Axonal conduction · Grey Matter

Axonal conduction is how an action potential travels from the cell body along the axon to the terminals, and its speed (from under a metre per second to over a hundred) decides how long a signal takes to cross the brain or reach a muscle.

Axonal conduction. Axonal conduction is how an action potential travels from the cell body along the axon to the terminals, and its speed (from under a metre per second to over a hundred) decides how long a signal takes to cross the brain or reach a muscle.

The axon is the neuron's single output cable. A spike started at its first segment depolarises the patch of membrane ahead of it, which reaches threshold and fires in turn, so the pulse regenerates itself at full size all the way along; the patch behind is refractory, which keeps it moving forward. How fast it moves depends on how far the current spreads ahead of each patch before leaking out, and two things extend that reach: a wider axon (less internal resistance) and an insulating wrap of myelin, which gives saltatory conduction.

Speeds span two orders of magnitude. Thin unmyelinated fibres (the C fibres of slow pain and temperature, under about a micrometre wide) conduct at about 0.5 to 2 m/s; the thickest myelinated fibres (A-alpha, about 13 to 20 micrometres, for muscle sense and motor commands) reach about 80 to 120 m/s.

Lengths span even more. Most axons of cortical neurons are short and branch locally within a millimetre or so; others run between areas in bundles of white matter; the longest, from spinal motor neurons to the foot, exceed a metre.

At the end, the axon branches into terminals (boutons), each a release site where the spike opens calcium channels and starts synaptic transmission.

Conduction speed is a design choice paid in space and fat.

Fast fibres are thick or myelinated, so the brain reserves speed for the paths where timing matters and leaves most local wiring thin and slow.

Questions: How long does a spike take to cross the brain or reach a foot? It depends on the fibre. The fastest myelinated motor and muscle-sense fibres conduct at about 80 to 120 m/s, so a command covering a metre to the foot takes around 10 ms; thin unmyelinated pain fibres at about 1 m/s need a second for the same distance, which is why a burn is felt after the first sharp prick. Inside the brain most axons are short and slower, and a signal between hemispheres through the corpus callosum takes from a few to a few tens of milliseconds, depending on the fibre. Synaptic delays at each relay add to the conduction time. If thick axons are faster, why are most axons in the brain thin and slow? Speed grows slowly with width while cost grows fast. In a bare axon conduction velocity rises roughly with the square root of the diameter, but the volume the axon takes up rises with the square of it, so doubling the speed means quadrupling the diameter and making the cable sixteen times bulkier. Wider axons also have more membrane to recharge, which costs pump energy with every spike. The brain therefore keeps most local wiring thin and slow (a millimetre takes about a millisecond even at 1 m/s) and spends myelin and width on the long paths where delay matters. Why does wrapping an axon in myelin make the signal so much faster? In a bare axon every patch of membrane must be charged to threshold in turn, and current leaks out through the wall as it spreads, so the spike crawls (about 0.5 to 2 m/s in thin unmyelinated fibres). Myelin wraps the axon in many layers of fatty membrane, which cuts the leak and the charge needed to move the voltage, so current from one node of Ranvier flows quickly along the insulated stretch to the next node. The spike is regenerated only at the nodes and seems to jump between them, reaching tens of metres per second in a fibre a few micrometres wide and up to about 120 m/s in the thickest ones. Matching that speed without myelin would need an axon hundreds of times thicker, which is how the squid gets its fast giant axon.