L5 Action Potentials

Question 1

Define the term action potential?

Answer 1

An action potential is transient and regenerative change in the membrane potential that transmits electrical signals along nerve fibers.

Question 2

What is the function of action potentials?

Answer 2

Allows for communication within the body by transmitting nerve signals.

Question 3

What are four characteristics of local potentials?

Answer 3

1. Graded response
2. Can be summated
3. Both depolarizing and hyperpolarizing
4. Conducted with decrement

Question 4

What is a local potential?

Answer 4

Small changes in the resting membrane potential that are not propagated.

Question 5

What are four characteristics of action potentials?

Answer 5

1. all or nothing response
2. cannot be summated
3. only depolarizing
4. conducted without decrement

Question 6

What are the four phases of an action potential?

Answer 6

1. Phase 0: resting
2. Phase 1: rising/depolarizing
3. Descending/ repolarizing
4. Hyperpolarization

Question 7

Describe phase 0.

Answer 7

The resting membrane potential before the action potential begins. The membrane is said to be polarized during this stage with a -90mV negative membrane potential.

Question 8

Describe phase 1.

Answer 8

An action potential is initiated when stimulation is strong enough to cause the RMP to reach threshold (about -50mV).

When this happens, it opens the voltage gated sodium channels.
Sodium moves down its conc grad (high outside, low inside) and enters the cell. There is an influx of positively charged sodium ions inside the cell, which neutralizes the negative interior of the cell. The membrane potential moves in a positive direction and this is called depolarization of the cell.

This process repeats itself thereby setting up a Positive Feedback cycle at threshold. Resultantly, there is the rapid opening of several voltage-gated Na channels and there is a rush of sodium into cell and making the cell more positive.

Rising phase stops when the voltage-gated sodium channels close.

Question 9

Describe the mechanism of Voltage gated sodium channels.

Answer 9

This channel has two gates—one near the outside of the channel called the activation gate, and another near the inside called the inactivation gate.

Activated state: Channel opens(for 1msec) because of a conformational change in the activation gate resulting from depolarisation of the membrane. (moving from -90mV towards zero). This occurs somewhere between -70 and -50mv). Sodium pours into cell.

Inactivated state: Channel at rest. Conformational change flips the inactivation gate to the closed state resulting in the stopage of sodium ions flowing into cell. The membrane potential begins to return toward the resting membrane state, which is the repolarization process.

The inactivation gate will not reopen until the membrane potential returns to or near the original resting membrane potential level.

Question 10

Describe phase 2.

Answer 10

Slow activating potassium channels open.
Potassium in high conc on inside of cell will move along its concentration gradient (high inside, low outside) causing positive charge to leave the cell. While there is virtually zero flow of sodium ions to the interior. The membrane potential then becomes more negative (Repolarization) and there is a return to the RMP.

Question 11

Describe the mechanism of the voltage gated potassium channels.

Answer 11

During the resting state, the gate of the potassium channel is closed, and potassium ions are prevented from passing through this channel to the exterior.

When the membrane potential rises from −90 millivolts toward zero, this voltage change causes a conformational opening of the gate and allows increased potassium diffusion outward through the channel.

There is a slight delay in opening of the potassium channels because they open slowly, for the most part, they open just while the sodium channels are beginning to close because of inactivation.

Question 12

Describe Phase 3.

Answer 12

The hyperpolarization phase is a result of the duration of time the K channels stay open. N.B. they do not inactivate but rather deactivate once membrane potential returns to normal

RMP ends up overshooting

Question 13

Describe the propagation of an action potential.

Answer 13

The depolarization process travels along the entire length of the fiber. The transmission of the depolarization process along a nerve or muscle fiber is called a nerve or muscle impulse.

At axon hillock there is the presence of many Na voltage gated channels, axon potential generated, inside +ve relative to outside, it is attracted to the negative charge next to it , causing depolarization

No backward movement of action potential

Question 14

What is the refractory period?

Answer 14

Period when another action potential can’t be generated no matter what is done to the membrane potential

Question 15

What is the absolute refractory period?

Answer 15

The period during which a second action potential cannot be elicited even with a strong stimulus.

Na channels inactivated state, inactivation gate is closed, no matter the depolarization another action potential can’t be generated.

Question 16

What is the relative refractory period?

Answer 16

An action potential can be generated but one must apply a bigger current than usual to get to threshold because cell is in a hyperpolarized state.

Question 17

Describe how action potentials are conducted along myelinated axons.

Answer 17

The myelin sheath is deposited around the axon by Schwann cells in which contains the lipid substance sphingomyelin.

Sphingomyelin is an insulator that decreases ion flow through the membrane.

The Node of Ranvier is an area along the axon that is not myelinated and in which ions can flow with ease, thus action potentials occur only at the nodes.

Saltatory conduction refers to the terms given to the conduction of action potentials from node to node.

Question 18

What are three benefits of myelinated axons?

Answer 18

1. Increases the velocity of nerve transmission in myelinated fibres 100m/sec vs 0.25m/sec unmyelinated fibres.
2. Saltatory conduction conserves energy for the axon because only the nodes depolarise.
3. Repolarization can occur with little transfer of ions.

Question 19

What are two factors that affect the speed of conduction of action potentials?

Answer 19

1. length constant

2. myelinated axon

Question 20

What is the length constant?

Formula?

Answer 20

distance over which response decays to 37% of regional size.

square root of (resistance of neuron membrane/ internal neuron resistance)

Question 21

8. Explain the physiological mechanisms underlying the action of local anesthetics.

Answer 21

Local anesthetics for example procaine and tetracaine act directly on the activation gates of the sodium channels, making it much more difficult for these gates to open, thereby reducing membrane excitability.

When excitability has been reduced low, nerve impulses fail to pass along the anesthetized nerves.