Summary of co-ordination processes

A RESTING POTENTIAL IS 65mV. POLARISED

1. Sodium ions are pumped OUT of the axon

2. Potassium ions are pumped IN to the axon

3. Sodium ions naturally diffuse back into the axon and potassium ions naturally diffuse back out of the axon

4. However, the axon membrane is more permeable to potassium ions than sodium ions (3 potassium ions for every 1 sodium ion)

5. This causes the inside of the axon to be negatively charged relative to the outside

AN ACTION POTENTIAL IS TEMPORARY REVERSAL OF CHARGES TO APPROXIMATELY +40mV. DEPOLARISATION

1. A stimulus causes sodium ion channels to open and sodium ions to diffuse IN to the axon, this triggers a reversal in the potential difference

2. At approximately +40mV the sodium ion channels close and potassium ion channels open

3. Potassium ions diffuse OUT of the axon, causing repolarisation

4. There is a temporary overshoot in the electrical gradient before it returns to resting potential, this is called hyperpoarisation

1. An action potential arrives at the synaptic knob, causing calcium ion channels to open and calcium ions to flood IN to the knob

2. The calcium causes the synaptic vesicles to fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft

3. Acetylcholine molecules fuse with receptor sites on the postsynaptic membrane, causing sodium ion channels to open and sodium ions to diffuse IN to the postsynaptic membrane

4. These sodium ions cause a new action potential to be created in the postsynaptic neurone

5. Acetylcholine is then hydrolysed by acetylcholinesterase to prevent it from creating continuous action potentials

There are two main co-ordination systems in the human body, the hormonal system and the nervous system

The nervous system is controlled by neurones, the structure of which includes:

• Cell body- containing a nucleus
• Dendrons- extensions away from the cell body
• Axon
• Schwann cells- wrap around the axon, protecting it
• Myelin sheath- an electrical insulator
• Nodes of Ranvier- gaps in the myelin sheath

The speed of an action potential is affected by several factors:

• Myelin sheath. Myelinated neurones transmit action potentials faster than unmyelinated neurones because the action potentials are forced to jump between the nodes of Ranvier, rather than travelling the whole way along the axon
• Diameter of the axon. An axon with a larger diameter will transmit action potentials faster
• Temperature. A high temperature increases the rate of diffusion and there increases the speed of the action potential