Action Potentials and Synaptic Transmission Flashcards
What is Ohm’s Law?
Rate of FLOW of charges across a membrane = current
I=V/R (current=voltage/resistance
charge, difference of charges across membrane, voltage, permeability
When is Ohm’s law most useful?
UNEQUAL distribution of charges VERY close on either side of membrane
Nernst potential
Membrane potential is which in/out movement of an ion is balanced/equal. EQUILIBRIUM
DIFFUSIONAL force/ energy gradient
NOT FLOW OR RESISTANCE
VOLTAGE across a membrane that is PERMEABLE to X given ratio of X in:out
At rest, neurons typically have a membrane potential of _______ which is close to the Nernst potential for ______ which is ________.
-75mV, K+, -90mV
At rest, which ion channels are the only ones open? What are they known as?
K+, leak channels
What does the membrane potential of any cell depend on?
relative permeability of the membrane to each ion AND the concentration of the ion on either side of membrane
What does it mean if the membrane potential is close to the Nernst potential of a particular ion?
That it is more permeable to that ion
What is the modified form of the Nernst equation, considering more than 1 ion involved? More accurate.
The Goldman Field equation
Why is the membrane potential of a neuron close to, but not the same, as the equilibrium (Nernst) potential for K+?
Because of the presence of other ions and the activity of ion pumps that contribute to the overall resting membrane potential.
Membrane potential depends on 2 things for each ion. Name them.
1.) permeability
2.) concentration gradients
Membranes are poorly permeable to what kind of particles? What does the membrane need for these particles?
charged, channels
What is a channel?
pores in the membrane that allow movements of an ion
Are channels selective to few ions which the same charge, or open to many ions with different charges?
few ions, same charge
Can membrane permeability change very quickly?
yes
What dictates how channels change their open/closed states?
What they are “built” to detect
What are the different types of channels/their stimuli? (4)
1.) voltage-gated
2.) stretch/mechanical deformation- mechanoreceptors/osmoreceptors
3.) intracellular messengers
4.) extracellular messengers -ionotropic receptors (ligand binds to a receptor which is also a channel, binding opens channel and allows ion to cross membrane
Which 3 parts of the neuron are involved in the action potential?
axon, axon hillocks, synaptic terminal
What kind of channel does an action potential require?
sodium voltage-gated
SOMETIMES calcium voltage-gated
What kind of feedback do action potentials rely on?
positive
Action potentials always result in a membrane voltage change that is the same size or gradually increasing in size?
same size
Action potentials occur very quickly/or relatively slowly? The membrane becomes more positive/negative?
Very quickly, positive (miliseconds!)
What helps to quickly terminate the action potential?
K+ VGC
For Na+/K+ ATPase channel, Na+ go in or out? How many? K+?
3 Na+ out, 2 K+ in
K+ concentrations are high inside/outside, so it diffuses in/out? This is why we need the pump
out
Step 1 is the _____ ________ _________
resting membrane potential
What is step 2?
depolarization
What does is depolarization activated by?
The inside of the membrane becoming more positive
What is the membrane potential that activates Na + VGC to open and Na+ to enter the cell?
~-55mV
What channels open in response to depolarization of the cell membrane?
Na+ VGC
How do Na+ initiate and propagate action potentials ?
By allowing rapid influx of sodium ions
When do K+ VGC open?
In response to depolarization but typically with slight delay
What is the role of K+ VGC?
contribute to REPOLARIZATION and RESTORATION of resting membrane potential by allowing efflux of potassium ions
What is the period called after an action potential has begun, when a second action potential cannot be triggered for about 1-2 msec, no matter how large the stimulus?
absolute refractory period- time required for the Na+ channel gates to reset to their resting positions
Why is absolute refractory period important?
so that action potentials moving from trigger zone to axon terminal cannon overlap and cannot travel backward
What’s happening in the relative refractory period?
some but not all Na+ have reset and K+ channels are still open.
Na+ that have not quite returned to their resting position CAN BE REOPENED by a stronger-than-normal graded potential.
What limits the rate at which signals can be transmitted down a neuron?
refractory periods
What are depolarizations or hyperpolarizations whose strength is directly proportional to the strength of triggering event. They lose strength through cell.
graded potential
_____________ made a neuron more likely to fire an action potential. ______________ make a neuron less likely to do so.
depolarizing graded potentials
hyderpolarizing graded potentials
Which is faster, activation gate or inactivation gate?
activation
What initiates an action potential?
depolarization
Difference in fiber size/ speed of impulse?
