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Slide 3

Recently, two properties of action potential generation in cortical neurons were highlighted:

  1. their rapid rate of rise at onset
  2. their variable threshold (Naundorf et al., Nature 440:1060)

Neither of these properties are predicted by Hodgkin-Huxley style single compartment models of action potential generation.

Could they come from the properties of spike initiation in the axon?

Figure 3. Spikes recorded simultaneously from the soma (a) and axon initial segment (d) of a cortical pyramidal neuron, as per the method of Shu et al. Nature 441: 761 and J. Neurophysiol, in press. Note the "kink" in the action potential in the soma and the smooth nature of the action potential in the axon initial segment. B, E. Phase plots of dV/dt versus membrane potential for the somatic and axonal action potentials. Insets illustrate the initial rate of rise of the action potentials. Note the biphasic nature of spike generation in the soma (parts labeled 1, 2). D, F. Injection of noise into the cell results in a more variable spike onset voltage in the soma than in the axon initial segment.