Hello everyone,
I have been having a lingering question regarding nerve impulses that I cannot seem to get an answer to, any help would be greatly appreciated.
As far as I understand, an action potential is an electrical phenomenon that occurs because of the difference in electrical charge between the inside and outside of a neural plasma membrane. This difference in electrical charge is caused by the differing concentrations of ions (Na+ and K+) inside and outside of the cell.
My question is, what causes the initial depolarisation (influx of Na+)? I know depolarisation can occur from electrical or chemical (neurotransmitter) stimulation from presynaptic neurons, but what causes the original action potential? What causes the initial influx of Na+?
Thank you in advance.
There are in fact many ways for the initial depolarization to occur... But the simplest answer is that many of the receptors for neurotransmitters are ion channels that open when the transmitter binds. i.e. Glutamate binds to AMPA (and NMDA) receptors and the channels (which are highly permeant to Na+ ions) open and depolarize the neuron (an excitatory postsynaptic potential EPSP). When enough AMPA receptors at enough synapses have been activated the depolarization is large enough to reach threshold for an action potential. The presynaptic axons you refer to also belong to neurons that receive synaptic input from other neurons...
'Pacemakers' are cells that have ion channels that open in a cyclical manner to periodically generate action potentials. These neurons can then also excite postsynaptic neurons.
Thank you for your answer. I was not aware of the existence of these 'pacemakers'.
A soma of a nuron N may receives several or many synapes. Each synape conveys excitatory neuonsmitters from a presynaptic neuron and make a small depolarization.. If many synapes come to the neuron N in about the same time and each synape gives excitatory neurotransmitters, each small depolarizaton from many synapes will make summation. If the sum of all depolarization in the axon hillock of the neuron N reach the firing level, this will stimulate the voltage gated Na+ channels to open and the first action potenial of the neuron N arise.
Dear Gjon Rakipi
“Action potentials are caused when different ions cross the neuron membrane. A stimulus first causes sodium channels to open. Because there are many more sodium ions on the outside, and the inside of the neuron is negative relative to the outside, sodium ions rush into the neuron.”
Please refer to the following web page:
http://faculty.washington.edu/chudler/ap.html