Chapter 12 [ EXAM #1 ] Flashcards
(85 cards)
action potentials (3)
To get electrical signal from dendrites and cell body to axon terminal
Rapid change in membrane potential that once starts moves along length of excitable membrane without loss of strength: “all or none” (-60mV does not work)
- Neurons and muscle cells
Voltage gated Na+ channels (VGSCs) open at axon’s initial segment
- Depolarizes and triggers more to open
- Action potential is nerve impulse: domino effect along membrane
threshold (4)
Depolarization of membrane to certain level: threshold (~60mV in neurons)
Result of local graded potentials produced by >1 stimuli
Locally summed at axon hillock
Spreads to initial segment
- VGSCs in initial segment open
- Chain reaction along membrane
threshold all-or-none principle (3)
Triggered when threshold reached -55mV: graded potentials result of stimuli added together
or not (subthreshold): only produces graded potential below threshold -55mV
Action Potential Generation (the 4 steps)
- ) Depolarization to threshold (-60mV)
- ) Rapid Depolarization: Activation of VGSCs (Na+ channels)
- ) Repolarization: Inactivation of VGSCs and activation of K+ channels (Kv)
- ) Hyperpolarization and return to normal resting potential and permeability
Channels involved in action potential generation
voltage gated Na+ (VGSCs) and K+ (Kv) channels
At resting potential, what channels are open?
Only leak channels are open at this phase
1.) Depolarization to threshold (-60mV)
local currents, graded potential summation at axon hillock
2.) Rapid Depolarization (3)
Activation of VGSCs (Na+ channels)
At threshold Na+ channel activation gates open, influx of Na+ occurs, opens more Na+ channels (up to +30mV)
Positive feedback loop
Is sodium positive or negative charge?
positive
3.) Repolarization (4)
Inactivation of VGSCs (close) and activation of K+ channels (open) (Kv)
Occurs at +30mV
Kv channels open slowly
K+ flows out of neuron
- Hyperpolarization (6)
return to normal
Kv channels begin to close at RMP (~ 70mV)
Kv finish closing slowly (~90 mV)
returns to rest (AP is over)
Na+/K+ ATPase works to restore ion concentrations
Na+ channels reset (ready)
Define refractory period
time between action potential start until normal resting potential regained
2nd action potential can not form in response to normal threshold stimulus
Absolute refractory period
Can not respond to any further stimulation
Maximum VGSCs open or inactivated
K+ flow-opposing force
Relative refractory period
Can produce another action potential in response to larger-than-normal stimuli….why?
Some VGSCs have returned to resting state
Propagation of action potentials
Propagation: message passed along axon by repeating same events over and over (open voltage-gated channels etc.)
Can be continuous or saltatory
From point of generation (axon hillock and initial segment) to axon terminal action potential keeps its strength
Where does continuous propagation occur
unmyelinated axons
is continuous propagation fast or slow
slow (2mph)
how does continuous propagation operate?
local Na+ current opens VGSCs in next axon segment
every axon segment affected
message is “retold” over and over until it reaches synapse
why can’t continuous propagation travel backward?
refractory periods
where does saltatory propagation occur?
myelinated axons of PNS and CNS
is saltatory propagation fast or slow?
fast (250 mph)
does saltatory propagation require more or less ATP than continuous propagation?
saltatory propagation uses less ATP than saltatory propagation
how does saltatory propagation operate?
local currents skip internodes, depolarize nodes of Ranvier
how does axon diameter affect absolute refractory period?
smaller axon diameter = longer abs. refract. period
