Excitable cells

Question 1

what is flux of ions?

Answer 1

number of molecules that cross a unit area per unit time

Question 2

what is a current?

Answer 2

movement of ions due to a voltage/potential difference

Question 3

what is zero volts in terms of membrane potential?

Answer 3

reference electrode placed outside of cell hence membrane potential is relative to how positive or negative it is inside the cell from the outside of the cell.

Question 4

what is electrochemical equilibrium?

Answer 4

as ions move across a membrane, there is a charge separation. electrical forces balance diffusion forces. this causes a stable transmembrane potential to form.

Question 5

what is equilibrium potential (Ek)?

Answer 5

membrane potential at which electrochemical equilibrium has been reached thus preventing further diffusion of ions down its concentration gradient

Question 6

how can Equilibrium potential be calculated?

Answer 6

Nerst Equation: (RT/zF)ln(Xinside)/(Xoutside)

Question 7

what is the easier nerst equation?

Answer 7

(-61/z)log([Xin]/[Xout]) where X is concentration and z is valency.

Question 8

main ions that contribute to the membrane potential?

Answer 8

potassium
sodium
(chloride)

Question 9

which equation takes into account variable permeabilities of many ions to work out the membrane potential?

Answer 9

Goldman-Hodgkin-Katz equation

Question 10

what does depolarisation mean?

Answer 10

membrane potential becomes more positive towards 0mV

Question 11

what does overshoot mean?

Answer 11

membrane potential becomes positive above 0mV

Question 12

what does repolarisation mean?

Answer 12

move towards a more negative membrane potential.

Question 13

what does hyperpolarisation mean?

Answer 13

membrane potential becomes more negative than resting potential.

Question 14

what are graded potentials?

Answer 14

potentials occurring at lower than threshold hence don’t trigger all or nothing AP. the larger the stimulus, the higher the potential. they are decremental as you move along a cell. this occurs at synapses and sensory receptors.

Question 15

where do action potentials occur?

Answer 15

in excitable cells like muscle and nervous tissue and some endocrine.

Question 16

describe what happens at resting potential (-70mV)?

Answer 16

permeability of potassium much higher than permeability of sodium as more ion channels open. hence nearer to potassium Ek.

Question 17

describe what happens in depolarisation?

Answer 17

If stimulus lets enough sodium in to increase potential to threshold, the permeability of sodium then increases rapidly as voltage gated sodium channels open allowing rapid influx of sodium down electrochemical gradient into the cell causing it to overshoot towards the sodium Ek.
As the excitable cell becomes more positive, permeability of potassium increases as more voltage gated potassium channels start to open SLOWLY. This means potassium slowly leaves the cell and therefore less sodium moves in as the electrochemical gradient changes.

Question 18

what happens in repolarisation from +30mV?

Answer 18

permeability of sodium decreases rapidly as voltage gated sodium ion channels close and inactivate meaning sodium entry into cell stops.
the permeability of potassium relatively increases as more voltage gated potassium channels open and remain open. This means potassium leaves cell down its electroconcentration gradient. meaning the cell becomes more negative.

Question 19

how many different gates does the Voltage gated sodium ion channel have?

Answer 19

2, activation gate and the inactivation gate.

Question 20

at the start of repolarisation, the absolute refractive period occurs, what is this?

Answer 20

in response to high potential, ball (with chain) blocks the sodium ion channel by closing the inactivating gate. this means a new AP cannot be triggered even if the stimulus is strong enough. it only reactivates when the membrane has been depolarised and the ball detaches/unblocks.

Question 21

what happens in late repolarisation?

Answer 21

the activation gate of the sodium ion channel closes

Question 22

what happens in hyperpolarisation?

Answer 22

potassium channels are still open hence K+ still leaves the cell meaning it becomes more negative than resting potential towards the potassium Ek. At this voltage, potassium ion channels start to close. resting potential is then restored using non voltage gated channels and NaK pump hence requires time.

Question 23

when does absolute refractor period stop?

Answer 23

at the start of hyperpolarisation.

Question 24

when does the relative refractory period start?

Answer 24

this means an action potential can be stimulated if stimulus is great enough. this occurs during hyperpolarisation. the inactivation gate opens but the activation gate is still closed but can be opened if stimulus great enough.

Question 25

what does all or nothing mean?

Answer 25

if threshold reached, same AP magnitude triggered no matter how big stimulus.

Question 26

what does the refractory state mean?

Answer 26

when unresponsive to threshold depolarisation.

Question 27

how does an AP propagate?

Answer 27

influx of sodium diffuses sideways causing local electrical current. the diffusion of sodium causes threshold to be reached adjacent to the neurone triggering new AP. An AP cannot be triggered behind the neurone as it is in refractory period. hence unidirectional.

Question 28

In myelinated neurones there is a saltatory conduction, what does this mean?

Answer 28

the action potentials jump from node to node meaning impulse transmission is faster. This is because there is voltage gated channels present at the nodes of ranvier. myelin provides electrical insulation so the APs jump.

Question 29

what affects impulse speed?

Answer 29

larger axon diameter means faster impulse transmission as less resistance
more myelination means faster as Abs jump.

Question 30

how is conduction velocity slowed?

Answer 30

• small axon diameter
• reduced myelination
• drugs, cold, hypoxia.