A synapse is a junction of an axon with either another neuron or an effector cell. At a synapse, the terminal bouton of the presynaptic neuron fits into a small depression on the postsynaptic neuron’s dendrite or cell body or on a cell within a muscle, a gland, or adipose tissue. There is a tiny space, the synaptic cleft, between the presynaptic and postsynaptic structures, so they are not in physical contact.
In neuron-to-neuron synaptic transmission, when a nerve impulse reaches the terminal bouton of the presynaptic neuron, it causes the terminal bouton to secrete neurotransmitters into the synaptic cleft. Then, the neurotransmitters bind to receptors on the postsynaptic neuron’s plasma membrane, which triggers a response in the postsynaptic neuron. Some neurotransmitters stimulate formation of a nerve impulse in the postsynaptic neuron, while others inhibit nerve impulse formation. If a nerve impulse is formed in the postsynaptic neuron, it is carried along the neuron’s axon to the next synapse where synaptic transmission takes place again.
Because only terminal boutons can release neurotransmitters, nerve impulses can pass in only one direction across a synapse-from the presynaptic neuron to the postsynaptic neuron. Thus, nerve impulses always pass in the “correct” direction, which maintains order in the nervous system.
Some neurotransmitters are reabsorbed into the terminal bouton for reuse. Others diffuse out of the synaptic cleft or are decomposed by enzymes released into the synaptic cleft. Some of the decomposition products are then reabsorbed into the bouton for reuse, while others diffuse away from the synaptic cleft. Quick removal of a neurotransmitter prevents continuous stimulation or inhibition of the postsynaptic neuron (or cell) and prepares the synapse for another transmission. From start to finish, synaptic transmission takes only a fraction of a second.