Neuroscience 3 - Resting and Action Potentials Flashcards
Define flux
- The rate of transfer of molecules
- The number of molecules that cross a unit area per unit of time
Define voltage.
- Potential difference
- Unit: Volts
- Generated by ions to produce a charge gradient (i.e. like a chemical battery)
Define current.
- Unit: Amps
- Movement of ions due to a potential difference
Define resistance.
- Unit: Ohms
- Barrier that prevents the movement of ions
How is the membrane potential measured?
- A reference electrode is placed outside the cell. This is the zero volt level.
- Another electrode is placed inside the cell. It measures a voltage difference that is negative compared with the outside (i.e. reference).
What is electrochemical equilibrium?
This is where the concentration gradient of ions is balanced by electrical forces, so there is no further diffusion across membranes. This generates a stable membrane potential.
What is the equilibrium potential?
The potential at which electrochemical equilibrium has been reached. It is the potential that prevents diffusion of the ion down its concentration gradient
What is the nernst equation used for?
Calculating the equilibrium potential.
What is the Nernst equation and what does everything stand for?
E= (RT/zF)ln([x2]/[x1])
R = gas constant T = Temp. Kelvin Z = charge on ion -1 for Cl-, +2 for Ca2+ F = Faraday’s number charge per mol of ion ln = log to base e
What is the composition of Na+ inside and outside the cell?
150mM outside and 10mM inside
What is the composition of K+ inside and outside the cell?
5mM outside and 150mM inside
What is the equilibrium potential for K+?
-90mV
What is the equilibrium potential for Na+?
+72mV
What is the Goldman-Hodgkin-Katz equation?
It is an equation used to determine the resting membrane potential.
Describe graded potentials.
- They can differ by stimulus type, either excitation or inhibitory.
- The second is the stimulus strength,which may be strong or weak, causing a different change in membrane potential.
- Finally, the magnitude of the change in membrane potential will decrease with distance, so if measured 1mm from the stimulus site the depolarisation will be less.
- This is decremental spread
Where do graded potentials occur and what is their function?
- They occur at synapses and sensory receptors
- The function is to contribute to initiating or preventing action potentials.
What are the ion pumps responsible for?
- Concentration gradients of potassium and sodium
- Not the membrane potential - this is due to potassium diffusing out of cells.
Why is depolarisation faster than repolarisation?
- Sodium ion channels open quickly, and voltage gated ion channels also open causing depolarisation.
- Voltage gated potassium ion channels open slowly, and the movement of potassium is less than the sodium entering.
What causes depolarisation?
Movement towards the equilibrium potential of sodium.
What causes repolarisation?
Movement towards the equilibrium potential of potassium.
What is the absolute refractory period, and what causes it?
- This is the time where a new action potential cannot be triggered, even with a very strong stimulus.
- Caused by sodium channel inactivation
What causes sodium channel inactivation?
- When depolarisation occurs, this is detected by sodium ion channels and an inactivation protein prevents entry of sodium into the cell.
- It does this by a ball and chain mechanism, which swings and blocks the site of entry until repolarisation occurs, when it swings off again.
What is the relative refractory period, and what causes it?
- This is where, due to hyperpolarisation, a larger stimulus is required to trigger an action potential than normal.
- The inactivation gate is open.
List the 5 stages of action potential.
1) Resting membrane potential
2) Depolarising stimulus
3) Upstroke (depolarisation)
4) Repolarisation
5) After hyperpolarisation
What is the threshold?
-55mV. Once this potential is reached an action potential is triggered.
What is the nature of action potentials?
All or nothing - once triggered, a full sized action potential occurs
How is positive feedback behaviour present in action potentials?
- Opening of sodium channels causes more sodium channels to open due to change in voltage
- This cycle contiunues until voltage gated sodium ion channels inactivate
To what extent are ion pumps involved during action potentials?
- Not directly, they exchange some ions but this is a slow process.
- Changes occur to small numbers of ions crossing the membrane and changing the membrane potential (less than 1%)
How is electrochemical equilibrium restored following an action potential?
Potassium and sodium ions moving through non-voltage gated ion channels.
What is passive propagation?
- This is where an impulse travels down the axon passively, as it is less than the threshold potential.
- Only the resting potassium channels are open
What affects passive propagation?
- Membrane resistance and internal resistance alter propagation distance and velocity
Describe why action potentials are regenerative
- The potential change induced by the opening of voltage gated channels decreases as you record further and further away from the Na+ channel.
- This depolarisation triggers the opening of neighboring Na+ voltage-dependent channels, which results in more Na+ entering the cell, and, in effect, regenerates the action potential.
Describe the active propagation of action potentials
- The local current flow of ions depolarises the adjacent region towards the threshold.
- This stimulates sodium ion channels to open, and the action potential moves along the axon
- This continues down the axon, in one direction as the old active region is in the refractory period
What is saltatory conduction?
This is where areas of myelin contain fewer voltage gated channels, so the action potential jumps between the nodes.
Which factors increase conduction velocity?
- There is an increase with axon diameter, as the resistance to flow decreases.
- It is higher in myelinated neurones, as the action potentials only occur at the nodes of Ranvier.
What causes conduction velocity to slow?
- Reduced axon diameter, for example caused by regrowth after injury
- Reduced myelination (caused by multiple sclerosis and diptheria)
- Cold, anoxia (lack of oxygen), and drugs (some anasthetics).
