Cable Theory - The Time Constant

The Time Constant

Neuroscientists are often interested in knowing how fast the membrane potential, of an axon changes in response to changes in the current injected into the axoplasm. The time constant, is an index that provides information about that value. can be calculated as:

(18)

which seem reasonable because the larger the membrane capacitance, the more current it takes to charge and discharge a patch of membrane and the longer this process will take. Thus membrane potential (voltage across the membrane) lags behind current injections. Response times vary from 1-2 milliseconds in neurons that are processing information that needs high temporal precision to 100 milliseconds or longer. A typical response time is around 20 milliseconds.