Action potentials Flashcards
How far can an electrical signal travel passively in a neuron
only a few mm
axons are very poor cables
What happens to current measured inside a cell hooked to an electrode but without ion channels?
Steadily increases
What happens to current measured inside a cell hooked to an electrode with ion channels?
Increases to a point and then plateaus
What are 2 important properties of action potentials?
- membrane potential must be depolarized to same level before action potential occurs EVERYTIME
- one action potential has begun: same SIZE and same SHAPE every time for the same neuron
ALL OR NONE EVENT
Does the action potential attenuate as it travels down the cell?
No
Is the cell membrane more permeable to Na+ or K+? What is the typical resting membrane potential?
K+
-70mV
How does the cell membrane change relative to Na during depolarization?
Becomes relatively more permeable to Na
What determines the threshold voltage?
It is the point at which the Na and K voltages are exactly equal and opposite
Once the threshold is reached, how is the action potential generated?
AP generated only if inward Na+ momentarily exceeds inward K+ - if it does not, no AP
Describe the positive feedback loop generated by Na+ during depolarization
Na entering the channels begin conducting -> further depolarize cell
With increased depolarization -> more Na channels start to open
ase more Na channels open -> more Na enters cell and cell becomes more depolarized
Why does repolarization occur?
- P Na declines back to resting level
2. P K undergoes transient increase
When is the incactivation gate closed? When is it open?
inactivation gate is open at rest, closes at depolarization
When is the activation gate open? When is it closed?
Activation gate is open at depolarization, closes at rest
What are the relative speeds of the Inactivation and activation gates?
Inactivation gate slower than activation gate
When are K channels open? When are they closed? How do they respond relative to Na channesl
K channels are closed at rest, open at depolarization, but respond with delay compared to Na
What causes the refractory period?
Na leaves cell, K enters, membrane potential builds back up
Describe the negative feedback that occurs during the refractory period
depolarization opens K channels-> K enters -> re-polarizes cell-> induces K channels to shut
What is the benefit of the voltage gated potassium channels?
Faster re-polarization -> more action potentials in a give period of time
What is the absolute refractory period?
Time when no stimulus can evoke an AP (inactivation Na gates are closed)
What is the relative refractory period?
Time when stronger than normal stimulus is reqired to evoke AP
What is the function of the Na/K pump?
Restore proper ion balance
- depends on surface to volume ratio
- big axons run down slowly
What causes the initial depolarization?
synaptic transmission
How does local anesthetic work?
Blocks sodium channels-> cannot open
- signal may spread passively by cannot travel length of axon to propagate signal
=> cannot reach threshold to propagate signal and send “pain” to brain
Explain the safety factor
the membranes of neurons contain 10-15 x the number of Na+ channels necessary to reach threshold depolarization
WHY:
- Branching axons: at each branch point the safety factor decreases
- Sodium channel inactivation: Na+ channels cannot propagate new signal until refractory period complete- with more Na channels, some did not depolarize, and axon is ready for next AP sooner
- Velocity: excess Na channels produce extra current -> next ring of membrane will be depolarized more quickly.
What does conduction velocity in an unmyelinated neuron depend on?
ONLY axon diameter:
- small diameter = depolarize shorter length ahead of active locus
- large diameter = depolarize longer length ahead of active locus
What is the effect of myelin on nerve conduction?
Myelin increases the effective resistance between axoplasm and extracellular fluid =>
ELECTRICAL INSULATOR!
- speeds signal propagation
Explain saltatory conduction
AP jumps from node (of ranvier) to node -> of myelinated axons (increased velocity
What does the conduction velocity in a myelinated neuron depend on?
directly proportional to diameter
=> 2x diameter = 2x velocity
What is the length constant?
Lambda
= pont (distance) by which AP has lost 37% of its original value
= 0.5x square root of (membrane resistance/internal resistance)
umyelinated = ~1mm
(Rm is low, Ri is high)
How does internal resistance Ri affect current propagation?
Increased internal resistance (Ri) = decreased spread of current
How does membrane resistance Rm affect current propagation?
increased membrane resistance (Rm) = increased spread of current
How does membrane capacitance affect current propagation?
increased membrane capacitance (Cm) = decreased spread of current
Why are axons poor passive conductors?
low Rm (less current spread) high Ri (less current spread) high Cm (less current spread)
How does lidocaine work?
Binds intracellular portion of voltage-gated Na+ channels, blocking Na+ entry
=> Channel must be open so it can enter cell
- most effective on smaller (low safety factor) & more active (open channel) axons
Preferentially affect pain over touch axons*
