5 - Action Potentials Flashcards
ions require ___ ____ to move across the membrane
ion channels
Na flows _____ through an Na channel because of the large concentration and electrical gradient
inwardly
what is the membrane potential for many neurones?
-80mV
what is the equilibrium membrane potential for Na?
+60mV
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 ?
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
K flows ____
outwardly
why does K flow outwardly?
the concentration gradient if outward and has an energy which exceeds the electrical gradient, which is inwards
the Vm is ___ to the equilibrium potential for K
positive
what is the the equilibrium (or reversal) potential of an ion
the equilibrium (or reversal) potential is the membrane potential where the net flow through any open channels is 0.
what is the equilibrium potential for K ?
-100mv
what is the equation that gives the equilibrium potential of an ion?
nernst
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)
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)
within a cell at membrane potential is more ___ than outside the cell membrane
negative
what is the effect of opening Na selective or K selective channels upon the membrane potnetial>?
the potential is driven towrds the equilibrium potential of the ion -
+ 16 for Na and -100 for K
ion channels can be ___- gated, ___ - gated or opened by ___ ___ e.g. ___, ___
voltage gates, ligand gated (opened by chemical substances) or opened by physical stimuli e.g. mechanical, thermal
what are the ion channels responsible for the action potential in neurones ?
Voltage-activated Na+ channels (Nav) (depolarizing)
Voltage-activated K+ channels (Kv) (hyperpolarizing)
what is responsible for the period of undershoot in an action potential?
K channels open and close slowly so too many K ions diffuse out of the cell causing hyperpolarisation
what happens to the magnitude and velocity of APs as they propagate along nerve cell axons?
stay the same
why is the ENa never reached during upstroke?
because the K pores dont allow this - by inward trickle
opening of voltage activates Na channels and depolarisation is a ____ feedback system.
Depolarisation causes an ___ in Na conductance causing the ___ movement of Na ions
positive, increase, inward
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 ______
negative, increase, outward, repolarisation
although VA Na channels initially open in response to depolarisation, what happens during maintained depolarisation?
they enter a non-conducting, inactivated state
what are the three states of the Na channels?
- open state (conducting)
- inactivated state (non- conducting)
- closed state (non-conducting)
____ is required for the Na channels to enter the closed state in readiness for opening and the generation of a further action potential
repolarisation
what causes the closed state of Na channel to become the open state
depolarisation
what causes the open state of Na channel to become the inactivated state?
maintained depolarisation
inactivation of the Na channel contributes to the ______ phase of the AP and is responsible for the ___ -__
repolarising phase, refractory period
what determines the time interval between APs
relative refractory period
what happens in the absolute refractory period - during downstroke
no stimulus, however strong, can elicit a second action potential (all Na+ channels inactivated)
what is the relative refractory period - during undershoot
a stronger than normal stimulus may elicit a second action potential (mixed population of inactivated and closed channels)
what does impulse conduction in an un-myelinated axon and a mylinated axon involve
passive spread of current - in un myelinated
saltartory conduction in myelinated
the longer the ___ ___ the greater the local current spread in an axon and the
greater local current spread in an axon increases AP ___ ____
length constant ,
conduction velocity
length constant = (rm/ri) ^0.5
You can increase length constant (lamda) by ____ ri and _____ rm
decreasing ri (maybe by increasing axon diameter)
Increasing rm (adding an insulating layer .
what are the cels that provide an insulating layer for axons in the PNS? and what are they for the CMS
schwann cells in PNS and oligodendrocytes in the CNS
what are the junctions between the myelinated sections of an axon called?
nodes of ranvier
what is saltatory conduction?
the AP jumps from one node of ranvier to the next
what is the point of saltatory conduction
speeds up transmission of signals
