Biomedical Engineering Reference
In-Depth Information
To indicate the direction of ionic or molecular movement in a neuron, two sets of terms have
been defined. When discussing motion within the axon, anterograde is movement from the soma to the
end of the axons and retrograde is movement from the axon to the soma. To discuss movement in the
entire neuron, orthodromic is in the direction from the axon to the dendrites, while antidromic is from the
dendrites to the axon.
1.2 GLIAL CELLS
Although often overlooked, glial cells in the brain outnumber neurons nearly two to one. Historically,
glial cells were thought to only impact electrical properties indirectly by maintaining extracellular ion
concentrations and by speeding electrically propagation through the growth of the myelin sheath . Recent
evidence, however, suggests that glial cells may play a more direct role in the propagation of impulses.
Some of the more common types of glial cells are listed in the table below.
Table 1.2:
Astroglia
Structural support and repairs
Oligodendroglia
Insulate and speed transmission in CNS
Schwann Cells
Insulate and speed transmission in PNS
Microglia
Perform phagocytosis of damaged or dead cells
Ependymal cells
Line the ventricles and produce cerebrospinal fluid
1.3 THE BRAIN
While our primary focus will be on quantifying the function of nerve cells, it is important not to lose
sight of the fact that our brain regulates nearly all voluntary and involuntary actions. It is often reported
that we only use 10% of our brain, with the implication that there is 90% of the brain that we do not
understand. The 10% number is the percentage of the brain that is active at any one time . Each structure
of the brain is known to perform some function. What is not clear is how these structures use neurons to
perform their functions. Below is a brief overview of the structure and function of the most significant
regions of the brain.
1.3.1 FromNeuron to Nuclei
Typically, cells that are close to one another are more densely connected by synapses and perform similar
functions. These groupings of cells form a nucleus . In histological slices, the dense tangle of cell bodies,
dendrites, and capillaries have a grayish color and so have been given the name grey matter . Several nuclei
may be connected together to form larger structures in the brain. The connections between nuclei are
made by bundles of long axons, called tracts , that may carry large amounts of information. Long axons
require fast propagation and so are covered in a fatty coating of myelin. In histological slices, these dense
regions of axons have a white color and so have been given the name white matter .
 
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