Neurochemistry refers to the chemical processes that occur in the brain and nervous system. The fact that one can read this text, remember what has been read, and even breathe during the entire time that these events take place relies on the amazing chemistry that occurs in the human brain and the nerve cells with which it communicates.
There are two broad categories of chemistry in nerve systems that are important. The first is the chemistry that generates electrical signals which propagate along nerve cells. The key chemicals involved in these signals are sodium and potassium ions. To see how they give rise to a signal, one must first look at a nerve cell that is at rest.
Like any other cell, a nerve cell has a membrane as its outer "wall." On the outside of the membrane, the concentration of sodium ions will be relatively high and that of potassium ions will be relatively low. The membrane maintains this concentration gradient by using channels and enzymes.
The channels are pores that may be opened or closed by enzymes which are associated with them. Some ion channels allow the movement of sodium ions and others allow potassium ions to cross the membrane. They are also called "gated" channels because they can open and close much like a gate in a fence. The voltage they experience dictates whether the gate is open or closed. Thus, for example, a gated sodium ion channel in a membrane opens at certain voltages to allow sodium ions to pass from regions of high concentration to regions of low concentration.
Active transport mechanisms are also present. Enzymes that span the membrane can actively pump sodium and potassium ions from one side of the membrane to another. When the nerve cell is at rest, these mechanisms maintain a high potassium and low sodium environment inside the cell.
Even when it is at rest, a nerve cell is in contact with many other nerve cells. When a neighboring cell passes on a signal to the resting cell (by a mechanism to be discussed shortly), a dramatic change occurs in the ion concentrations. Once the nerve cell at rest has received a sufficient signal from a neighbor to surpass a threshold level, some of the sodium ion channels near the connection point open and sodium ions flow into the cell. This flow of charge results in an electrical potential that is called the action potential. The action potential does not stay localized, however. Farther down the nerve cell, more sodium ion channels surpass their threshold and open so that the sodium ions flow into them as well. Thus, the action potential moves down the nerve. After the sodium ion gates open, the potassium ion gates also open and potassium ions flow out of the cell. This flow of ions offsets the charge from sodium ions flowing into the cell and the signal has receded in that region (and has moved on).
Once the cell propagates a signal, how does that cell send its signal to a neighbor? This question leads to the second broad category of neurochemistry: the chemistry at the synapse. Nerve cells do not actually touch their neighbors, but rather form a small gap called the synapse. The signal is transferred across this gap by chemicals called neurotransmitters.
The communication that occurs across the synapse may either excite or inhibit the action of the neighboring nerve cell. Thus, synapses are further categorized as either excitatory synapses or inhibitory synapses. The cell that is propagating the signal is called the presynaptic cell, and the cell that receives the signal is the postsynaptic cell.
The end of the presynaptic cell contains small vesicles , spherical collections of the same lipid molecules that make up the cell membrane. Inside these vesicles, neurotransmitters exist in high concentrations. When the action potential reaches the end of the presynaptic cell, some of the vesicles merge with the cell membrane and release their contents (a process called exocytosis). The released neurotransmitters experience an immediate concentration gradient. They diffuse away from the release point to counteract the gradient, and in doing this, they cross the synapse and arrive at the neighboring cell.
On the postsynaptic cell, there are receptors that are capable of interacting with the neurotransmitters. Once these messenger molecules cross the synapse, they connect with the receptors and the two cells have successfully communicated. The proteins of the receptors are capable of opening sodium gated ion channels, and a new action potential is engaged in the postsynaptic cell.
The remaining step in the process is also a critical one. Somehow the action of the neurotransmitters must cease. If they continue to cross the synapse, or are not removed from the receptors of the postsynaptic cell, they will continue to activate that cell. An overexcited or inhibited nerve cell is not capable of proper function. For example, schizophrenia is a mental disease that is caused by the brain's inability to eliminate excitatory neurotransmitters. The nerve cells continue firing, even when they need not, and the incorrect brain chemistry results in debilitating symptoms such as auditory hallucinations—hearing voices that are not actually there.
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Bradford, H. F. (1986). Chemical Neurobiology. New York: W.H. Freeman.
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