Flashcards in Action potentials Deck (34)
What causes changes in membrane potentials?
Ionic movements acrosss the cell membrane
What does the direction of the change in potential (hyperpolarization and depolarisation ) depend on?
-The direction of the movement of the ion
-the charge carried by the ion
What is the driving force for Na and what happens when it is negative?
Vm (membrane potential) - Ena ( equilibrium potential for Na+)
-when its negative, inward movement of Na+ occurs
How is the current carried by Na+ calculated?
Ina= gna ( Vm - Ena)
gna- Na+ conductance
Why does Na+ generate inward current?
-the concentration gradient is inward
-electrical gradient is inward
Why does K+ generate an outwards current?
-the concentration gradient is outward and has an energy which exceeds the electrical gradient which is inward
What is the driving force of K+ efflux and what happens when its positive?
-when positive, outward movement of K+ occurs
How is the K+ current calculated?
Ik= gk (Vm - Ek)
gk- K+ conductance
What happens to the membrane potential when Na+ channels open?
The membrane potential is driven towards ENa (equilibrium potential of Na)
What happens to the membrane potential when K+ channels open?
The membrane potential is driven towards EK (equilibrium potential of K)
What are ion channels?
protein complexes that span the lipid bilayer to form a central pathway that allows rapid flow of selected ions
What are the different ways that ion channels are gated?
-membrane voltage (voltage-gated channels)
-Chemical substances ( ligand-gated)
-physical stimuli- mechanical / thermal
What are the ions channels responsible for the action potentials in neurones?
-Voltage -activated Na+ channels (depolarising)
-volatge-activated K+ channels (hyperpolarizing )
What is the resting potential of a membrane?
Describe an action potential in neurone
-Na+ flows down electrical gradient so membrane potential moves to ENa+
-hyperpolarization occurs when K+ channels open and reverse system
What cause the undershoot in an action potential of a neurone?
Delayed closure of K+ channels
What causes the overshoot in an action potential of a neurone?
Delayed closure of Na+ channels
What are action potentials?
Brief electrical signals which the polarity of the nerve cell membrane is momentarily (2 sec) reversed
How do action potentials travel long distances?
They propagate along nerve cell axons with constant magnitude and velocity for a given axon
What does "all or none" in terms of action potentials mean?
Action potential are generated when threshold is reached, if the threshold is not reached (sub threshold), no action potential is created
What does it mean by the activation of Na+ channels is self-reinforcing?
The opening of a few channels causes further channels to open which causes further depolarisation etc..- POSITIVE FEEDBACK
What does it mean by the activation of K+ channels is self-limiting?
The outward movement of K+ causes repolarisation which turns off the stimulus for opening - NEGATIVE FEEDBACK
What are both Na+ and K+ channels activated by ?
Describe the conducting state of Na+ channels?
-depolarization initially opens Na channels into an open state
What are the 2 non-conducting states of Na channels?
-during maintained depolarisation, the channels are open but in an inactivated state
-after repolarisation, channels close
What is the refractory period and what causes it?
-There is no secondary action potential even when a nerve is stimulated
-Inactivation contributes to the repolarizing phase of the action potential and is responsible for the refractory period
what are the 2 types of refractory periods?
Describe the absolute refractory period.
no stimulus, however strong, can elicit a second action potential (all Na+ channels inactivated)
Describe relative refractory period.
a stronger than normal stimulus may elicit a second action potential (mixed population of inactivated and closed channels)