NeuroPhys 2 Flashcards
the movement of a dissolved, charged particle across a lipid membrane depends on:
- the charge of the particle
- the difference in distribution of charges across the membrane
- the permeability of the membrane to the charged particle
the rate of flow of charges across a membrane is known as ?
current (I) and is defined by Ohm’s law I = V/R
what is the nerst potential?
the membrane potential at which the inward and outward movement of an ion through a channel is balanced and equal
at rest, neurons typically have a membrane potential that is close to?
Nerst potential for K+
at rest, the only ion channels that are open are what?
K+ channels
the membrane potential of any cell depends on?
- the relative permeability of the membrane to each ion
- the concentration of the ion on either side of the membrane
an action potential:
○ Requires the presence of sodium voltage-gated channels (or sometimes calcium voltage-gated channels)
○ Relies on positive feedback
○ Always results in a membrane voltage change that is the same size
○ Occurs very quickly - the membrane becomes more positive (depolarized) in a matter of milliseconds
where do action potential occur?
the axon hillock, the axon, and the synaptic terminals
what is the first step of action potential?
- The Na+/K+ ATPase uses ATP to pump Na+ out of the axon, and K+ in
○ K+ concentrations are high inside the axon, and low outside (vice-versa for Na+) - K+ is high inside the axon, so it diffuses out
○ Diffusional, or chemical, force acting on K+ - Membrane becomes negative inside the axon
○ Negatively-charged proteins, ions cannot leave the cell with K+ - The attractive force of the negatively-charged membrane balances out the diffusional force driving K+ out
○ This balance establishes the resting membrane potential at about -70 mV (inside membrane negative)
what is step 2 (depolarization) of action potential?
- The inside of the axonal membrane becomes more positive, and a Na+ VGC opens
○ Channels are opened by more positive charges inside membrane
○ Threshold = membrane potential at which all Na+ VGC will end up opening (~ -55 mV) - Na+ VGC opening leads to other Na+ VGC opening, eventually all open
○ Positive feedback, Na+ diffuses into the cell, making membrane more positive, allowing more Na+ in - Inside of the axon becomes completely depolarized
○ Diffusion gradient (high Na+ outside, low inside) as well as electrical force (inside negative) drives Na+ into the cell - K+ VGC open, Na+ VGC close after ~ 1 millisecond
what is step 3 (repolarization) of the action potential?
- Na+ VGC are closed, no further Na+ entering the axon
○ Close after about 1 millisecond
○ Are unable to open for 1-2 millisecond - they are “locked”
○ After 1-2 millisecond, Na+ VGC will “unlock” - but only if the membrane is repolarized (becomes inside-negative again) - K+ rapidly leaves the axon
○ High K+ inside axon and positive charge inside the membrane strongly drive K+ out
○ K+ VGC and regular K+ channels are both open, allowing rapid K+ exit - Na+ VGC are ready to re-open:
○ When the membrane potential is -70 mV (repolarization)
○ After they’re “unlocked” (1-2 millisecond after closing)
the sodium voltage gated channel has 2 gates which are?
the activation gate and inactivation gate
when does the activation gate of the NaVGC open?
as soon as threshold is reached (the membrane depolarizes to -55 mV)
what does the inactivation gate do for the NaVGC?
this gate closes very soon after the activate gate opens, after Na+ has rushed into the cell
the potassium voltage gated channel does not have what?
an inactivation gate, it opens when the cell depolarizes, and closes once the cell is inside-negative again, much slower to open than the sodium voltage-gated channel
what is the absolute refractory period?
inactivation gate of the Na+ VGC is closed (at the peak of depolarization)
what happens after the absolute refractory period?
relative refractory period
what is the relative refractory period?
inactivation gate is open, activation gate is closed for the Na+ VGC
- the cell is hyperpolarized - the membrane potential is lower than resting membrane potential
what are the properties of action potentials?
- all or none event
- initiated by depolarization
- have constant amplitude
- have constant conduction velocity along a fiber
explain continuous conduction
when one part of the membrane depolarizes, it reaches threshold and an action potential occurs, the neighboring part of the axon needs to depolarize -> reach threshold -> before the action potential progress further down the axon
explain saltatory conduction
in a myelinated axon, the nodes of ranvier are the only parts of the axon expressing voltage-gated channels, the myelin insulation allows the electrical field from the depolarization to jump to the next node ranvier hence being faster than continuous conduction
which fibers are the largest?
