Action Potentials Flashcards
What are action potentials?
Action potentials are a change in voltage across a membrane. They depend in ionic gradients and the relative permeability of the membrane. They have an all or nothing principle so they only occur when threshold is reached. They are propagated without a loss of amplitude.
What two action potentials are fast and what two action potentials take longer?
Axon: Fast (0.5ms)
Skeletal Muscle: Fast (0.5ms)
SAN: Slower (100ms)
Cardiac Ventricle: Slower (100ms)
What happens if conductance to an ion is increase?
If conductance to any ion is increased, the membrane potential (Vm) will move closer to the equilibrium potential for that ion.
What is the conductance of the membrane to a particular ion dependant on?
The conductance of the membrane to a particular ion is dependant on the number of channels for the ion that are open.
What effect does the movement of a small amount of ions have?
The movement of a small amount of ions will produce a relatively large change in the membrane potential.
Experimentally, how can we show that Na+ is responsible for action potential depolarisation?
- Draw a straight line to show the predicted change in E na as the external concentration of sodium is reduced.
- The peaks of the action potential changes in a manner parallel to the changes in E Na.
- The supports the idea that the upstroke of the action potential is due to a large increase in permeability to Na ions.
What can be used to measure membrane currents at a set membrane potential?
Voltage clamps.
When looking at the curve of Na+ moving into the cell and causing depolarisation, Why does it show a quick increase and a quick decrease?
Quick increase as all channels open and, because of conc gradient, Na+ rushes in. But, also quick decrease as the channels close and, they undergo Na+ inactivation.
What is the consequence of K+ channels closing slowly?
K+ channels open slowly but, they take even longer to close. This causes hypoerpolarisation so, the membrane potential goes lower than resting.
What two things happen when a membrane depolarises?
Voltage gated Na+ channels inactivate (they are unable to open) and so, Na+ influx stops.
Voltage gated K+ channels opne and cause an efflux of K+ ions.
Both these actions cause the membrane to depolarise.
THE Na,K PUMP IS NOT INVOLVED.
What happens after an action potential has occurred?
A refractory period occurs. First, an absolute refractory period occurs where nearly all the Na+ channels are in the inactivated state.
Then, a relative refractory period occurs. This is when the Na+ channels are recovering from inactivation, the excitability returns towards normal as the number of channels in the inactivated state decrease.
What is the basic structure of a voltage gated Na+ channel?
A voltage gated Na+ channel consists of one alpha subunit containing four similar repeats forming a ring structure.
Each repeat contains six membrane spanning segments. The 4th membrane spanning segment is the voltage sensor and there is a pore between the 5th and 6th segments in which ions travel through.
What is the basic structure of a voltage gated K+ channel?
A voltage gated K= channel consists of four alpha subunits arranged in a ring each containing only one repeat.
Each subunit contains six membrane spanning segments. The 4th membrane spanning segment is the voltage sensor and there is a pore between the 5th and 6th segments in which ions travel through.
How do local anaesthetics work?
The block local Na+ channels. eg Procaine. They either go into a cell through the membrane or they, in the charge state, travel though the membrane of the cell.
What does use dependant mean?
Local aesthetics are more effective if Na+ channels are open. So, if patient feels a little bit of pain at first, anaesthetic will be more effective as more Na+ channels will be open so more places for it to bind.
