The propagation of neural impulses or action potentials down the axon is how signals are transferred within an individual nerve cell or neuron. Ultimately, that signal will be transferred to another cell such as a muscle cell, the secretory cell of a gland, or in this case, another neuron. This signal transfer from one cell to another occurs at a junction called the synapse. There are two types of synapses, electrical and chemical. Here we will discuss the events at a chemical synapse. In this illustration we see a synapse between two neurons, the presynaptic neuron, or the neuron that sends impulses towards the synapse, and a postsynpaptic neuron, or the neuron that carries the signal away from the synapse. The neurons appear to be touching, but there is a space of about 30 to 50 nanometers or about one one-millionth of an inch, between the two cells. Let’s take a closer look at that synapse. Here you see the axon terminal of the synaptic neuron. This enlarged structure at the end of the presynaptic axon contains a large number of membranous sacs called synaptic vesicles. Each vesicle is filled with thousands of neurotransmitter molecules. Below the presynaptic terminal is the membrane of the postsynaptic neuron. And between the two neurons is the previously mentioned space, or gap, called the synaptic cleft. Now let’s look at the detailed sequence of events of how a signal is transmitted at a chemical synapse. First, an action potential arrives at the axon terminal, depolarizing the presynaptic membrane. This depolarization opens voltage-gated calcium channels embedded in the presynaptic membrane. Calcium ions then diffuse into these open channels down their electrochemical gradient, increasing the concentration of calcium within the terminal. The increase in calcium triggers synaptic vesicles to fuse with the presynaptic membrane, releasing their neurotransmitter contents via exocytosis. Neurotransmitter molecules quickly diffuse across the synaptic cleft and bind to receptors embedded in the membrane of the postsynaptic cell. Often these postsynaptic receptors are chemically gated ion channels. When neurotransmitter binds the channel opens. Once open, these channels allow the movement of ions across the postsynaptic membrane, which generates a graded potential in the postsynaptic cell. The graded potential can be either excitatory or inhibitory. If it’s excitatory, the postsynaptic neuron will be more likely to generate an action potential. If the graded potential is inhibitory, a postsynaptic action potential is less likely. The neurotransmitter released by a single presynaptic action potential is like a single word you might say to your friend. That single word doesn’t last forever. You say it and then you’re done. So too, at a synapse. The chemical message from a single action potential is very brief, neurotransmitter only remains in the synaptic cleft for a few milliseconds after it’s released. The removal of neurotransmitter occurs in three ways: reuptake into the presynaptic cell by transport proteins, degradation by enzymes, or diffusion away from the synapse. Once the neurotransmitter is removed the postsynaptic receptors close, the graded potential ends, and the synapse returns to its resting state. Let’s now summarize the events that occur at a synapse. First, recall the components of a chemical synapse. A synapse includes the axon terminal of a presynaptic neuron, a postsynaptic cell, and a synaptic cleft between them. The presynaptic membrane contains voltage-gated calcium channels, and within the terminal are vesicles filled with neurotransmitter. On the postsynaptic membrane are chemically gated channels that bind neurotransmitter. The events at a chemical synapse occur in six steps. Number one, an action potential arrives at the axon terminal, depolarizing the presynaptic membrane. Number two, depolarization opens voltage-gated calcium channels, calcium enters the axon terminal, and increases intracellular calcium. Number three, increased calcium in the axon terminal causes exocytosis, vesicles containing neurotransmitter fuse to the presynaptic membrane, and release neurotransmitter into the synaptic cleft. Number four, neurotransmitter diffuses across the cleft and binds with the postsynaptic receptors. Number five, binding of neurotransmitter opens channels on the postsynaptic membrane causing a graded potential in the postsynaptic neuron. Number six, neurotransmitter effects are terminated by reuptake, degradation, or diffusion of the neurotransmitter molecules clearing the way for a new signal to arrive at the postsynaptic cell.