Nerve Impulses
Sensory Receptors
Refractory Period + Action Potential
Resting Potential
Transmission of an Action Potential
Neurons have very specialised cell-surface membranes filled with many channel proteins. These are specialised to sodium and potassium channel proteins.
A receptor converts an external or internal stimulus into an electrical signal. Sensory receptors detect changes in their surroundings.
Voltage-gated channels are channel proteins which have a gate which, when open, allows a certain type of ion through.
Sensory neurons are energy transducers. They convert anything from pressure, to heat, and convert this into electrical energy.
The sodium potassium pump pumps out three sodium ions for every two potassium ions which it pumps in.
The neural membrane is often described as leaky as potassium ions will diffuse back out of the cell.
In this state there are more potassium ions inside of the cell and sodium ions outside of the cell. Using ATP, both ions can be actively transported, against their concentration gradient, across the membrane.
The inside of the cell has a negative overall charge compared to the exterior, as there are many different negative ions in the cytoplasm.
During the refractory period the cell is given the chance to recover.from the action potential and restore the correct balance of ions either side of the membrane. It is the role of the sodium potassium pump to actively transport ions across the membrane.
If enough ions enter a cell a threshold potential is generated. When a threshold potential is generated, an action potential is generated as a result of this (an impulse).
After hyperpolarization, the ionic movements during the action potential have left the sodium ions inside of the cell and the potassium ions outside of the cell.
This is an all or nothing response. If the voltage is below a certain threshold potential, then nothing will happen. If the depolarisation is large enough, even if its only just over the threshold potential, then it will produce the same action potential as a voltage twice the size of it.
This causes the cell exterior to become more negative due to the loss of Na plus ions. As the ions enter the cell, the concentration will increase, creating a concentration gradient to adjacent areas of the cell. This causes ions to diffuse into these areas.
This depolarises these parts of the membrane, causing the sodium voltage-gated channels to open in these areas. This induces an action potential. This leads to another concentration gradient of sodium ions to the next area along the membrane and the process then repeats.
An action potential is generated at one point in the membrane as sodium voltage-gated channels open and Na plus ions flow into the cell, depolarising the membrane at that point.