The spinal cord is made up of 31 segments, and each segment
gives rise to one pair of spinal nerves.
In cross-section, the spinal cord segments appear to be divided into two zones.
The inner zone looks somewhat like the letter "H" and is darker in color than
the outer zone.
The outer zone contains many myelinated axons. Because myelin (= lipid) is light
in color, the structures in this zone are usually referred to as white matter.
Axons in the white matter rapidly conduct action potentials (= electro-chemical
impulses) up and down the spinal cord.
The inner zone contains many dendrites and cell bodies, which are not covered
by myelin. These structures are thus often called gray matter. Axons enter the
gray matter from the spinal nerves and white matter. In the gray matter, the
axons synapse with the dendrites and cell bodies. Many of the gray matter neurons
send axons back into the white matter and spinal nerves.
