Action Potential Practical

Question 1

What are the 2 types of electrical recording?

Answer 1

1. Transcellular recording
2. Extracellular recording

Question 2

How does transcellular recording work? What does it measure?

Answer 2

• The potential difference across a membrane may be recorded by inserting one electrode into a cell and placing the other on the surface of the cell or in the extracellular fluid.
• It measues the electrical activity of a single cell.

Question 3

How does extracellular recording work? What does it measure?

Answer 3

• Electrical activity can be recorded by placing two electrodes on the surface of a nerve trunk
• It measures the electrical activity of all cells adjacent to the electrodes

Question 4

What are the 4 reasons while the extracellular recording is used?

Answer 4

1. the measurement of nerve conduction velocity.
2. the effect of temperature on the excitability of the nerve.
3. the changes in excitability of the nerve that occur after it has been stimulated (refractory periods).
4. some relationships between nerve and muscle activity. (Here, the muscle compound action potential and resulting contraction are also recorded).

Question 5

What is recorded when a microelectrode pierces the nerve cell membrane? Why?

Answer 5

A potential between the inside and outside of the cell = transmembrane potential.

Question 6

What happens to this transmembrane potential when the nerve is stimulated? What ions are involved?

Answer 6

Action potential occurs, once reaching the threshold.

Na2+ and K+

Question 7

Why does the passage of a nerve impulse normally take place only in one direction?

Answer 7

Even though the impulse travels in both directions in the first nerve, it can only be transmitted across a synapse where the neurotransmitter is released (only neurotransmitter on one side)

Question 8

What sort of record is produced by recording from the extracellular fluid surrounding a nerve? Why?

Answer 8

A small current (through extra-cellular recording)

If two simple wire electrodes are placed close to the outside surface of the nerve, any change in polarity of charge across a membrane can be detected.

Question 9

What is a compound action potential? What is the magnitude of the potential related to?

Answer 9

The sum of action potentials in one nerve

Number of axons that respond with an action potential.

The sum of electrical activity resulting from the action potentials occurring simultaneously in a number of axons.

Question 10

How does COLD Ringer affect the compound action potential?

Answer 10

Cold- delays action potential (channels open later, threshold later)

Question 11

What is meant by the refractory period of a nerve? What happens to the recorded compound action potential if the nerve is stimulated in its refractory period?

Answer 11

During much of the action potential the membrane of a nerve is completely refractory to further stimulation i.e. will not fire a second action potential in response to a stimulus no matter how strong = absolute refractory period

During the last part of the action potential, a second action potential can be triggered but the stimulus must be stronger than normal to elicit it = relative refractory period

Question 12

What is the stimulus artefact?

Answer 12

The time of first stimulation

Question 13

What does the amplitude and duration of the stimulus artefact reflect?

Answer 13

The amplitude and duration of the stimulus artefact reflect the strength and duration of the stimulus applied to the nerve.

Question 14

What happens to the transmembrane potential, when a cell is depolarised?

Answer 14

made less negative

Question 15

An active response (the action potential) occurs when the membrane potential reaches a critical (threshold) level. This is due to the ______ (opening/closing) of sodium channels in the membrane by the ______ in the voltage field across them (that is, the channels are voltage sensitive). Sodium ions enter the cell through these channels making it even ______ (more/less) negative inside, until the transmembrane potential reverses so that the inside is ______ (positive/negative) relative to the external solution.

Answer 15

opening; change; less; positive

Question 16

This change of making the inside of the cell more positive than the outside only occurs for a short period because of 3 reasons. List them

Answer 16

(i) the sodium equilibrium potential (ENa) is approached, (ii) the sodium channels close (they inactivate), and (iii) voltage - sensitive potassium channels open, K+ ions exit and the membrane repolarizes.

Question 17

How can changes in voltage be recorded?

Answer 17

inserting a micropipette inside the nerve or muscle cells

Question 18

How can changes in polarity of charge across the active membrane be detected?

Answer 18

Extracellularly- as a small current is generated in the extracellular fluid

If two simple wire electrodes are placed close to the outside surface of the active membrane, this current can be detected as a voltage difference between the two points in the extracellular space.

Question 19

If many nerve or muscle fibres discharge (that is, fire action potentials) at the same time, then the response will be _____ (large/small).

Answer 19

large

Question 20

What is the threshold stimulus?

Answer 20

the minimum stimulus required to initiate an action potential in the axon

Question 21

As the stimulus voltage applied to a compound nerve is increased, ______ (more/less) axons, with _____ (higher/lower) thresholds, will be stimulated

Answer 21

more; higher

Question 22

As more individual action potentials reach the recording electrode, a compound action potential of ______ (higher/lower) amplitude is recorded, consequently there is a relationship between the strength of the stimulus applied to the compound nerve and the _____ of the response.

Answer 22

higher; size

Question 23

Why is there an upper limit to how big the compound action potential can get?

Answer 23

This is the point at which the stimulation threshold for all available axons has been exceeded and thus all axons have been recruited. Each axon arising from a motoneuron makes synaptic contact with a number of muscle fibres. The actual number of muscle fibres innervated by a motor neuron varies with the size of the muscle.

Question 24

What is the principle of a strength duration curve?

Answer 24

Stimuli of very short duration will not evoke an action potential in an axon no matter how intense the stimulus is. With stimuli of longer duration, the strength of the stimulus which excites a population of axons is related to its duration.

Question 25

In a strength duration curve, stimuli of very short duration ______ (will always/ will never) evoke an action potential in an axon no matter how intense the stimulus is. With stimuli of longer duration, the strength of the stimulus which excites a population of axons is related to its duration.

Answer 25

will never

Question 26

Briefly explain why there is a minimum voltage (1) and duration of stimulus (2) required to generate an action potential.

Answer 26

2 step process

1. need min voltage –> activate gated voltage channels –> Na+ channels open –> depol –> enough depol –> action potential
2. need duration to be processed -> activate gated voltage channels –> Na+ channels open –> depol –> enough depol –> action potential

Question 27

What is the principle of an absolute refractory period?

Answer 27

During much of the action potential the membrane is completely refractory to further stimulation i.e. it will not fire a second action potential in response to a stimulus no matter how strong.

Question 28

What is the principle of a relative refractory period?

Answer 28

During the last part of the action potential, a second action potential can be triggered but the stimulus must be stronger than normal to elicit it

Question 29

During which part of the action potential is the second stimulus delivered?

Answer 29

1. during relative refractory period
2. during relative refractory period
3. outside the refractory period
4. during the absolute refractory period