Action Potential

Question 1

What is the range of resting membrane potentials across all cells?

Answer 1

20-90mV

Question 2

What charge does the intra cellular fluid have with respect to the extra cellular fluid?

Answer 2

Negative charge

Question 3

List the concentration values of sodium, potassium and chloride ions in EXF and ICF

Answer 3

1. Sodium:
   •ECF = 145mM
   •ICF = 15mM
2. Potassium:
   •ECF = 4mM
   •ICF = 150mM
3. Chloride:
   •ECF = 110mM
   •ICF = 10mM

Question 4

What ion is easily able to diffuse out of the ‘resting’ cell membrane?

Answer 4

Potassium

Question 5

What creates resting membrane potential?

Answer 5

Potassium ions constantly diffuse out of the cell and leave excess -ve charge in the cell membrane.
The potential gradient arising from diffusion is the ‘resting membrane potential’

Question 6

Summarise resting membrane potential

Answer 6

1. RMP arises from the separation of charges on either side of the membrane
2. The RMP is due mainly to diffusion of potassium ions from the cell interior through potassium ion channels
3. The small amount of sodium ions that leak into the cell are expelled by the sodium/potassium pump
4. The pump also contributes to RMP as it moves 3 K+ ions in and 2 Na+ ions out

Question 7

What is the Action potential threshold value?

Why is this?

Answer 7

•55mV

• at this point the voltage gated sodium channels open due to influx of sodium
• this is the start of the rising phase

Question 8

What causes the falling phase in action potential?

Answer 8

Efflux of potassium ions through voltage gated K- channels

Question 9

Describe the three possible stages of a voltage-gated sodium channel

Answer 9

1. Channel closed
   - m-gate closed
   - h-gate open
   (RMP)
2. Channel open
   - m-gate open
   - h-gate open
   (Threshold level)
3. Channel closed (refractory)
   - m-gate open
   - h-gate closed
   (Hyperpolerisation)

Question 10

Summarise action potential

Answer 10

1. At ‘threshold’:
   - voltage-gated Na+ channels open
   - Na+ diffuse in = further depolarisation
   - positive feedback involved here
2. Peak
   - Na+ channels close; voltage-gated K+ channels open
   - potassium ions diffuse out = repolarisation
3. Return to resting membrane potential

Question 11

What name is given to the period of inexcitability that occurs after an AP is initiated?

Answer 11

Refractory period

Question 12

What causes the refractory period to occur?

Answer 12

In activation of voltage gated sodium channels

- the inactivation (‘h’) gates are shut, so Na+ can’t diffuse into the neuron

Question 13

What consequences are caused by the refractory period?

Answer 13

1. Limits firing frequency of action potentials in axons
2. Ensures unidirectional propagation of action potentials
3. Prevents summation of action potentials
4. Prevents summation of contractions in cardiac muscle - the cardiac AP lasts as longs as the ventricular contraction

Question 14

Explain how action potential travels along axons

Answer 14

1. An AP in one section of the axon sets up longitudinal current flow
2. This depolarises adjacent ‘resting’ parts of the axon
3. The AP is regenerated further along the axon
4. More current flows and the next region of the axon is activated
5. Action potentials travel along the axon as waves of depolarisation

Question 15

In what direction does current flow along the axon?

Answer 15

Positive to negative regions

Question 16

What factors increase the speed of AP propagation?

Answer 16

1. Increased axon diameter
   - however a very large diameter increase is needed to significantly increase conduction speed in unmyelinated axons
2. Myelin

Question 17

Briefly describe myelin and how it relates to nerve axons

I.e appearance, function

Answer 17

1. Myelin consists of many layers of cell membranes wrapped around the axon
2. Myelin inlaid down by glial cells
3. Myelin forms an insulating layer, reducing leakage of current from axon
4. Myelin sheath is interrupted at intervals (nodes of ranvier)

Question 18

What is the main function of the nodes of Ranvier?

Answer 18

Here, the axon membrane is exposed to the ECF and ion flow can occur to generate AP

Question 19

Why does AP propagate more rapidly in myelinated axons?

Answer 19

1. In myelinated nerves, the passive currents spread further along the axon
2. There are fewer regeneration steps per unit length of axon
3. Therefore AP propagates more rapidly

Question 20

What type of functions do peripheral nerve axons have?

Answer 20

Sensory

Motor

Question 21

What axon types are myelinated?

Answer 21

1. A beta and A delta fibres

2. C fibres are unmyelinated

Question 22

What function do A beta axons have?

Answer 22

Mechanoreceptors

Question 23

What functions do A delta axons have?

Answer 23

1. Mechanoreceptors
2. Thermoreceptors
3. Nociceptors
4. Chemoreceptors

Question 24

What functions do C axons have?

Answer 24

1. Mechanoreceptors
2. Thermoreceptors
3. Nociceptors