action potentials Flashcards
Definition of an action potential
a rapid, transient, and reversible depolarization of an electrically excitable plasma cell membrane in response to a threshold depolarization
The membrane structures (and mechanisms) that produce the action potential are distributed equally along the membrane so that the AP does not decrease in amplitutde with distance as it propogates along the membrane.
It is produced by a series of identical, transient, and reversible changes in transmembrane properties as it propagates along the membrane
There is no decrease in amplitude of the action potential of the action potential when its propagated past a bifurcation into two or more branches of a nerve fiber (not like electricity)
it causes a decrease in transmembrane electrical resistance (increase in conductance)
ionic contribution to AP
Transmembrane ion permeabilities (and conductances) increase transiently with membrane depolarization in the region of the action potential
Specific ion permeabilities and conductances must be goverened by the transmembrane Vm (they have to be voltage dependent)
Velocity of propagation of the AP
Cooling the axon decreases the conduction velocity
cooling increases the time duration of the AP at any one site on the membrane
Cooling decreases the rate of rise and fall of the AP
Membrane mechanisms regulating the ion permeabilities involve conformational changes in the molecular structure within the cell membrane and these conformational changes are temperature dependent
Effects of a reduction of the extracellular Na on the properties of the action potential
A reduction of extracellular Na o-> decreases the following properties of the AP:
- Rate of rise
- Amplitude
- magnitude of positive overshoot (the Vm that goes above 0)
- propogation velocity along the nerve fiber
the positive overshoot is due to:
- net increase in positive transmembrane current flows into cell
- transient increase in membrane permeability for a positive ion
- positive ion has an inwardly directed electrochemical gradient
Positive overshoot is due to Na (b/c alpha is increased in GHK equation and the Vm is going to the positive Em of Na)
Time rates of change of g na and g k during an AP
- First, a local depolarization generated by application of a stimulus (due to the removal of capacitance from the membrane) moves the Vm to a level more positive than the threshold potential
- This causes initial, transient, self limiting rise in both gNA and gK due to an increase in the number of open Na and K channels
- Transient rise in gNa and Gk is followed by a spontaneous return to resting levels due to automatic closure of the Na and K channels
gNa is much faster than gK so the peak happens due to Na channels and the fall is due to K channels
Threshold potential
a level of depolarization of the Vm (in response to a depolarizing stimulus) where the increasing inward positive current (due to Na current) just equals outward positive current (due to K current), then the Na current increases rapidly compared to the outward K –> AP
electrical excitability refers to the ease of generating an AP. Electrical excitability is inversly proportional to the difference between Vth and Vm
The smaller the difference, the more excitable the cell
Ca effect on the excitability of the cell
Extracellular Ca concentrations regulate membrane excitability (not by diffusion into the Cell but) by binding to negative charges in glycoprotein moietes in the immediate vicinity of the cell membrane
high levels of Ca make it hard to discharge the membrane capacitance (need to discharge to reach threshold) low levels of Ca (hypocalcemia) increase the excitability of the cell membrane -> seizures, tetany, arrythmias
Structure and function of voltage gated ion channels within the nerve cell membrane
specific amino acids regulate 3 important properties of voltage gated ion channels:
Ion selectivity: they like certain ions
Activation: the mechanism for opening an ion channel, A Specific membrane bound segment of the polypeptide chain is a gate and is similar for all voltage gates
Inactivation: involves membrane bound segment
The specificity of the amino acids in particular segments of the polypeptide chain determines the major functions
Voltage gated channel= 6 transmembrane lipophilic polypeptide chain segments termed a sequence. S4 sequence has a lot of positively charged residues, and are voltage sensors that open the channel
The pore determines the ion selectivity
Structure of the channel pore
4 domains in circular geometry to form a central pore. Protruding loop in each domain represent the P region of the extracellular side to form the ion selectivity filter
When depolarized the four subunits rise and carry with it the gate
Inactivation gates
The channel is closed and opens upon depolarization
Hyper polarization closes the inactivation gate
Membrane potential repolarization opens the inactivation gate and closes the activation gate
Na channel: hinged lid inactivation (closes on hyperpolarization)
K channel: ball and chain (closes on repolarization)
depolarization blockade
cant make an Ap if the cell is depolarized (no inactivation)
how to keep a cell depolarized:
bath in NT
anesthetic (keep the Na inactivation closed)
increase H+ concentration
