Communication Between Neurons - Synaptic Transmission



 By the end of this section, the student will be able to:


Neurons can communicate with three types of cells: muscle cells, glandular cells, and other neurons. These cells that neurons communicate with are called effector cells. Some ways that the cells communicate with each other are either through chemical synapses or electrical synapses called gap junctions, which send signals faster since they are connected by their cytoplasm. Gap junctions as you may recall are formed by proteins called connexons that make a gaping hole in the cell membranes of adjacent cells. Because of this opening, the cytoplasm between the two cells can be shared directly. The action potential can spread much more rapidly as the ions flow from one cell to the other through the gap junction. This can be seen in the heart muscle cells, which have to rapidly signal each other in order to work together as a unit in what is termed a syncytium.   Visceral smooth muscle cells have gap junctions as does the developing embryo. 

Electrical synapses need to communicate through their watery cytoplasm, directly sending ions over to the adjacent cell. In most cases, however, the neuron is separated from its effector cell by an actual space called the synaptic cleft. This cleft is very important because it allows for one-way conduction of impulses only. For example, if two neurons were actually in contact with one another, when one neuron fired an impulse, the other neuron would also fire an impulse because their membranes were in contact. If you touch an electric fence and touch a friend at the same time, both you and your friend feel the effect. However, if you touch an electric fence and are not in direct contact with your friend, only you get shocked, because there is no connection between the two of you. The synaptic cleft between cells allows the presynaptic cell to communicate with its effector only and not vice versa, by bridging the gap with a chemical signal.



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