Resting Neurons - Do not have an action potential and the membranes are polarised
Polarised Neuron - The outer surface of the membrane is positively charged and the inner surface of the membrane is negatively charged. The difference between the membranes is -70mV and is known as Resting Potential. An impulse can only travel when the resting potential is achieved.
The resting potential is generated by sodium potassium pumps in the cell membrane. They move 3 sodium ions and 2 potassium ions by active transport (requires ATP). Potassium gets moved into the neuron and sodium gets moved out of the neuron. The Potassium within the cell then gets taken out of the cell via potassium channels (facilitated diffusion). This allows more sodium to be pumped out by the sodium potassium pump. At -70mV there is no net movement of potassium.
This causes the inner membrane to become more negatively charged than the outer membrane because more positively charged ions move out. There are also lots of negatively charged proteins within the cell
Depolarisation of the Membrane
- when a neuron is stimulated, sodium channels open and sodium diffuses into the cell
- the increase in positive charge in the cell causes the potential difference to change from -70mV to +40mV
- +40mV is known as threshold, and it's at this point that adjacent sodium channels open
- an action potential is generated
- depolarisation can only occur at the nodes of ranvier, a process known as saltatory conduction
Repolarisation (recovery of the resting potential)
1- Sodium channels close and potassium channels open
2- Potassium diffuses out by facilitated diffusion
3- Outside of cell is more positive, making the the potential difference more negative
4- Potential difference drops below resting potential (-70mV), which is known as hyperpolarisation
At this point there is lots of potassium outside the cell and lots of sodium inside the cell. This is reversed by the sodium potassium pump. This occurs until resting potential is reached, only then can another action potential travel along the neuron. The refractory period is when the membrane is returning to it's resting potential.