NEUR 0010 - Chapter4 Flashcards
What is the rising phase of an action potential?
Depolarization, peaking at 40 mV
What is the falling phase of an action potential?
Repolarization
What is overshoot?
When the inside of the neuron is so depolarized that it’s actually positive
What is undershoot?
When repolarization is so intense that the membrane potential is more negative than usual
How does an action potential start?
Depolarization: if it reaches the threshold level of depolarization (if inside gets less negative enough), will trigger an action potential
How are firing frequency and depolarizing current related?
The higher the magnitude of the depolarizing current, the higher the firing frequency
What is the approximate maximum firing frequency of a neuron, and why?
Approximately 1000 Hz, because of the absolute refractory period
How does the relative refractory period differ from the absolute refractory period?
Relative refractory period is at the end of the absolute refractory period, and requires a higher depolarization than normal to activate another action potential
What ions cause depolarization and repolarization?
Depolarization is caused by the influx of Na+ ions, and repolarization is caused by the efflux of K+ ions
What allows the entry of Na+ ions after depolarization to threshold?
The transient increase in g(Na) [sodium conductivity], which helps drives sodium ions in, allowing the real depolarization to set in, which activates the voltage-gated sodium channels
How does a voltage-gated sodium channel work, basically?
When depolarized above threshold, the voltage opens the channel; when potential reaches a certain positive value, voltage closes the channel
What is the peptide structure of a voltage-gated Na channel?
Long polypeptide with four domains (six subunits each), clumped together to form a pore: domains twist after depolarization to threshold, opening the pore; has pore loops assembled into a selectivity filter that allow Na+ but not K+
How does the selectivity filter in the voltage-gated Na channel operate?
Strips most companion water molecules from the Na+ ion : excess water serves as a molecular chaperone, allows the partially-hydrated Na+ to pass through the filter
What is the approximate voltage of the threshold in a neuronal membrane?
-40 mV
What are the characteristic behaviors of the voltage-gated sodium channel?
Open after depolarization to -40 mV, only for 1 msec, can’t be opened again until membrane potential has been repolarized again to a value below threshold
How does tetrodoxin affect sodium channels?
Blocks: binds tightly to a specific site on the outside of the channel
What are two toxins that block sodium channels?
TTX and saxitoxin
How does bactrachotoxin affect sodium channels?
Causes them to open inappropriately: open at more negative potentials, stay open too long (doesn’t inactivate)
What are three toxins that cause sodium channels to open at subthreshold potentials and prevent inactivation?
Batrachotoxin (Colombian frog), veratridine (lilies), and aconitine (buttercups)
What is the “delayed rectifier?”
When potassium conductance increases to reset the membrane potential
When do voltage-gated K+ channels typically open?
After the membrane’s been depolarized beyond threshold, to try to bring it back down to threshold potential
What does it mean that action potentials are orthodromic?
Can only travel in one direction (normally, action potentials are orthodromic)
Why does action potential conduction increase with axonal diameter?
Consider the axon like a water hose: if it’s narrow and there are too many pores open, it’ll leak and not conduct well; if it’s wider and has fewer open pores, then it will conduct well
How does axonal size affect axon excitability?
Smaller axons require more depolarization to reach threshold, more sensitive to being blocked by local anesthetics
How are our neurons effective if they’re not wide enough to prevent “leakage?”
Myelin!
Why are nodes of Ranvier useful?
Allow ions to enter the axon to initiate action potentials
What kind of channel is concentrated in nodes of Ranvier?
Voltage-gated sodium channels
What is a good metaphor for saltatory conduction?
Skipping down the sidewalk
Can neurites initiate action potentials?
Generally, NO!
What is the spike-initiation zone?
Where the axon can start producing action potentials
Where is the spike-initiation zone for CNS vs PNS neurons?
CNS: often at the axon hillock; PNS (sensory neurons); at sensory nerve endings, where depolarization is caused by sensory stimulation
