Necessary for allowing communication between certain areas of the brain, nerve fibers are actually extensions of nerve cells referred to as axons. Also called u-fibers, interconnecting nerve fibers are divided into two types: association fibers and commissural nerve fibers. While association nerve fibers are necessary for communication between parts of the same brain hemisphere, commissural fibers are responsible for connections across the two hemispheres of the cerebrum.
A neuron is a type of cell that transmits sensory and motor information to, from, and between brain structures, including operations involving decision-making, perception, and emotion. Each nerve cell consists of a body, or soma, and spider-like extensions projecting out of from it, referred to as dendrites. Axons are long, thin fibers attached to the soma of each cell, having several tiny branches with bulbs at each end called terminals, which contain neurotransmitters, chemicals responsible for the communication between nerve cells. Dendrites receive information as axons transmit it away from the cell body in the form of electrical impulses. Of all of the neurons within the brain, those of association fibers are the most abundant type.
Many kinds of nerve fibers exist in the human body; however, it is the interconnecting fibers that are responsible for transmitting information within areas of the brain only. Association fibers are myelinated axons located just beneath the cerebral cortex that are covered with glial cells working to insulate and conducting impulses very rapidly. As the association fibers are necessary for connections on the same side of the brain, short association fibers specifically associate parts within a particular lobe, such as Wernicke's area located within the temporal lobe necessary for the understanding of language. Chiefly responsible for connecting one lobe with another, long association fibers like the inferior longitudinal fasciculus make it possible for the temporal and occipital lobes to communicate with each other.
Certain diseases destroy association fibers, including the myelin sheaths that surround them. As of 2011, multiple sclerosis (MS) is the number one illness causing demyelination, a breakdown of white matter resulting in slowed or complete cessation of nerve impulses, producing symptoms indicating central nervous system (CNS) impairment. Early onset is characterized by periods of sensory loss of sensation, or paresthesia, a condition in which a limb feels as if it is prickling, itching, and tingling. Manifestations in the beginning generally come and go, occurring months, even years, between them. Thought to be caused by a combination of genetics and environmental factors, MS attacks on the CNS are due to an autoimmune response where the body reacts against its own tissues, resulting in inflammation and leading to nerve cell damage. Even though there is no cure for MS, symptoms can be managed and its progression slowed.