Lectures 7 & 8: Action Potentials Flashcards
1
Q
Action potential
A
- Rapid change in membrane potential of a cell
- Followed by a return to the resting Vm
2
Q
Action potential characteristics
A
- 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
3
Q
Action potentials are produced and conducted by
A
- The plasma membrane of cells
- Used for communication over long distances
4
Q
Chronaxie is useful as index of
A
- Membrane excitability
- The larger the chronaxie, the less excitable the preparation
5
Q
After depolarization, Vm does not return to rest immediately, but stays depolarized to some degree
A
- For 1-2 msec
- Seen in muscle usually, not nerve
- May be due to K accumulation in T- tubules
6
Q
Post spike hyperpolarization
A
- 3 to 5 msec in duration
- Until increased PK turns off
- Amplitude around 5 mV
7
Q
Negative afterpotential
A
- Around 30 msec in duration
- Due to K accumulation outside membrane
- Stays until K can diffuse away
8
Q
Positive afterpotential
A
- 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
9
Q
Activation/inactivation
A
- Na current turns on
- This is followed by Na current turning off
10
Q
Same time as Na current turns off,
A
- K current turns on (activation)
- Will be maintained as long as membrane is clamped
11
Q
Hodgkin cycle
A
- The self-sustaining entry and circular role of Na during depolarization
12
Q
Blockers of sodium channels
A
- TTX
- Saxitoxin
13
Q
Cardiac muscle ion currents during action potential
A
- 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
14
Q
Smooth muscle ion currents during action potential
A
- 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
15
Q
Voltage inactivation
A
- 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
16
Q
Accommodation
A
- Slow depolarization
- Threshold is passed without an action potential being fired
17
Q
Accommodation is due to
A
- 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
18
Q
K channels open in response to
A
- Depolarization
- Make the membrane more refractory to depolarization
19
Q
Because of accommodation, a weak stimulus
A
- Will not elicit an action potential no matter how long applied
- Hence, the rheobase on strength-duration curve
20
Q
Depolarizing drugs such as potassium cause nerve blocks or muscle paralysis due to
A
- Accommodation
- Voltage inactivation
21
Q
Conduction velocity is determined by
A
- Cm
- Electrical resistance to current flow
22
Q
Typical value of Cm
A
- Around 10 farads/cm2 membrane
23
Q
To depolarize membrane from -100 mV to 0 mV,
A
- 10 coulombs of charge must flow across each square centimeter of membrane
24
Q
Cm determines
A
- 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
25
Resistance also determines how rapidly charge can flow (two components)
- Rm: membrane resistance
| - Rin: the resistance to longitudinal current flow in cytoplasm
26
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
27
The larger the length constant,
- The further the electronic conduction spreads
28
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
29
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)
30
Saltatory conduction occurs
- Impulses (action potentials) occur only at nodal membranes
31
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
32
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
33
Increasing [Ko] or decreasing [Ki] causes
- Depolarization
| - Mainly affects the resting membrane potential
34
Changing [Nao]/[Nai] mainly affects
- Peak of the action potentia
| - If there were no overshoot then [Nao] = [Nai]
35
Metabolic poisons inhibit
- Sodium/potassium pump
| - Neurons can still conduct millions of action potentials before failure
36
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)
37
Propagated with decremen
- Diminish in size as it moves away from site of stimulation
38
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
39
Propagated non-decrementally
- Size unchanged as you move away from site of stimulation
40
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)
41
Absolute refractory period
- No AP possible
42
Relative refractory period
- Attenuated AP possible upon greater than normal stimulation
43
During accommodation, action potentials fail to occur due to
- Voltage inactivation of Na channels because of slow depolarization of membrane
- Opening of K channels
44
Electronic spread is dependent upon
- Charge storage capacity of membrane, Cm
| - Ratio of Membrane resistance (Rm) to internal resistance (Rin) to current flow
45
Small Cm and large (Rm/Rin)^1/2 means
- Greater spread of current
| - Higher AP conduction velocity
46
Myelination increases conduction velocity
- Reduces Cm
| - Increases Rm/Rin
