What you'll learn

• How neurones are adapted to carry information quickly around the body.
• How ion movements create a resting potential and an action potential.
• Why action potentials are all-or-nothing and travel in one direction.
• How myelin, nodes of Ranvier and refractory periods affect nerve impulse speed.

The big idea: rapid communication

Your nervous system uses neurones to carry information between receptors, the central nervous system, and effectors such as muscles and glands.

A typical pathway is:

• Receptor detects a stimulus, such as pressure, light or temperature.
• Sensory neurone carries impulses from the receptor to the central nervous system.
• Relay neurone carries impulses within the central nervous system.
• Motor neurone carries impulses from the central nervous system to an effector.
• Effector produces a response, such as muscle contraction or gland secretion.

Neurone structure and adaptation

A neurone has a cell body, which contains the nucleus and many organelles. Branching dendrites receive impulses from other neurones or receptors. A long axon carries impulses away from the cell body towards the axon terminals, where the neurone can pass the signal on.

Many axons are surrounded by a myelin sheath, made by Schwann cells in the peripheral nervous system. Gaps between myelinated sections are called nodes of Ranvier.

The structure fits the function:

• Long axon: carries impulses over long distances.
• Dendrites: provide a large surface area for receiving signals.
• Myelin sheath: electrically insulates the axon membrane.
• Nodes of Ranvier: contain many voltage-gated ion channels, allowing rapid regeneration of action potentials.
• Many mitochondria at terminals: provide ATP for processes such as neurotransmitter release.

Membrane potential: the starting point

Before you can understand an action potential, you need the idea of a membrane potential.

In a resting neurone, the inside of the axon is usually about -70 mV compared with the outside. This is called the resting potential.

The resting potential is maintained mainly by:

1. The sodium-potassium pump, which uses ATP to move 3 sodium ions out of the neurone and 2 potassium ions into the neurone.
2. Potassium ion leak channels, which allow some potassium ions to diffuse back out.
3. The membrane being much less permeable to sodium ions at rest, so sodium ions cannot easily diffuse back in.

This means there are more positive ions outside than inside, so the inside becomes negative relative to the outside.

Threshold and the all-or-nothing principle

A stimulus can make the inside of the axon less negative. This is called depolarisation.

If depolarisation reaches a certain value, usually around -55 mV, an action potential is triggered. This value is the threshold potential.

Action potentials are all-or-nothing. If threshold is reached, a full action potential occurs. If threshold is not reached, no action potential occurs.

A stronger stimulus does not produce a bigger action potential. Instead, stronger stimuli usually produce action potentials at a higher frequency, meaning more impulses per second.

The action potential sequence

An action potential is a rapid, temporary reversal of the membrane potential across a neurone membrane.

1. Resting state

At rest, the membrane potential is about -70 mV. Voltage-gated sodium and potassium channels are closed, although potassium leak channels remain open.

2. Depolarisation

If threshold is reached, voltage-gated sodium channels open. Sodium ions diffuse into the axon down their electrochemical gradient. The inside becomes less negative, then positive, reaching around +30 mV.

This is a positive feedback process: initial sodium ion entry causes more sodium channels to open, causing even more sodium ion entry.

3. Repolarisation

Voltage-gated sodium channels become inactivated. Voltage-gated potassium channels open. Potassium ions diffuse out of the axon, making the inside more negative again.

4. Hyperpolarisation

Potassium channels are slow to close, so too many potassium ions may leave. The membrane potential briefly becomes more negative than the resting potential. This is hyperpolarisation.

5. Return to resting potential

The sodium-potassium pump and ion diffusion restore the normal ion distribution. The membrane returns to about -70 mV.

Refractory period and one-way transmission

After an action potential, the neurone enters a refractory period. During this time, sodium channels are recovering from inactivation, so another action potential cannot immediately be generated in the same region of membrane.

The refractory period is useful because it:

• Ensures action potentials travel in one direction along the axon.
• Separates action potentials from each other.
• Limits the maximum frequency of impulses.

How an impulse moves along an axon

When one region of axon membrane depolarises, local currents spread to the next region of membrane. If that next region reaches threshold, it generates its own action potential.

So the action potential is not the same ions travelling all the way down the axon. It is repeatedly regenerated along the membrane.

In an unmyelinated axon, action potentials are generated along the whole axon membrane. In a myelinated axon, action potentials are generated mainly at the nodes of Ranvier. This is called saltatory conduction.

Myelination increases speed because less membrane needs to depolarise. A larger axon diameter also increases speed because there is less resistance to local current flow inside the axon.

What happens at the axon terminal?

When an action potential reaches an axon terminal, it usually causes a chemical signal to be released across a synapse, which is the junction between two neurones or between a neurone and an effector.

At many synapses, depolarisation of the axon terminal opens voltage-gated calcium ion channels. Calcium ions enter, causing vesicles containing neurotransmitter to fuse with the presynaptic membrane. The neurotransmitter diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane.

You will usually study synapses in more detail separately, but for this topic remember: the impulse travels along the axon as an action potential, then is converted into a chemical signal at the synapse.