Lectures 7 & 8: Action Potentials

Question 1

Action potential

Answer 1

• Rapid change in membrane potential of a cell

- Followed by a return to the resting Vm

Question 2

Action potential characteristics

Answer 2

• 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

Question 3

Action potentials are produced and conducted by

Answer 3

• The plasma membrane of cells

- Used for communication over long distances

Question 4

Chronaxie is useful as index of

Answer 4

• Membrane excitability

- The larger the chronaxie, the less excitable the preparation

Question 5

After depolarization, Vm does not return to rest immediately, but stays depolarized to some degree

Answer 5

• For 1-2 msec
• Seen in muscle usually, not nerve
• May be due to K accumulation in T- tubules

Question 6

Post spike hyperpolarization

Answer 6

• 3 to 5 msec in duration
• Until increased PK turns off
• Amplitude around 5 mV

Question 7

Negative afterpotential

Answer 7

• Around 30 msec in duration
• Due to K accumulation outside membrane
• Stays until K can diffuse away

Question 8

Positive afterpotential

Answer 8

• 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

Question 9

Activation/inactivation

Answer 9

• Na current turns on

- This is followed by Na current turning off

Question 10

Same time as Na current turns off,

Answer 10

• K current turns on (activation)

- Will be maintained as long as membrane is clamped

Question 11

Hodgkin cycle

Answer 11

• The self-sustaining entry and circular role of Na during depolarization

Question 12

Blockers of sodium channels

Answer 12

• TTX

- Saxitoxin

Question 13

Cardiac muscle ion currents during action potential

Answer 13

• 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

Question 14

Smooth muscle ion currents during action potential

Answer 14

• 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

Question 15

Voltage inactivation

Answer 15

• 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

Question 16

Accommodation

Answer 16

• Slow depolarization

- Threshold is passed without an action potential being fired

Question 17

Accommodation is due to

Answer 17

• 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

Question 18

K channels open in response to

Answer 18

• Depolarization

- Make the membrane more refractory to depolarization

Question 19

Because of accommodation, a weak stimulus

Answer 19

• Will not elicit an action potential no matter how long applied
• Hence, the rheobase on strength-duration curve

Question 20

Depolarizing drugs such as potassium cause nerve blocks or muscle paralysis due to

Answer 20

• Accommodation

- Voltage inactivation

Question 21

Conduction velocity is determined by

Answer 21

• Cm

- Electrical resistance to current flow

Question 22

Typical value of Cm

Answer 22

• Around 10 farads/cm2 membrane

Question 23

To depolarize membrane from -100 mV to 0 mV,

Answer 23

• 10 coulombs of charge must flow across each square centimeter of membrane

Question 24

Cm determines

Answer 24

• 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

Question 25

Resistance also determines how rapidly charge can flow (two components)

Answer 25

- Rm: membrane resistance | - Rin: the resistance to longitudinal current flow in cytoplasm

Question 26

Length constant

Answer 26

- √(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

Question 27

The larger the length constant,

Answer 27

- The further the electronic conduction spreads

Question 28

Effect of fiber size

Answer 28

- The larger the fiber diameter, the greater the conduction velocity - Diameter affects resistances and capacitances in manner so this relationship holds

Question 29

Myelin increases the conduction velocity by

Answer 29

- 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)

Question 30

Saltatory conduction occurs

Answer 30

- Impulses (action potentials) occur only at nodal membranes

Question 31

The Na pump is more metabolically efficient because

Answer 31

- 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

Question 32

With certain "demyelinating diseases" conduction velocities may be sufficiently reduced to cause

Answer 32

- Significant impairment of sensory or motor function | - As part of patient assessment measurements of nerve conduction velocity may be conducted

Question 33

Increasing [Ko] or decreasing [Ki] causes

Answer 33

- Depolarization | - Mainly affects the resting membrane potential

Question 34

Changing [Nao]/[Nai] mainly affects

Answer 34

- Peak of the action potentia | - If there were no overshoot then [Nao] = [Nai]

Question 35

Metabolic poisons inhibit

Answer 35

- Sodium/potassium pump | - Neurons can still conduct millions of action potentials before failure

Question 36

Graded potential features (also called passive or electrotonic potentials)

Answer 36

- All are sub-threshold - Magnitude varies with stimulus - Propagated with decrement - Persist only as long as stimulus is present (sometimes fail to occur)

Question 37

Propagated with decremen

Answer 37

- Diminish in size as it moves away from site of stimulation

Question 38

Action potential features

Answer 38

- 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

Question 39

Propagated non-decrementally

Answer 39

- Size unchanged as you move away from site of stimulation

Question 40

Refractory behavior

Answer 40

- 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)

Question 41

Absolute refractory period

Answer 41

- No AP possible

Question 42

Relative refractory period

Answer 42

- Attenuated AP possible upon greater than normal stimulation

Question 43

During accommodation, action potentials fail to occur due to

Answer 43

- Voltage inactivation of Na channels because of slow depolarization of membrane - Opening of K channels

Question 44

Electronic spread is dependent upon

Answer 44

- Charge storage capacity of membrane, Cm | - Ratio of Membrane resistance (Rm) to internal resistance (Rin) to current flow

Question 45

Small Cm and large (Rm/Rin)^1/2 means

Answer 45

- Greater spread of current | - Higher AP conduction velocity

Question 46

Myelination increases conduction velocity

Answer 46

- Reduces Cm | - Increases Rm/Rin