Neuronal conduction and myelin Flashcards

1
Q

what is the speed of action potential propagation determined by?

A

the ‘reaction time’ of the voltage-gated Na+ channels of the membrane.

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2
Q

how fast the next segment of membrane gets depolarised to threshold level is determined by 2 factors:

A

space constant and time constant.

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3
Q

what is the space constant?

A

a no. that describes (mathematically) how far the signal travels passively down an axon before it decays by a certain amount.

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4
Q

how is space constant calculated?

A

√ (membrane resistance ÷ internal resistance).

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5
Q

how does the axon impact space constant?

A

membrane resistance depends on the axon’s circumference, the internal resistance depends on the area - meaning wider axons have a longer space space constant.

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6
Q

what is time constant?

A

the time it takes for a voltage to build up to a certain level.

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7
Q

how is the time constant calculated?

A

(membrane resistance) x (membrane capacitance).

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8
Q

oligodendrocytes / Schwann cells wrap around the axon many times, creating myelin.
how does myelin affect resistance?

A

the insulation increases resistance, making it difficult for charges to cross membrane.

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9
Q

how to action potentials propagate?

A

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.

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10
Q

what happens if myelination is lost?

A

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.

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11
Q

what happens if myelination is lost?

A

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.

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12
Q

what does demyelination cause?

A

decreased conduction velocity,
loss of action potentials,
ectopic impulse generation (generation of random action potentials).

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13
Q

what are 2 syndromes of failure in neural conduction?

A

multiple sclerosis and Guillain-Barré syndrome.

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14
Q

what is multiple sclerosis?

A

auto-immune disorder where the immune system attacks myelin,
causes: vision problems,
numbness / tingling,
or muscle spasms / weakness.

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15
Q

what is Guillain-Barré syndrome?

A

auto-immune disorder affecting myelin in the peripheral nervous system with symptoms ranging from numbness, tingling, or weakness.

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