The Electrical Synapse (Gap Junction)

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Cell Junctions

• Tight Junctions– Tight junctions are present when two cells are close together and can form a watertight seal. The closeness of these cells becomes important in parts of the body such as the bladder, intestines, and the kidney because these are areas that must be able to seal in order to perform their proper function.
• Adheren Junctions- Adheren junctions are anchoring junctions that hold cells together in epithelial tissues. They are large multiprotien complexes that are comprised of cadherins, actin, and catenins.
• Desmosomes– Desmosomes are cells linked together similar to tight junctions, but they are not as close together and are not watertight.  Water and ions are able to pass between these bonds without any problem. Desmosomes are found in parts of the body that undergo stress, such as skin and intestines. Even though tight junctions are also prominent in the intestines, desmosomes are still scattered around in the intestines as well.
• Gap junctions– Gap junctions are the type of bond between cells that we will most focus on in this chapter. Gap junctions are a connection between cells that create a tunnel-like effect that allows water and ions to flow through the tunnel to get from one cell to the next. Gap junctions are found prominently in cells or tissue that spread action potential and electrical coupling. Parts of the body that have gap junctions are the heart; the action potential is what keeps the heart beating. Gap junctions are also found in neurons.

Gap junction structure

Gap junctions are made of two half-channels called connexons. If the gap junction is between two adjacent cells, each cell contributes one connexon to the gap junction. Each connexon is made of 6 subunits called connexin. Each connexin is integrated within the cell membrane and stretches from the cytoplasm to the extracellular space. This makes it so that the gap junction can allow the passage of ions and other molecules between two adjacent cells.

1 Connexon  = 6 Connexin, this means that gap junctions are made of 2 connexons or 12 connexin.

The Electrical Synapse

• Presynaptic cell
• Synapse (synaptic cleft is the space between the synapse)
• Postsynaptic cell

The electrical synapse consists of cells that are joined together by connexons, which are made of proteins called connexin. When the cell at the top depolarizes, then the ions that flow into the presynaptic cell can continue diffusing into the postsynaptic cell through the connexon proteins of the electrical synapse.

Electrical synapse are found between neurons and glial cells. There is a cytoplasmic continuity between the pre and postsynaptic cells. The electrical synapse has a large and nonspecific core that allows many different molecules to pass through it, such as cAMP, small peptides, anions, and cations. Some gap junctions allow flow in both directions, but others only flow in one direction. Gap junctions can close due to low pH, high calcium, phosphorylation, and voltage.

The Chemical Synapse

The chemical synapse has similarities and differences in comparison to the electrical synapse. The chemical synapse has no cytoplasmic continuity between the presynaptic and the postsynaptic cells. Chemical synapses are also up to 10 times larger than electrical synapses. The order of events starts with action potentials reaching the presynaptic terminals. Next, the neurotransmitters are then synthesized and they are stored in synaptic vesicles. Then, calcium channels are opened from the depolarization of action potentials. The calcium then helps to fuse the vesicles with the membrane and neurotransmitters are released in the synaptic cleft. The neurotransmitters are then sent to the postsynaptic cleft.

Gap junctions in different parts of the body

• Cardiac Muscle

Synchronized firing is an important task that gap junctions perform in the cardiac muscles. There are gap junctions connecting the cells to each other in the heart. There is a small delay and a rapid transmission of the signal is what produces the rhythmic contractions of the heart. Cardiac myocytes are the contracting cells of the heart. The signal for the heart to contract in unison comes in the form of ions.

• Neurons

Electrical synapses occurring in both directions are important in the central nervous system. It is important for there to be no delay in the transmission on neurotransmitters. Some examples of parts of the brain that have electrical synapses are in the retina, thalamus, and the cortex. This is important for regulating oscillatory rhythm as well as sensory processing and coordination. Electrical synapses in the thalamus regulate slow wave sleep and if there are any deformities in this process it can cause seizures in the brain to occur.