5.1 Action Potential And Its Properties

Question 1

What is an action potential?

Answer 1

Change in electrical potential (voltage across membrane) associated with an impulse along membrane of muscle/nerve cell

Question 2

What does an action potential depend on?

Answer 2

1. Ionic gradients of membrane
2. Relative permeability of membrane
3. Membrane potential reaching threshold (all or nothing)

Question 3

What is the threshold?

Answer 3

To generate an AP, axon requires a stimulus of a certain minimum strength raising MP to a certain level

Question 4

What is the axon hillock?

Answer 4

The point along the axon that needs to depolarise to threshold for the entire axon to depolarise

Question 5

What happens to cause a membrane potential?

Answer 5

Movement of ions dictated by no. Of channels open for that particular ion

Question 6

What is conductance?

Answer 6

E.g. Opening channels for a particular ion = conductance for that ion increasing.
This leads to membrane potential being closer to eq potential for that ion

Question 7

How can we measure how channels open and close with time (membrane potential)? What were the results?

Answer 7

Need a technique to hold MP but see ion flow.
Voltage clamp enables membrane currents to be measured at a set membrane potential

1. Decide MP (e.g start at -70 and suddenly change to -20, then back to -70)

We find:
-70=>-20 - increased sodium ion flow inward, so inc sodium channel
Maintained depolarisation - current carried by sodium wanes to 0 v quickly - INACTIVATION
Potassium current increases more slowly and slight delay when turned on. No inactivation.
-20=>-70 - K+ do not close immediately, take time

Question 8

Stages of an AP?

Answer 8

1. Resting membrane potential
2. Depolarising stimulus
3. Voltage gated Na+ channels open, Na+ in, voltage gated K+ channels open SLOWLY. Membrane depolarises to threshold
4. Rapid Na+ entry depolarises cell
5. Once Na+ channels are open, prone to inactivation (close).
6. K+ channels still open - efflux causing repolarisation
7. K+ channels do not close immediately and so hyperpolarisation
8. Voltage gated K+ channels close, K+ enters cell again through leaky channels
9. Cell returns to resting ion permeability and resting membrane potential

Question 9

What happens when sodium channels inactivate and what consequences does it have?

Answer 9

Sodium channels inactivate shortly after opening so stop working (they do not close though). Cannot start working again until they recover, and cannot recover until MP negative again.

It allows refractory periods:
Absolute: doesn’t matter how strong stimulus is at this point, AP will not be initiated
Relative: if you give a strong enough stimulus, AP initiated

Question 10

What is the structure of a voltage gated Na+ channel?

Answer 10

1. 1 peptide
2. 4 similar parts (i.e. Repeats)
3. P region - pore region - AA sequence allowing Na+ through more than other ions
4. 4th membrane spanning domain has lot of +ve AA - creates voltage field - if you change MP, voltage field changes causing conformational change - pore opens
5. Channel susceptible to inactivation - between 3rd and 4th domain, inactivation particle which is like a plug. Doesn’t come out of the pore until hyperpolarisation.

Question 11

How do local anaesthetics have their effect?

Answer 11

1. They work by block voltage gated Na+ channels
2. If you block nerve fibres, no AP’s, no pain
3. Blocked in a use-dependent manner (blocker gets to channel more easily when opens - so may feel pain initially but then eventually pain decreases)
4. Block in following order - small myelinated axons, un-myelinated axons, large myelinated axons

Question 12

How does a nerve impulse travel along axon from axon hillock to other end?

Answer 12

Local circuit theory

• when Na+ flows in, +ve charge repels any other +vely charged ions and attracts -vely.
• sets up local current
• once you get depolarisation at one point, passive/instantaneous depolarisation spreads by pushing it to threshold

Question 13

What is conduction velocity?

Answer 13

1. Speed at which an electrochemical impulse propagates down a pathway

Question 14

What is capacitance (Cm)? What does high capacitance lead to?

Answer 14

Ability to store charge (rises with size as larger tissue can hold more electrons so more charge)

High capacitance means voltage changes more slowly in response to current injection

Question 15

What does conduction velocity vary with? Why?

Answer 15

Diameter of axons

Larger diameter, larger conduction velocity

Question 16

What is membrane resistance (Rm)? What does high resistance result in?

Answer 16

Dependent upon number of ion channels open
Lower the resistance, more ion channels open

High resistance - change in voltage spreads further along axon

Question 17

What does the spread of local current depend upon?

Answer 17

Membrane resistance (Rm)
Capacitance (Cm)

Question 18

What is myelination?

Answer 18

Folding of schwann cell over axon.
Gaps are nodes of ranvier (where high conc of Na+ channels are found)
Allows large increase in membrane resistance = shut channels so AP spreads further
Large decrease in membrane capacitance so voltage changes very quickly

Question 19

Myelin sheath improves conduction by (4)

Answer 19

1. Large increase in membrane resistance (Rm)
2. Decrease in membrane capacitance (Cm)
3. Increase length constant (due to above)
4. Decrease in time constant

Question 20

What is saltatory conduction?

Answer 20

AP jumps from node to node resulting in much faster conduction velocity.
Myelin is a good insultator
Causes local circuit current to depolarise next node above threshold

Question 21

Describe demyelination and what it can lead to.

Answer 21

1. AP arrives at damaged myelin, poor working local current, not depolarised to threshold so no AP (Stops saltatory conduction)
2. Multiple Sclerosis (Immune system targets proteins in myelin sheath CNS)
3. Landry-Guillain-Barre syndrome (peripheral NS)

Question 22

What is the structure of K+ channels?

Answer 22

1 subunit making functional channel
P region specific to K+ channel (AA sequence)
voltage sensing domain on 4th transmembrane region