Chemical Synapses
At chemical synapses, cells meet at a synaptic cleft with no direct contact between them. In response to a nerve impulse, the presynaptic neuron releases a neurotransmitter that diffuses across the synaptic cleft and binds to receptors in the plasma membrane of the postsynaptic neuron. This type of transmission involves the conversion of an electrical signal to a chemical one, followed by conversion back to an electrical signal.
e.g. multi-unit smooth muscle, skeletal muscle, PNS neurons
Disadvantage:
a) Synaptic delay: signal must be converted back and forth, making it slower than electrical signals
Sequence of Events at a Chemical Synapse
1.
Action potential arrives at synaptic end bulbs of a presynaptic axon
2. Depolarization: voltage-gated Ca ++ channels in membrane of synaptic end bulbs open and Ca ++ diffuses into the presynaptic neuron 3. Increased Ca ++ concentration in ICF triggers exocytosis of acetylcholine (ACh) from synaptic vesicles into the synaptic cleft 4. ACh diffuses across the cleft and binds to receptors on the plasma membrane of the postsynaptic neuron
5. Ligand-gated channels of receptors open and allow Na + ions to flow into the cell
Na + inflow triggers a postsynaptic potential
6.
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