Human Physiology: Action Potential

The Action Potential

- the action potential (or “spike”)

- action potential: propagating waves of electrical activity along the plasma membrane

- culminate in a secretory or contractile event

- result from momentary changes in membrane ionic permeability

- generally initiated by influx of Na⁺ or Ca²⁺

- physiological role

- amplify and transmit information rapidly over long distances, without signal degradation

- abnormalities: basis of numerous pathological conditions

- durations and shapes: different between cell types, but similar within cell types

- motor neuron: sharp spike 2 msec

- skeletal muscle: sharp spike, slower decline 5 msec

- cardiac ventricle: sharp spike, very slow decline 200 msec

features of the action potential

- passive graded potential: small hyperpolarizing or depolarizing change in membrane potential

- passive: depend only on the resting resistance and capacitance

- graded: proportional to the size of the stimuli

- action potential: all or nothing depolarization of the membrane

- threshold: membrane potential at which there is a 50% probability that an action potential will be generated

- generally 10-30 mV from rest, depending on cell type

- physiological role: ensures transmission of meaningful signals, rather than random noise

- overshoot: phase of the action potential in which the membrane potential becomes positive

- afterhyperpolarization: phase of the action potential in which the membrane potential becomes sub-resting

ionic basis for the action potential

- mathematical interpretation

- GHK constant field equation:

- resting membrane: high P_K/P_Na ratio

- action potential peak: very low P_K/P_Na ratio (transient permeability increase for Na⁺)

- chord conductance equation:

- resting membrane: baseline conductances

- polarity reversal: dominant increase in g_Na drives V_m­ toward E_Na

- afterhyperpolarization: g_Na subsides, g_K increases, driving V_m toward E_K

- the voltage clamp technique

- thermostat analogy: senses temperature, compares to desired temperature, and signals the heater to turn on/off

- voltage clamp: senses V_m, compares to desired V_m, and signals current injection/removal

- current injected by voltage clamp is equal, opposite to current flowing across the membrane

- consequently, is a measure of the total membrane currents

- patch clamp: voltage clamp technique, confined to a single ion channel

- macroscopic membrane current consists of capacitive and ionic components

- membrane current consistency:

- I_C: capacitive current amount of current that goes into charging capacitance

- I_x: ionic (flux) current current that flows through time- and voltage-dependent ion channels

- can be described by a variation of Ohm’s law:

- ionic current:

- membrane current is a function of:

- gating properties of conducting channels (g_x, encompassing both voltage- and time-dependent properties)

- driving force on the permeating ion species ([V_m – E_x], based on the existing gradient)

- demonstration of channels in isolation: use of toxins

- tetraethylammonium (TEA): K⁺ channel blocker

- tetrodotoxin (TTX): Na⁺ channel blocker

- found in puffer fish, a delicacy; require specially-trained chefs to extract food

- Homer thought he was going to die because of this

- time dependence of Na⁺ and K⁺ currents: application of potential

- Na⁺: spikes, is quickly inactivated, allowing current to fall below baseline before reestablishment of resting

- K⁺: slowly activated, remains at plateau for duration of voltage application

- voltage dependence of Na⁺ and K⁺ currents: application of potential

- Na⁺: increase with voltage to a point, but begin to diminish due to reduced driving force

ionic current:

- low potential: conductance term causes expected current behavior (negative, influx)

- increasing potential: reduction in current due to reduced driving force (V_m – E_x)

- high potential: driving force term is positive, causing current reversal (positive, efflux)

- K⁺: increase with voltage in an ohmic fashion

ion channels – the elements of conductance

- important features of ion channels

- selective permeability: only allow passage of a specific ion

- different kinetics: sequential activation of Na⁺, K⁺ channels mediates rise and fall, respectively

- charges on the channel protein sense changes in V_m

- voltage-gated ion channels use energy stored in V_m to drive conformational changes

- electric field approximation

- assumptions: V_m = -100 mV, r_m = 10 nM

- electric field: 10⁷ V/m, or 10 megavolts/meter

- proteins: assume conformations that minimize energy in the presence of a strong electric field

- ion channels have specialized voltage sensors

- ion channel structure overview

- K⁺ channels: four subunits

- Na⁺ channels: four repeats

- each subunit or repeat contains six transmembrane domains, numbered S1-S6

- S4 subunit: sensor

- amphipathic, with (fixed charged-X-X) repeats

- charges sense electric field, and movement drives opening and closing of ion channels during V_m changes

- S6 subunit: activation gate

- activation gate: region lining the channel pore that blocks ion channel

- by a poorly understood mechanism, S4 communicates with S6 to open the pore during an AP

- depolarization increases the probability of opening

- patch clamp: modification of voltage clamp where the electrical activity of a single ion channel is isolated

