at rest, there is a potential difference across the membrane of a neuron. it will be -70mV (inside's negative in respect to the outside)
you have high K+ ions inside the cell and high Na+ outside the cell
the gradient is maintained by the Na+/K+ pumps which because of active transport use ATP to actively move ions against the concentration gradient.
the membrane needs some stimulation above the threshold value.
changes in the membranes potential difference alters the shape of the Na+ gates, causing some to open. Na+ therefore, moves down the concentration gradient; causing further Na+ channels to open. this is an example of Positive Feedback. The flow of ions entering leads to +40mV.
repolarisation occurs after 0.5ms when the Na+ voltage dependent gates close. Once these have closed, the voltage dependent K+ channels open, this allows for the K+ ions to flow out of the cell following the electrochemical graident and concentraion gradient. hence the falling phase on a oscilloscope.
action potential ...continued...
whilst the voltage dependent K+ channels are open the membrane is very permeable to K+ ions. More ions move out of the cell then normal, making the membrane slightly negative (hyperpolarisation), the resting potential is re-established when the voltage dependent channels close.
Things to remember~
some K+ channels don't close. (low voltage dependent channels)
at rest, the Na+/K+ pumps maintain a concentration gradient.
theres both low Na+ and K+ voltage dependent channels.
stage one: When an action potential arrives at the presynaptic neurone, the membrane depolarises. These depolarisations open the calcium ion channels.
stage two: Calcium ions flood into the presynaptic neurone through the open calcium channels, they fuse to the vesicles containing the neurotransmitter. this causes the vesicles to fuse the cell membrane and release their load via exocytosis.
stage three: once in the synapitc cleft the neurotransmitter diffuses from the area of high concentration to low concentration and binds to the receptors on the post synaptic membrane. this opens the cation channels that allow Na+ to enter the postsynaptic neuron and trigger another action potential (once the thresold has been reached)
stage four: binding of transmitters to the post synaptic neuron is irreversible. typically the transmitter will be broken down by enzymes in the synaptivc cleft to stop the continuous depolarisation of the pst synaptic neuron an example of an enzyme is Acetylcholinesterase.