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An axon is a long, branching cell structure that is unique to nerve cells. Like all animal cells, nerve cells — also known as neurons — are covered with a semi-permeable membrane, and it is this membrane that makes up the axons. These structures are responsible for carrying information from the nerve cells to all the other cells of the body. Interference with signals as they travel through the axons has been identified as a cause of certain degenerative neurological disorders.
The neuron itself is composed of three basic structures: the cell body, the axon, and numerous branching dendrites. The cell body houses the nucleus and other organelles. The dendrites collect information from other parts of the body and carry it into the neuron. The axon carries electrical impulses from the neuron to all the other cells of the body. A fatty sheath that covers the structure for the entirety of its length serves to insulate the electrical signals from interference. Known as the myelin sheath, this protective covering is composed primarily of fat cells, and is responsible for the characteristic whitish color of neural tissue.
The many branches of the structure allow the neuron the enervate multiple cells with a single signal. Each branch is able to produce thousands of specialized structures, known as synaptic terminals. These interact with other cells through electrical signals, or by the release of chemical messengers known as neurotransmitters. Each synaptic terminal makes contact with a target cell, which may be either a nerve cell or a functional cell, such as a muscle fiber or white blood cell. The point of contact between an axon and a target cell is known as a synapse.
Neurons send information along the axons in the form of electrical impulses. The point at which the base of the structure originates from the cell body is known as the axon hillock. It is in this region that electrical signals are generated. Called the action potential, these electrical impulses convey information by varying the length and frequency of signals in a kind of neurological Morse code.
The neuron is able to create the voltage necessary for the action potential by controlling the concentration of various ions, both inside and outside of the cell. By channeling positive ions along the length of the axon, the neuron is able to generate brief electrical signals to activate the enervated cell or cells on the receiving end of the message.
The myelin sheath is a critical component of this process. By insulating axons from interference, myelin sheaths ensure that neurons are able to quickly and accurately convey information. Degeneration of the myelin sheath is associated with the disrupted neuronal communication often seen in patients diagnosed with multiple sclerosis. Similarly, Alzheimer’s disease is thought to be related to the destruction of myelin tissue in certain parts of the brain.