Axons come in all lengths, with some spanning the entire length of your body from your spinal cord to your toes. Axons are generally thinner than a piece of human hair.
Structure
Every nerve has axons. The larger the diameter of the axon, the more quickly it can transmit messages. In the innermost part of the nerve are axons that can be typically found inside a myelin sheath.
Myelin is a fatty protective substance that acts as insulation for axons, helping to send signals over long distances. For this reason, myelin is mostly found in neurons that connect different brain regions, rather than in the neurons whose axons remain in the local region.
Function
Axons help with the cable transmission between neurons. They form side branches called axon collaterals so they can send messages to several neurons at once.
These branches split into smaller extensions known as axon terminal branches, or nerve terminals. Each terminal holds a synapse where neurotransmitters send their messages and where messages are received.
Simply put, axons allow nerve cells to send electrical and chemical messages to other nerve, gland, and muscle cells using this internal communication process.
Axon vs. Dendrite
Dendrite is another part of a neuron. It is where a neuron receives input from another cell. Axons and dendrites are both made of fibrous root-resembling materials, but they differ in several ways:
Length: Axons are generally much longer than dendrites. Cell location: Axons are found at the specialized location on a cell body called the axon hillock. Dendrites are seen as branching away from the cell body into what’s called dendritic trees due to their appearance. Function: The two work together. Axons help messages move through your body systems, and dendrites receive and process those messages from the axons. Quantity: A neuron may have just one axon, while it may have more than one set of dendrites.
Types
A nerve contains bundles of nerve fibers, either axons or dendrites, surrounded by connective tissue. Different types of nerves contain different types of fibers.
Sensory Fibers
Sensory fibers pass impulses or messages from sensors to the brain and toward the central nervous system. These fibers are responsible for sensations like interpreting touch, pressure, temperature, and pain.
Motor Fibers
Motor fibers are behind why you tense your shoulders in response to a potential threat. They send messages to your muscles and glands in response to stimuli, including damage or physical traumas like accidents.
Damage
Acute axon damage is serious and life changing. Severe and diffuse axonal injuries can explain why people with head injury may be limited by a vegetative state. Axonal tears have been linked to lesions responsible for loss of consciousness in people who experience mild head injuries or concussions. Axon damage can result in axon degeneration (loss) and can eventually kill the underlying nerve.
Axon loss is an early sign of neurodegenerative diseases like:
Physical impact from an event like a motor-vehicle accident or falling from a height Injury from an assault or sport injury hemorrhage, contusion, or hematomaScattered brain bruising (contusion)Internal bleeding outside of the blood vessel (hematoma)
Alzheimer’s disease and other memory disorders Huntington’s disease Parkinson’s disease Amyotrophic lateral sclerosis (ALS)
Demyelination
When the fatty myelin sheath begins to thin, a process known as demyelination, the axon’s ability to send signals may become impaired. Some disease states can cause this myelin breakdown.
While the sheath can technically repair itself, damage can be severe enough to kill the underlying nerve fiber. These nerve fibers in the central nervous system cannot fully regenerate.
Conditions that can cause demyelination include:
Multiple sclerosis (MS): MS occurs when the immune system attacks myelin in the brain and spinal cord. Acute disseminated encephalomyelitis (ADEM): This is characterized by a brief but widespread attack of inflammation in the brain and spinal cord that damages myelin.
What is the axon hillock?
In the nervous system, the axon hillock is a specialized location on a cell body (soma) where the neuron connects to an axon. It controls the firing of neurons.
What are axon terminals?
Axon terminals are located at the end of an axon. This is where messages from neurotransmitters are sent and received.
How does myelin “insulate” an axon?
Myelin insulates an axon by surrounding the thin fiber with a layer of fatty substance protection. This layer is located between the axon and its covering (the endoneurium).
Summary
An axon is a thin fiber that extends from a neuron, or nerve cell, and is responsible for transmitting electrical signals to help with sensory perception and movement. Each axon is surrounded by a myelin sheath, a fatty layer that insulates the axon and helps it transmit signals over long distances.
