Neuronal conduction Flashcards
What channels open for the downstroke of an action potential?
Voltage gate K+ channels
What is the speed of action potential determined by?
How fast the next segment of the membrane takes to get depolarised to the threshold
- Determined by space constant and time constant
What is the space constant?
How far a current can passively spread along the axon
- How far a current spreads before it decays to a certain fraction of initial value
What is the space constant equation?
Space constant = membrane resistance / internal axial resistance
Define internal resistance
Resistance to current flowing along the length of an axon
Define membrane resistance
Indication of how much current leaks out of the membrane
What is the relationship between membrane resistance and surface area of the membrane?
Membrane resistance is inversely proportional to the surface area of the membrane
What is the relationship between internal resistance and cross sectional area?
Internal resistance is inversely proportional to the cross sectional area of the axon
What do internal and membrane resistance depend on?
Internal - area
Membrane - circumference
If an axon is narrow, what will the space constant be?
Short
What is a capacitor?
Two plates with a gap between them
- Charge can build up on one side and create a voltage
What is capacitance?
The store of how much charge can push against the membrane
What is time constant?
A measure of how long it takes the membrane to “charge” up
What is the effect of myelin on membrane resistance and membrane capacitance?
Increases resistance
Decreases capacitance
How does myelin decrease capacitance?
It increases the distance between the extracellular and intracellular fluid
What is the equation for time constant?
T = rm x Cm
T = time constant
rm = membrane resistance
Cm= membrane capacitance
How does myelin increase the time constant?
Myelin increases membrane resistance so longer space constant so current is able to spread further down the axon
What type of animals have myelin?
Vertebrates
Describe saltatory conduction.
Myelin doesn’t wrap the entire axon ->
Non myelinated zones are called nodes of ranvier ->
A current enters through Na+ channels at the node of ranvier ->
Depolarisation spreads passively down the axon ->
At the next node of ranvier, depolarisation triggers voltage gated Na+ channels to regenerate the action potential.
Why does saltatory conduction save energy?
Sodium only enters at the nodes of ranvier, therefore there is less work for the Na+/K+ pump to restore the Na+ gradient
What sense (and the axons used) requires myelinated axons?
Proprioception
Motor axons
What sensations don’t require myelinated axons to form a response?
Pain
Temperature
Why do demyelinating diseases impair neuronal conduction?
Myelin removal reduces the propagation of the action potential
- less membrane resistance and less space constant
Name a demyelinating disease
Multiple sclerosis
Name a disease that affects PNS myelin
Guillain Barre syndrome
Name the symptoms of multiple sclerosis
Vision problems
Numbness, tingling
Muscle spasms/weakness
When may symptoms worsen in multiple sclerosis? Why?
Under high stress or at high temperatures
- Neuronal conduction is safer at lower temps due to Na+ channels inactivating more slowly
What are the symptoms of Guillain Barre syndrome?
Numbness
Tingling
Weakness
Which type of myelin can be regenerated?
PNS myelin
