Saltatory conduction

Saltatory conduction is the way a nerve impulse travels along a myelinated axon: the action potential jumps from one node of Ranvier to the next instead of moving along every part of the membrane. The fatty myelin sheath insulates the axon between the nodes, so depolarisation only happens at the gaps. This makes impulses travel much faster than along an axon without myelin.

What is saltatory conduction?

The word comes from the Latin for to leap, because the impulse seems to leap along the axon. It only happens in myelinated neurones. In AQA's A-level Biology specification it is one of three factors affecting the speed of conductance, alongside axon diameter and temperature.

Many axons are wrapped in a myelin sheath, a lipid-rich layer made from the membranes of glial cells coiled tightly round the axon. In the peripheral nervous system the myelin is made by Schwann cells, each covering one segment of one axon. In the brain and spinal cord it is made by oligodendrocytes, which can each cover segments of several axons. Between the segments are short gaps, about one micrometre long, called nodes of Ranvier.

How saltatory conduction works

1. An action potential happens at one node of Ranvier. Voltage-gated sodium ion channels open and Na⁺ ions flood into the axon, so that patch of membrane depolarises.
2. The incoming positive charge spreads along the inside of the axon, under the myelin, as a local current.
3. Myelin is an electrical insulator, so very little charge leaks out on the way. The current reaches the next node and depolarises it to the threshold, about −55 mV.
4. Voltage-gated channels at that node open, and a new action potential is generated there.
5. The process repeats node after node. Behind the impulse, each node is in its refractory period, so the impulse can only travel forwards.

The key fact is where the channels are. Along a myelinated axon, voltage-gated sodium channels sit at the nodes, not under the myelin. So the membrane can only depolarise at the nodes, and those are the only places an action potential can happen.

Why is saltatory conduction faster?

In an axon without myelin, the impulse moves by continuous conduction: every small patch of membrane along the whole length has to depolarise in turn, which means channels opening all the way along. Opening channels takes time, so this is slow. In a myelinated axon, action potentials only have to be generated at the nodes, and the current between them travels quickly under the insulating myelin. Fewer depolarisations over the same distance means the impulse arrives much sooner.

Other factors that affect conduction speed

• Myelination: myelinated axons conduct much faster, by saltatory conduction.
• Axon diameter: depolarisation spreads faster down a wide axon than a narrow one, because there is less resistance to the flow of ions inside it.
• Temperature: warmer axons conduct faster, because ions diffuse faster, up to the temperature at which the proteins in the membrane start to denature.

Worked examples

Example 1: conduction speed

An impulse travels 0.75 m along a motor neurone in 12.5 ms. What is its speed?

1. Convert the time to seconds: 12.5 ms = 0.0125 s.
2. Speed = distance ÷ time = 0.75 ÷ 0.0125 = 60 m/s.

Example 2: comparing two axons

Suppose a myelinated axon conducts at 50 m/s and a thin unmyelinated axon at 1.25 m/s. How long does an impulse take to travel 1.0 m along each?

• Myelinated: 1.0 ÷ 50 = 0.02 s = 20 ms.
• Unmyelinated: 1.0 ÷ 1.25 = 0.8 s, which is 40 times longer.

That difference matters for reflexes: a signal from your foot to your spinal cord has to arrive in a fraction of a second.

What happens when myelin is damaged?

In demyelinating diseases such as multiple sclerosis, the myelin sheath is damaged. Current leaks out of the axon between nodes, so impulses slow down and may fail to reach the next node at all. The symptoms depend on which neurones are affected.

Common mistakes

• Saying the impulse jumps across the myelin. The action potential is regenerated at each node; between nodes the charge flows inside the axon as a local current.
• Saying myelin conducts electricity. It does the opposite: it insulates the axon.
• Saying action potentials are bigger in myelinated axons. They are all-or-nothing and the same size; they are just generated less often along the length, so they arrive sooner.
• Mixing up the cells. Schwann cells make myelin in the peripheral nervous system; oligodendrocytes make it in the brain and spinal cord.
• Forgetting units. Convert milliseconds to seconds (÷ 1000) and millimetres to metres before working out a speed in m/s.

Nerve impulses sit with reflexes and synapses in the Human Physiology course, and the reflex arc guide shows how neurones link together. To practise in a short timed paper, try today's A-level Biology: Year 2 paper.

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