Nerve Impulses: Movement Of Action Potential Flashcards
Nerve impulse
Movement of the action potential (pos charge inside axon relative to outside a neurone due to Na+ ions) along the whole length of an axon
In a non myelinated neurone, what is the first step to move the action potential towards the axon terminal end?
Within axon, Na+ (that had entered using voltage gated Na+ channel proteins) will diffuse sideways down electrochemical gradient to an area with low pos charge INSIDE the axon
Local current
Diffusion of Na+ ions sideways down electrochemical gradient within an axon
Once a local current has been generated, what are we left with in the axon?
Accumulation of Na+ ions thus pos charge in an adjacent patch in the neurone under neurone membrane
Taking this patch to threshold potential
What happens when the adjacent patch in the neurone under the neurone membrane is taken to threshold potential?
Voltage gated Na+ channel proteins open allowing more Na+ ions to diffuse in to neurone and increase pos charge causing further depolarisation over the neurone membrane
Generates a new local current
Local currents can move within a neurone…
Backwards and forwards: diffusion of Na+ ions down electrochemical grad toward cell body and toward axon terminal end
Why can the action potential only move forward?
Due to the refractory period
What is the refractory period?
The patch of neurone which just had the area of pos charge (but Na+ions diffused to next patch in the local current) is now repolarising from pos action potential back to resting neg potential
At this point voltage gated Na+ channel proteins SHUT
Why does the refractory period prevent action potential flowing backwards?
Because action potential cannot be generated backward in the neurone as the voltage gated Na+ channel proteins SHUT so Na+ ions cannot flow into neurone at this point
When can a new action potential be generated again?
After refractory period once restored back to resting potential, however the action potential would have moved forward at this point so therefore it cannot flow backward
‘Discrete’
Resting potential must be restored in a patch of neurone under the neurone membrane before generating a new action potential
So the action potentials cannot merge
What does discrete nature of neurone membranes ensure?
There is a limit to the number of action potentials transmitted in a neurone (limit to frequency)
So this places a limit on the strength of the stimulus that can be perceived
Passage of the local current (thus generated action potentials) in a myelinated neurone only occurs at…
Nodes of Ranvier
Patches on the neurone that are not covered by Schwann cells thus no myelin
Why does myelin maintain the strength of a local current?
Because it physically blocks leak of Na+ ions out of neurone as the Na+ diffuses down electrochemical gradient sideways within the neurone
Maintains strength of local current therefore so can travel a longer distance without needing to generate a new action potential
Why can the generation of a new action potential only occur at nodes of ranvier?
No myelin surrounds neurone membrane for exchange of ions to surrounding fluid:
Na+ ions into neurone
K+ ions leave neurone
When is a new action potential generated?
When the local current weakens due to leak of Na+ ion through leak channel proteins
Why are myelinated neurones better at sending an impulse?
Local current travels longer before weakening due to leak of Na+ by leak channel proteins
Occurring only at nodes of ranvier means impulses are quicker
Why is passage of an action potential in unmyelinated neurone quite slow?
the whole length of the neurone membrane needs to be progressively depolarised to pass an impulse from one end of an axon to another
Saltatory conduction
Depolarisation only occurs at the nodes of Ranvier and so the nerve impulse essentially jumps from node to node so transmission of an action potential is FASTER
Does saltatory conduction maintain strength of local current?
No, while both are due to myelination of neuron:
-Myelin insulates neurone so strength of local current maintained
-Myelin causes saltatory conduction so action potential is FASTER
How does axon diameter effect speed of transmission?
Wider the axon, the faster transmission of impulse
Why do wider axons have faster speeds of transmission in terms of resistance)
Wider axons have more alternative pathways for transmission of action potential = less resistance to flow of local current (Na+ diffusion)
Resistance caused by organelles
Why do wider axons have faster speeds of transmission (in terms of surface area to volume ratio)
Wider axons have lower surface area to volume ratios
This reduces the leakage of local current ions Na+ per unit volume of axons and therefore strengthens these currents = can travel longer before weakening
What are the 2 reasons for why wider axons have faster transmission of action potential?
Less resistance due to more alternative pathways for transmission of action potential
Wider axons have lower surface area to volume ratio
