18.11:
The Synapse
The cells of your nervous system are constantly receiving and transmitting information, from basic bodily function to sensory stimulus. Neurons communicate with electrical signals called action potentials. These action potentials originate in the cell body and travel along the axon to the axon terminal where they are passed on to the next cell.
The point at which two neurons meet is called the synapse. Electrical synapses allow direct communication between cells using gap junctions, and are often involved in coordination of rapid activity. However, most synapses are chemical synapses that contain a synaptic cleft, the physical space that exists between the neuron sending the signal, known as the presynaptic cell, and the neuron receiving it, called the postsynaptic cell.
Action potentials cannot travel across the synaptic cleft, so neurons convert the electrical signal into a chemical signal at the synapse. This is accomplished by the release of molecules known as neurotransmitters. There are many neurotransmitters, each with different effects on the postsynaptic neurons, including the excitatory glutamate and the inhibitory GABA, among others.
When the action potential reaches the presynaptic terminal, voltage-gated calcium channels on the presynaptic membrane open. Calcium rushes into the cell, which triggers the fusion of vesicles with the membrane and the release of neurotransmitters into the synaptic cleft. These are then able to bind to receptors on the postsynaptic cell.
The binding of neurotransmitters to receptors may result in and increased or decreased postsynaptic membrane potential, changing the likelihood of an action potential initiating in the postsynaptic cell. Neurons can have thousands of synapses and receive information from many cells. These signals are combined in the soma of the postsynaptic neuron where the cell determines whether or not to pass the message forward.
After briefly binding to postsynaptic receptors, neurotransmitters may diffuse away, be degraded, or recycled. Reuptake proteins on the presynaptic cell are often responsible for recycling neurotransmitters. The release and binding of neurotransmitters across synapses allow the electrical signals of action potentials to be communicated to adjacent neurons. This multi-step process is critical to neuron function.
18.11:
The Synapse
Neurons communicate with one another by passing on their electrical signals to other neurons. A synapse is the location where two neurons meet to exchange signals. At the synapse, the neuron that sends the signal is called the presynaptic cell, while the neuron that receives the message is called the postsynaptic cell. Note that most neurons can be both presynaptic and postsynaptic, as they both transmit and receive information.
An electrical synapse is one type of synapse in which the pre- and postsynaptic cells are physically coupled by proteins called gap junctions. This allows electrical signals to be directly transmitted to the postsynaptic cell. One feature of these synapses is that they can transmit electrical signals extremely quickly—sometimes at a fraction of a millisecond—and do not require any energy input. This is often useful in circuits that are part of escape behaviors, such as that found in the crayfish that couples the sensation of a predator with the activation of the motor response.
In contrast, transmission at chemical synapses is a stepwise process. When an action potential reaches the end of the axonal terminal, voltage-gated calcium channels open and allows calcium ions to enter. These ions trigger fusion of neurotransmitter-containing vesicles with the cellular membrane, releasing neurotransmitters into the small space between the two neurons, called the synaptic cleft. These neurotransmitters—including glutamate, GABA, dopamine, and serotonin—are then available to bind to specific receptors on the postsynaptic cell membrane. After binding to the receptors, neurotransmitters can be recycled, degraded, or diffuse away from the synaptic cleft.
Chemical synapses predominate the human brain and, due to the delay associated with neurotransmitter release, have advantages over electrical synapses. First, a few or many vesicles may be released, resulting in a variety of postsynaptic responses. Second, binding to different receptors may cause an increase or decrease membrane potential in the postsynaptic cell. Additionally, the availability of neurotransmitters in the synaptic cleft is regulated by recycling and diffusion. In this way, chemical synapses achieve neuronal signaling that can be highly regulated and fine-tuned.
Suggested Reading
Xu-Friedman, Matthew A. “Measuring the Basic Physiological Properties of Synapses.” Cold Spring Harbor Protocols 2017, no. 1 (January 1, 2017): pdb.top089680. [Source]
Sheng, Morgan, and Eunjoon Kim. “The Postsynaptic Organization of Synapses.” Cold Spring Harbor Perspectives in Biology 3, no. 12 (December 1, 2011): a005678. [Source]