Neuroscience/Cellular Neurobiology/Synapses

Synapses refer to the cell-cell interaction usually found among neurons, immune cells, and epithelial cells. In order to function properly, some synapses require adhesive proteins such as the neurotransmitter receptors. There are two types of synapses and the difference is based on how the signal is transmitted from one cell to the next. In the electrical synapse, there is a connection between the cytosol of one cell and the next, where as in the chemical synapse, the action potential is converted to a chemical signal which is then converted to an electrical signal in the post synaptic cell.

Electrical Synapses

Electrical synapses are formed when two cells meet and form gap junctions in between their membranes. The gap junctions are formed from protein subunits called connexins, and six of these connexin subunits will form a trans-membrane protein named connexon. When two of these connexons join, one from the membrane of each of the two cells, a gap junction is formed, and there is a connection of the cytosol. An electrical synapse consists of many of these gap junctions. Because these gap junctions connect the cytosol of the two cells, there is an electrical continuity between the cells that facilitates instantaneous communication in both direction (although sometimes one direction is favored.) The connection of the cytosol makes the synapse fail-proof and also gives multiple neurons that signal each other via electrical synapses the ability to synchronize rapid activity. Also, cells that share cytosol, such as those connected by two electrical synapses, can share chemical signals as well, so a cell growth signal picked up by one cell might be found in a neighboring cell. Electrical synapses are found in the highest proportion in invertebrate nervous systems. They are also found throughout the mammalian central nervous system but in a lower ratio to chemical synapses.

Chemical Synapses

In contrast to the quick electrical cytosolic transmission of the electrical synapse, the chemical synapse distinctly separates the cytosol of the two cells. Action potentials that arrive in the axon terminal cause the release of chemical signals which then cause changes in membrane potential on the post synaptic membrane.

Axon Terminal

In the axon terminal, neurotransmitters are first stored in a large 'pool' called the endosome. As the demand for neurotransmitter goes up, the membrane of the endosome buds to form vesicles containing an amount (this amount doesn't vary much) of neurotransmitter. These vesicles will then move closer to the active zones, the places where the neurotransmitter is released, in a synapse, waiting to be used. Once a vesicle has been used, it will be taken back to the membrane of the endosome and used again.

Presynaptic Membrane

The presynaptic membrane has many vesicles bound to it, containing neurotransmitter ready to be released. When an action potential arrives at the presynaptic membrane, voltage-gated calcium channels open, allowing an influx of calcium. Calcium ions will bind to proteins on the membrane of the vesicles and cause it to move closely to and fuse with the presynaptic membrane. As this continues, the vesicle will open up, releasing its contents into the synaptic cleft. When this is finished, the proteins surrounding the vesicle in the presynaptic membrane will cause the membrane to bud inside the cell, essentially reforming the vesicle and allowing it to return to the endosome.

Synaptic Cleft

In between the presynaptic membrane and the postsynaptic membrane is a space 20-50nm wide filled with fluid and structural proteins that hold the two synaptic membranes together. This is the synaptic cleft, and it is the space that the neurotransmitters diffuse through to bind to receptors on the postsynaptic membrane after being released.

Postsynaptic Membrane

This refers to the area that receives the chemical signals from a presynaptic membrane. So, axons are found to synapse (that is, adhere and signal) on dendrites, but also the membrane on a cell body or other axon terminal. The postsynaptic membrane will have receptors which the neurotransmitter can bind to. Depending on the type of receptor, a channel will be opened quickly or further chemical messages will be generated, affecting the synapse over longer periods of time.

Neuronal Synapse

Neurons utilize synapses exclusively for cell-cell interaction. Whenever a stimulus triggers an action potential, a neuron transmits the signal to the next through their synapses. First, an action potential reaches at the presynaptic membrane (axon terminal), where it contains several neurotransmitters packed in synaptic vesicles. The action potential triggers the release of the neurotransmitters to the synapse as the synaptic vesicles diffuse into the cellular membrane. The neurotransmitters released to the synapses reach the neurotransmitter receptors located on the postsynaptic membrane (dendrite terminal). This triggers an action potential at the postsynaptic membrane which will be transmitted to the next neuron and so on. The following diagram displays the overall picture of a neuronal synapse.