- population conductance equation:

- g_x: total conductance

- N: number of channels

- P_o: probability of each channel being open

- γ_x: single channel ion conductance (isolated by patch clamp technique)

- ion channel behavior

- resting voltage: voltage-gated K⁺, Na⁺ channels spend most of their time closed

- depolarized voltage: probability of being open increases

- highly positive voltage: above approximately +50 mV, P_o is saturated

- probability (P_o) or conductance vs. V_m (mV)

- h curve: Na⁺ inactivation curve starts at P_o ≈ 1, falls sigmoidally to 0 with increasing V_m

- m curve: Na⁺ activation curve starts at P_o ≈ 0, rises sigmoidally to 1 with increasing V_m

- n curve: K⁺ activation curve similar behavior to m curve, but begins increase at higher V_m

ion channel gating and generation of the action potential

- negative and positive feedback during an action potential

- K⁺ channel: negative feedback

- K⁺ gradient: driven to flow out of the cell

- channel states: closed, open

- C à O: depolarization increases probability of channel opening, causing efflux of K⁺ and repolarization

- O à C: repolarization increases probability of channel closing

- Na⁺ channel: positive feedback, subsequent inactivation

- Na⁺ gradient: driven to flow into the cell

- channel states: closed, open, inactive

- C à O: at threshold, channel opens; Na⁺ influx further depolarizes, increasing probability of being open

- O à I: depolarization also increases probability of inactivation, which happens shortly after opening

- I à C: mechanism still debated; may be time- or voltage-based

- combined effects: summary of the phases of the action potential

- threshold

- graded potentials bring membrane potential to threshold, where Na⁺ channels begin opening strongly

- general threshold: usually -50 to -55 mV

- upstroke

- P_Na (permeability) increases rapidly as channels open, giving a resulting depolarization

- positive feedback of depolarization causes Na⁺ channels to open more rapidly

- V_m: nearly reaches E_Na at the peak of the action potential

- repolarization

- Na⁺ channels quickly inactivate, causing repolarization

- K⁺ channels begin to open, also contributing to repolarization

- afterhyperpolarization

- Na⁺ channels are still inactivated

- K⁺ channels are still open in greater abundance than at rest

- V_m: driven more negative than rest, towards E_K, helping to close K⁺ channels and recover Na⁺ channels

- general considerations

- only a small number of Na⁺, K⁺ ions must cross the membrane to produce an action potential (several million)

- Na⁺ and K⁺ concentration gradients are not disrupted by a single AP

- Na⁺/K⁺ ATPase counteracts effects of repeated action potentials over long periods of time

- the refractory period

- refractory period: period when firing of another action potential is more difficult

- absolute refractory period

- no AP can be generated regardless of stimulus strength

- results from Na⁺ channel inactivation, high P_K

- relative refractory period

- AP can be generated, but requires a stimulus stronger than the original stimulus strength

- results from partial Na⁺ channel inactivation, hyperpolarization (high P_K)

- absolute vs. relative

- absolute: cannot launch another action potential due to high Na⁺ channel inactivation

- relative: more difficult to launch another action potential until K⁺ channels turn off

- distinction

- visual

- absolute: upstroke, early repolarization (extending to shortly before afterhyperpolarization phase)

- relative: late repolarization, afterhyperpolarization

- mathematical (rule of thumb)

- absolute: available g_Na <>K

- relative: g_K > g_{K, rest}

- steady state inactivation

- steady state inactivation: failure of AP firing due to slow depolarization

- results from time difference between Na⁺ activation (CàO), inactivation (OàI)

- activation gates work rapidly, while inactivation gates work more slowly

- if depolarization occurs slowly, inactivation catches up with activation, reducing channel availability

- sufficient reduction in channel availability will prevent spikes

- depolarizing block: steady state inactivation resulting from long-term depolarization

- results from accumulation of inactive Na⁺ channels at high potentials

- inactivation of Na⁺ channels prevents further spikes from being fired

a few additional points

- bulk ion concentrations do not change significantly during an action potential

- relatively few ions must cross the membrane to generate the large differences in potential

- changes in E_K and E_Na are negligible, except in pathology

- many types of ion channels exist

- ionic specificity

- channels discussed: Na⁺ and K⁺ channels in axons

- other channels: K⁺ channels with inactivation, Ca²⁺ channels, Ca²⁺ channels with inactivation, others

- structural differences

- most: tetrameric structure with 6 transmembrane domains and an S4 sensor

- others: K⁺ channels with 2 transmembrane domains, Cl^‑ channels with 12 transmembrane domains

- ion channel diseases and the human genome project

- cystic fibrosis: first recognized ion channelopathy (1989)

- since the discovery of CFTR defects, however, numerous other channelopathies have been linked to disease