1 - excitable membranes lecture Flashcards
what are excitable membranes?
membranes that allow action potentials to occur eg. nerves, skeletal/cardiac muscle etc
what is the resting potential usually inside a cell with respect to the outside?
negative
what does a resting cell membrane have a high permeability to?
K+ — K+ crosses much more easily than Na+
what does the direction of diffusion of uncharged substances depend only on?
conc gradient — move down conc grads
what does the direction of diffusion of charged substances depend on?
- conc grad
- electrical gradient
how to +ve and -ve ions move according to electrical gradient?
- +ve ions move from +ve to -ve
- -ve ions move from -ve to +ve
what is resting potential established by?
K+ conc grad
when is equilibrium reached?
when the force produced by the conc grad is equal to the force produced by the electrical gradient
why is the membrane potential actually less -ve than the number given by the nernst equation?
Na+ can also cross the membrane
what could cause the RP to become less -ve in the cell and why?
kidney failure or tissue damage — K+ released from cells
what does ‘all or none’ mean when referring to action potentials?
- APs are all or none
- small (subthreshold) stimulus — no AP
- larger stimulus — AP of a fixed size
- the body codes stimulus intensity by changes of frequency not size of APs
larger stimulus = higher ___ of AP
frequency
bigger stimulus = bigger electrical charge in the nerve
what underlies the upstroke of the AP?
Na influx
how do local anaesthetics work?
inhibit Na channels — blocks nerve AP — blocks pain fibres
why does depolarising the cell membrane initiate the AP?
makes the Na+ channels open
descirbe the positive feedback of Na and depolarisation?
larger depolarisation makes Na+ channels open — Na+ influx —more depolarisation — more Na channels open — more na enters etc
what stops +ve feedback and allows repolarisation to RP?
- inactivation of Na channel
- activation of K channel
what does a simple Na channel look like?
what is the h gate like at rest? how is it affected by the MP?
it is open — when MP becomes less -ve it closes — stops Na+ entry
- close more slowly than m gate opening
describe the time course of permeability changes during AP
what is the absolute refractory period?
- no stimulus can provide a big enough AP
- cannot produce an AP
- because Na channels are inactivated
absolute vs relative refrac period
what does the refractory period result from?
- inactivation of Na channel
- activation of K channel
why is conduction velocity lower in multiple sclerosis?
myelin loss
