What sets how far ahead the depolarisation wave can reach, and so how fast it moves? · Lobeworks/17
The length constant λ=rm/ri\lambda = \sqrt{r_m / r_i}λ=rm/ri.
What sets how far ahead the depolarisation wave can reach, and so how fast it moves?
The length constant λ=rm/ri\lambda = \sqrt{r_m / r_i}λ=rm/ri. Ahead of the front, current spreads passively and the voltage falls as V(x)=V0 e−x/λV(x) = V_0,e^{-x/\lambda}V(x)=V0e−x/λ, so λ\lambdaλ is the distance at which a depolarisation has dropped to 37 % of its size. A larger λ\lambdaλ brings membrane further ahead to threshold at once, and the wave moves faster: widening the axon lowers the internal resistance rir_iri, and myelin raises the membrane resistance rmr_mrm so much that the wave jumps from node to node. Typical values run from about 0.2 mm in a thin mammalian fibre to about 13 mm in the squid giant axon.