NAS L1&L3

Question 1

What is an electrical signal?

Answer 1

Change in balance of positive and negative charges

Question 2

How is an electrical signal generated?

Answer 2

By transfer of ions through ion channels

Question 3

Conc of K+ in and out of cell?

Answer 3

Inside: 100 high
Outside: 5 low

Question 4

Conc of Na+ in and out of cell?

Answer 4

Inside: 15 low
Outside: 150 high

Question 5

Conc of Ca2+ in and out of cell?

Answer 5

Inside: 0.0002 low
Outside: 2 high

Question 6

Conc of Cl- in and out of cell?

Answer 6

Inside: 13 low
Outside: 150 high

Question 7

What maintains ionic gradients?

Answer 7

Na+/K+ pump. Exchanges Na+ from inside out and K+ from outside in, against conc gradients. Ca2+ pump transports Ca2+ out of cell. These pumps operate in the background continuously, over long time periods. Constant K+ efflux will lead to significant change in conc. PUMPS MAINTAIN CONCS OVER LONG TERMS NOT FOR CONTROLLING Em.

Question 8

What is resting membrane potential?

Answer 8

Voltage inside a cell.

Question 9

What is the RMP for neurons?

Answer 9

-65mV

Question 10

What determines RMP?

Answer 10

Leak channels, ion movement is passive. Non-gated (leak) - set Em for resting membrane.
There is an excess of negative charge inside.

Question 11

How does RMP remain stable?

Answer 11

By the movement of ions across the membrane.

Question 12

At RMP how much Na influx and how much K efflux?

Answer 12

Na influx = K efflux

Question 13

What factors influence movement of the ions?

Answer 13

Chemical gradient = unequal ion distribution and electrical force = if attracted/repelled by voltage inside cell.
When electrical and chemical influence balance there is no net movement of ions.

Question 14

What controls ion movement across a resting membrane?

Answer 14

Present of ion channels.
Non-gated: K, Na channels
Therefore membrane has permeability to each ion. Permeability is in part dependent on number of channels for that ion.

Question 15

Nernst - equilibrium potential of an ion…

Answer 15

the voltage at which equilibrium of ions takes place depends on permeability and conc gradient.

Question 16

What equil potential for Na and K

Answer 16

ENa = +62mV
EK = -80mV

Question 17

What is ionic driving force?

Answer 17

net force from chemical and electrical influences. Ionic force is present whenever Em is different from equilibrium potential for the ion

Question 18

Why is Em -65mV in neurons?

Answer 18

Rests between equilibrum potentials for EK and ENa (and other ions; Cl- and Ca2+). But actual value depends on relative permeabilities. PK = 40 x PNa, so Em closer to Ek

Question 19

How do you achieve positive charge?

Answer 19

Need positive charge influx

So need to open more Na channels i.e. increase in PNa

Question 20

What causes repolarisation?

Answer 20

Increase in K+ efflux and terminating activity of ‘extra’ Na+ channels

Question 21

What is conductance (g) of ion channels

Answer 21

= permeability (conductance easily measured). Membrane acts as an electrical resistor (R). conductance (g) = 1/R. Each ion has its own conductance
g = number of open ion channels. Change in g for one ion –> change in Em. This is due to changes in amounts of charge inside the cells.

Question 22

Which channels open during depolarisation?

Answer 22

Na v.g. and leak Na. V.g. are ‘extra’?

Question 23

What causes the initial depolarisation?

Answer 23

Synapses and generator potential

Question 24

Describe the events underlying an action potential.

Answer 24

1) opening of v.g. Na channels (increase in Na conductance). Na entering leads to further depolarisation leading tofurther increase in Na conductance. Em approaches ENa.
2) Na channels inactivate (decrease in Na conductance). Na influx stops despite maintained stimulus. Delayed opening of v.g. K channels leads to increase in K conductance, K efflux, repolarisation (but K conductance increases after Em returns to rest, so Em approaches Ek - hyperpolarisation.
3) V.g. K channels close leading to decrease in conductance
4) Em returns to resting value (leak channels)

Question 25

During AP why does Em approach ENa?

Answer 25

At rest PNa = PK/40. During AP PNa = 1000 x PK

Question 26

What are action potentials described as as events?

Answer 26

All or none events. If Na influx by leak and v.g. > K efflux by leak then you get depolarisation. Threshold is the Em value where this condition is satisfied. A stimulus needs to bring cell to threshold. Na influx leads to depo which leads to opening of Na channels which leads to Na influx etc

Question 27

How does the ap spread in a myelinated axon?

Answer 27

Myelin sheath increases speed of A conduction: ions flow to next node. Depo and A.P. at each node.

Question 28

How does ap spread in an unmyelinated axon?

Answer 28

Passive spread of depolarisation and v.g. Na channels. Positively charged ions inside repel negative ones out, depo, opening of na channels and this moves along the axon.