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Let's say the axon lies along the x axis, and voltage-gated sodium channels lie parallel to the y axis. When a channel is opened, sodium ions will flow along the y direction into the cellular fluid. My question is, how does this flow of ions in the y direction cause a flow of electric current in an entirely different direction (i.e., the x direction along the axon)?

Both layman and technical explanation would be appreciated.

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The flow of current during action potential generation is perpendicular to the axon length. The channels that transmit ions in neurons are often voltage-gated. Voltage-gated Na+ channels (VGSCs) open when the membrane depolarizes. So when excitatory neurotranmitters (e.g., glutamate) open up channels (e.g., AMPA receptors) in the dendritic region, anions (e.g., Na+) flow in. The ensuing membrane depolarization opens up VGSCs in the axon hillock, that subsequently opens VGSCs adjacently in the axon, that opens up VGSCs a little further up in the axon etc etc. Hence, while the flow of Na+ is perpendicular to the axonal membrane, the net effect is a wave of VGSCs opening up along the axon length. With some delay, voltage-gated K+ channels open that allow K+ to flow in, repolarizing the membrane, preparing the neuron for another action potential (fig. 1).

AP
Fig. 1. Action potential mediated by step wise activation of voltage-gated ion channels. Source: Human Medical Physiology

Hence, although neuronal ion fluxes and networks can be modeled by electric currents flowing in electronic circuitry (Fig. 2), nerve conduction is not anything like it. Current flow in electronic circuits is a flow of electrons. In action potentials in neurons it is ions that carry the transported charge.

![Hodgkin & Huxley model of an axon.
Fig. 2. Hodgkin & Huxley's axon model. The power sources are the electric gradients of Na+, K+ and a leak current source. The conductances are represented by those same ions. The cell membrane is modeled by a capacitor. Source: Bonabi et al. (2014)

Reference
- Bonabi et al., Front Neurosci (2014)

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  • $\begingroup$ Thanks Alice for taking the time to reply. I think you meant to say that potassium ions flow OUT of the membrane during re-polarization. Also Na+ is a cation not anion. Lastly, while I understand that the net effect of opening of channels "is a wave of VGSCs opening up along the axon length", I don't understand how this wave can be interpreted as electrical conduction per se. To my understanding, electrical conduction along a particular direction means we have a net flow of charges (usually electrons or ions) along that direction. But here there seems to be no such flow along the axon. $\endgroup$ – Amin Nov 9 '15 at 8:22
  • $\begingroup$ Maybe I'm mixing up the notion of electrical current with nerve conduction? My impression has always been that nerve conduction is a kind of electrical current, Is it incorrect to think of nerve conduction as an electrical current? $\endgroup$ – Amin Nov 9 '15 at 8:30
  • $\begingroup$ @Amin - I added an additional paragraph below the figure. Hope this helps. $\endgroup$ – AliceD Nov 9 '15 at 8:54
  • $\begingroup$ @Christiaan I have got a question with respect to this topic- What causes the adjacent VGSC to open? Is it the flow of Na+ along the x axis that cause conformational change in the adjacent VGSCs and thus they open ? Could we call this motion of Na+ ion, electric current? $\endgroup$ – Tyto alba Mar 12 '16 at 8:25

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