Synaptic Transmission Flashcards
What are 3 roles of electrical synapses?
1) network activity (synchronization of electrical activity/info across many neurones)
2) communication between glia
3) coordination of growth in embryonic development (Bear et al)
What protein forms a gap junction?
Connexin (6 subunits from a connexon = hemichannel)
Which type of synapse has a role in epilepsy?
Electrical
Which type of synapse is the most common form of communication in the CNS?
Chemical
Chemical synapses occur between a presynaptic element, most commonly _______, and postsynaptic elements such as ________
Axon terminals
Dendrites, soma, axon terminals
(I.e. Axoaxonic, axosomatic, axodendritic)
Why is chemical transmission greatly flexible?
Many different NTs and receptors
Can modulate expression of NT and receptors
=> huge modulatory activity
What is the distance across the synapse for a) electrical synapses?
and b) chemical synapses?
Electrical - 0.35nm
Chemical - 30-50nm
What is the synaptic delay at:
a) electrical synapse?
b) chemical synapse?
a) none
b) min 0.3ms, usually 1-5ms
What is the direction of transmission for chemical and electrical synapses?
Chemical - unidirectional
Electrical - bidirectional
What are the 4 types of major NT?
Ach
Amines
AAs
Peptides
Small molecule NTs are synthesised in______ and stored in _____?
Synaptic terminal using enzymes from cell body
Small clear vesicles (40-60nm)
Large NTs are synthesised in _____ and stored in _____?
Cell body using RER and Golgi
Large dense core vesicles (90-250nm)
Where does Ca enter the presynaptic terminal?
Active zone
What is a MEPP?
Miniature end plate potential
=> spontaneous release of a single vesicle (quanta) (not due to AP)
What is/what causes an EPP?
Multiple MEPPs due to AP dependent release of many vesicles
Are axo-dendritic synaptic connections usually inhibitory or excitatory?
Excitatory
Are axo-somatic synaptic connections usually inhibitory or excitatory?
Inhibitory
Synapses where influence neurone firing the greatest?
At the initial segment / axon hillock
Which type of synaptic connection is modulatory and controls NT release?
How does it control NT release?
Axo-axonic
Controls Ca influx in presynaptic neurone
What is the name of the axo-axonic connection that increases NT release?
Presynaptic facilitation
Decreases K+ current, increases Ca current
What is integration?
Summation of all EPSPs and IPSPs
Where does integration occur?
Axon hillock
What is convergence?
Many neurones synapse onto one neurone
What is divergence?
One neurone communicates with many via axon collaterals
What is temporal summation?
Summation of PSPs arriving in quick succession (only needs one synaptic input)
What is spatial summation?
Summation of PSPs from different inputs across membrane surface
How does frequency affect NT release?
Increased frequency = increased release
What NTs require higher frequency APs for release?
Peptides
Why is an AP?
Self-propagating wave of depolarization
What is the normal direction of an AP?
Orthodromic
Synaptobrevin is what type of SNARE and what does it do?
V-SNARE
Binds T-SNAREs: SNAP-25 and syntaxin forming SNARE complex (docking and priming)
Which SNARE protein is a Ca sensor?
What does it trigger?
Synaptotagmin
SNARE complex formation and Fusion (Ca dependent)
What do synapsins do?
Restrain vesicles - bind NT to cytoskeleton
How are vesicle released from synapsins?
Ca dependent phosphorylation of synapsin
What occurs in targeting and docking?
Vesicle moves towards and binds to active zone
What is priming?
Vesicles are prepared for fusion
What is fusion?
Pore opening
What is the process (each step) of NT release from vesicles?
1) vesicles restrained (synaspsin)
2) targeting + docking
3) priming
4) fusion
5) exocytosis
6) endocytosis
How long does it take for vesicles to be recycled after release?
30-60 sec
What is the role of Clathrin proteins and dynamin?
Endocytosis;
- Clathrin binds and distorts vesicle membrane, pulls it into cell
- once fully invaginated dynamin “pinches off” vesicle
(Energy dependent GTP -> GDP)
What do zinc endonucleases and clostridium toxins do?
Cleave SNARE proteins actively involved in fusion
What are clostridium toxins?
Botulinum and tetanus
Which toxins is synaptobrevin cleaved by?
Tetanus and botulinum B, E, F + G
Which toxins are snap-25 and syntaxin cleaved by?
Snap25: botulinum A + E
Syntaxin: botulinum C
What does botulinum toxicity cause?
Blocks neuro transmission at peripheral Cholinergic synapses
Do vesicle transporters have high or low specificity?
LOW (many NTs use same transporter)
Do membrane tansporters have high or low specificity?
High (each NT has dif transporter)
What are vesicular transporters?
What else do they require to function?
Proton dependent antiporters
(NT in, H+ out)
ATPase: transports H+ into vesicle creates electro chem gradient
What is a membrane transporter?
Which NTs have these?
Na- dependent symporter
Choline, 5-HT, DA, GABA, glutamate
What is disinhibition?
Inhibition of inhibitory neurones
Generation of excitation by transient inhibition of a tonically active neurone
Describe how disinhibition works in the basal ganglia to excite the motor cortex
1) striatum excited (by excitatory input from cortex)
2) increased inhibition of globes pallidus
3) decreased inhibition of VL/VA of thalamus (it is disinhibited)
4) allows excitation of motor cortex
