Lecture 3 (Revised) Flashcards
Action Potential is a?
Rapid + transient change in membrane potential (Vm) of a cell (ex. neuron)
(basis of electrical signaling in body)
AP usually initiate at?
Axon Hillock
AP usually propagated by?
Voltage-gated ion channels along axon
“All-or-None” Phenomenon?
(Action Potential)
-Minimum depolarization needed to fire AP
-AP will NOT fire if Vm is under threshold
-Na+ channels blocked by TTX and Riluzole
Voltage-gated Ion Channels?
“Voltage sensors” with ion selectivity
1) Na+ Channel
2) K+ Channel (Tetramer (4 subunits))
(S4 segment senses change in voltage)
S4 segment?
Senses change in voltage and is what will change our ion channel from open to close
Voltage-gated Na+ Channels?
-3 Conformational States:
1) Open (“a” or “m” gate)
2) Inactivated (“I’ or “h” gate)
3) Closed
-Activation (increased flow of ions (depolarization))
-Inactivation (stops flow of ions (repolarization))
Voltage-gated K+ Channels?
1) Depolarization triggers opening of voltage-gated K+ channels
2) K+ ions rush OUT OF cell
3) Transient efflux (high Pk) leads to hyper polarization
-Hyperpolarization is built in defense against hyperexcitability (shooting another too soon)
Negative feedback decreases probability of?
Activating more channels
Na+ and K+ Channel Synchrony?
1) Rapid activation of Na+ channels causes depolarization
2) Na+ channel inactivation STOPS depolarization
3) Slower activation of K+ channels contribute to repolarization and eventual hyperpolarization
Propagation means?
Movement down axon
Non-Propagated Depolarization?
If minimum depolarization to reach threshold is not met, AP will not propagate, instead spreads passively through local current flow (“Electronic Conduction”)
Key Concepts of AP Propagation?
1) Depolarization decreases as it moves down axon
2) Peak membrane potential during an AP remains consistently depolarized
3) AP propagation takes time (to charge membrane)
4) Myelination increases conduction velocity
Propagated Depolarization (Concept 1)?
Depolarization decreases as it down axon, caused by:
1) Membrane resistance (Rm) (barricade on cell surface that is preventing passive flow of ions out)
2) Interaxonal resistance (Ri) (referring to resistance inside axon (like “traffic”))
Length Constant?
Distance it takes to reach 37% of Vmax
-Directly proportional to Rm
-Inversely proportional to Ri
Propagated Depolarization (Concept 2)?
Peak membrane potential along an axon during an AP remains consistently depolarized (caused by refractory periods)
Absolute Refractory?
A 2nd AP can NOT fire (Na+ channels can not be reactivated (either in open or inactivated state))
Refractory Period makes sure AP?
Doesn’t stay in 1 spot
Relative Refractory?
A much greater depolarization required for an AP to fire (active K+ channels hyperpolarizing cell)
Propagated Depolarization (Concept 3)?
AP propagation takes time
-Time is needed for membrane to charge
-Time needed to charge = rate limiting event of AP propagation
Time Constant?
Time it takes for the charge in Vm to reach 63% of its final value
-Directly proportional to Rm
-Directly proportional to Cm
Greater the size of the axon?
Greater the time it takes for membrane to charge
Capacitance is?
Ability for something to hold a charge
Time Constant Equation?
t = RmCm
Propagated Depolarization (Concept 4)?
Myelination increases conduction velocity by:
1) Increasing Rm –> Increases length constant
2) Decreasing Cm –> Decreases time constant
Saltatory Conduction?
Propagation of EPs along myelinated axons from 1 node of Ranvier to next node
Multiple Sclerosis?
An autoimmune, degenerative disease of axon demyelination
-Increases passive current flow
-Decreases AP conduction velocity
Length Constant Equation?
