Nervous transmission Flashcards
(21 cards)
What is the cell-surface resting potential and what is it’s normal value?
When a neurone is not transmitting signals it maintains a state of polarisation
meaning there’s a difference in voltage across the membrane
resting potential = -70 mV
How is resting potential achieved?
Sodium potassium pumps = active transporters moving 3 Na+ ions out the neurone for every 2 K+ ions they move in
Potassium ion channels = channels allow diffusion of K+ out of neurone down its concentration gradient.
Sodium ion channels = channels are closed preventing the movement of Na+ into the neurone
What does the membrane being polarised mean?
Extracellular space outside axon accumulates more positive ions making the axon cytoplasm negatively charged in comparison
What are the stages of action potential propagation?
- resting potential
- stimulus
- depolarisation
- repolarisation
- hyperpolarisation
- refractory period
What happens when a stimulus is detected?
VG Na+ channels open causing more Na+ flows into the axon making the inside less negative
What happens during depolarisation?
If threshold potential of around -55 mV is reached due to the stimuli more Na+ channels open causing an influx of Na+
What happens during repolarisation?
At around +30 mV Na+ channels close and K+ channels open so K+ flows out of the axon and the membrane starts repolarising
What happens during hyperpolarisation?
Excess of K+ leaves the axon dropping the potential below the -70 mV resting level
What happens during the refractory period?
Various ion pumps and channels work together to restore the membrane back to the resting potential
Why is action potential propagation an example of positive feedback?
Initial Na+ influx depolarises the axon membrane opening more Na+ channels meaning a greater influx of Na+ further depolarising membrane
What can an action potential be described as?
Uniform self-propagating fashion
What is the all-or-nothing principle of action potential propagation?
Threshold phenomenon = once threshold potential is reached an action potential is always triggered regardless of stimulus’ strength
No partial response = without reaching threshold potential no action potential is initiated.
Action potentials always same size = stronger stimulus doesn’t increase the size of the action potential but does increase propagation frequency
What is the refractory period?
Recovery period where neurone’s membrane can’t generate another action potential due to Na+ channels remain closed during repolarisation
Why is the refractory period essential?
- ensures action potentials don’t overlap
- limits frequency at which impulses are transmitted
- guarantees impulses travel in only one direction
How can action potentials be described as travelling?
As a wave of depolarisation
How does an action potential travel as a wave of depolarisation?
1) Na+ channel opening results in local depolarisation allowing positive ions to spread sideways
2) adjacent VG Na+ channels open in response to this change
3) action leads to depolarisation of nearby membrane areas
4) as each patch of membrane activates the next advancing a wave
5) areas of membrane that have just experienced depolarisation are in the refractory period and remain unresponsive while they repolarise
6) ensures that the wave moves in one direction preventing the backward flow of the nerve impulse
Once triggered what happens to an action potential?
Self-propagates through local currents along axon without any size decrease
What factors affect transmission speed of an action potential?
- myelination
- axon diameter
- temperature
How does myelination affect transmission speed?
Myelinated neurones transmit impulses faster than unmyelinated neurones due to insulation by a myelin sheath enabling saltatory conduction
How does axon diameter affect transmission speed?
Broader axons transmit impulses faster due to a larger diameter meaning there is less resistance to ion flow so wave of depolarisation travels faster along the axon
How does temperature affect transmission speed?
Faster impulse transmission due to faster depolarisation as higher temperatures accelerate diffusion of ions
However temperatures above 40°C can cause proteins to denature, which results in slower impulse transmission due to membrane damage
