Explanation:
single neuron, or nerve cell, can do a lot! It can maintain a resting potential—voltage across the membrane. It can fire nerve impulses, or action potentials. And it can carry out the metabolic processes required to stay alive.
A neuron’s signaling, however, is much more exciting—no pun intended!—when we consider its interactions with other neurons. Individual neurons make connections to target neurons and stimulate or inhibit their activity, forming circuits that can process incoming information and carry out a response.
How do neurons "talk" to one another? The action happens at the synapse, the point of communication between two neurons or between a neuron and a target cell, like a muscle or a gland. At the synapse, the firing of an action potential in one neuron—the presynaptic, or sending, neuron—causes the transmission of a signal to another neuron—the postsynaptic, or receiving, neuron—making the postsynaptic neuron either more or less likely to fire its own action potential.
Schematic of synaptic transmission. An action potential travels down the axon of the pre-synaptic—sending—cell and arrives at the axon terminal. The axon terminal is adjacent to the dendrite of the post-synaptic—receiving—cell. This spot of close connection between axon and dendrite is the synapse.
Schematic of synaptic transmission. An action potential travels down the axon of the pre-synaptic—sending—cell and arrives at the axon terminal. The axon terminal is adjacent to the dendrite of the post-synaptic—receiving—cell. This spot of close connection between axon and dendrite is the synapse.
In this article, we'll take a closer look at the synapse and the mechanisms neurons use to send signals across it. To get the most out of this article, you may want to learn about neuron structure and action potentials first.
Electrical or chemical transmission?
At the end of the 19th and beginning of the 20th century, there was a lot of controversy about whether synaptic transmission was electrical or chemical.
Some people thought that signaling across a synapse involved the flow of ions directly from one neuron into another—electrical transmission.
Other people thought it depended on the release of a chemical from one neuron, causing a response in the receiving neuron—chemical transmission.
We now know that synaptic transmission can be either electrical or chemical—in some cases, both at the same synapse!
Chemical transmission is more common, and more complicated, than electrical transmission. So, let's take a look at chemical transmission first.
Overview of transmission at chemical synapses
Chemical transmission involves release of chemical messengers known as neurotransmitters. Neurotransmitters carry information from the pre-synaptic—sending—neuron to the post-synaptic—receiving—cell.
As you may remember from the article on neuron structure and function, synapses are usually formed between nerve terminals—axon terminals—on the sending neuron and the cell body or dendrites of the receiving neuron.
Schematic of synaptic transmission. An action potential travels down the axon of the presynaptic—sending—cell and arrives at multiple axon terminals branching off from the axon. The axon terminal is adjacent to the dendrite of the postsynaptic—receiving—cell. This spot of close connection between axon and dendrite is the synapse.
