Resting Potential
Potential difference across the membrane when an impulse in not being conducted (-70mV)
- Na/K pump maintains it
- Uses active transport to pump 3Na+ ions out of neurone and 2K+ ions in
- Active process = ATP used
- Membrane has higher permeability to K+ ions as some K+ channels are open
- K+ ions diffuse down the concentration gradient out of the axoplasm
- Organic anions like proteins are negatively charged
- Higher concentration of positive ions outside and anions inside so there is a p.d of -70mV across the membrane
Depolarisation
Rapid rise and fall of electrical potential across neurone membrane as an impulse passes
(Stimulus causes voltage-gated Na+ channels to open)
- Threshold value/ potential of -55mV is reached
- Voltage-gated sodium channels open
- Sudden increase in permeability of membrane to Na+ ions
- Influx of Na+ ions into cytoplasm in the axon as they rapidly diffuse/ flood in down the conc gradient
- Charge becomes more positive inside the axon, so the pd is +40mV
- Membrane is depolarised through depolarisation
Repolarisation
- At +40mV, voltage-gated sodium channels close to stop influx of Na+ ions
- Voltage-gated potassium channels open
- K+ ions rapidly diffuse out , down the concentration gradient
- pd inside axon becomes more negative so the membrane is repolarised
Hyperpolariation
- More K+ ions diffuse out of the axon than Na+ ions diffuse in
- pd across membrane becomes more negative the resting potential
- This is an overshoot and the membrane is hyperpolarised at -85 mV
- Na/K pump restores ion balance to return to resting potential
All or nothing law
- If threshold value/ potential of -55mV is not reached
- There is a failed initiation
- Stimulus/ depolarisation not enough to open Na+ channels
- Too few Na+ channels opened
- Action potential not generated- all or nothing response
Intensity and size of action potential
- Always same size of +40mV and energy not lost in propagation
- Increase in stimulus intensity = higher FREQUENCY of action potential NOT higher PD
Passing of action potential (unmyelinated)
- Flood of Na+ ions diffuse along the membrane and cause local currents
- Stimulates next part of membrane to open voltage-gated Na+ channels
- Next part of membrane is depolarised and Na+ ions continue diffusing down
Refractory period
Period during which a new action potential cannot be generated- voltage-gated Na+ channels are inactivated
Importance of refractory period (3)
1) Impulse travels in 1 direction only- area that generated original action potential is in refractory period after stimulating the next region. No action potential generated as it only occurs down the membrane where resting potential is reached
2) Action potentials discrete and separate- new action potential cannot be generated immediately behind one
3) Limits frequency of action potentials- certain number fit along as discrete and in 1 direction
Factors affecting impulse transmission (3)
1- Myelination
2- Diameter of axon
3- Temperature
Diameter of axon
- Speed of action potential transmission depends on resistance of axoplasm
- Larger diameter = less resistance to ion flow so membrane potential easily maintained
- Larger diameter = greater speed of transmissions
Temperature
- Higher temperature = more kinetic energy
- Ions move faster = higher rate of diffusion
- Higher respiration rate as it increases rate of enzymic action
- More ATP for Na/K pump
- Above 40˚C, protein would denature
Myelination
- Myelin electrically insulates the axon and inhibits movement of ions and thus depolarisation
- Gaps between Schwann cells called Nodes of Ranvier
- No myelin is present in these nodes
- Depolarisation is only possible at Nodes of Ranvier, so that is where action potentials can form
- Action potential jumps from one node to the next through saltatory conduction
- Thus, nerve impulse transmission faster
- If no myelin sheath = local circuits which are much slower than saltatory conduction
Multiple Sclerosis
Disease of nervous system where myelin is destroyed. Muscle weakness and lack of vision.
Saltatory conduction is stopped- impulses can’t jump from node to node
Action potential cannot be generated/ speed of conduction reduced
Treatment- remyelinate axon and prevent further demyelination
Why myelinated axon uses less ATP?
ATP required for active transport
Na+ ions actively moved out only at the Nodes of Ravier in myelinated but along the whole length of axon in myelinated
Why impulses only in 1 direction?
1) Sensitivity of membrane lost during refractory period
2) Neurotransmitters/ synaptic vesicles only occur at axon terminal of the pre-synaptic membrane
3) Neurotransmitter receptors only occur on post-synaptic membrane
Effect of higher temperature on action potential
- Action potential returns to resting potential more quickly
- K+ channels open more quickly at high temp so more K+ ions diffuse out = faster repolarisation
- Shorter refractory period/ less hyperpolarisation
- K+ channels close moe rapidly at higher temp
