L7 Neuronal Conduction Flashcards
The speed of action potential gradient propagation is determined by?
It’s determined by how fast the next segment of membrane gets depolarized to threshold.
How is an action potential like a moving wave?
An action potential isn’t simply one event at a single point; it’s a chain reaction.
When one part of an axon’s membrane depolarizes (becomes positively charged), it triggers the depolarization of the adjacent membrane section
Explain continuous action potential vs saltatory Conduction
- Continuous - Happens in unmyelinated axons and slow
- Saltatory conduction - Myelinated axons, contains gaps (nodes of Ranvier) so action potential “jumps” from one node to the next, much faster
What 4 things influence the speed of aciton potential propagation?
- Myelination
- Axon diameter
- Space constant
- Time constant
What is space constant?
It describes how far an electrical current spreads passively along the axon
How does membrane resistance (Rm) affect space constant?
Higher membrane resistance (fewer open ion channels) allows the signal to travel further. Myelin increases membrane resistance
What is membrane resistance (Rm)?
Its a measure of much the cell membrane resists the flow of electrical current.
It prevents ions from leaking out hence the resistance
What is internal resistance?
How hard it is for current to pass along the axon
What does a higher membrane resistance mean?
A higher membrane resistance means that ions are less likely to leak out, allowing the eletrical signal to travel further along the axon
What does a lower internal resistance mean?
A lower internal resistance means that ions can flow more easily along the axon, allowing the electrical signal to travel faster
What affects time constant?
- Membrane resistance (Rm): How un-leaky the hose wall is
- Membrane Capacitance (Cm): how stretchy the hose wall is
Why is it better to have a lower membrane capacitance (strechiness)?
- If the rubber sheet is very stretchy, it takes a lot of force (charge) to change it shape. It stores a lot of the force instead of quickly changing.
- If the rubber sheet is stiff ( less stretchy), it takes less force to change its shape. It responds quickly to applied force -
What is the relationship between membrane resistance and surface area?
Membrane resistance is inversely proportional to the axon’s circumference of the membrane (more area, more leaks)
Circumference = 2πr
What is the relationship between internal resistance and cross sectional area?
Internal resistance is inversely proportional to the area
Area = πr²
What is the time constant formula?
Time constant = RmCm (membrane resistance X capacitance)
Rm - membrane resistance - how un-leaky is the hose wall
Cm - membrane capacitance - how stretchy is the hose wall
Can a cell membrane be both a resistor and a capacitor?
Yes - A cell membrane is both a resistor (current can pass through but not very well) and a capacitor (charge can build up on one side)
What happens if there’s loads of Oligodendrocytes/ Schwann cells wrapped around an axon?
This is like wrapping a wire with insulating tape. - it increases membrane resistance.
It also increases the distance between extra and cellular solution
What is the formula for space constant?
λ = / (rm / ri)
rm - membrane resistance
ri - internal resistance
What affects does Myelin have on membrane resistance and membrane capacitance?
Myelin increases membrane resistance and decreases membrane capacitance
What would happen to space + time constant if myelin increased membrane resistance and decreases membrane capacitance?
Myelin increases the space constant while keeping the time constant the same
What happens in the node of ranvier?
Current enters through Na+ channels at a node of ranvier. This depolarisation spreads along passively down the axon
What happens when the current reaches another node of ranvier ?
At the next node of ranvier, depolarisation triggers voltage - gated Na+ channels to regenerate the action potential
How can you increase the conduction speed without myelin?
Without myelin, you can only increase conduction speed by widening the axon following [λ ∝ √radius]
To increase the conduction speed by 10x, how much do you need to increase the radius and axon volume by?
You need to increase the radius by 100x and the axon volume by 10,000x
- speed ∝ √radius : 10= √100
- volume ∝ radius² : 10,000 = (100)²
What is a demyelinated neuron?
A demyelinated neuron is a nerve cell that has lost its myelin sheath
What are the 5 biological effects of demyelinated conduction?
- Decreased conduction velocity
- Frequency- related block
- Total conduction block
- Ectopic impulse generation
- Increase in mechanosensitivity
Give an example showing myelinations saves space.
Example: the human optic nerve is 3 mm wide. Without myelin, to conduct at the same speed, it would have to be 300 mm wide (1 foot)!
Name two disease caused by demyelinating.
- Multiple sclerosis
- Guillain - Barre syndrome
Name some symptoms of multiple sclerosis.
- auto-immune disorder, immune system attacks myelin
- episodic: symptoms get worse, then better, then worse, etc.
- diverse neurological symptoms, e.g., vision problems, numbness/tingling, muscle spasms/weakness.
- symptoms might be worse when under stress or at high temperatures – neuronal conduction is “safer” at low temperatures, because Na+ channels inactivate more slowly.
Name some symptoms of guillain barre syndrome.
auto-immune disorder affecting PNS myelin
symptoms: numbness, tingling, weakness
patients usually recover because PNS myelin can regenerate (unlike CNS myelin)
