ACTION POTENTIALS

Question 1

What are action potentials?

Answer 1

Rapid change in electrical potential across the plasma membrane of a cell

Question 2

describe how action potentials are generated?

Answer 2

Signals arrive from presynaptic neurons and generate positive/negative potentials in the environment around the dendrites
Electrotonic potentials spread passively from dendrites to soma where they are summed.
If net positive electric potential is greater than the threshold at axon hillock: an action potential occurs

Question 3

what is membrane potential?

Answer 3

The difference in electric potential between the interior and exterior of a cell, caused by a separation of electric charges. (mV)

Question 4

what is the resting potential? how is it set up?

Answer 4

-70mV
the membrane has a low protein permeability sot he negatively charged proteins stay within the cell, the Na+/K+ pump pumps out 3 Na+ and 2K+ in
and the leak channels are always open so K+ move freely through them out of the cell.

Question 5

what are leak channels?

Answer 5

channels that are always open and continually leak ions across the membrane in either direction

Question 6

what are voltage gated channels?

Answer 6

channels that open and close in response to voltage changes

Question 7

what are ligand gated channels?

Answer 7

channels which open and close in response to a chemical or drug

Question 8

do we tend to have more sodium inside or outside the cell? what about potassium?

Answer 8

normally there is more Na+ outside the cell and more K+ inside the cell.

Question 9

what is the equilibrium potential?

Answer 9

Voltage at which ion’s net flow across membrane is 0.

Question 10

what does the Nernst equation aim to do?
what does it take into account?
what does it not take into account?

Answer 10

enables the determination of cell potential under non-standard conditions. it considers the valence (charge) and concentration gradient.
it does not take into account permeability

Question 11

What is the Goldman equation? what does it take into account?

Answer 11

an equation used to calculate the electrical equilibium potential across the cell’s membrane in the presence of more than one ions.
it takes into account the valency, concentration and permeability of multiple ions.

Question 12

what happens at cellular level in the resting state of the action potential?

Answer 12

-70mV

all voltage gated Na+ and K+ channels are closed but leak channels are open so K+ can flow freely in and out

Question 13

what happens at cellular level in the depolarising state of the action potential?

Answer 13

-55mV (threshold)
Voltage gated sodium channels open but voltage gated potassium channels are closed so sodium flows into the cell very quickly increasing the charge within the cell.

Question 14

what happens at cellular level in the re-polarising state of the action potential?

Answer 14

voltage gated potassium ion channels open and voltage gated sodium channels are inactivated. This allows potassium to flow out of the cell and no more sodium can enter the cell which decreases the positive charge within the cell

Question 15

what happens at cellular level in the after-hyperpolarisation state of the action potential?

Answer 15

-80mV (relative refractory period)
Voltage gated potassium channels begin to slowly close and voltage gated sodium channels are closed like before. This causes an under shoot.
leaky potassium channels restore the resting membrane potential

Question 16

what is threshold membrane potential?

Answer 16

-55mV (can vary)

Question 17

how is the stimulus strength encoded?

Answer 17

by the frequency of action potentials as they are an all or nothing response.

Question 18

why do neurones only travel in 1 direction?

Answer 18

nerve impulses cannot travel in the opposite direction, because nerve cells only have neurotransmitter storage vesicles going one way, and receptors in one place. and previous Na+ channel are inactivated during the absolute refractory period.

Question 19

describe conduction in a myelinated axon?

Answer 19

Na+ entry at axon hillock locally depolarises the axon but no ion leakage happens due to the myelin sheath insulating the axon. Na+ and K+ channels are only at the Nodes of Ranvier so the action potential appears to jump- much faster

Question 20

what is saltatory conduction?

Answer 20

the way an electrical impulse skips from node to node down the full length of an axon, speeding the arrival of the impulse at the nerve terminal

Question 21

what does axon conduction velocity depend on?

Answer 21

1. axon diameter

2. myelination

Question 22

what is carbamazepine?

Answer 22

a drug to treat epilepsy and nerve pain by prolonging the inactivated state of voltage gated Na+ channels and absolute refractory period

Question 23

What is tetrodotoxin?

Answer 23

a Na+ channel blocker- present in pufferfish. victims die of respiratory paralysis

Question 24

outline the pathophysiology of multiple sclerosis?

Answer 24

an auto-immune disease where T-cells attack myelin sheaths causing inflammation, lesions and scarring in axon bundles.

Question 25

how do local anaesthetics work?

Answer 25

they reversible prevent transmission of action potentials within a local region by blocking voltage gated Na+ channels and thereby decreasing the rate of depolarisation

Question 26

what are the 2 classes of local anaesthetics?

Answer 26

esters and amides

Question 27

why are local anaesthetics sometimes administered with vasoconstrictors?

Answer 27

to prolong the efect