Your brain may look like a clump of gray slime, but it is much more complex than that. Actually, every human nervous system is tissue comprised of billions of cells, working in unison to help you function. Nervous system tissue is made of nerve cells and supporting cells.
Neurons are the primary nerve cells responsible for receiving and sending information. Large networks of neurons integrate information and produce your thoughts, feelings, actions and sensations. There are about 100 billion neurons within your brain. Neurons are made of a cell body (which contains the nucleus and other cellular components), dendrites, and an axon.
Dendrites are twisted extensions from a nerve cell. Dendrites receive information from other neurons. Usually, neurons have dendritic trees, a mass of dendrites emanating from the cell body, much like branches from a tree trunk. Such extensive branching creates greater surface area, allowing space for many more connections with other neurons. In dendrites, signals from other neurons travel only from the connection down the dendrite toward the neuronal body.
Axons are long, thin extensions from nerve cells that, in contrast to dendrites, send electrical signals from the cell body toward other neurons. Axons are wrapped in bundles of insulating membrane called myelin. Like duct tape around a leaky garden hose, myelin keeps the flow (an electrical signal in this case) from spilling out and increases efficiency. Without myelin, signals would leak and dissipate before they reached the end of long axons.
Neurons use synapses, a type of cellular connection, to communicate with each other. The very tip of an axon, called the axon terminal, comes into close contact with cell bodies, dendrites, or other axons to form a synapse. There are two types of synapses, electrical and chemical.
Electrical synapses consist of the membranes of two neurons abutting each other. One membrane is the axon terminal, and the other membrane is that of a cell body, dendrite, or another axon. A protein junction forms a hole between the two membranes, allowing the electrical signal to pass directly from one cell to the other. Electrical synapses are much faster than chemical synapses, but unlike chemical synapses, cannot be regulated or controlled. Electrical synapses are unaffected by methamphetamine.
[Insert image of chemical synapse]
Unlike electrical synapses, chemical synapses may be regulated and are affected by methamphetamine. Chemical synapses have a presynaptic membrane (the axon terminal), a postsynaptic membrane (a dendrite or neuronal cell body), and a small space between the two membranes called the synaptic cleft. Signals always travel from the presynaptic membrane, through the synaptic cleft, and to the postsynaptic membrane.
An electric signal travelling down the axon stimulates merger of enclosed neurotransmitter bundles called neurotransmitter vesicles with the presynaptic membrane. Fusion of vesicles with the tip of the axon terminal releases neurotransmitter molecules into the synaptic cleft.
After passing through the synaptic cleft, neurotransmitter molecules then bind to receptors on the surface of the postsynaptic membrane, similar to keys fitting into locks. Binding causes proteins called ion channels, in the same fashion as a door, to open or close. Charged particles, called ions, enter the postsynaptic membrane through the opened ion channels. The ions generate either increased or reduced electrical flow, leading to either excitation or inhibition of signal.
Neurotransmitters then quickly break their bonds with the receptors and flow backwards into the synaptic cleft to be recycled into the axon as vesicles, ready for the cycle to start again.
[Insert fig 5.1 and 5.3 Neurophysiology]
In addition to neurons, synapses and blood vessels, your nervous system contains around 300 billion supporting cells, known as glial (glue) cells. There are many different types of glial cells in the nervous system with varying functions. Astrocytes serve as structural support for the nervous system, and provide neurons with nutrients. Oligodendrocytes in the central nervous system and Schwann Cells in the peripheral nervous system produce myelin and wrap the myelin around axons. Microglia recycle dying neurons and, like white blood cells, destroy foreign invaders.
Now that you know some fundamentals about the nervous system, feel free to read more about the neurology behind methamphetamine use [Link to Angela, Jace and Tucker].