BTEC Applied Science Unit 1 - Transmission of Action Potentials and Saltatory Conduction

Nerve cells communicating

The transmission of action potentials is a key topic in BTEC Applied Science Unit 1 and is also highly relevant to Unit 9 coursework, particularly when studying the nervous system and communication within the body. Understanding how nerve impulses travel is essential for explaining how the body responds quickly to stimuli.

What Is an Action Potential?

An action potential is a rapid, temporary change in the electrical charge across the membrane of a neurone. It allows information to be transmitted quickly over long distances in the nervous system. These "nerve impulses" or "spikes" act as signals that travel along neurons and muscle cells to communicate information throughout the body.

Stages of an Action Potential

When a big enough stimulus occurs at a nerve cell, an action potential is triggered. We usually represent this using a graph like this. There are 4 stages of the process that you need to know.

Graphical representation of an action potential

1. Threshold potential

At rest, a neurone has a resting potential of about –70 mV. This means the inside of the nerve cell axon is more negative than the outside - to a difference of -70mV. This is because:

• The sodium–potassium pump, which uses ATP, pumps these positive ions in and out of the nerve cell

• The membrane of the nerve cell has different permeability to the ions - allowing 3 sodium ions out for every 2 potassium ions in

Nerve cell axons are more negative on the inside - to a value of -70mV

2. Depolarisation

Here, special channels in the nerve membrane called Voltage-gated sodium ion channels, will open. This causes:

• Na⁺ ions rush into the neurone

• The inside of the membrane becomes more positive than the outside

3. Repolarisation

After a short while, the Sodium channels close so sodium no longer enters. At this point:

• Voltage-gated potassium ion channels open

• K⁺ ions leave the neurone

• The inside of the membrane returns towards negative

4. Hyperpolarisation and Recovery

Too much K⁺ may leave the neurone, making it temporarily more negative than resting therefore

• The sodium–potassium pump restores the normal resting potential

This process is all-or-nothing — if the threshold potential of -70mVbis reached, a full action potential occurs.

How Action Potentials Are Transmitted Along a Neurone

Once an action potential is generated, it travels along the axon because:

• Depolarisation in one region triggers depolarisation in the next

• Each electrical current stimulates adjacent voltage-gated sodium channels

This ensures the impulse travels in one direction only, helped by the refractory period, when ion channels cannot immediately reopen.

What Is Saltatory Conduction?

Saltatory conduction occurs in myelinated neurones. These neurones are covered by a myelin sheath, formed by Schwann cells, which acts as an electrical insulator.

The myelin sheath prevents ion movement along most of the axon. Instead, action potentials only occur at gaps called the nodes of Ranvier.

Saltatory conduction

As a result:

• The impulse appears to “jump” from node to node

• Fewer ions move across the membrane

• Transmission is much faster and more energy efficient

Advantages of Saltatory Conduction

Saltatory conduction provides several important benefits:

• Increased speed of impulse transmission

• Reduced energy use (less ATP needed for ion pumping)

• Allows rapid responses, such as reflex actions

This is why many sensory and motor neurones are myelinated.

Key Exam Tip

High-mark answers clearly:

• Use correct terms (depolarisation, repolarisation, nodes of Ranvier)

• Explain why myelination increases speed

• Link ion movement to changes in membrane potential

Key Takeaway

Action potentials allow rapid communication in the nervous system, while saltatory conduction makes this process faster and more efficient in myelinated neurones. Together, they show how biological structure is closely linked to function — a core theme across BTEC Applied Science.