Action potential 1
Action potential 1
Action potentials are a sequence of changes in the membrane potential that affect an entire excitable membrane. An action potential is generated in the first segment of the axon (the axon hillock) when the input of signals entering the neuron via the dendrites has reached a certain threshold. Usually a depolarisation of 60mV is enough to initiate the processes of an action potential according to the all-or-none principle. Then voltage dependent ion channels play the main role in the events during an action potential. How these channels open or close depending on the membrane potential is explained across a four step sequence.
An inactive neuron always has a membrane potential difference of about -70 milliVolts and is considered at rest. This rest potential is due to a difference in ion concentration at both sides of the membrane. Inside the cell,
potassium ions are more abundant than sodium ions. In contrast, the extracellular fluid contains much more sodium ions than potassium ions. Via leak channels in the membrane, potassium ions and sodium ions go down their concentration gradient. This means that potassium ions leak out of the neuron and sodium ions leak into the cell. These leak channels are always open. Potassium diffuses more easily out of the cell than sodium ions enter the cell. If this leakage process would go on without compensation, the membrane potential would be zero. The sodium-potassium pump stabilizes the rest potential by pumping 2 potassium ions into the cell and 3 sodium ions outwards. This process of pumping ions against their concentration gradient, of course, requires energy. So, the rest potential is a balance of passive and active mechanisms, which transport ions over the membrane.
The primary determinant of the resting membrane potential is:
Extra info: Negatively charged ions do not play a role in the formation of the transmembrane potential. Potassium diffuses more easily out of the cell than sodium ions enter the cell, and thus the permeability of potassium is in the first place responsible for the membrane potential in rest.
Gated channels that open or close in response to a change in the membrane potential are:
Extra info: Voltage dependent channels react on changes in membrane potential. Chemically regulated channels are controlled by chemical compounds. Mechanically regulated channels act upon mechanical stimuli.