Chemical Synapses

A chemical synapse is a junction between a neuron and an adjacent cell, separated by a synaptic cleft into which a neurotransmitter substance is released.

The neuron that releases the neurotransmitter molecules into the cleft is called the presynaptic cell. First, gated protein channels open to allow calcium ions to enter the neuron. Calcium causes vesicles to fuse with the membrane and release the transmitter substance into the cleft. The neurotransmitter binds to receptors on the membrane of the postsynaptic cell. Binding of neurotransmitter to receptors opens ion channels in the membrane of the postsynaptic cell.

Types of Neurotransmitters

• Acetylcholine is the transmitter at neuromuscular junctions.
• Serotonin acts on brain cells to govern sleeping, sensory perception, temperature regulation, and emotional states.
• Norepinephrine apparently affects brain regions concerned with emotions, dreaming, and awaking.
• Dopamine is the specialty of neurons in brain regions dealing with emotions.
• GABA is the most common inhibitory signal in the brain
• Neuromodulators are substances that enhance or reduce membrane responses in target neurons. Endorphins, for example, inhibit perceptions of pain.

Synaptic Integration

Neurotransmitters may have excitatory effects if they drive a cell's membrane to the threshold of an action potential. Neurotransmitters may have inhibitory effects if they help drive the membrane away from threshold.

Excitatory and inhibitory signals compete at the input zone. An excitatory postsynaptic potential (EPSP) is a summation of signals that brings the membrane closer to threshold (depolarizing effect). An inhibitory postsynaptic potential (IPSP) drives the membrane away from threshold by a hyperpolarizing effect. In synaptic integration, competing signals that reach the input zone are reinforced or dampened (sent on or suppressed).

Neurotransmitter molecules must be removed promptly from the synaptic cleft. Some molecules diffuse out. Acetylcholinesterase degrades. Many others are actively pumped back into the presynaptic cells by membrane transport proteins.

These two animations (Audio - Important) describe the function of a nerve synapse:
nerve synapse 1,
nerve synapse 2.

Look at the Synaptic Transmission Tutorial .