The Nervous System
Reflex Arc
Above is a reflex arc, it is a simple example of nervous coordination. It begins with a stimulus at the receptor, this causes an impulse. In the spinal chord (part of the central nervous system), relay neurones send the impulse to an effector.
Reflexes are useful because they are quick and can help us adapt very quickly to the environment without the need for conscious control. For example, the knee jerk reaction where the leg jumps forwards if the patellar tendon (just below the knee) is stimulated.
The diagram below shows the structured of a myelinated (covered in fatty insulator) motor neurone. Pay particular attention to the Schwaan cells, these are wrapped around the axon and provide the insulation, there are gaps between these called the nodes of Ranvier.
Action Potentials
The graph below shows the stages of an impulse in the nerves. Note how the potential goes from negative to positive.
Firstly we will look at what the nerve does when it is not being stimulated, and is in a state called resting potential. In this state the membrane is polarized where the inside is negative and outside positive, this is maintained by sodium-potassium pumps which move 3Na+ ions out for every 2K+ in.
Protein channels for K+ are open so that some potassium will diffuse out, but sodium (Na) channels are closed. So overall there are more positive ions outside and therefore a potential difference is created.
When a neurone is stimulated an action potential is generated. The neurone is depolarized, this is done by sodium channels opening (the membrane is more permeable to Na+), and since there is a high concentration of sodium ions from the pumps, lots move in and push up the charge to about +40mv. The pumps actions stays the same.
Once the charge has reached a certain level, the Na+ channels close and K+ open allowing potassium out. Meanwhile the pumps continue to remove sodium so the overall charge in the neurone falls. This stage is repolarisation since we are returning to the polarised state.
However, notice on the graph how the charge falls below the resting level, this is called hyperpolarisation, and the time it occupies is the refractory period. It results from the time delay in closing channels. This is useful because it prevents any impulse being directly after this one.
In an unmyelinated neurone, this process will happen along the entire length: which is relativly slow. However, in a myelinated neurone (see above) it only happens at the nodes of Ranvier, therefore the charge jumps from node to node when each charge is generated. This is known as saltatory conduction and means that the impulse is several times faster.
Synapses
A synapse is a junction between two neurones, there is a gap called the cleft that the signal must cross. The presynaptic neurone is where the impulse comes from, and it travels to the postsynaptic neurone.
When the impulse arrives at the presynaptic neurone, calcium ion channels open, and the membrane becomes more permeable to calcium. This enters the cell and causes the vesicles to release the neurotransmitter (acetylcholine) by exocytosis in to the cleft.
On the postsynaptic membrane are neuroreceptors, where the acetylcholine binds and and is broken down by acetylcholinesterase [ah-seat-iyl-ko-lyn-es-ter-ayse] in to ethanoic acid and choline. These reenter the presynaptic neurone and are resynthesised by ATP.
The neurotransmitter binding causes the depolarisation of the postsynaptic neurone, which initiates an actional potential there.