Nerve Impulses

• Nodes of Ranvier - (situated between each Schwann cell -> a tiny gap where the axon is exposed). Nodes are only places that ions can pass between the tissue fluid and the axon through the cell surface membrane. Play an important part in speeding up the conduction of impulses along the axon
• Myelin Sheath - Surrounds the axon and acts as an insulator. Made up of highly specialised cells (Schwann cells)
• Schwann Cells - lie alongside axon and as axon grows they wrap around it. Shields axon from surrounding tissue fluids and electrically insulating it from other neurones.
• Location of ion channels - found within cell membrane. The channels extend from one side of the plasma membrane to the other and have a 'tunnel' through the middle.
• Saltatory Conduction - In myelinated axons, action potentials only occur at the nodes of Ranvier, with charge diffusing along the cell where myelin is present

(When neurone is not electrically active at -70mv)

• Neurones have a negative resting potential across their cell membrane. The outside is more positively charged than the inside (+ ions want to enter and - ions want to leave because opposites attract). Kept by keeping more positive ions outside the neurone than inside

• 1) Membrane more permeable to loss of K+ ions
• 2) Limits the entry of Na+ ions
• 3) Na-K pump actively transports more Na+ out than K+ in (3 Na out and 2 K in) due to electrochemical gradient
• 4) - charged ions inside

- Does not use ATP for active transport (K+ moving down its conc. gradient provides energy to move Na+ down its conc. gradient)

• 1) Resting Potential (Na-K pump, active transport, Na+ diffuses out and K diffuses in, Some K+ diffuses out of ion channels)

• 2) Generator Potential (weak stimulus, some Na+ channels open, some Na+ diffuses in then Na-K pump restores resting potential)

• 3) Threshold (voltage-gated Na+ channels open, Na+ diffuses into axon, creates positive feedback)

• 4) Depolarisation (Na+ channels open, Na+ diffuses in)

• 5) Repolarisation (voltage-gated K+ channels open, K+ diffuses out and voltage-gated Na+ channels close

• 6) Hyperpolarisation (membrane potential is more negative than the resting potential)

• Is the period of time following a nerve impulse before the neurone is able to fire again
• Action potentials prevented as the neurones are hyperpolarised - ion channels are inactive

Discrete Impulse:

• 1) During the refractory period ion channels cannot be open
• 2) RP acts as time delay between 1 action potential and another (ensures no overlap)
• 3) RP also ensures action potentials are unidirectional

Axon Diameter:

• Faster along bigger diameters. Less resistance to flow of ions in cytoplasm. Less resistance then means that depolarisation occurs quicker

Myelination:

• Impulses pass through Nodes of Ranvier. Creates a faster rate as it reduces the distances impulses have to travel
• Speeds up conduction of action potentials.

Temperature:

• Ions diffuse faster at higher temperatures. As temperature increases, speed of conduction increases.
• However, it only increases to around 80 as then proteins begin to denature. Speed would then decrease.

Non-Myelinated Neurones:

• Change in voltage in membrane stimulates adjacent sodium ion channels to open
• The action potential travels like a wave along the neurone (nerve impulse) which travels to the next synapse

Myelinated Neurones:

• Ions only pass through the cell-surface membrane at the Nodes of Ranvier. Action potentials can then only occur at these nodes
• effect of depolarisation causes almost immediate action potential at the next node (Saltatory conduction)
• At each node there are a large number of sodium and potassium channels and, since nodes are about 1mm apart, saltatory conduction has the great advantage of being much faster