6.021/Notes/2006-11-17

Myelinated axons

 * Potential is not just at nodes
 * Is greatly spread out
 * Reacts very slowly
 * Voltage clamp
 * 3 barriers to separate 3 nodes
 * Test one node by shorting out the 2 adjacent nodes with KCl in bath
 * $$V_K = 0 \rightarrow V_m = 0 \rightarrow G_K \uparrow$$
 * Inject current in one, force zero voltage drop across other side via feedback to measure node in middle
 * Fitting data
 * Frog
 * $$I_{Na}=\overline{I_{Na}}(V_m)m^2h$$
 * $$I_K=\overline{I_K}(V_m)n^2$$
 * $$I_p=\overline{I_p}(V_m)p^2$$ (different sodium)
 * Rabbit
 * $$I_{Na}=\overline{I_{Na}}(V_m)m^2h$$
 * $$I_K=$$small
 * Model of myelinated fiber
 * Core conductor model with cable model at internodes and Hodgkin-Huxley like model at nodes
 * Internode parameters
 * inner diameter d, outer diameter with myelin D, length L
 * Experimentally find $$d \propto D \propto L$$ in many different cells
 * Why does this scaling hold?
 * $$L \approx 0.5 \lambda_c$$ tends to give the fastest conduction
 * Internode is electrically small, but just barely
 * The entire internode is roughly isopotential
 * $$v = \frac{L}{\tau_m}$$ and $$\tau_m$$ is independent of shape
 * Fault tolerant: safety factor
 * Several nodes (amplifiers) can fail and still give an action potential (but takes longer -- more delay)
 * Myelinated fiber advantages
 * myelinated fibers also use less energy (only nodes use energy)