The Five Senses and Beyond: The Encyclopedia of Perception - Jennifer L. Hellier 2017

Axon

An axon is a single, dedicated, long cellular process of a neuron that transmits electrical signals from the neuron to its target cell. In general, most neurons have just a single axon that extends out from the main body of the neuron. The axon consists of the cell membrane with cytoplasm and microtubule fibers running the length of the projection. The particular location where the axon extends from the soma (cell body) is called the axon hillock. It is this area where the cell body is connected to the axon.

In addition to an axon, neurons have numerous other shorter cell processes called dendrites that extend out of the soma. Axons are distinguishable from dendrites as both projections are structurally and functionally very different. While dendrites are specialized to receive signals from other neurons and transmit them toward the soma, axons are specialized to conduct the signals from the soma to the branched terminal ends of the axon. These terminal ends are called the axon terminals or boutons, while the electrical signals are called action potentials.

In both the peripheral and central nervous systems (PNS and CNS, respectively), axons may be myelinated or unmyelinated. Myelin is a coating of “insulation” surrounding axons. Glial cells, a specialized group of support cells, myelinate these axons. Specifically, glial cells that myelinate axons in the PNS are called Schwann cells, while glial cells in the CNS are called oligodendrocytes. The cytoplasm and plasma membrane of these glial cells flatten into a thin sheet, which wraps around a segment of an axon at regular intervals to form myelin sheaths. These cells protect, support, and insulate axons so that action potentials may skip the myelinated regions of the axon, making the conduction of signals through the entire length of axon fast and efficient. Small, unmyelinated gaps in the myelin sheath, present at regular intervals throughout the length of the axon, are called the nodes of Ranvier. These structures ensure the signal reaches the terminal end of the myelinated axon without degradation. Nonmyelinated axons do not have the myelin sheath, hence the action potential must travel down the entire length of the axon membrane, which results in a slower transmission speed of the action potential compared to its myelinated counterpart.

History and Function

The axon was first described and distinguished from dendrites by a German neuroanatomist, Otto Friedrich Karl Deiters (1834—1863). Since then, numerous studies were conducted to decipher its exact function and mechanism. In 1952, details of the axon’s functionality and its physiology were elucidated by Alan Hodgkin (1914—1998) and Andrew Huxley (1917—2012). These two scientists used giant axons of squids to measure the electrical signals transmitted through the axon. The squid giant axon served as an excellent experimental model due to its large size (up to 1 millimeter in diameter), length, and ease of access. Hodgkin and Huxley’s work resulted in a comprehensive mathematical model of the action potential and its electrical mechanism that is known today as the Hodgkin-Huxley model. For their work, Hodgkin and Huxley were awarded the Nobel Prize in 1963. In 1978, Louis-Antoine Ranvier noted and described a pattern of regularly spaced gaps in the myelin sheath of an axon, which is known today as the nodes of Ranvier.

Anatomy and Physiology

An axon is a single projection of the neuron that varies in length and diameter depending on the location, function, and type of neuron. Some axons of sensory neurons located in the body can conduct action potentials from the peripheral sensory receptors, such as in the hand, toward their cell bodies located in the brain or spinal cord. In this way, the axons deliver critical information about the state of the body, such as its position and environment, to other neurons. In general, axons travel together both outside of (nerves) and within (tracts) the CNS.

The diameter of axons also affects the speed of action potential transmission. The bigger the axon diameter, the faster an action potential can travel. Numerous microtubule fibers are present within the axon and function as a set of cable systems by which vesicles containing neurotransmitters and other cellular materials are transported between the cell body and axon terminals. Neurotransmitter vesicles and other secretory products are made and packaged in the neuron cell body, which are then hooked onto the microtubule fibers and transported to the axon terminal by sliding down the microtubule fibers. Once they reach the axon terminal, neurotransmitter vesicles are stored here until the action potential arrives and triggers their release. Other cellular materials or vesicles may be transported from the axon terminal to the cell body using the microtubules as well, but in a reverse direction. In this way, the cell body and the axon terminals that may be several feet apart are still able to communicate with each other effectively.

Lisa M. J. Lee

See also: Action Potential; Nerves

Further Reading

Hodgkin, Alan L., & Andrew F. Huxley. (1939). Action potentials recorded from inside a nerve fiber. Nature, 144(3651), 710—711.

Poliak, Sebastian, & Elior Peles. (2003). The local differentiation of myelinated axons at nodes of Ranvier. Nature Reviews Neuroscience, 4, 968—980. Retrieved from http://www.nature.com/nrn/journal/v4/n12/pdf/nrn1253.pdf