Neuronal conduction and myelin Flashcards
what is the speed of action potential propagation determined by?
the ‘reaction time’ of the voltage-gated Na+ channels of the membrane.
how fast the next segment of membrane gets depolarised to threshold level is determined by 2 factors:
space constant and time constant.
what is the space constant?
a no. that describes (mathematically) how far the signal travels passively down an axon before it decays by a certain amount.
how is space constant calculated?
√ (membrane resistance ÷ internal resistance).
how does the axon impact space constant?
membrane resistance depends on the axon’s circumference, the internal resistance depends on the area - meaning wider axons have a longer space space constant.
what is time constant?
the time it takes for a voltage to build up to a certain level.
how is the time constant calculated?
(membrane resistance) x (membrane capacitance).
oligodendrocytes / Schwann cells wrap around the axon many times, creating myelin.
how does myelin affect resistance?
the insulation increases resistance, making it difficult for charges to cross membrane.
how to action potentials propagate?
current enters through Na+ channels at node of Ranvier,
depolarisation spreads passively down the axon (sped up by space constant),
at the next node of Ranvier depolarisation triggers voltage-gated Na+ channels to regenerate the action potential,
saltatory conduction means Na+ only enters at the nodes instead of all the way down.
what happens if myelination is lost?
the space constant is lower - the generated action potential will decay more, meaning when it reaches the next node of Ranvier it won’t be depolarised enough to trigger voltage-gated Na+ channels.
what happens if myelination is lost?
the space constant is lower - the generated action potential will decay more, meaning when it reaches the next node of Ranvier it won’t be depolarised enough to trigger voltage-gated Na+ channels.
what does demyelination cause?
decreased conduction velocity,
loss of action potentials,
ectopic impulse generation (generation of random action potentials).
what are 2 syndromes of failure in neural conduction?
multiple sclerosis and Guillain-Barré syndrome.
what is multiple sclerosis?
auto-immune disorder where the immune system attacks myelin,
causes: vision problems,
numbness / tingling,
or muscle spasms / weakness.
what is Guillain-Barré syndrome?
auto-immune disorder affecting myelin in the peripheral nervous system with symptoms ranging from numbness, tingling, or weakness.
