Cellular
Neuroanatomy
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.
Structure of
Neurons
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.
Neuronal
Communication
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].