1-Action Potential Flashcards
excitable cells
nerve + muscle
types of electrical signals
- local/passive: receptor potential, synaptic potential
- active: action potential (stim in brain)
main difference that active does not decay over distance like passive
circuit of excitable membrane
bilayer acts as capacitor to slow the charge/time constant*
*amount of time for voltage to change by 63% of eventual new steady state value
passive conduction
-signal at the site of stimulation
-will decay over distance
voltage gated Na channels
2 gates: activation + inactivation
3 states: closed (resting), open (activated), inactivated
-always in that order
tetrodotoxin from puffer fish blocks Na channel
voltage gated K channel
1 gate
2 states: closed (resting), open (slow activation)
action potential steps
- resting state- both Na and K channels closed
- depolarization- Na channel opens, K stays closed
- rising of action potential- Na open, K closed
- action potential- peak of voltage, overshoot
- falling phase- Na inactivates, K opens, repolarize
- undershoot- K stay open prolonged
Na cycle fast regeneration, K cycle slower
membrane permeability during AP
- resting- K>Na
- rising- Na inc
- AP- Na>K
- falling- Na dec
shape of action potentials
- cardiac- AP w/ Ca plateau, slower
- skeletal muscle- no plateau, sharper peaks
- neurons- vary based on type
local anesthesia
blocks Na channels to inhibit action potentials and reduce pain
voltage threshold
AP’s are all or nothing so indicates a threshold
lowest voltage/minimal depolarization required to drive Na channels into fast feedback loop
above threshold stimulation not needed
factors affecting AP threshold
- Na channel
- K channel
- external Ca- indirectly thru Na channels open probability, direct relation to threshold
hypocalcemia symptoms
- neuropsychiatric
- neuromuscular irritability (Chvostek, Trousseau)
- cardiovascular
- autonomic
bc lower threshold required so more spasms/contractions
Chvostek sign
contraction of muscles @ eye, nose, mouth but not very sensitive or specific
Trousseau sign
muscle spasm of hand and forearm, more specific and sensitive
hypercalcemia
increases threshold so larger amount of depolarization needed to reach threshold
fatigue, muscle weakness, diminished reflexes
refractory periods
absolute: no response to second stimulus bc Na channels inactivate
relative: second response possible but greater cost, bc K channel prolonged open
set up upper limit of firing frequency and unidirectional propogation
AP propagation
if axon stimulated in the middle the current flow in both directions so AP goes both directions theoretically
unidirectional propagation
stimulus at initial segment of axon with lowest threshold to open Na channels then flows down axon
some will flow back upstream but since Na channels inactive nothing happens
axon size
larger axon diameter = faster axon conducts potential
-electrical conductance inc faster than capacitance
velocity = square root of radius
myelination
greatly dec axon capacitance (bc capacitors connected in series) and inc membrane resistance = effective insulator
inc action potential conduction speed (saltatory conduction)
myelin sheath composition
cholesterol
lipids
proteins
Nodes of Ranvier
where Na channels are located high density so where AP generated, unmyelinated segments
saltatory conduction steps
- AP at one node = current flow to next node
- new node reaches threshold = new AP jump to next node
fast bc no time wasted generating nodes in myelin covered and capacitance is lower
energy efficient bc less membrane to generate AP so less Na flows into cell aka less work for pump
length not matter
Schwann Cells
myelinate a single axon in PNS
oligodendrocytes
myelinate multiple axons in CNS
Multiple Sclerosis
demylelination disease of CNS that damages axons
autoimmune attack oligodendrocytes
Guillain-Barre Syndrome
demyelination of PNS, autoimmune
muscle weakness and paralysis but can recover bc remyelinaton/regeneration
maybe after infection or stomach flu
