Follow one nerve signal through a healthy fibre, an immune attack on its myelin, conduction block, repair, axon loss and heat. Press play, or step through the seven stages, and watch the far-end recording change.
The story warms the body in stage 7 unless you set it yourself.
Schematic, not to scale. Reference speeds: myelinated fibres 50–100 m/s, unmyelinated under 1 m/s. 246 Travel times, the safety margin and the effect of temperature are drawn to show direction, not measured values.
Seeing the delay: the visual evoked potential
Show a flashing pattern to one eye at a time and record over the back of the head. A positive wave arrives about 100 ms after the flash, named P100. When one optic nerve is demyelinated its wave comes late. Pick a state for the right optic nerve and press play.
Schematic traces. P100 is named for its timing; the size of the delay and the drop in amplitude shown for each state are illustrative.
Who makes the myelin: brain and spinal cord versus peripheral nerves
In the CNS one oligodendrocyte myelinates 40–60 different axons, one internode on each, so every axon depends on many oligodendrocytes. In the PNS each Schwann cell myelinates one internode of one axon. 246 Oligodendrocytes also feed the axon through myelinic channels, so losing them can harm axons. 246
Exam hookSaltatory conduction needs myelin. Demyelination causes slowing (a delayed VEP), conduction block and temperature sensitivity (Uhthoff). 160,177,438 CNS myelin = oligodendrocyte; PNS myelin = Schwann cell, and MS spares the PNS. 246
In plain wordsMyelin lets nerve signals jump quickly along the wire. Where it is stripped, signals crawl or stop, which is why symptoms can come and go and can get worse in the heat. Some repair happens, but fibres that are cut through are lost.