A fibers
which fibers are the medium sized ones?
B fibers
which fibers are the smallest?
C fibers
which fibers are myelinated and which ones aren’t
A fibers are myelinated, and B & C aren’t
which fibers are large sensory nerves for touch, pressure, position, heat, and cold?
A fibers
which fibers are the final common pathway for motor system
A fibers
which fiber is from viscera to brain and spinal cord, autonomic efferents to autonomic ganglia?
B fibers
which fibers is for impulses for pain, touch, pressure, heat, cold from skin and pain impulses from viscera
C fibers
which fibers is visceral efferents to heart, smooth muscle and glands?
C fibers
what releases the neurotransmitters?
the presynaptic neuron
where does the neurotransmitter goes after it gets released from the presynaptic neuron?
to receptors embedded in the post-synaptic cell membrane
how does the neurotransmitters cross the synaptic cleft from the presynaptic neuron to the post synatic neuron?
by diffusion
where are neurotransmitter vesicles synthesized?
in the rER and golgi
what is fast axonal transport
the molecule motor kinesin transports the vesicles towards the synaptic terminal
how does calcium enter the presynaptic terminal
through the opening of Ca+2 VGC
basic steps of NT release?
- AP arrives at the presynaptic terminal
- Depolarization leads to opening of voltage-gated calcium channels
- Calcium enters the presynaptic terminal (as per its Nernst potential)
- Calcium binds to a protein associated with neurotransmitter-filled vesicles
- Neurotransmitter is released into the cleft as the vesicles fuse with the presynaptic membrane
- Neurotransmitter binds to a receptor
what is the whole point of the action potential when it gets to the presynaptic terminal?
to open the Ca+2 VGC causing the release of NT to the synaptic cleaft
what are V-SNARES?
protein complex that attaches to the vesicles and force the vesicle to fuse with the presynaptic membrane
- dock with t-SNARES
synaptobrevin is a ?
v-SNARE
what is a t-SNARE?
a protein complex attached to the pre-synaptic membrane and it grabs the v-SNARES
syntaxin and SNAP-25 are?
t-SNAREs
what are complexin?
a molecule that prevents premature release after v-SNAREs and t-SNAREs engage with each other
what is synaptotagmin?
a calcium-binding protein and when calcium binds, it knocks complexin off the v-SNARE-t-SNARE complex
steps of vesicle release
- V-SNARES and t-SNARES “zipper” together
- Synaptotagmin and complexin prevent premature fusion and release after zippering
- AP -> depolarization -> Ca2+ VGC opening -> calcium influx into the pre-synaptic terminal
- Calcium binds to synaptotagmin -> disengagement of complexin
- The synaptic vesicle fuses when complexin disengages -> release of NT into the synapse
- The v-SNAREs and t-SNARES disengage, and the vesicle is re-used
- This occurs after intracellular calcium levels decrease
what does botox impair?
impair the assembly and function of v-SNAREs and t-SNAREs and the fusion of vesicles with the presynaptic membrane
which botox is used for medical use?
botox A
what does Botox A bind to
SNAP-25, a v-SNARE
how do NT get removed from the receptor afterwards?
degraded by enzymes in the synapse
Acetylcholine - excite
receptor: Nicotinic, M1, M3, M5
signal:
- ionotropic, sodium channel
- increases in calcium (metabotropic)
acetylcholine - inhibit
receptor: M2, M4
signal:
- ionotropic, chloride channel
GABA - inhibit
Receptor: GABAa
signal: ionotropic, chloride channel
glutamate - excite
receptor: NMDA, AMPA
signal: ionotropic sodium + calcium channels
Glucine - inhibit
receptor: strychnine- sensitive
signal: ionotropic, chloride channel
norepinephrine - excite
receptor: alpha -1 and beta -1
signal:
- increased IP3 and calcium
- increased cAMP
if the NT binds to an inhibitory receptor, what happens?
results in dendrite hyperpolarization (membranes becomes more negative)
if the NT binds to an excitatory receptor, what happens?
results in dendrite depolarization (membrane becomes more positive)
what is a graded potential?
any change in membrane potential that doesn’t result in an action potential
