Axon - Anatomy

Anatomy

Axons are the primary transmission lines of the nervous system, and as bundles they form nerves. Some axons can extend up to one meter or more while others extend as little as one millimeter. The longest axons in the human body are those of the sciatic nerve, which run from the base of the spinal cord to the big toe of each foot. The diameter of axons is also variable. Most individual axons are microscopic in diameter (typically about 1 micron across). The largest mammalian axons can reach a diameter of up to 20 microns. The squid giant axon, which is specialized to conduct signals very rapidly, is close to 1 millimetre in diameter, the size of a small pencil lead. Axonal arborization (the branching structure at the end of a nerve fiber) also differs from one nerve fiber to the next. Axons in the central nervous system typically show complex trees with many branch points. In comparison, the cerebellar granule cell axon is characterized by a single T-shaped branch node from which two parallel fibers extend. Elaborate arborization allows for the simultaneous transmission of messages to a large number of target neurons within a single region of the brain. In vertebrates, the axons of many neurons are sheathed in layers of a fatty insulating substance called myelin, which is formed by two types of glial cells: Schwann cells ensheathing peripheral neurons and oligodendrocytes insulating those of the central nervous system. Along myelinated nerve fibers, gaps in the myelin sheath known as nodes of Ranvier occur at evenly-spaced intervals. The myelination enables an especially rapid mode of electrical impulse propagation called saltatory conduction. Demyelination of axons causes the multitude of neurological symptoms found in the disease Multiple sclerosis.

If the brain of a vertebrate is extracted and sliced into thin sections, some parts of each section appear dark and other parts lighter in color. The dark parts are known as grey matter and the lighter parts as white matter. White matter gets its light color from the myelin sheaths of axons: the white matter parts of the brain are characterized by a high density of myelinated axons passing through them, and a low density of cell bodies of neurons. The cerebral cortex has a thick layer of grey matter on the surface and a large volume of white matter underneath: what this means is that most of the surface is filled with neuron cell bodies, whereas much of the area underneath is filled with myelinated axons that connect these neurons to each other.