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39 Cards in this Set
- Front
- Back
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Scheme of AP action
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AP-->depolarizes nerve terminal-->open voltage gated calcium channels-->local calcium levels rise in nerve terminal-->transmitter release
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Synaptic Delay
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time between presynaptic AP and postsynaptic effect,about 1 msec, hal of which is until calcium channels open
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Normal concentration of Ca
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normal extracellular calcium=2 mM
resting intracellular calcium = 10-100 nM 5 orders of magnitude difference, bigger difference than Na or K |
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What allows for Ca to be a signaling ion?
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low intracellular calcium concentration
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Experiments done by Katz
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Katz hypothesized that calcium entered nerve terminal and regulated vesicular release
1) remove calcium from bath --> no release, although AP activity and postsynaptic sensitivity same 2)Katz calcium pulse experiment 3)Add ionic blocker (Mg or Cd) block release (blocks calcium flux by competeition) Concluded calcium is necessary for release |
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New experiments to show calcium is sufficient to trigger release
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calcium ionophore causes release
load terminal with caged calcium, liberation induces transmitter release calcium liposomes |
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Why doesnt injected free calcium work?
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rapid buffering
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Two major transport systems
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1. Ca-Mg-ATPase - activated by intracellular calcium, high affinity for calcium(> 300 nM), Mg is needed as a co factor for ATP building, one calcium moved out of the cell for every ATP hydrolyzed
2. Na-Ca exchanger--> activated by intracellular calcium, lower affinity for Ca, driven by electrochemical gradient for Na, one calcium extruded for three Na ions entering cell, doesnt have rapid or potent effect on membrane potential |
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Buffers of Ca
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Endoplasmic reticulum, mitochondria, proteins. Buffering - regulatory point
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EGTA
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high affinity, low speed of binding --> no effect on release
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BAPTA
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high affinity, fast speed --> blocks release. Need faster buffer to grab calcium after it enters the nerve terminal and before it binds to secretory apparatus, so calcium entry and release apparatus must be very close to one another
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What concentration of calcium do we need to get transmitter release
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10- 100 micro molar
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Microdomains
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microdomains are hypothesized to be calcium just under calcium channel that triggers transmitter release
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Paired pulse facilitation
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when two stimuli are delivered in rapid succession, second stimuli evokes larger response the first
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Tetanic potentiation
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during a train of multiple stimuli at high frequency, there is a gradual increase in the amount of transmitter released
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Post tetanic potentiaiton
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after a train of stimuli has ended, there is an increase in the amount of transmitter release with single stimuli at low frequency for some time
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Model systems that allow to patch a nerve terminal
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squid giant synapse, chick calyx, auditory calyx of Held, Xenopus nerve muscle cultures
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Ca I-V relationship
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similar to Na due to similar DF and activation curves
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Tail current
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deactivation at micro and macro current scale, not seen much with Na channels because they inactivate too fast
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What is the relationship between calcium and NT release
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Release is very sensitive to Ca--> drop Ca-drop release
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what is the equation to describe relationship between Ca and release
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fold change in postsynaptic measure of release = fold change in calcium concentration raised to 3-4
On a log log plot straight line with slope of 3-4 ex ca change 0.15=0.3 2 fold change, postsynaptic change 0.25 - 4 (16 fold ) 2^4 = 16 |
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What does this relationship mean
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It is important for modulation of synapses, a small change in calcium current can have a big effect on the release, easier for plasticity to occur
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Name types of Ca channels that have been shown to regulate transmitter release
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mammalian NMJ = P type
amphibian/chick NMJ = N type |
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What is the role of Ca activated K channels
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Kca channels can shape action potential even with single APs and thus indirectly change transmitter release by changing voltage pulse (AP) that triggers Ca entry. Need to be close to Ca channels and respond very quickly.
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What happens when you block voltage gated K channels
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broaden AP from peak and increase peak calcium and release
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What happens when you block Ca activated K channels
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broaden AP from half way down and decrease peak calcium and release
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Describe LEMS
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LAMBERT EATON MYASTHENIC SYNDROME
autoimmune disease in which body attacks its own Ca channels at active zones of NMJ, clinically related to small lung cancer, tumor cells are like neuroendocrine cells and recognized as foreign by immune system, body makes antibodies to calcium channels on these cells (P/Q type). Antibodies reduce number of Ca channels in nerve terminal by crosslinking them and removing them from plasma membrane, patient feels weak because not all of nerve terminals release enough Ach to cause muscle AP. |
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What is the treatment of LEMS
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block K channels, prolong AP to increase Ca entry, increase release
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Synapsin 1
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keeps vesicles in the meshwork of actin, has a head domain that binds to phospholipids in the vesicle membrane and inserts into hydrophobic regioon. Tail associates with a specific vesicle membrane protein, also has two high affinity binding sites for actin in head region. Regulated by phosphorylation
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What phosphorylates synapsin
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cAMP- PK and specific CaM kinase I and CaM kinase II
Phosphorylation at site 1 diminishes binding to actin, at sites 2 and 3 reduces affinity of synapsin 1 to vesicles and abolishes binding to acti |
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Unoccupied vesicle docking area
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syntaxin (associated with nsec 1) binds calcium channel. At rest syntaxin is bound by nsec 1 and VAMP is bound by other vesicle proteins. This normally inhibits docking of vesicles
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Targeting of vesicles to active zone area
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Exocyst (complex of proteins) interacts with cytoskeleton to transport vesicles to active zone area after they released by synapsin, acts as chaperon for vesicle to find active zone.
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Tethering of vesicles to docking area
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Rab 3 (GTPase) may regulate the alignment of vesicles in the docking area and may assist in the displacement of nsec 1 from syntaxin.
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Docking
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Syntaxin and SNAP 25 bind to VAMP to form CORE complex
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T Snare
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terminal membrane receptors for these cytoplasmic proteins
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V Snare
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vesicle membrane receptors for cytoplasmic proteins
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Priming
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CORE complex binds synaptotagmin and forms 4 protein docked complex - VAMP and synaptotagmin on the vesilce bind to syntaxin and SNAP 25 on plasma membrane, 7S complex - stable on denaturing gels
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Fusion
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Synaptotagmin hold this configuration as a clamp waiting for calcium entry
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Recycling/Recovery
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Cytoplasmic NSF/alpha SNAP break up CORE complex so that it can be recycled by joining association with VAMP, SNAP 25 and syntaxin, hydrolyzing ATP and kicking out synaptotagmin, 20 S complex - stable on denaturing gels
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