Neurobio 2.6 Flashcards
Method used in first experiment suggesting presence of vesicles.
Bernard Katz recorded from endplate of frog neuromuscular junction w/o any stimulation.
What did Katz observe?
Spontaneous mini EPPs (minis) at regular intervals. Usually 1 mv, but sometimes ones two or three times larger.
Size of a mini in a frog neuromuscular junctino
0.7 mV
Typical duration of mini
10ms
Quanta
Packets of neurotransmitters
Each quantum produces
1 mini
According to quantum hypothesis, a full size EPP occurs when
an action potential causes the synchronous release of many quanta
Quantum size
Size of a mini
Quantum size of most central synapses
0.1-0.2 mV
Quantum content
number of packets (quantums) released due to an action potential
Quantum content at neuromuscular junction
About 200
Quantum content in CNS
between zero and 2
Physical correlates of quantums are
vesicles
Why is an AP in muscle cells guaranteed to generate a response?
Quantum number is so high
Places vesicles might be found in presynaptic terminal
- Readily releasable pool
2. Storage pool
Tends to be anchored to cytoskeleton
storage pool
Docked at active zones of presynaptic terminal
Readily releasable pool
Anchors vesicles to cytoskeleton
Synapsin I
Synapsin I is located
in vesicle membrane
Effect of calcium on synapsin I
interaction between vesicles and cytoskelton becomes weaker, freeing vesicles
Two proteins calcium binds upon entering terminal of presynaptic cell
Synapsin I (storage pool) Synaptotagmin I (readily releasable pool)
Two effects of calcium entering terminals of presynaptic cell
Readily releasable vesicles released and storage pool vesicles mobilized
Function of the SNARE complex
Docking of vesicles at active zone of presynaptic membrane.
How calcium relates to SNARE complex
Binds synaptotagmin I on vesicle. Somehow the membranes of vesicle and presynaptic terminal active zone then fuse together.
Where are the alpha helices of the SNARE complex?
Protrude into cytoplasm.
Describe SNARE complex
Synaptobrevin alpha helix protrudes from vesicle. SNAP-25 and and syntaxin alpha helices protrude from presynaptic membrane. The alpha helices are like velcro.
Synaptotagmin I is on the vesicle.
VSNARE
synaptobrevin (on vesicle)
TSNARE
Syntaxin and SNAP-25 (targets of vesicle fusion)
Why is curare useful in studying neuromuscular junction?
Competitive inhibitor of acetyl choline which prevents action potential but not end plate potential. Now you can study end plate potential alone and without the muscle moving.
Some bacterial toxins that target SNARE
Tetanus toxin
Botulinum toxin A and C
How toxins that target SNARE work (how are they classified)
Proteases that hydrolyze the proteins
Tetanus toxin targets
synaptobrevin
Targets SNAP-25
Botulinum toxin A
A = apple = SNAPple
Targets syntaxin
Botulinum toxin C
C for Syn
Calcium pump takes about _____
100 ms
This is the time needed for calcium in presynaptic cells to return to normal calcium levels
Clears away calcium after AP causes entry into presynaptic cell
calcium pumps
Two types of synaptic plasticity
- Paired pulse facilitation
2. LTP (long term potentiation)
How paired pulse facilitation works
When AP’s are fired in succession, residual calcium from previous AP’s causes increase in amplitude of next EPSP’s
Short term synaptic plasticity
Paired pulse
Neurons thought responsible for long-term memory
CA1 and CA3 neurons of the hippocampus
100 Hz corresponds to ____ interval between pulses
10ms
Result of one tetanus stimulation
The second AP generates EPSP up to three times larger. After first 1/2 hour, EPSP is larger but not as much, goes down after about another 1/2 hour, and after about 2 hours, decays back to normal.
Result of four tetanus stimulations
EPSP elevation lasts for up to 10 hours
Early LTP
One tetanus stimulation
Late LTP
Multiple (four was example given in class) trains of tetanus stimulations
Two glutamate receptors on post-synaptic membrane involved in LTP are:
AMPA
NMDA
Two phases of LTP
- Induction phase
2. Expression phase
What happens during induction phase of LTP?
- Tetanus causes summation of EPSP’s
- Sustained depolarization removes magnesium block from from NMDA receptors
- NMDA conducts calcium through post-synaptic membrane and so both AMPA and NMDA receptors are bringing in calcium
What happens during expression phase of LTP?
Three mechanisms are involved
- Ca increase activates calmodulin, which activates CaM kinase II, which phosphorylates AMPAR’s, causing them to generate a larger inward current.
- CaM kinase II somehow causes vesicles whose membranes contain AMPAR’s to fuse with post-synaptic membrane
- Activation of silent synapses
How are silent synapses activated during LTP?
At first, they have no AMPA receptors on membrane. Depolarization from neighboring synapses is strong enough to remove magnesium block from NMDA. Calcium enters and, through insertion mechanism, AMPA receptors are inserted into membrane.
Two properties of LTP
Cooperativity and associativity
Cooperativity
Many CA3 neurons must be activated in order for the spatial summation of PSP’s to be strong enough to remove magnesium block from NMDAr’s.
Associativtity
Both pre and post-synaptic neurons must be activate at same time: pre for generating depolarization, and post-synaptic for NMDA magnesium block to get removed
Cooperativity and associativity are due to teh special properties of
NMDA receptors
Following LTP, longer term changes include
transcription of new proteins and formation of new synapses
Describe a “repetitive” stimulation of CA3
100 stimulates pulses at 100Hz (10ms between pulses)
Stimulus artifact
Recorded by electrode in bath which contains the cell (not a direct biological response)
How expression phase relates to IV curve equation
Increases Po and N
I = N * Po * gamma * (Vm-Erev)
After first of paired-pulse facilitation, Ca++ levels stay elevated about
20-100ms
