Speed of Nerve Impulses

The speed at which an impulse travels along a neurone, its conduction velocity, depends on three main factors.1. Myelination. Myelinated axons conduct much faster than unmyelinated axons, because depolarisation happens only at the nodes of Ranvier.2. Axon diameter. The wider the axon, the faster the impulse. A wider axon has less resistance to the flow of ions in its cytoplasm, so local currents spread further and depolarise the next section of membrane sooner. Some invertebrates, such as the squid, have unmyelinated giant axons up to about 1 mm across that conduct fast enough for escape responses.3. Temperature. Up to about 40 °C, a higher temperature increases the speed, because ions diffuse faster and the proteins involved work faster. At very high temperatures proteins such as channel proteins denature and conduction stops. Mammals keep their body temperature near 37 °C, so their conduction velocity stays stable.Conduction velocity is calculated from:speed = distance ÷ timeWorked example: two recording electrodes are placed 45 mm apart on an axon. An action potential reaches the second electrode 0.75 ms after the first.distance = 45 mm = 0.045 m; time = 0.75 ms = 0.00075 sspeed = 0.045 ÷ 0.00075 = 60 m s⁻¹This value is typical of a myelinated mammalian neurone. Unmyelinated axons of normal diameter usually conduct at less than about 2 m s⁻¹. Always convert millimetres to metres and milliseconds to seconds before dividing.