neuronal conduction Flashcards
what determines the speed of action potential propagation?
how fast the next segment of membrane gets depolarized to threshold, this is determined by:
- space constant
- time constant
what is the space constant?
how far current can spread passively along the axon (not by opening or closing of ion channels)
how does membrane resistance effect space constant?
current spreads further if the membrane is less leaky
higher membrane resistance = higher space constant
how does internal resistance effect space constant?
current spreads further if there is little resistance to it moving down the axon
higher internal resistance = smaller space constant
how does the size of the axon effect space constant?
membrane resistance is inversely proportional the the surface area (circumference) of the membrane (more area = more leaks)
internal resistance is inversely proportional to the cross-sectional area (area) of the axon (wider hose = water travels more easily)
therefore space constant is proportional to the square root of the radius
what is the time constant?
how long does it take the membrane to ‘charge up’?
what is the formula for time constant
membrane resistance x membrane capacitance
(how un-leaky is the hose wall x how stretchy is the hose wall)
how does myelin effect membrane resistance and membrane capacitance?
increases membrane resistance
decreases capacitance (because it increases distance between extracellular and intracellular solution)
how does myelin effect space constant and time constant?
increases space constant
time constant stays the same
therefore speeds up conduction of action potentials
how does saltatory conduction save energy?
it means that Na+ only enters at nodes of ranvier (instead of all along axon) so less work for Na+/K+ pump to restore Na+ gradient
why do demyelinating diseases impair neuronal conduction?
ion channels are distributed with myelin in mind, they are only present at nodes of ranvier so if myelin disappears signals cant travel correctly
which neurons are myelinated?
vertebrate neurons
neurons that need to carry information quickly e.g. motor axons (brain to muscles), proprioception (how to tell how body is arranged and where everything is)
