Exam #1: Synaptic Transmission Flashcards
Describe the morphology of an electrical synapse.
An electrical synapse is two neurons connected by one or more gap junctions, which is permeable to ions and small molecules.
- Each opposite cell contributes half a channel, called a semi-channel or connexon
What is a gap junction composed of?
- Each gap junction contains a connexon
- A connexon is formed by six protein subunits, called connexins
How are signals passed between cells at a gap junction?
Electrical & chemical coupling
- Bidirectional
- Fast
- Low selectivity
What are connexins?
Protein subunits of the connexon that makes a gap junction
What are the two types of chemical synapses?
1) Hormone
2) Neurotransmitter
Describe the morphology of a CNS synapse.
- Synaptic cleft is 20-40nm apart
- Pre & post-synaptic membranes are held together by ECM & transmembrane proteins
- Unidirectional transmission
- Can have excitatory or inhibitory effects post-synaptically
Describe the morphology of a neuromuscular junction.
Much larger than the nerve terminal
What is the mechanism by which action potentials cause transmitter release?
1) Transmitter is synthesized & stored in vesicles
2) Action potential invades presynaptic terminal
3) Depolarization causes opening of voltage-gated Ca++ channels
4) Influx of Ca++
5) Ca++ causes vesicles to fuse with presynaptic membrane
6) Transmitter is released into synaptic cleft via exocytosis
7) Transmitter binds to receptor on post-synaptic membrane
8) Opening or closing of post-synaptic channels
9) EPSP or IPSP
10) Removal of NT by glial uptake or enzymatic degradation
11) Retrieval of vesicular membrane from plasma membrane
What is the general role of SNARE proteins?
Several specific transmembrane proteins located at vesicles & on the presynaptic plasma membrane that form a helix complex for vesicle docking & fusion
- Some are Ca++ binding proteins that “sense” Ca++ binding
What is the active zone of a chemical synapse? How do active zones differ in the CNS & NMJ?
The part of the pre-synaptic membrane that is specialized for the vesicular release of neurotransmitter
- Multiple in NMJ
- Single in CNS
Where is the docking complex located & what is its function?
Presynaptic density where vesicles containing neurotransmitters dock
What is synaptic delay?
The pause between the arrival of an action potential in the pre-synaptic terminal and the onset of potential change in the post-synaptic cell
What are the causes of synaptic delay?
- Events leading to fusion of synaptic vesicle with presynaptic membrane
- Diffusion of transmitter across synaptic cleft
- Activation of postsynaptic channels
What type of receptor is associated with short synaptic delay times?
Electrical synapses have a shorter delay than chemical ones
Is the synaptic delay shorter at chemical or electrical synapses?
Electrical
What are the three mechanisms of removing neurotransmitters from the synaptic cleft?
1) Diffusion
2) Enzymatic Degradation
3) Transport back to the pre-synaptic terminal or into neighboring astrocytes (glial cells)
Describe neuromuscular transmission.
1) Motor neuron action potential
2) Ca++ enters voltage-gated channels
3) ACh release
4) Binds NAChR
5) Na+ entry
6) Depolarization of the muscle fiber
7) Depolarization of voltage-gated Na+ channels
8) AP generated & propagated
9) ACh breakdown
What is the role of AChE in neuromuscular transmission?
Breakdown of ACh into acetate & choline
What is an EPSP?
Excitatory post-synaptic potential
Which ion permeabilities are affected during an EPSP?
Both Na+ & K+, but there is a higher permeability to Na+