Lectures 7 & 8: Action Potentials Flashcards
Action potential
- Rapid change in membrane potential of a cell
- Followed by a return to the resting Vm
Action potential characteristics
- Produced by excitable cells; definition of excitability
- Size and shape vary from one excitable cell to another
- Propagated/conducted from one part of a cell to another, unchanged (non- decremental)
- Are all-or-none
Action potentials are produced and conducted by
- The plasma membrane of cells
- Used for communication over long distances
Chronaxie is useful as index of
- Membrane excitability
- The larger the chronaxie, the less excitable the preparation
After depolarization, Vm does not return to rest immediately, but stays depolarized to some degree
- For 1-2 msec
- Seen in muscle usually, not nerve
- May be due to K accumulation in T- tubules
Post spike hyperpolarization
- 3 to 5 msec in duration
- Until increased PK turns off
- Amplitude around 5 mV
Negative afterpotential
- Around 30 msec in duration
- Due to K accumulation outside membrane
- Stays until K can diffuse away
Positive afterpotential
- Around 200 msec
- 2 mV in amplitude
- Due to stimulation of Na pump by K or Na
- Pump is electrogenic and creates a hyperpolarization
- Seen in skeletal muscle
Activation/inactivation
- Na current turns on
- This is followed by Na current turning off
Same time as Na current turns off,
- K current turns on (activation)
- Will be maintained as long as membrane is clamped
Hodgkin cycle
- The self-sustaining entry and circular role of Na during depolarization
Blockers of sodium channels
- TTX
- Saxitoxin
Cardiac muscle ion currents during action potential
- Fast Na channels carry fast inward current (depolarization)
- Slow inward Ca current produces plateau along with turning on of delayed K current
- Leads to repolarization
Smooth muscle ion currents during action potential
- Lacks fast Na channels
- Have slow Na and Ca channels (L - type calcium channels) for depolarization
- For repolarization: K activation, along with inactivation of slow channels
Voltage inactivation
- Once Na channels are inactivated, membrane must be repolarized toward normal
- Resting Vm before they can be reopened
- If membrane is too depolarized (high Ko or low Ki or use of depolarizing drugs) a considerable number of Na channels are inactivated and enough can’t open to produce an action potential
Accommodation
- Slow depolarization
- Threshold is passed without an action potential being fired
Accommodation is due to
- Inactivation of a significant number of Na channels before threshold potential reached
- Sodium channels have undergone voltage inactivation and will not open again unless the membrane is significantly repolarized
- If depolarization is slow enough, the critical number of Na channels needed to produce an action potential may not be achieved
K channels open in response to
- Depolarization
- Make the membrane more refractory to depolarization
Because of accommodation, a weak stimulus
- Will not elicit an action potential no matter how long applied
- Hence, the rheobase on strength-duration curve
Depolarizing drugs such as potassium cause nerve blocks or muscle paralysis due to
- Accommodation
- Voltage inactivation
Conduction velocity is determined by
- Cm
- Electrical resistance to current flow
Typical value of Cm
- Around 10 farads/cm2 membrane
To depolarize membrane from -100 mV to 0 mV,
- 10 coulombs of charge must flow across each square centimeter of membrane
Cm determines
- How much charge must flow to depolarize membrane
- The greater the Cm, the greater the charge that must flow and the slower the rate of electrotonic spread
Resistance also determines how rapidly charge can flow (two components)
- Rm: membrane resistance
- Rin: the resistance to longitudinal current flow in cytoplasm
Length constant
- √(Rm/Rin): distance over which an electrotonically conducted signal falls to 37% (l/e) of its initial voltage
- Typical mammalian is 2-3 mm
- Also determined by capacitance and resistance
The larger the length constant,
- The further the electronic conduction spreads
Effect of fiber size
- The larger the fiber diameter, the greater the conduction velocity
- Diameter affects resistances and capacitances in manner so this relationship holds
Myelin increases the conduction velocity by
- Decreasing Cm of axon and by allowing action potential to be generated only at nodes of Ranvier
- It effectively decreases the decremental loss of potential with distance (effectively increases the length constant)
Saltatory conduction occurs
- Impulses (action potentials) occur only at nodal membranes
The Na pump is more metabolically efficient because
- Ionic currents are restricted to nodal membrane
- Less Na enters and K leaves per unit area of membrane
- Requiring less pumping to maintain Na and K gradients
With certain “demyelinating diseases” conduction velocities may be sufficiently reduced to cause
- Significant impairment of sensory or motor function
- As part of patient assessment measurements of nerve conduction velocity may be conducted
Increasing [Ko] or decreasing [Ki] causes
- Depolarization
- Mainly affects the resting membrane potential
Changing [Nao]/[Nai] mainly affects
- Peak of the action potentia
- If there were no overshoot then [Nao] = [Nai]
Metabolic poisons inhibit
- Sodium/potassium pump
- Neurons can still conduct millions of action potentials before failure
Graded potential features (also called passive or electrotonic potentials)
- All are sub-threshold
- Magnitude varies with stimulus
- Propagated with decrement
- Persist only as long as stimulus is present (sometimes fail to occur)
Propagated with decremen
- Diminish in size as it moves away from site of stimulation
Action potential features
- Produced by depolarizing excitable cells to threshold Vm (supra-threshold)
- Magnitude same regardless of stimulus
- At peak, Vm usually reverses with regard to exterior
- Propagated non-decrementally
- All-or-None
- Persist long after stimulus ends
Propagated non-decrementally
- Size unchanged as you move away from site of stimulation
Refractory behavior
- Occurs immediately following the firing of an AP
- Period of time during which additional AP cannot occur
- Varies from one excitable cell to another
- Incorporates two periods of time (absolute and relative)
Absolute refractory period
- No AP possible
Relative refractory period
- Attenuated AP possible upon greater than normal stimulation
During accommodation, action potentials fail to occur due to
- Voltage inactivation of Na channels because of slow depolarization of membrane
- Opening of K channels
Electronic spread is dependent upon
- Charge storage capacity of membrane, Cm
- Ratio of Membrane resistance (Rm) to internal resistance (Rin) to current flow
Small Cm and large (Rm/Rin)^1/2 means
- Greater spread of current
- Higher AP conduction velocity
Myelination increases conduction velocity
- Reduces Cm
- Increases Rm/Rin
