Primary Somatosenory Cortex
Chemical Synapse – Basic Structure
- The most common type of neuron synapse is the chemical synapse.
- At chemical synapses, the presynaptic neuron is separated from the postsynaptic neuron by a narrow (20 nm), water-filled space called the synaptic cleft.
- Neurotransmitter molecules are used by the presynaptic neuron to send a message across the cleft to the postsynaptic neuron.
- The neurotransmitter molecules are stored in synaptic vesicles located in the expanded tips (or synaptic knobs) of the presynaptic neuron.
- Action potentials (or electrochemical impulses) at the synaptic knob signal the synaptic vesicles to fuse with the presynaptic membrane and release their contents into the synaptic cleft. Some of the transmitter molecules diffuse across the synaptic cleft and bind with receptors in the postsynaptic membrane.
- The bound transmitter causes channels in the postsynaptic membrane to open.
- Ions diffuse through the open channels, and the exchange either excites (depolarizes) or inhibits (hyper-polarizes) the postsynaptic neuron.
- Transmission at chemical synapses is slower than at electrical synapses. It is also slower than the rate at which action potentials move down axons. The reduced rate of current flow at the chemcial synapses is known as synaptic delay.