Neurotransmission - Some Brain Facts

The brain is made of cells (called neurons) which communicate with each other at places called synapses. A synapse is a functional (but not physical) contact between two neurons. There are about 100 billion neurons in the human brain and each has about 10,000 contacts with other neurons. The number of synapses in the human brain is about 10 to the 15th power. The neurons of one human cerebral cortex would reach over 250,000 miles if placed end to end. The complexity of this organ is obviously enormous!

Parts of a Neuron [the Soma or Cell Body, the Nucleus, and the Dendrites]

The soma of a neuron contains the nucleus (DNA) of the cell. Along with the dendrites the soma constitutes the "receiving" surface of the neuron. The continuous chemical input to the cell causes the formation of PSPs (postsynaptic potentials). These electrical events are integrated (by summation) in the soma and the decision to conduct an action potential (an output signal) is based on the soma reaching the threshold for conduction. If a soma is damaged a neuron will not recover. Neurons are the only cells of the body which do not continuously regenerate; the brain is thus highly vulnerable to damage.

Along with the soma, the dendrites constitute the "receiving" surface of the neuron. Dendrites may be few or they may branch extensively forming a dendrite arborization. Dendrite branching is one of the properties of a neuron which is influenced by the environment during development, both pre and postnatal. Stimulation can enhance a neuron's dendrite arborization making available more receiving sites for a neuron. In conditions such as Down Syndrome and Fetal Alcohol Syndrome the dendrites are few and sparsely branched. Laboratory animals who have received stimulation as infants show more dendritic branching which suggests to some the possibility that early human stimulation might play a role in shaping the architecture of the brain.

How does one neuron communicate with another neuron?

Neurons generate electrical events called action potentials which consist of brief reversals in the polarity (electrical state) of the axon (transmitting region) of the cell. These action potentials cause the release of a chemical messenger from a storage vesicle in the axon terminal. The chemical messenger (called a neurotransmitter) travels across a synapse to bind to a postsynaptic receptor protein. The act of binding to the receptor protein sets in motion a series of events which eventually brings about a change in the electrical state of the postsynaptic cell. Some neurotransmitter-receptor bindings excite the cell and others inhibit it. At any given moment a neuron receives thousands of these messages and integrates this input to bring about only one of two possible outcomes - the neuron stays in a resting state or it generates an action potential to communicate with another neuron.

Parts of a Synapse [the Postsynaptic Neuron, the Presynaptic Neuron, the Vesicle with Neurotransmitter (NT) Molecules, the Mitochondrion (for energy production from glucose), the Synaptic Cleft, the Neurotransmitter (NT) Molecules, and the Postsynaptic Membrane (with NT receptors)]

The synapse is a very dynamic region consisting of a presynaptic axon terminal, a synaptic cleft or space, and a postsynaptic neuron (dendrite, soma, sometimes an axon or another dendrite).

Neurotransmitters

Neurotransmitters are chemicals which are released into the synaptic space whenever a neuron conducts an action potential to the axon terminals. There are perhaps 100 or so different neurotransmitter varieties in the brain. Some of them are shown in the flow chart below. Each neurotransmitter plays some role in most behaviors, but often we identify a key behavior with each neurotransmitter.

Neurotransmitters are released from storage vesicles and diffuse across the synaptic cleft to bind to a specific receptor site on the postsynaptic neuron. The neurotransmitter then activates (excites or inhibits) the next neuron and is then reuptaken into the sending neuron or destroyed either in the cleft or in the presynaptic neuron. It is at the synapse that most drugs or chemicals work to alter the brain and thus the mental state of the person. As an example, cocaine blocks the reuptake of dopamine into its presynaptic neuron making it more available in the synaptic cleft for binding to the postsynaptic receptor. The effects of cocaine then are really due to the persistent dopamine binding. Drugs which prevent dopamine from binding (at DA1 receptors) will block the rewarding effects of cocaine.