how does action potenital happen ?

Basically, though a series of opening/closing Na+ channels which allow an influx of Na+ into the cell which changes the membrane potential.

Presynaptic cell recieves "signal" down at terminal button relases Neurotransmitter into synaptic cleft (space between presynapic neuron and post synaptic neuron) this neurotransmitter binds to receptors located on the motor end plate of the post synaptic neuron causing the opening of chemically gated channels. When these channels along the membrane open, they allow the positively charged ions of Na into the cell. This slight influx drives membrane potential (seperation of charge across a membrane usually -70mV for a cell) increase membrane potential bringing it closer to threshold. Finally, enough Na enters the cell, the membrane potential reaches threshold (normally around -55mV) and now at -55mV ALL of the Na channels can open up allowing in a HUGE influx of sodium into the cell. This huge influx skyrockets the action potential and the charge moves all down the axon until it reaches the end. At which point, when the signal reaches the end, it signals neurotransmitter to be released again, and then the process occurs once more and the next neuron.

I hope this helps! I left out all the technical names & stuff because I didn't want to confuse you. Best of Luck!

I will try to make it easy to understand.In order to have an action potential, first thing that you need to understand is the rest membrane potential which sets charge gradient. Resting membrane potential is created by the sodium and potasium ATPase, and it usually has electric potential around-70mV. Now you have a charge gradient which is ready to create action potnetial, however, you also need stimuli to reach certian threshold. There are two summation that cause this happen:1, Temperal summation which means the repeated stimuli happen at in a very short time; 2, Spatial summation, which means multiple happen at once. Either one of them will be a strong enough stimuli to reach the threshold which is around -40mV. Once the threshold is reached, the neuron will directly go to a stage called depolarization which caused by the opening of Voltage gated sodium channel, sodium ions flow into the cell, down its concentration gradien, respectively, the cell become more polar from -70mV to +30mV, this voltage trigerred a different responsed , called a action potential, followed by this stage are repolarization, hyperpolarization, restoration, final refactory peroid, then finish the stimulating process. Then the action will travel through axon by saltatoty conduction, which means the action potential will travel form nobe of ranvier to nobe of ranvier. Finally it will reach teh presynaptic terminal, where there is anothe channel called voltage gated Ca++ channel will open by the depolarization of the membrane, then Ca++ will flow into the cell, cause synaptic vesicles to empty the synaptic cleft, then the neurotransmitters which are released by presynaptic terminal will diffuse through the cleft, finally bind to the receptor of the Ligand gated sodium channel which situates on the postsynaptic cell(target cell), once the signal is properly bind with its receptor, the target cell will conduct accordingly. That is the whole process of nerve transmission. It may sound tedious, but I ahvealready shortened a lot.

presynaptic cells are activated into synaptic cells.