Like wires in your home's electrical system, nerve cells make connections with one another in circuits called neural pathways. Unlike wires in your home, nerve cells do not touch, but come close together at synapses. At the synapse, the two nerve cells are separated by a tiny gap, or synaptic cleft. The sending neuron is called the presynaptic cell, while the receiving one is called the postsynaptic cell. Nerve cells send chemical messages with neurotransmitters in a one-way direction across the synapse from presynaptic cell to postsynaptic cell.
Let's look at this process in a neuron that uses the neurotransmitter serotonin:
- The presynaptic cell (sending cell) makes serotonin (5-hydroxytryptamine, 5HT) from the amino acid tryptophan and packages it in vesicles in its end terminals.
- An action potential passes down the presynaptic cell into its end terminals.
- Serotonin passes across the synaptic cleft, binds with special proteins called receptors on the membrane of the postsynaptic cell (receiving cell) and sets up a depolarization in the postsynaptic cell. If the depolarizations reach a threshold level, a new action potential will be propagated in that cell. Some neurotransmitters cause the postsynaptic cell to hyperpolarize (the membrane potential becomes more negative, which would inhibit the formation of action potentials in the postsynaptic cell). Serotonin fits with its receptor like a lock and key.
- The remaining serotonin molecules in the cleft and those released by the receptors after use get destroyed by enzymes in the cleft (monoamine oxidase (MAO), catechol-o-methyl transferase (COMT)). Some get taken up by specific transporters on the presynaptic cell (reuptake). In the presynaptic cell, MAO and COMT destroy the absorbed serotonin molecules. This enables the nerve signal to be turned "off" and readies the synapse to receive another action potential.
- There are several types of neurotransmitters besides serotonin, including acetylcholine, norepinephrine, dopamine and gamma-amino butyric acid (GABA). Any given neuron produces only one type of neurotransmitter. Any one nerve cell may have synapses on it from excitatory presynaptic neurons and from inhibitory presynaptic neurons. In this way, the nervous system can turn various cells (and subsequent neural pathways) "on" and "off." Finally, nerve cells synapse on effector cells (muscles, glands, etc.) to evoke or inhibit responses.
Next, we'll learn about the different types of sensory neurons.