Lecture 3 - Chapter 5: Synaptic Transmission Flashcards
What two classes of synapses are there?
- Electrical synapse
- Chemical synapse
What is an electrical synapse?
Here, the membranes of two communicating neurons come extremely close at the synapse are are linked together by gap junctions (aligned, paired channels in the membrane of the pre- and postsynaptic neuron).
What are characteristics of an electrical synapse?
- Fast transmission
- Bi-directional (allow transmission in both directions)
- No plasticity
What is a connexon?
An assembly of six proteins (connexins) that form the pore for a gap junction between the cytoplasm of two adjacent cells.
Electrical synapses are characterized by the ability for bidirectional transmission. What function is enhanced by this characteristic?
Synchronization of electrical activity among populations of neurons. As can be seen in the picture.
What is a chemical synapse?
Compared to electrical synapses, the space between the pre- and postsynaptic neurons is greater in chemical synapses. This space is called the synaptic cleft. Within the presynaptic terminal, synaptic vesicles are located that are filled with neurotransmitters. Upon presynaptic stimulation, these vesicles fuse with the presynaptic membrane so that neurotransmitters are released in the synaptic cleft.
What are characteristics for chemical synapses?
- Slower transmission
- Uni-directional
- Plasticity
What are neurotransmitters?
Neurotransmitters are present in synaptic vesicles in the presynaptic neuron. They’re released into the synaptic cleft, upon arrival of an action potential in a calcium-dependent manner. Here, neurotransmitters can bind to postsynaptic (and sometimes also presynaptic) receptors.
What two categories of neurotransmitters are there?
- Small-molecule neurotransmitters
- Neuropeptides
Neurotransmitters are carried inside vesicles. There are two types of vesicles, name these.
- Synaptic vesicles
- Dense core vesicles
Name characteristics of synaptic vesicles.
- 40-60 nm
- Carry small molecule neurotransmitters
- Local recycling of synaptic vesicles
- Found in synapses
- Rapid action
- Can only fuse at the active zone
Name characteristics of dense core vesicles.
- 60-120 nm
- Carry (neuro)peptides
- Produced inside golgi
- Found everywhere
- Slow action
- Can fuse anywhere
How can the pH be used to visualize secretion?
The intracellular environment of vesicles have a pH of about 5,0-5,5, while the extracellular environment (cerebrospinal fluid, blood, etc.) has a pH of 7,4. This difference in pH can be used to visualize secretion.
This is done through a modified version of GFP, pHluorin. This GFP can either bind to a transmembrane domain of a synaptic vesicle or it can bind to a neuropeptide of a dense core vesicle and via this way can be taken up inside the vesicle.
Note: modified in the sense of that the GFP is more sensitive to pH changes.
What fluorescence intensity is expected when pHluorin is taken up inside a vesicle and when the vesicle with pHluorin fuses with the membrane?
The fluorescence intensity of pHluorin increases when the pH increases.
- Since the intracellular environment of a vesicle has a low pH, pHluorin is expected to have a low fluorescence intensity.
- When the vesicle fuses with the membrane, it is exposed to the extracellular environment which has a higher pH (7,4). Thus, upon fusion with the membrane the fluorescence intensity of pHluorin is expected to increase.
What’s the relationship between the fusion of a synaptic vesicle and electrical events?
That fusion of one synaptic vesicle corresponds to one spontaneous electrical event.