Action Potential

Question 1

At Rest

Answer 1

Na+/K+ pump actively transports 3Na+ out of cell and 2K+ in. Some K+ diffuse back out of cell through channgels, however Na+ channels are closed so cell impermeable to Na+. (-60mV)

Question 2

Depolarisation

Answer 2

Na+ channels open and Na+ diffuse into cell until it reaches threshold potential of -50mV. If the threshold potential is reached, VG Na+ channels open and more Na+ diffuse in resluting in postive +40mV.

Question 3

Repolarisation

Answer 3

The postive potential difference shuts Na+ channels and open K+ channels. K+ diffuse out of cell bring potential difference back to negative.

Question 4

Hyperpolarisation and At Rest

Answer 4

The potential difference overshoots -80mV, however Na+/K+ pump returns the potential difference to normal.

Question 5

How does an action potential move along a neurone?

Answer 5

When Na+ diffuse into cell they increase the local concentration of Na+ in that region of the cell. The Na+ diffuse down a concentration gradient away from region producing a local current. This causes Na+ channels further along the membrane to open so depolarisation can occur in the next region.

Question 6

Depolarisation at Synapse

Answer 6

When action potential arrives at synapse it causes Ca2+ channels to open and diffusion of Ca2+ into pre-synaptic bulb. This causes vesicles to move and fuse to the pre-synaptic membrane releasing ACh via exocytosis. Ach diffuses across synaptic cleft and binds to specific receptors on Na+ channels causing them to open. Na+ diffuse into post-synaptic bulb and if the generator potential is sufficent and threshold potential is reached, a new action potential is created. ACh is hydrolysed by acetylcholinesterase into choline and ethanoic acid to be recycled and repackaged into vesicles.