5 - Synaptic Transmission & Integration Flashcards
What’s saltatory conduction?
AP leaping from one node of ranvier to the next
T/F - it’s faster for AP to occur at every point in axon than passive diffusion
False - faster for passive diffusion to occur between Nodes than for AP to occur at every pt in axon
Is the Na/K-ATPase pump involved in changing mempot during an AP?
NO
What does the Na/K-ATPase pump help with then?
Helps reset Na and K+ concentrations after AP
What are the 2 classes of synapses?
- Electrical
2. Chemical
Electrical synapses
Direct electrical coupling between cytosol of 2 neurons
Chemical synapses
Electrical signals converted into chemical signal
Gap junctions used in which synapses?
Electrical - connect cytosol of 2 neurons so changes in Vm of one immediately affect the other
How many subunits form cone on in each cell membrane with a gap junction?
6
Are gap junctions selective?
No - allow ions and small molecules through
Gap junctions electrically….
Couple neurons
Cytosol connections are limited by what?
Number of ions that can pass through gap junctions
Are gap junctions unidirectional?
No - bidirectional; change in Vm in post-synaptic neuron reflected in pre-synaptic and vice versa
T/F - chemical synapses are fast but weak
False - slow but powerful
Pre-synaptic active zones
Location of specialised proteins that bind vesicles, releasing neurotransmitter
What are the different types of chemical synapses based on axon location?
Axo-dendritic
Axo-somatic
Axo-axonic
Axo-dendritic
Most common; axon-dendrite connection on dendrite spines or shaft
Axo-somatic
Axon-soma connection; commonly inhibitory
Axo-axonic
Axon-axon connection; can module signal before transmission
Different types of chemical synapses based on microscopic structure
Gray’s type I and Gray’s type II
Gray’s type I
- asymmetrical
- round vesicles
- e- dense
- usually excitatory
- contact dendritic spines
Gray’s type II
- symmetrical
- oval vesicles
- less e- dense
- usually inhibitory
- contact dendritic shaft/soma
What does number & size of synapses connecting 2 neurons indicate?
Strength and importance of the signalling
4 criteria of a neurotransmitter
- Synthesised in pre-synaptic neuron
- Defined action on post-synaptic neuron/effect after release
- Exogenous administration mimics actions of endogenous transmitter
- Specific mechanism exists for removing substance from synaptic cleft
What are the 3 types of neurotransmitters?
Amino acids
Amines
Peptides
Amino acids
Small organic molecules
Eg: glutamate (excitatory - opens cation channels), GABA (usually opens Cl- channels)
Amines
Small organic molecules
Eg: dopamine, acetylcholine
Peptides
Short amino acid chains stored in and released from secretory granules
Neuromuscular junction
Special synapse in periphery connecting moto neurons to muscle fibres
What are the steps for communication via a chemical synapse?
- Neurotransmitter synthesis
- Neurotransmitter loading into synaptic vesicles
- Vesicles fuse w/ pre-synaptic terminal
- Neurotransmitter binds post-synaptic receptors
- Post-synaptic response
- Removal of neurotransmitter from synaptic cleft
- Neurotransmitter synthesis
Must be synthesised in neuron from which it will be released
Most neurons only release one type
A) simple amino acids available in all cells
B) GABA & amines need special enzymes for synthesis
C) peptides packaged into secretory granules in nucleus then transported to terminal
- Neurotransmitter loading into vesicles
Vesicles from phospholipid bilayer, approximately all same size and contain similar amounts of neurotransmitter
Are secretory granules smaller than vesicles?
Nope - larger
- Vesicles binding & neurotransmitter release
Exocytosis to release vesicle contents & endocytosis to recover vesicle membrane & reuse
When AP reaches axon terminal, Cav channels in active zone open.
Intracellular Ca2+ triggers fusion of synaptic vesicle w/ membrane, releasing neurotransmitter into cleft
- Neurotransmitter binding to post-synaptic receptors
What are the 2 mechanisms of neurotransmitter action at post-synaptic membrane?
- Ionotropic
2. Metabotropic
Ionotropic
Ion channel gated by ligand (neurotransmitter), fast & brief
Metabotropic
Receptor indirectly linked w/ ion channel, need signalling cascade, slower and longer action, broader effects
- Response to neurotransmitter
Post-synaptic potential
Post-synaptic potential (PSP)
Transient change in mempot due to neurotransmitter-mediated channel opening
Excitatory PSP
Transient depolarisation (eg: Na+ channels opened)
Inhibitory PSP
Transient hyperpolarisation (eg: Cl- channels opened)
Size of PSP depends on what?
Amount of neurotransmitter released, no. Binding sites that can receive it, no. Neurotransmitter molecules that successfully bind
- Recycling the neurotransmitter
Signal turned off by diffusion, reuptake or enzymatic destruction
How do we get Ca+2 out of axon terminal?
Na/Ca exchanger (NCX) moves 3 Na+ into cell & 1 Ca+2 out) uses [Na+] gradient
Dendrites ___________ inputs from each synapse
Integrate
Spatial summation
Summation across input from many axons
Temporal summation
Summation across time from single axon
