neuroscience - action potentials

excitatory postsynaptic potentials (EPSPs)

sodium channels open. driven by the electrochemical gradient, NA+ ions bring their positive charges into the cell, where the cell depolarizes or moves closer to zero to become more positive. this increases the likelihood that the neuron will be able to fire an action potential

inhibitory postsynaptic potentials (IPSPs)

chloride channels open. equilibrium potential of chloride is -65V, so if cell is at rest at -60V, this will cause the cell to become more negative, or hyperpolarize. as a result, it decreases the likelihood that the neuron will be able to fire an action potential

temporal summation

postsynaptic potentials that result from the same input and can be added together if they occur close in time

spatial summation

potentials that occur near each other on the cell membrane can add together

resting potential

the neuron is resting at -70V

• sodium pump is closed

• potassium pump is closed

• small amount of potassium leaks out

• 
  

depolarization

voltage-gated Na+ channels open, leading to a rapid influx of positive Na+ ions to enter the cell

repolarization

the voltage-gated Na+ channels inactivate, and the voltage-gated K+ channels open, allowing for positive K+ ions to flow out of the cell (potassium efflux), making the cell more negative again

hyperpolarization

some voltage-gated K+ channels stay open/are slow to close, leading to excess positive K+ ions to continue leaving the cell. this leads to an “undershoot” where the cell polarizes beyond the resting threshold.

returning to rest

voltage-gated K+ channels close, and the sodium-potassium pump works to restore original balance; uses ATP to move 3 K+ out and 2 Na+ in

absolute refractory

during the rising and falling phase; a second action potential cannot be fired, Na+ channels are either open or inactivated

relative refractory

while the neuron is hyperpolarized. a larger than normal stimulus is needed to reach threshold to fire an action potential than when the cell is at rest