Nerve Transmission

Neurones transmit electrical impulses along the cell membrane. This happens by changing the electrical charge inside and outside the axon of the neurone.
Resting Potential 

• No impulse is being sent to the inside of the cell - so the cell has a negative charge inside the cell compared to the outside. 
• Resting potential = minus values
• Membrane is polarised
• 3 sodium ions are pumped out of the axon for every 2 potassium ions in by the Na+/k+ pump
• The potassium channels allow 2 K+ to flow back out of the axon. 
• As Na+ channels are closed the membrane is much more permeable to K+ than Na+

Action Potential
When the nerve impulse is initated the membrane is depolarised and the action potential is created.

• The impulse causes the sodium channels to open 
• This causes a sudden increase in permeability of the membrane to Na+
• The sodium diffuses into the axon and this depolarises the membrane. 

Repolarisation

• After depolarisation occurs the sodium channels close 
• Potassium ions diffuse out of the axon 
• The membrane is repolarised. 

Refactory Period = the time it takes for the membrane to be repolarised (1 millisecond). During this no other action potential can be generated. 

Time delay ensures unidirectional impulse and limits frequency of impulses. 

All or Nothing Law: Action potential is only produced if the stimulus is above the threshold value.

Factors affecting the speed of impulse conduction: 

• Myelination - Depolarisation only occurs at the nodes of Ranvier and speeds up nerve transmission
• Diameter of Axon - Increasing diameter of axon speeds up nerve transmission as resistance decreases.

• Na+/K+ pumps are working: 3 sodium ions are transported out  and 2 potassium ions are transported in. This needs energy in the form of ATP as transport is against the concentration gradient. The membrane is polarised and the membrane potential is -70mV.

• Sodium channels open and there is a rapid diffusion of Na+ into the cell. The membrane is depolarised.

• The membrane potential is now +40mV and this is the action potential

• Extra potassium channels open, Na+ channels close and K+ rapifly diffuse outside the cell to repolarise the membrane

• The amount of K+ ions that diffuse outside is a slight overhsoot - the membrane potential falls to about-75mV (refractory period). This ensures the nerve impulse continues in one direction. After this the Na+/K+ pumps start to work again and the membrane is restored to a resting potential of -70mV.

• When a neurone is excited to a threshold potential is reached at +40mV - so if enough sodium ions have diffused into the neurone so that an action potential can be produced. If the threshold is not reached an action potential will not take place (All or Nothing law)

Non-myelinated neurone

• Sodium ions rapidly diffuse into the axon during the depolaristion phase of action potential (Sodium channels are open). 
• Movement of sodium ions across the axon membrane actually initates the action potential in the next part of the membrane. 
• This is due to the local current effect caused by Na+ ions diffusing along the inside of the axon - down their concentration gradient.

Myelinated neurone - SALATATORY CONDUCTION 

• In myelinated neurones the axon is surrounded by schwann cells that forma myelin sheath which acts as an electrical insulator.
• The nerve impulse appears to jump from one node of Ranvier to the next.
• Myelin is a lipid type sbstance that is impermebale to ions, therefore action potentials can only take place at the nodes where there is an abundance of Na+ channels and Na+/K+ pumps.
• The local currents caused by the diffusion of sodium ions are elongated and the speed of transmission is increased. 
• This is salatory conduction. 

MYELINATION - speeds up rate of transmission through being an electrical insulator