A fibers=largest/fastest/myelinated (touch, pressure, position, heat, cold) final common pathway (12-130 m/sec)
B fibers=medium/medium speed/Non-myelinated15m/sec)
viscera to brain/SC, autonomic efferents to auto. ganglia
C fibers= smallest, non-myelinated (.5-2m/sec) pain, touch, pressure, heat, cold from skin, pain impulses from viscera, efferents to heart/smooth muscle/glands
In a chemical synapse, which neuron releases a neurotransmitter?
presynaptic
In a chemical synapse, where are the receptors?
post-synaptic cell membrane
What transport mechanism is used in chemical synapses for NT to cross the synaptic cleft from pre-synaptic to post-synaptic membrane?
diffusion
Where is synapse located? What expresses the receptor for NT?
between a dendritic spine or an axon terminal
dendritic spine
NT vesicles are synthesized and packaged where? Transported down the axon via what?
What is this called??
rER and Golgi
microtubules
fast axonal transport
Where are NT synthesized?
cytosol of presynaptic terminal
How are the NTs transported into the viscle?
proton pump
In the presynaptic terminal cytoskeleton, vesicles bind to ______ and are transported to release sites aka _________ close to synapse
actin, active zone
Which type of channels are used in NT release?
calcium VGC which leads to release and diffusion across synaptic cleft
What does the entry of calcium trigger?
exocytosis of synaptic vesicle contents
5 steps of neurotransmitter release
1.) AP depolarizes axon terminal
2.) depolarization opens Ca2+ VGC and Ca2+ enters the cell.
3.) Ca2+ triggers exocytosis of synaptic vesicle contents
4.) NT diffuses across syn. cleft and binds with receptors of the postsyn. cell.
5.) NT binding initiates response in the postsyn. cell.
Key players in vesicle release?
v-SNARES, t-SNARES, complexin, synaptotagmin
v-SNARES are attached to what? What do they do?
vesicles, they “force” the vesicles to fuse with the presynaptic membrane and dock with t-SNARES
What type of protein is synaptobrevin?
v-SNARE
What protein complex is attached to the pre-synaptic membrane and “grabs” the v-SNARE?
t-SNARE (target)
What are 2 t-SNARES?
syntaxin and SNAP-25
Which molecule prevents premature release after v-SNARE and t-SNARES engage?
complexin
Name the calcium-binding protein that “knocks” complexin off the v-SNARE/t-SNARE complex
synaptotagmin
What does the toxin released by Clostridium botulinum do?
impairs assembly and function of v-SNARES and t-SNARES
What does Botox A bind to?
SNAP-25 (v-SNARE)
Which enzymes degrades acetylcholine to acetate and choline?
acetylcholinesterase
Once NTs are degraded, what reabsorbs them? Where are they then diffused out?
astrocytes and presynaptic terminal. Out of cleft and carried by blood
What is the result of NTs causing cation channels to open?
depolarization for sodium and calcium
hyperpolarization for potasium
What is the result of NTs causing anion channels opening?
hyperpolarization
Many NTs cause what kind of effect? (ex. G-protein)
intracellular cascade of second messengers. Can open/close for longer periods of time. Change kinase activity, and gene expression
What do ionotropic receptors do?
open an ion channel when they bind to their ligand
Which type of receptor binds to the NT glutamate which causes Na+ and Ca2+ channel to open
NMDA receptor
Which receptor binds to acetylcholine, causing Na+ channels to open?
Nicotinic acetylcholine receptor
Which receptors bind to GABA and glycine causing Cl- channels to open?
GABA(a) and glycine receptors
Most ____________ receptors are linked to G-protein signaling.
metabotropic receptors
What is the NT of the neuromuscular junction and expressed through the brain?
acetylcholine
What is the most important inhibitory NT of the “intracranial” CNS?
GABA
What is the most important inhibitory NT of the spinal cord?
glycine
What is are the most common excitatory NT of the CNS?
glutamate
NMDA receptors are very important for what?
learning and memory
Which NT has autonomic NS functions, cortical and limbic system roles
norepinephrine
If NT binds to an inhibitory receptor what happens? excitatory?
hyperpolarization (membrane becomes more negative)
depolarization (membrane becomes more positive)
What brings about graded potentials in the dendrites and cell bodies?
graded potentials
Graded potential is any change in membrane potential that DOES NOT result in what?
action potential
Include changes in membrane potential that are below the threshold for an action potential or occur in areas of the cell that do not have Na+ VGCs
Which lasts longer, graded or action potential?
graded
If multiple graded potentials add up in a “staircase” fashion over time, this is called what?
temporal summation
What does it take for graded potentials to initiate an action potential in a Na+ VGC?
The net result of all EPSPs and IPSPs are integrated and the axon hillock and if it brings the hillock to threshold, an AP (or string of APs) will fire
What kind of receptors can have very long-lasting effects including protein synthesis and long-lasting intracellular signals?
metabotropic
