Synaptic Transmission Flashcards
Synaptic Transmission: Neuron Terminates Where?
- Another neuron (synapse)
- A muscle (neuromuscular junction)
- A gland
Synapses
- Junction between a presynaptic neuron and a postsynaptic neuron
- Two general types: electrical (gap junctions; can go both directions) and chemical (unidirectional)
- Operation is unidirectional (pre –> postsynaptic neuron
Electrical Synapse: Gap Junctions
- Gap between two adjacent cells linked by small connecting tunnels formed by connexons
- Especially abundant in cardiac and smooth muscle
- Permit unrestricted passage of small nutrient molecules between cells in non-muscle tissues
- Action potential in presynaptic cell passes through GJ and depolarizes postsynaptic cell leading to action potential in PSC
Chemical Synapse
- Neurotransmitter carries signal from a synaptic vesicle in the synaptic knob across the synaptic cleft to the postsynaptic neuron
- Synaptic delay: time (`0.5-1 ms) for conversion of the electrical signal to move from pre to postsynaptic neuron by chemical means
Excitation-Secretion Coupling
- Action potential reaches axon terminal
- Ca+ enters synaptic knob
- Neurotransmitter is released by exocytosis
- Neurotransmitter binds to receptors that are an integral part of chemically gated channels on sub-synaptic membrane
- Binding of neurotransmitter to receptor opens that specific channel
Post-synaptic Rececptors
- Ligand-gated ion channels: FASTEST neurotransmission (changes membrane potential)
- G protein-coupled: SLOWER transmission
*Result: graded synaptic potential in post-synaptic membrane
Removal of neurotransmitter
Ends the signal. How?
1. Diffusion: movement away from synaptic cleft
2. Enzymatic degradation: inactivation by specific enzymes w/in the cleft
3. Reuptake: actively taken back up into the pre-synaptic axon terminal where its recycled (stored and released another time) or destroyed (by enzymes w/in synaptic knob)
Common Neurotransmitters
Acetylcholine (at neuromuscular junctions)
Norepinephrine
G-aminobutyric acid (GABA): inhibitory in CNS
Cholinergic Synapse
Miniature transducer that converts a presynaptic electrical signal into a chemical signal (ACh), which diffuses across cleft, triggering another electrical signal on postsynaptic side by interacting w/ ACh receptors
Synaptic Vesicle Fusion and Exocytosis
Membrane proteins on the vesicles bind membrane proteins at the axon terminal to tether the vesicles in place. Ca binds to protein triggering the vesicle to fuse with presynaptic cell membrane
All the proteins make up the SNAP/SNARE complex
Neuromuscular Junction: A Chemical Synapse
- Postsynaptic organization in skeletal muscle includes several folds with ACh receptors at the crest and voltage-gated sodium channel in the troughs and folds
-Opening of ACh receptors results in an end-plate potential (EPP). the EPP depolarizes the membrane and initiates action potential
Excitatory Adrenergic Synapse
- Employs norepinephrine (NE; aka noradrenaline)
- Receptor is NOT an ion gate, but a transmembrane protein associated with a G protein
- Unstimulated NE receptor is bound to a G protein causing G protein to be released
- G protein binds to adenylate cyclase which activates the enzyme and converts ATP to cAMP (2ND MESSENGER)
- cAMP produces an internal chemical that binds to a ligand-gated sodium channel depolarizing the cell
Inhibitory GABA-ergic Synapse
**Inhibit in order to help maintain homeostasis
- GABA receptors are channel receptors
- Employs GABA as neurotransmitter, the gate is a Cl- channel
- GABA binds, change shape and allow Cl- to pass causing hyperpolarization and reducing neuron’s excitability (Cl- influx makes the RMP more neg inhibiting neuron firing)
*common in mammalian nervous system
