The brain and spinal cord are made up of gray matter and white matter.
Gray matter is made up of nerve cell bodies, dendrites and axons, glial cells, and capillaries (the smallest blood vessels in the body).
The white matter contains relatively few neurons and is composed primarily of axons lined with many layers of myelin and oligodendrocytes that make up myelin. Myelin is what makes the white matter white. (Myelin accelerates the conduction of nerve impulses, see Nerves.)
Neurons routinely go increasing or decreasing the number of connections between them. This process would partly explain how we learn, adapt, and form memories. However, the brain and spinal cord rarely produce new neurons. An exception is the hippocampus, an area of the brain that is involved in the formation of memories.
The nervous system is an extraordinarily complex communication system that simultaneously sends and receives a considerable volume of information. However, it is vulnerable to illness and injury, as noted in the following examples:
Neurons can undergo a degenerative process that causes Alzheimer’s disease, Huntington’s disease, or Parkinson’s disease.
Oligodendrocytes can become inflamed and lost, causing Multiple Sclerosis.
Bacteria or viruses can infect the brain or spinal cord, leading to encephalitis or meningitis.
If the blood supply to the brain is obstructed, a cerebrovascular accident will occur.
Trauma or tumors can also cause structural damage to the brain or spinal cord.