5 - Action Potentials

Question 1

ions require ___ ____ to move across the membrane

Answer 1

ion channels

Question 2

Na flows _____ through an Na channel because of the large concentration and electrical gradient

Answer 2

inwardly

Question 3

what is the membrane potential for many neurones?

Answer 3

-80mV

Question 4

what is the equilibrium membrane potential for Na?

Answer 4

+60mV

Question 5

The driving force for Na influx is simply (___-____). When this is _____ (in this case __mV) Na moves __wardly

The current carried by Na (Ina) is given by the expression:
INa = GNa (Vm - ENa)

what is Ina and Gna ?

Answer 5

The driving force for Na influx is simply (Vm - ENa). When this is negative (in this case - 140 mV) Na moves inwardly

The current carried by Na (Ina) is given by the expression:
INa = GNa (Vm - ENa)

Ina is Na current

Gna is Na conductance

Question 6

K flows ____

Answer 6

outwardly

Question 7

why does K flow outwardly?

Answer 7

the concentration gradient if outward and has an energy which exceeds the electrical gradient, which is inwards

Question 8

the Vm is ___ to the equilibrium potential for K

Answer 8

positive

Question 9

what is the the equilibrium (or reversal) potential of an ion

Answer 9

the equilibrium (or reversal) potential is the membrane potential where the net flow through any open channels is 0.

Question 10

what is the equilibrium potential for K ?

Answer 10

-100mv

Question 11

what is the equation that gives the equilibrium potential of an ion?

Answer 11

nernst

Question 12

The driving force for K efflux is simply (___-____). When this is _____ (in this case __mV) K moves __wardly

The current carried by Na (Ina) is given by the expression:
IK = GK (Vm - EK)

Answer 12

The driving force for K efflux is simply (Vm- Ek). When this is positive (in this case 20mV) K moves outwardly

The current carried by Na (Ina) is given by the expression:
IK = GK (Vm - EK)

Question 13

within a cell at membrane potential is more ___ than outside the cell membrane

Answer 13

negative

Question 14

what is the effect of opening Na selective or K selective channels upon the membrane potnetial>?

Answer 14

the potential is driven towrds the equilibrium potential of the ion -

+ 16 for Na and -100 for K

Question 15

ion channels can be ___- gated, ___ - gated or opened by ___ ___ e.g. ___, ___

Answer 15

voltage gates, ligand gated (opened by chemical substances) or opened by physical stimuli e.g. mechanical, thermal

Question 16

what are the ion channels responsible for the action potential in neurones ?

Answer 16

Voltage-activated Na+ channels (Nav) (depolarizing)

Voltage-activated K+ channels (Kv) (hyperpolarizing)

Question 17

what is responsible for the period of undershoot in an action potential?

Answer 17

K channels open and close slowly so too many K ions diffuse out of the cell causing hyperpolarisation

Question 18

what happens to the magnitude and velocity of APs as they propagate along nerve cell axons?

Answer 18

stay the same

Question 19

why is the ENa never reached during upstroke?

Answer 19

because the K pores dont allow this - by inward trickle

Question 20

opening of voltage activates Na channels and depolarisation is a ____ feedback system.

Depolarisation causes an ___ in Na conductance causing the ___ movement of Na ions

Answer 20

positive, increase, inward

Question 21

opening of voltage activates K channels and depolarisation is a ____ feedback system.

depolarisation causes an ___ in K conductance causing the ___ movement of K and thus ______

Answer 21

negative, increase, outward, repolarisation

Question 22

although VA Na channels initially open in response to depolarisation, what happens during maintained depolarisation?

Answer 22

they enter a non-conducting, inactivated state

Question 23

what are the three states of the Na channels?

Answer 23

• open state (conducting)
• inactivated state (non- conducting)
• closed state (non-conducting)

Question 24

____ is required for the Na channels to enter the closed state in readiness for opening and the generation of a further action potential

Answer 24

repolarisation

Question 25

what causes the closed state of Na channel to become the open state

Answer 25

depolarisation

Question 26

what causes the open state of Na channel to become the inactivated state?

Answer 26

maintained depolarisation

Question 27

inactivation of the Na channel contributes to the ______ phase of the AP and is responsible for the ___ -__

Answer 27

repolarising phase, refractory period

Question 28

what determines the time interval between APs

Answer 28

relative refractory period

Question 29

what happens in the absolute refractory period - during downstroke

Answer 29

no stimulus, however strong, can elicit a second action potential (all Na+ channels inactivated)

Question 30

what is the relative refractory period - during undershoot

Answer 30

a stronger than normal stimulus may elicit a second action potential (mixed population of inactivated and closed channels)

Question 31

what does impulse conduction in an un-myelinated axon and a mylinated axon involve

Answer 31

passive spread of current - in un myelinated

saltartory conduction in myelinated

Question 32

the longer the ___ ___ the greater the local current spread in an axon and the

greater local current spread in an axon increases AP ___ ____

Answer 32

length constant ,

conduction velocity

Question 33

length constant = (rm/ri) ^0.5

You can increase length constant (lamda) by ____ ri and _____ rm

Answer 33

decreasing ri (maybe by increasing axon diameter)

Increasing rm (adding an insulating layer .

Question 34

what are the cels that provide an insulating layer for axons in the PNS? and what are they for the CMS

Answer 34

schwann cells in PNS and oligodendrocytes in the CNS

Question 35

what are the junctions between the myelinated sections of an axon called?

Answer 35

nodes of ranvier

Question 36

what is saltatory conduction?

Answer 36

the AP jumps from one node of ranvier to the next

Question 37

what is the point of saltatory conduction

Answer 37

speeds up transmission of signals