Monday, August 29, 2005

Chapter 2 (1): Classical Biophysics of the Action Potential

Last updated 9/01/05
In Chapter 2, Hille provides an excellent summary of the classical period in biophysics that culminated in the Hodgkin-Huxley papers of 1952. This work on the squid giant axon laid the foundation for thinking about the electrical properties of neurons that has persisted to this day.

Bernstein's membrane hypothesis
Bernstein was the first physiologist to hypothesize that action potentials result from transient changes in membrane permeability to ions. Based partly on the work of Nernst, he hypothesized that at rest the neuronal membrane was selectively permeable to K ions, and that during an action potential there was a reversible 'breakdown' of the cell membrane so that it became nonselectively permeable to all ions. Such a breakdown would cause Vm to shoot up to zero.

Right before WWII, when Cole and Hodgkin's groups were finally able to accurately measure membrane voltage with intracellular electrodes, they discovered that something had to be wrong with Bernstein's hypothesis: Vm shot up past zero during an action potential (Figure 2.1). After the war, Hodgkin's group was finally able to experimentally attack the cause of the overshoot. They surmised that, instead of becoming permeable to all ions, the membrane became selectively permeable to sodium ions, which they knew from Nernst had a resting potential well over zero mV.

Hodgkin and Huxley crack the action potential
Hodgkin and Katz, in 1949, confirmed their sodium hypothesis by decreasing the Na-concentration in the extracellular fluid and observing a reduction in the amplitude of the action potential. They also showed that, when the extracellular K concentration was reduced, the resting potential of the cell became less hyperpolarized.

The development of the technology to clamp the voltage across the membrane of the squid giant axon was the driving force behind their solution to the action potential problem. It allowed them to hold Vm at a particular voltage (Vclamp) while measuring the current across the membrane (Im) (Figure 2.6). By combining voltage clamp with ionic substitution experiments they were able to more precisely characterize the ionic basis of the action potential. For instance, they predicted and confirmed that when Vclamp was above Ena, the current would switch from negative (inward) to positive (outward) (Figure 2.7). They hypothesized that the action potential could be generated solely by voltage-dependent Na and K currents (i.e., Im=Ina+Ik). When they replaced extracellular Na with an impermeant cation, clamping the cell at any voltage above Ek generated only outward currents, which they took to be Ik. By subtracting Ik from the membrane current Im observed in normal conditions, they were able to infer Ina during an action potential (Figure 2.8). They applied this method at multiple values of Vclamp, generating Figure 2.9, which plots steady-state Ik and peak Ina as a function of Vclamp.

Since from Ohm's law we know that for ion species i, gi=Ii/(Vm-Ei), they were able to track the voltage and time-dependent changes in gna and gk during the action potential (Figure 2.11). The story that emerged from such experiments is now well-known to all neurophysiologists. After a suprathreshold voltage step, gna quickly increases(and then decreases), while gk increases to a steady state (Figure 2.11). This causes Vm to quickly approach Ena, but then when gk increases (and gna decreases), the cell quickly comes back down to the resting potential near Ek.

What causes these changes in gk and gna? HH hypothesized that the gna change is actually due to two types of processes in the channel: activation (m) and inactivation (h). These processes are referred to as 'gates' or 'gating variables'or 'particles'. Each of these gates can be in a permissive (activated/de-inactivated) or nonpermissive (de-activated/inactivated) state: a channel is open only when all of the gates are in the permissive state. Unlike the sodium channel, the voltage-gated K channels have only one type of gate, which is either activated or deactivated (n). At Erest, most Na channels are closed: while h tends to be in the permissive (deinactivated) state, the m gate tends to be in the nonpermissive, deactivated state. When the voltage rises, m tends to become activated very quickly. At this time, when m and h are both in the permissive state, the Na channel opens and sodium flows into the cell. This action current generates a brief and fast rise in Vm: the action potential. This rapid depolarization causes the Na channel to inactivate (i.e., h tends toward the nonpermissive state), shutting off the current flow. Compounding this insult to Na, the depolarization also causes m, the gating element in the potassium channel, to enter the permissive state (i.e., the K channel is activated), which causes Vm to drop back down toward Erest. This hyperpolarization resets the whole process. Back down at baseline, the K channels inactivate and the Na channels become de-inactivated (while remaining deactivated), so the cell is poised to spike once again. The above menagerie of permissive and nonpermissive gate states can be confusing, but it's the biological reality evolution stuck us with, and is so important in neuroscience it is worth taking the time to grok.

In the next post, I will summarize Hille's more quantitative description of the Hodgkin-Huxley model and add more details to the above qualitative summary. Then, I will describe gating currents, about which two Nature papers were published just last month, which settles an open question at the time of publication of ICEM.

1 comment:

cara_megalie said...

Cool blog you have got going here :-).