Synapse

For a neuron to communicate with another neuron, first in action potential must occur. Once the action potential occurs, it travels the full length of the axon, ultimately reaching the axon terminals. How does the action potential move from the neuron in which it starts to another neuron?

Neurons communicate with each other across synapses. A synapse is the site of action potential transmission from one neuron to another. There are both chemical and mechanical synapses, but the most common type is the chemical synapse, which is our focus. As seen in the figure up, a synapse between two neurons includes:

• The axon terminals of the neuron sending the action potential;
• Receptors on the neuron receiving the action potential, and
• The space between these structures.

The neuron sending the action potential across the synapse is called the presynaptic neuron, so axon terminals are presynaptic terminals. Similarly, the neuron receiving the action potential on the opposite side of the synapse is called the postsynaptic receptors. The axon terminals and postsynaptic receptors are not physically in contact with each other. A narrow gap, the synaptic cleft, separates them.

The action potential can be transmitted across a synapse in only one direction: from the axon terminals of the presynaptic neuron to the postsynaptic receptors, usually on the dendrites, of the postsynaptic neuron. Impulses also can go directly to receptors on the cell body: about 5% to 20% of the axon terminals are adjacent to the cell body instead of the dendrites. Why can the action potential go in only one direction?

The presynaptic terminals of the axon contain a large number of saclike structures, called synaptic vesicles. These sacs contain neurotransmitter chemicals. When the impulse reaches the presynaptic axon terminals, the synaptic vesicles respond by dumping their chemicals into the synaptic cleft. These neurotransmitters then diffuse across the synaptic cleft to the postsynaptic neuron’s receptors. The postsynaptic receptors bind the neurotransmitter once it diffuses across the synaptic cleft. When this binding occurs, the impulse has been transmitted successfully to the next neuron and can be transmitted onward.