Chemical Signaling | Neurochemistry Research

Chemical signaling between nerve cells and their targets occurs at anatomically specialized domains called synapses. When an electrical impulse reaches the terminus of a nerve cell axon, balloon-like synaptic vesicles inside the cell and filled with chemical neurotransmitter fuse with the plasma membrane at the axon terminal. This fusion at one point creates "omega" structures. As synaptic vesicles explode into the extracellular space, neurotransmitters are released. These chemical messengers cross the narrow synaptic cleft.

Some of these chemical messengers dock at binding sites on a structurally matched neurotransmitter receptor (neuroreceptor; binding sites are at depressions or in caverns on the surface of the receptor). These receptors lie in the plasma membrane of the target cell. The membrane is a two-sided bed of phospholipids with polar heads facing the inside or outside of the cell and linked in the middle of the membrane via interactions between their hydrophobic tails.  

At rest, the channel at the center of each receptor is closed, the plasma membrane is impermeable to ions such as sodium (Na+), potassium (K+), or calcium (Ca++) ions, and the target cell is electrically silent. Upon interacting with neurotransmitter, the shape of the neurotransmitter receptor changes.  The channel widens, allowing ions to flow through it. If enough sodium ions flow inward, electrical activity in the target cell is triggered. If electrical activity is adequate, an electrical impulse is propagated in the target cells, and the entire process can begin again. Outward flow of positive charges (K+ ions flowing through specialized channels) neutralizes electrical activity in the cell, allowing it to return to its resting state. Inward flow of Ca++ ions can have many effects on signaling within the cell.

Acetylcholine is the natural chemical neurotransmitter made by many nerve cells. Nicotine from tobacco has many of the same effects as acetylcholine. Both acetylcholine and nicotine exert their effects on the nervous system by acting on nAChR, which are Na+ (and Ca++) ion-permeable, neurotransmitter receptors that function as acetylcholine- or nicotine-gated ion channels.

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Barrow Neurological Institute of St. Joseph's Hospital and Medical Center, Phoenix, Arizona