Saltatory conduction · Grey Matter

Saltatory conduction is the way an action potential travels along a myelinated axon, jumping from one gap in the insulation to the next, and it is how the nervous system gets fast signals from thin fibres.

Saltatory conduction. Saltatory conduction is the way an action potential travels along a myelinated axon, jumping from one gap in the insulation to the next, and it is how the nervous system gets fast signals from thin fibres.

Myelin is a sheath of glial membrane wound many times around the axon, laid by oligodendrocytes in the brain and spinal cord and by Schwann cells in the nerves. It is mostly lipid, so it insulates: it reduces the current that leaks out across the wall and the charge needed to move the voltage of each stretch. The sheath is interrupted every so often by a short bare gap, the node of Ranvier, about a micrometre long, where sodium channels are packed densely. The current from a spike at one node flows quickly inside the insulated stretch (the internode, typically on the order of a hundred times the fibre's diameter) and brings the next node to threshold, so the spike is regenerated only at the nodes.

It is fast and cheap. A myelinated axon a few micrometres wide conducts as fast as a bare axon hundreds of times thicker, and because sodium enters only at the nodes, each spike costs the pump less work.

It is tunable. Node length, internode length and sheath thickness vary along and between axons, and adjusting them changes arrival times, which matters where signals from different sources must coincide.

It is fragile. When myelin is lost (as in multiple sclerosis), current leaks out along the bare stretches, conduction slows, becomes unreliable or fails, and the symptoms depend on which tracts are affected.

A stone skipping across a pond is the picture of saltatory conduction.

It touches the water only at intervals, and it crosses far faster than it would by ploughing through.

Questions: How does one oligodendrocyte myelinate many axons at once? An oligodendrocyte sends out several long processes, and the tip of each one flattens into a sheet that winds around a segment of a different axon, layer after layer, forming one internode of myelin. Depending on the region a single cell makes roughly 20 to 60 such segments, up to about 50 in the optic nerve, so one cell serves many axons and each axon is served by many cells along its length. In the peripheral nerves Schwann cells do the same job one segment at a time. The economy has a cost: losing one oligodendrocyte strips myelin from many axons at once. What happens to conduction when an axon loses its myelin? Without the insulation, current from each node leaks out along the bare stretch before it reaches the next node, and the sodium channels it needs are concentrated at the nodes, with few under the old sheath. The spike may slow, fail to cross the gap, or get through only at low firing rates, and signals that should arrive together lose their timing. In multiple sclerosis the immune system damages myelin in the brain and spinal cord, and the symptoms (blurred vision, weakness, numbness) depend on which tracts are hit. Partial recovery comes from redistributing sodium channels along the bare axon and from new myelin made by precursor cells, which is often thinner than the original. 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. Why are sodium channels packed into the nodes of Ranvier? In a myelinated axon the spike is regenerated only at the nodes, short gaps about a micrometre long, so each node must produce enough inward current to charge the next internode to threshold. Clustering sodium channels at high density there, held in place by scaffold proteins and flanked by junctions that seal the sheath to the axon, provides that current from a very small area of membrane. It also saves energy, since sodium enters only at the nodes and the pump has less to restore. Varying node length and channel number is one of the ways axons tune their conduction speed.