5 - Synapse Structure Flashcards
What is a synapse?
A specialized functional contact zone for communication between nerve cells or between a nerve cell and its effector organ.
What is the current flow like at electrical synapses? What type of junction is this?
Bridged/gap junction .
The current flow can be bidirectional or unidirectional.
What is the current flow like at chemical synapses? What type of junction is this?
Unbridged junction (presence of a synaptic cleft)
Current flow is always unidirectional from pre-synaptic to post-synaptic cells.
Describe the structure of gap junctions in electrical transmission?
Gap onl 3.5 nm wide.
Apposing cells contribute a hemi-channel = connexon
How many connexons form an ion channel in gap junctions (electrical transmission)? How do they open?
2 connexons, each made of 6 connexins that each have 4 membrane-spanning regions.
Open via a conformational change in the pore.
What is the most rapid form of synapse between neurons with virtually no synaptic delay? Describe the pathway current takes.
Gap junctions with electrical synapses.
They are low resistance, high conductance channels that bridge two cells.
Current flows through the junction and depolarizes the pot-synaptic cell.
What are some properties of electrical synapses? What are some down-sides?
Very fast.
Synchronization of transmission between 2 or more electrically coupled neurons.
All-or-none.
No mechanism for inhibitory actions or long-lasting changes in effectiveness (modulation) of synapses.
Where are electrical synapses typically found?
In invertebrates.
Also present in the retina and inner ear. More prevalent during development.
What is the most common type of transmission in the CNS and PNS? What neurotransmitter is found at the neuromuscular junction? What effect does this have on skeletal muscle vs cardiac muscle?
Chemical transmission.
Acetylcholine: slows contractions of cardiac muscle and conversely causes contractions in skeletal muscle.
How is it possible that Ach can have two different effects on cardiac and skeletal muscle?
They have two different types of receptors: muscarinic receptors in cardiac muscle and nicotinic receptors in skeletal muscle.
What is chemical transmission mediated by?
Release and diffusion of nts into the synaptic cleft.
What are the components of the presynaptic (prejunctional) region of the neuron?
Synaptic vesicles, active zones with presynaptic dense projections that contain Ca dependent calmodulin, and autoreceptors where nts bind to be released.
What is located at the postsynaptic region (end-plate; postjunctional)?
Receptors for neurotransmitters, postsynaptic density(PSD), and actin molecules.
What is the difference between Gray’s type I and Gray’s type II CNS chemical synapses?
I: Asymmetric with round synaptic vesicles, wide synaptic cleft, prominent post synaptic densities, and is often associated with excitatory synapses (
cholinergic or glutamatergic).
II: Symmetric with flat or pleomorphic vesicles, a narrow cleft, less prominent Postsynaptic densities, and often associated with inhibitory synapses (gaba or glycine)
What are characteristics of monoaminergic synapses?
Dense-core vesicles, axonal varicosities, wide synaptic cleft, and no prominent pre- and postsynaptic densities.
Norepi, epi, serotonin, and 5HT.
What do peptidergic synapses contain?
Large dense-core vesicles that are often of different sizes and shapes.
What is the structure of neuromuscular junctions? What is located there?
Presynaptic dense bars, Ca++ channels, primary and secondary synaptic clefts, AchE and basal lamina clefts, postsynaptic junctional folds, Ach receptors, and voltage gated Na channels along sides and depths of folds.
What is located within the synaptic cleft itself?
Enzymes that degrade Ach called acetylcholine esterases.
Basal lamina also there, which is important for regeneration.
What is Myasthenia Gravis (MG)? What are symptoms and causes?
Disease of chemical transmission at the neuro-muscular junction causing severe weakness of muscles.
Most common cause is autoimmunity: antibodies against one’s own nicotinic ach receptors in muscles.
Congenital and heritable is rqare.
What is the treatment for myasthenia gravis (MG)?
Inhibitors of acetycholinesterases.
What is Lambert-Eaton Myasthenic Syndrome (LEMS)?
Antibodies against one’s own presynaptic voltage-gated calcium channels.
What are the steps involved in a synaptic vesicle releasing its contents in the synaptic cleft?
- Approach
- Attach
- Contact the PM
- Fusion
- Fission - opens so nts released
- Collapse - of vesicles
- Retrieval - for recycling
- Recycling
Where is the tetanus toxin found and what are its effects?
Soil and rusty wires.
It cleaves VAMP and doesn’t allow the release of GABA, resulting in reduced inhibition.
Excitatory nts take over, leading to generalized muscle spasms (spastic paralysis).
Where is botulinum toxin found and what effect does it have?
In improperly processes meats.
Cleaves VAMP (v-snare) or t-snare and prevents the release of nts so muscles become weak and flaccid paralysis occurs.
