AQA Biol5: The Nervous System

• In a neurones resting potential the outside of the membrane is more postively charged than the inside (it is polarised).
• This resting potential is maintained by sodium-potassium pumps and potassium ion channels
• The sodium-potassium pumps use active transport to move 3 Na+ ions out of the neurone for every 2 K+ ions it moves in
• These sodium ions are unable to move back into the neurone so there is a concentration gradient
• Potassium ion channels allow the diffusion of the K+ ions back out of the neurone making the outside of the cell positively charged

If a stimulus is big enough there is a rapid change in potential difference which leads to an action potential which travels along the neurone by a wave of depolarisation:

• Stimulus - this excites the cell membrane causing sodium channels to open allowing Na+ ions to enter the neurone making the inside of the neurone less negative
• Depolarisation - if the threshold is reached more sodium channels open an more Na+ ions flood into the neurone
• Repolarisation - at a potential difference of around +30mV the sodium channels close and potassium channels open allowing K+ ions to diffuse out of the neurone returning the membrane to the resting potential
• Hyperpolarisation - potassium ion channels close slowly so there is an 'overshoot' where the potential becomes more negative than the resting potential
• Resting potential - the sodium-potassium pumps return the neurone to its resting potential

• After an action potential the neurone can't be excited straight away this is called the refractory period
• This is as ion channels are recovering and can't be opened
• It acts as a time delay between action potentials so they don't overlap but pass along as separate impulses
• It also ensures action potentials are unidirectional

There are three factors which affect the speed of conduction of action potentials:

• Myelination - some neurones have a myelin sheath made of schwann cells, there are small gaps between the schwann cells called the nodes of Ranvier. Depolarisation only happens at these nodes so the conduction speed is increased as the action potential jumps from node to node and doesn't have to travel the whole length of the axon. This is called saltatory conduction.
• Axon Diameter - action potentials travel down axons with wider diameters more quickly as there is less resistance to the flow of ions.
• Temperature - the speed of conduction increases as temperature increases as the ions have more energy so diffuse faster. The speed only increases up to 40C though as after that proteins start to denature

A synapse is a junction between a neurone and the next neurone or an effector cell, synaptic transmission happens in the following steps:

• an action potential arrives at the synaptic knob of the presynaptic neurone
• the action potential causes calcium ions to be released into the synaptic knob
• the calcium ions cause the vesicles to fuse with the presynaptic membrane releasing neurotransmitter into the synaptic cleft
• the neurotransmitter molecules (e.g. acetylcholine) diffuse accross the cleft and bind to specific receptors on the postsynaptic membrane
• this causes sodium channels in the postsynaptic neurone to open allowing Na+ ions to flood into it
• if the threshold is reached an action potential is created
• the neurotransmitter molecules are then either taken back into the presynaptic neurone or broken down and then taken in