Cells in your brain communicate with each other using chemicals called neurotransmitters. Molecules of neurotransmitters are stored in small bubbles, called vescicles, that are located at the connections between brain cells, the synapse. A brain cell generates an electrical signal, or action potential, when other cells have stimulated it with neurotransmitters in turn. When the action potential reaches the synapse, calcium rushes into the cell, causing neurotransmitter release.
Signals in Your Brain
Each of the tens of billions of cells in your brain, or neurons, connects to as many as a thousand other cells. Neurons send out thin input and output processes, dendrites and axons respectively, in a branched structure similar to a tree. Where the axon of one cell comes into contact with the dendrite of another cell, a connection called a synapse is formed. Although it is also possible to have different connections between parts of a neuron, the axon-dendrite connection is the most common in most areas of the brain.
The Synapse
The synapse has three parts: the presynaptic cell, the postsynaptic cell and the synaptic cleft. The presynaptic cell has a swelling of the axon, the presynaptic terminal or axonal bouton, that contains vescicles of neurotransmitter. The synaptic cleft is the space between the presynaptic and postsynaptic cells that contains structural proteins and special enzymes to degrade excess neurotransmitter. The postsynaptic cell contains neurotransmitter receptors that cause a small electrical signal in the postsynaptic cell. Enough of these small electrical signals causes the postsynaptic cell to fire an action potential.
Neurotransmitter Release
When the action potential reaches the axonal bouton, voltage-gated calcium channels are opened allowing calcium to rush into the cell. Neurotransmitter vescicles are anchored near the synaptic membrane by proteins called VAMPs, vescicle-associated membrane proteins. VAMPs appear as two rods twisted around each other and stuck through the membrane of the synapse and vescicle. Calcium causes VAMPS to twist around each other, forcing the membrane of the vescicle to merge with the synaptic membrane, releasing neurotransmitter into the synaptic cleft.
Neurotransmitter Effects
Neurotransmitters can produce excitatory -- positive -- or inhibitory -- negative -- electrical effects on the postsynaptic cell, depending on the identity of the neurotransmitter and the receptor proteins. Glutamate is the primary excitatory neurotransmitter in the brain. GABA, gamma-aminobutyric acid, is the primary inhibitory neurotransmitter in the brain. There are other neurotransmitters that can cause cells to be more or less responsive to glutamate or GABA. These include acetylcholine, serotonin, dopamine, epinephrine and norepinephrine.
References
- Williams College: Neuroscience; Neurotransmitter Release; 1998
- "Principles of Neural Science," 4th ed.; E. R. Kandel , J. H. Schwarz and T.M. Jessel; 2000
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