channels to open.
4. If enough ions enter to reach the threshold, an action potential is triggered in the next neurone.

This chemical process allows the nerve impulse to continue its journey.

The Role of Neurones in the Body

Nerve impulses allow neurones to carry out their essential functions, which include:

• Sensory neurones – Transmitting information from sensory organs (e.g., skin, eyes, ears) to the central nervous system.
• Motor neurones – Sending signals from the central nervous system to muscles and glands.
• Interneurones – Connecting sensory and motor neurones within the central nervous system.

Disruptions and Disorders

Disruptions in nerve impulse generation or conduction can lead to medical issues. For example:

• Multiple sclerosis (MS) – A condition where the immune system attacks the myelin sheath, slowing or stopping nerve conduction.
• Epilepsy – Abnormal electrical activity in the brain causing seizures.
• Neuropathies – Damage to neurones, often causing impaired sensation or movement.

Final Thoughts

The generation and conduction of nerve impulses are central to how the body communicates and responds. This complex electrical and chemical process occurs with incredible speed and precision. From enabling movement to processing sensory information, nerve impulses underpin many essential functions of life.