Neuron Conduction Velocity Calculator

Nerve impulses travel along axons as action potentials, which are brief reversals of membrane polarity. How fast they travel depends critically on axon diameter and whether the axon is myelinated.

Unmyelinated axons (Type C fibers). Action potentials propagate by local circuit currents spreading continuously along the membrane. Velocity scales with the square root of diameter:

v ≈ 1.1 × sqrt(d) m/s

where d is the axon diameter in micrometers. Typical velocities: 0.5-2 m/s. These fibers carry pain (slow, burning), temperature, and autonomic signals.

That constant is worth pinning down, because square-root scaling is easy to state and easy to get wrong by a factor of two. Check it against the squid giant axon, which is unmyelinated, about 500 µm across, and the most-measured nerve fibre in biology: 1.1 × sqrt(500) = 24.6 m/s, against a measured 20 to 25. Check it again at the other end of the range, a 1 µm human C fibre: 1.1 m/s, sitting in the middle of the 0.5 to 2 m/s band those fibres are known to occupy. A constant near 0.57, which some sources quote, fails both checks by about half.

Myelinated axons (A and B fibers). Myelin sheath acts as electrical insulation. The action potential jumps from one node of Ranvier to the next (saltatory conduction), dramatically increasing speed while reducing energy cost. Velocity scales linearly with diameter:

v ≈ 6 × d m/s

where d is the total fiber diameter (axon + myelin) in micrometers. Typical velocities: 3-120 m/s for B and A fibers.

Why the difference matters. Touch and proprioception (A-alpha, A-beta fibers, 6-20 µm) conduct at 36-120 m/s. A tap on your knee reaches your spinal cord in about 10 ms. Slow pain signals (C fibers, 0.5-1.5 µm) travel at 0.5-2 m/s, and that is the whole story behind first and second pain. Stub your toe and the sharp signal arrives over A-delta fibres in well under a tenth of a second; the dull, spreading ache follows over C fibres about a second and a half later, having covered the same metre and a half of leg at walking pace.

Demyelinating diseases like multiple sclerosis slow or block conduction in myelinated fibers, causing the characteristic patchy neurological deficits.