Algonquin College
7.4 - Synaptic Transmission
The Synapse

At the end of each neuron there are one or several synaptic terminals each housing many neurotransmitter (synaptic) vesicles. At the synaptic terminal the presynaptic neuron connects to the postsynaptic neuron across the synaptic cleft, a small extracellular space between the two neurons. The combination of presynaptic neuron, postsynaptic neuron and synaptic cleft is called a synapse.

When an action potential arrives at a synapse the electrical signal is converted to a chemical signal through a series of events called synaptic transmission.

Synaptic Transmission

Click here for a tutorial about synaptic transmission



Part 1 - Action Potential Arrival and Vesicle Fusion

1. An action potential arrives at the presynaptic axon terminal, triggering the opening of calcium ion channels on the presynaptic membrane

2. Calcium ions flood into the presynaptic neuron and bind to nearby synaptic vesicles

3. Synaptic vesicles that have been activated by the binding of Calcium ions fuse with the presynaptic membrane and empty their contents into the synaptic cleft

Part 2 - Neurotransmitter Binding

4. The neurotransmitters cross the synaptic cleft and bind to receptor proteins on the postsynaptic membrane

Part 3 - Postsynaptic Response

5. a) If the neurotransmitter binds to sodium ion channels, the membrane potential of the postsynaptic neuron is brought closer to the threshold for generating an action potential. This is the activation or stimulation of the postsynaptic neuron.

b) If the neurotransmitter binds to potassium ion channels, the membrane potential of the postsynaptic neuron is brought further away from the threshold for generating an action potential. This is the inhibition of the postsynaptic neuron.

Part 4 - Neurotransmitter "recycling"

6. After the channels on the postsynaptic membrane close and the neurotransmitters are released back into the synaptic cleft, they are quickly destroyed by enzymes or taken back into the presynaptic terminal by endocytosis in a process called reuptake.

7. The destruction or reuptake of neurotransmitters prevents the effects of the neurotransmitters from being too long-lasting.