Action potential
Upon stimulation, the axon membrane becomes depolarised. This occurs as follows: firstly, the excitation of the neurone cell, triggered by a stimulus, causes the sodium ion channels to open, as a result making it more permeable to sodium ions. These subsequently diffuse into the axon down the electrochemical gradient, as a result making the inside less negative.
Upon reaching the threshold of -55mV, even more sodium ion channels open eventually giving a potential difference of +40mV. This is the end of depolarisation and now repolarisation starts. Repolarisation is achieved as a result of sodium ion channels closing and potassium ion channels opening. The potassium ions diffuse out of the neurone down the concentration gradient and eventually restore the resting potential. However, as the closing of potassium ion channels is slightly delayed, this leads to hyperpolarisation i.e. when the potential difference becomes more negative than the resting potential. The resting potential is then achieved with the help of sodium-potassium pump which returns the potential difference to the value of -70mV.
Passage of action potential
If an action potential travels along an unmyelinated axon the wave of depolarisation moves to the adjacent resting region where sodium ions go on trigger a change in potential difference, thus stimulating another action potential. The presence of myelin sheath speeds up the passage of an action potential down an axon. Due to the presence of myelin no action potential can be generated. Therefore the action potential has to jump between gaps in the myelin called nodes of Ranvier. The mechanism they do this by is called saltatory conduction
Speed of nerve impulsesThe speed of an action potential is affected by three main factors:
- Presence or absence of myelin sheath - if an axon is myelinated then saltatory conduction can occur which is much faster than generating an action potential at every point along the axon.
- Diameter of the axon - the greater the diameter of the axon the faster the conduction e.g. the giant squids axon is 1mm in diameter compared to a humans at 22um.
- Temperature - if the temperature is increased then the ions will diffuse more rapidly. It will also affect the rate of respiration and therefore the production of ATP needed in the sodium-potassium pump.
Afterwards, there is a short period during which the neurone membrane cannot be excited as the sodium channels enter a recovery stage. This period is known as the refractory period and serves an important role in ensuring that an action potential can only pass in one direction as discrete signals. Finally the all-or-nothing principle means that either an action potential is produced or it is not. A threshold value must be reached in order for an action potential to be created, with all action potentials being of the same strength.
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