Nervous Coordination
Senior 6 · Biology
Nervous Coordination
Lesson 3 of 5
25 min
Stub your toe on a stone and you feel two things: first a sharp, well-placed pain, and about a second later a dull, spreading ache. Both start at the same moment in the same toe. The difference is in the axons that carry them. The sharp pain travels along thin myelinated axons; the ache along even thinner axons with no myelin, which conduct roughly twenty times more slowly. This lesson explains how an action potential moves along an axon, and why myelin, axon diameter and temperature change its speed.
Definition
Local circuits (local currents): the movement of ions along the inside and outside of the axon, between a depolarised region and the resting region next to it.
Saltatory conduction: conduction in a myelinated axon, in which action potentials occur only at the nodes of Ranvier, so the impulse appears to "jump" from node to node (from Latin saltare, to jump).
Conduction velocity: the speed at which an impulse travels along an axon, in m s⁻¹: .
Note that the impulse does not lose strength as it travels: each region generates a fresh, full-sized action potential.
Myelin is an electrical insulator, and voltage-gated channels are concentrated at the nodes. Ions can cross the membrane only at the nodes, so action potentials happen only there. The local circuits run from one node to the next, 1 to 2 mm away, under the myelin. The impulse "jumps" from node to node, and is much faster than in an unmyelinated axon of the same diameter. Because ions cross the membrane only at the nodes, the pump has far fewer ions to move back, so myelinated axons also use less ATP per impulse.
| Factor | Effect | Reason |
|---|---|---|
| Myelination | Myelinated axons are much faster | Saltatory conduction: depolarisation happens only at nodes, so fewer, longer steps |
| Axon diameter | Wider axons are faster | Less resistance to the flow of ions along the inside of the axon, so the local currents spread further and faster; a wide axon also loses fewer ions through its membrane relative to its volume |
| Temperature | Faster at higher temperatures, up to about 40 °C | Ions diffuse faster and channels open and close faster; above about 40 °C proteins, including channels and pumps, begin to denature |
| Axon (typical values) | Diameter | Myelin | Speed |
|---|---|---|---|
| Human C fibre (dull pain, warmth) | About 1 µm | No | About 0.5 to 2 m s⁻¹ |
| Squid giant axon | About 500 µm | No | About 25 m s⁻¹ |
| Human motor neurone to a leg muscle | About 15 µm | Yes | Up to about 120 m s⁻¹ |
Vertebrates achieve high speeds with thin myelinated axons, so thousands fit in one nerve. Invertebrates such as the squid have no myelin and can gain speed only by growing a few very wide axons.
When myelin is damaged. In multiple sclerosis, the body's immune system attacks the myelin in the brain and spinal cord. Impulses slow down or fail, causing symptoms such as numbness, muscle weakness, poor balance and blurred vision.
Worked example 1: measuring conduction velocity
Two recording electrodes are placed 45 mm apart on a nerve. An impulse reaches the second electrode 0.90 ms after the first. Calculate the conduction velocity, and suggest whether the axons are myelinated.
Worked example 2: the two pains, explained
The distance from the toe to the spinal cord is 1.1 m. Sharp pain travels in thin myelinated fibres at about 20 m s⁻¹; dull pain in unmyelinated C fibres at about 1.0 m s⁻¹. Calculate the time each takes to reach the spinal cord, and the gap between them.
Interpretation: the gap of about one second matches the delay people notice between the sharp "first pain" and the dull "second pain".
Warning
Common mistakes
Nonstop Neuron: "Propagation of Action Potential". Watch for how local currents depolarise the next region, and how myelin makes the impulse jump between nodes.
Practice: conduction of impulses
Key points
Sources and credits
Figures 1 and 2, the tables, the worked examples and the practice questions were written by Ganzaa.
