neural conduction and myelin Flashcards
in propagation of an action potential, are the Na+ channels open for long?
the Na+ channels in a given area are only actually open for like a milliseconds - the wave can only move forward because the Na+ ion channels, once closed, have a refractory period where they are inactive
explain propagation of an action potential
An action potential is like a moving wave.
Voltage-gated Na+ channels are open in a small zone. The positive charge entering the cell via voltage-gated Na+ channels is like positive current entering a cable, which spreads passively in each direction. “Behind” the open zone, the current goes out via the K+ channels that open on the “downstroke” of the action potential.
“In front” of the open zone, the positive charge depolarises the membrane potential, and this triggers more voltage-gated Na+ channels to open.
So at the next time step, the wave has moved forward, because voltage-gated Na+ channels in the next section of membrane have opened. The wave doesn’t move backward because voltage-gated Na+ channels are inactivated for a couple ms after they open, plus the open K+ channels “behind” the wave hyperpolarize the membrane
what is the space or length constant?
how far an AP can spread along the axon passively
does high resistance make the space constant smaller or bigger?
higher internal resistance makes the space constant lower
a higher membrane resistance means it’s harder for ions to leak out of the axon, making the space constant higher
what is formula for space constant?
SQRT of membrane resistance/internal resistance
why do wider axons have a longer space constant?
membrane resistance does decrease as radius (and therefore circumference) increases
however internal resistance decreases as radius (and therefore cross sectional area) increases because ions flow more easily, and this change is more drastic as its r^2
how is the neuron cell membrane a capacitor?
it separates ions, and therefore charge can build up on either side
what is the time constant?
its to do with how quickly a neuron’s potential decay’s to it’s resting
formula for time constant?
membrane resistance x membrane capacitance
therefore an increase in either causes an increase in the time constant
how does myelin effect membrane resistance and capacitance?
myelin acts like wrapping a wire in insulating tape - it raises membrane resistance by making it less leaky
myelin increases membrane capacitance because it increases the size of the gap between the separated charge areas, inside and outside of the cell
how does myelination effect space and time constant?
increases time constant and space constant
explain saltatory conduction
Na+ channels at nodes of Ranvier allows current to enter - propagates passively, faster if space constant is longer, at next node of Ranvier the depolarisation triggers V-gated Na+ channels to regenerate the action potential
what is the point in saltatory conduction? another good thing about myelination?
faster and saves energy as Na+/K+ pump has less work to to restore the Na+ gradient later
myelination the only other way to speed up conduction is wider axons - to increase speed by 10X you gotta increase radius by x10^2, meaning axon volume increased by 10,000x
why aren’t all axons myelinated?
its costly, so only done if the speed is needed like in proprioception and motor axons, but not in pain or temperature ones for example
what is the issue with demyelinating disease?
distribution of Na+ channels is made with myelin in mind, without the myelin the signals doesn’t work properly - fizzle out, extra signals when there shouldn’t be
