Lecture 5: Synaptic Transmission Flashcards
What is synaptic modulation?
Changes in synaptic function, allowing both robust function and higher complexity (learning, memory, etc.)
Gap junction structure
6 connexin transmembrane proteins form a connexon hemi-channel. Pairs of hemi-channels cross both cell membranes. Arrays of these pairs form a gap junction.
Properties of gap junctions
-Channels can be modulated open/closed
-Post-synaptic response is very fast, <1 ms
-Transmitted signal depends on relative size/electrical properties of pre/post synaptic neurons + gap junctions
-No amplification or polarity reversal possible
How do botulinum toxins paralyze?
Botulinum cleaves SNARE proteins, inhibiting the release of NTs
What is 1 quantum of neurotransmitter?
1 quantum of NT = 1 vesicle, which contains 1000s of NT molecules
Miniature End Plate Potential (MEPP)
The post-synaptic potential caused by spontaneous (not induced) fusion and release of 1 NT vesicle.
End Plate Potential (EPP)
The summation of the many MEPPs stimulated in response to an action potential
How many vesicles are released per action potential?
Vesicles per action potential depend on how much Ca2+ influx occurs. At the neuromuscular junction, 1 AP = ~150 vesicles, much more than enough to trigger post-synaptic firing. In the CNS, 1 AP = 1-4 vesicles.
Different types of neurotransmitter receptors
- Ionotropic (1 NT opens 1 channel directly)
- Metabotropic (GPCR or other secondary messenger)
Ionotropic receptor
One type of post-synaptic receptor. Ligand gated ion channel that opens when 1 NT binds, fast response
Metabotropic receptor
One type of post-synaptic receptor. GPCR that can open many ion channels in response to 1 NT, amplifying the signal. Response can be ~100 ms to minutes; plays role in modulating function or sustained effects.
Chemical synapse properties
Neuronal tuning, synaptic strength modulation, signal amplification, excitatory/inhibitory signals
Neuromuscular junction properties
No summation occurs; 1 action potential = 1 muscle activation (strong synapse)
1 AP -> ~150 vesicles, creating EPP up to 70 mV; ensures 1:1 transmission with 1.5-4 safety factor
Typical EPSP value
~0.2-0.4 mV on average; ~25 summed for 10 mV threshold change
Synapse
Anatomically specialized junction between 2 neurons
Convergence
A single post-synaptic cell can receive signals from 100s or 1000s of synapses
Divergence
1 pre-synaptic cell can branch to synapse on many post-synaptic cells
What affects the excitability of a post-synaptic cell?
Number of active synapses and number of excitatory/inhibitory signals received
Types of synapses
- Electrical
- Chemical
Electrical synapse
Gap junction joining plasma membranes of 2 neurons, allowing local currents from arriving APs to flow directly through
Chemical synapse
Synaptic vesicles containing NTs (can be multiple different) are released to the synaptic cleft and bind to the post-synaptic cell, stimulating a post-synaptic potential via ion channels. Prevents direct current propagation but permits signal summation.
EPSP
Excitatory Post-Synaptic Potential. Created by opening Na+/K+ nonselective channels, allowing Na+ influx/small K+ efflux and local depolarization
IPSP
Inhibitory Post-Synaptic Potential. Created by opening K+ or Cl- channels, reducing chance of reaching AP threshold. K+ hyperpolarizes. If Cl- is actively exported, Cl- influx hyperpolarizes. If not, Cl- stabilizes resting membrane potential and counters positive ion movement.
Post-synaptic density
The post-synaptic membrane adjacent to the axon terminal is a specialized area with a high density of membrane proteins
How are neurotransmitters stored and released?
- NTs stored in vesicles docked at active zones or dispersed in axon terminal
- AP opens voltage-gated Ca2+ channels, allowing Ca2+ influx.
- Ca2+ binds synaptotagmins, causing SNARE protein conformational changes -> membrane fusion and NT release.
Post-fusion vesicle fates
- Endocytosis recycling beyond active zone.
- “Kiss and run” fusion; resealing/withdrawal after brief fusion. Common in high frequency neurons.
What happens to NTs after receptor binding?
Unbound NTs are removed by:
1. Reuptake; active transport and recycling to pre-synaptic axon.
2. Transport to nearby glial cells for degradation.
3. Diffusion away from receptors.
4. Enzymatic inactivation (some reused).
Types of summation
- Temporal
- Spatial
Temporal summation
Summation based on input frequency, where a new PSP is added on before the previous graded potential leaks away.
Spatial summation
Summation of 2 inputs arriving simultaneously at different locations on the cell.
Axon hillock
Most responsive area of neuron due to higher density of voltage-gated Na+ channels, = more negative threshold vs. dendrites or cell body. Synapses closer to the hillock will induce a greater EPSP.
How long do post-synaptic potentials last?
Always longer than 1 AP duration; neuronal responses almost always come in bursts, not as single events.
