Action Potentials

Question 1

What is hyper polarisation

Answer 1

The mem pot becomes more negative

Question 2

What does ionic movements across the cell mem cause

Answer 2

Changes in mem pot

Question 3

The direction of the change in potential depends on

Answer 3

The direction of the movement of the ion or out of the cell

The charge carried by the ion

Question 4

What does the movement of ions across the mem need

Answer 4

Ion channels

Question 5

What is passive movement of an ion dept on

Answer 5

Driven by electrochem grad for that ion

Question 6

Why does sodium move inward

Answer 6

Conc grad is inward
Electoral grad inward
Mem potential -80mv negative to equilibrium potential for sodium
Driving force for sodium influx simply vm- Ena

Question 7

What are ion channels

Answer 7

Protein complexes that span lipid bilayer to form a central pathway that allows rapid flow of selected ions

Either open or closed

May be gated - mem voltage, chem subs, physical stimuli

Ion channels responsible for action potential in neurones

• voltage activated sodium channels depo
• voltage activated pott channels hyperporlarising

Question 8

What are the properties of action potentials in neurones

Answer 8

APs are brief electrical signals which polarity of nerve cell mem momentarily reversed

APs propagate along nerve cell axons with constant magnitude and velocity allowing signalling over long distances

APs are gen when threshold is reached therefore are all or none

Question 9

What are voltage activated sodium and pottasium channels

Answer 9

Both highly selective for their respectable cations
Both activated by mem depo - sodium channels rapidly, pottasium changes with a slight delay

Activation of sodium channels self reinforcing positive feedback
Activation of pott channels self limiting negative feedback

Question 10

What are the refractory period and sodium inactivatin.

Answer 10

After sodium channels being open state they enter non conducting inactivated state

Repo required for channel to enter closed state

Inactivation contributes to the repo phase of action potentail and is responsible to refractory period

Question 11

What is an absolute and relative refractory period

Answer 11

Absolute refractory peirod - no stimulus however strong can elicit a second action potential, all sodium channels inactivated

Relative refractory period - a stronger than normal stimulus may elicit a second action potential, mixed pop of closed and inactive channels

Question 12

How do local anaesthetics work

Answer 12

Such as lignocaine
Are very frequently employed to prevent action potentials discharge in sensory neurones in repsonse to noxious stimuli that ultimately give rise to perception of pain by the Brian

Block pore of voltage activated sodium channels
Increase amount of time that sodium channels sends in inactivated state

Net effect is to prevent the upstroke of the AP

Question 13

What does it mean the cell mem being leaky

Answer 13

Nerve cell mem is leaky hence passive signal do not spread far from site of origin due to current loss across mem

Question 14

Passive conduction and action pot velocity

Answer 14

Passive conduction - is a factor in propagation of AP
The longer the length constant the greater the local current spread
Greater local current spread increases AP condition velocity

Question 15

What are some strategies to increase passive current speed

Answer 15

Decrease ri ( by increase diameter)
Increase rm ( add insulation myelination) 
Conduction in myelinated axons much faster than non myelinated

Question 16

What happens at a node of ranvier

Answer 16

AP jumps from one node to the next

Question 17

What is depolarisation

Answer 17

The mem pot becomes less negative