Electrical Properties of cells- the action potential

Question 1

What is the Vm

Answer 1

The amount of work done to separate the ion pairs

Question 2

Define Compacitance

Answer 2

The ability to store charges on the inner and outer surfaces

Question 3

4 stages of AP

Answer 3

Small depolarisation triggers-

1. Rest
2. Depolarisation
   3.Repolarisation
3. Hyperpolarisation- the membrane goes more negative than resting potential
   The period at which no AP can be triggered- refractory period.

Absolute refractory period is when no action potential can be triggered

Question 4

Properties of AP

Answer 4

1. Triggered by depolarisation
2. Threshold of depolarisation required for AP- ALL or NOTHING
3. Propagation without decrement- stays the same amplitude
4. At peak Vm reach E na
5. The membrane at AP is inevitable during the refractory period - which limits the frequency of the AP

Question 5

What causes there to be changes in Vm which trigger AP

Answer 5

AP caused due to flow of Na and K through voltage gated channels

They can either be open or closed
The probability of opening and closing is determined by voltage across the channel

Question 6

What happens when a cell becomes permeable got a a particular ion

Answer 6

The ion will move down its concentration gradient driving Vm towards the equilibrium potential for that ion

During AP, membrane becomes more permeable to NA first then K- E Na then Ek

Question 7

Depolarisation

Answer 7

opens NA voltage gated channels- FAST. Due to positive feedback- causes more Na to open

Na influx

It abruptly finished due to prolonged depolarisation (sodium inactivation)

Na channels close

Vm-E Na

Question 8

Repolarisation

Answer 8

Open K+voltage gated channels- SLOW -

K+ efflux

Repolarised

Vm- E k

Question 9

Why is there a threshold for excitation

Answer 9

At rest the permeability of K is larger than permeability of Na

Flow of K+ counteracts flow of Na+ in

Question 10

How much charge separation is required for 100 mV

Answer 10

1 picomole per cm^3 for 100mV

few ions are separated in the AP- practically no change in electrochemical gradient and no osmotic effect

Question 11

Propagation of AP

Answer 11

R (longitudinal) along membrane
R (membrane) across the membrane

Transmembrane current is either - RESISTIVE- ions flow through the channel or CAPACITATIVE- ions approach one surface of the membrane- another is expelled from the other side

Question 12

What is saltatory conduction

Answer 12

Activation at a distance- myelin is an electrical insulator- the electrical waves will go straight to the next node of Renvier

Theres a delay because takes time to change the shape of the proteins

Question 13

What does myelination result in

Answer 13

Increase in conduction velocity

As axon diameter increase, the R L decreases

R M increased as current forces through to the next axoplasm to the next node

Decreased C M, charge is not stored at the membrane

Question 14

What does unmyelination result in

Answer 14

Axons conduct slowly

Thinner axons- more resistance- high longitudinal axoplasmic resistance

Question 15

Why else is hyper polarisation useful

Answer 15

prevents the AP from travelling backwards