Understanding the Structure and Function of an Axon

Axons are thin fibers that enable communications between neurons (nerve cells). The function of axons is to transmit information in the form of electrical impulses between neurons.

From the broadest perspective, axons act like transmission cables. They enable the passage of electrical impulses within the brain and between the brain and the rest of the body.

The article looks at the structure, function, and types of axons in the body's nervous system. It also describes conditions that can damage axons and lead to nerve dysfunction.

Structure and Types of Axons

With a few exceptions, every neuron in the nervous system has only one axon. They come in different sizes and lengths.

Some axons are as long as one meter, while others are less than one millimeter. The longest are those of the sciatic nerve which run from the base of the spinal cord down each leg and end at your big toes.

As a general rule, the larger the diameter of the axon, the more quickly it can transmit (conduct) nerve impulses.

There are two types of axons found in the nervous system:

• Myelinated axons: These axons are covered with a fatty insulated coating called a myelin sheath. Myelinated axons connect neurons in the somatic nervous system which directs the voluntary movement of skeletal muscles in the body.
• Unmyelinated axons: These axons are not covered with a myelin sheath. Unmyelinated axons connect neurons in the autonomic nervous system which direct the involuntary movement of smooth muscles, like those of the heart, blood vessels, and intestines.

Myelinated and unmyelinated axons differ in how they work (and why).

Myelinated axons service the peripheral nervous system. This is part of the nervous system that carries nerve signals from your brain to skeletal (voluntary) muscles so that you can move. Nerve impulses move faster with myelinated axons, enabling rapid and intricate variations in movement.

Unmyelinated axons service the autonomic nervous system. This is part of the nervous system that regulates smooth (involuntary) muscles involved with things like heart rate, blood pressure, respiration, and digestion. Nerve impulses move slower with unmyelinated axons, enabling steady, consistent movement with less variation.

Function of Axons

The function of an axon is to transmit information between neurons and away from muscles and glands.

Each neuron has an axon that directly connects it with another neuron. There are three types of neurons it can connect to:

• Sensory neurons: These are nerve cells that relay information that allow us to hear, touch, smell, see, or feel things like temperature or pain.
• Motor neurons: These are nerve cells that direct muscle contractions and gland functions.
• Interneurons: These connect nerve cells to nerve cells within the same region of the brain or spinal cord. Neurons connected in this way create a network known as a neural circuit.

How Electrical Impulses Become Communication

The translation of electrical impulses into actual communication involves chemical messengers known as neurotransmitters. These are what neurons use to communicate with each other. It also involves the movement of electrically charged particles called ions across the neuron's membrane.

The process starts when a signal, also called an action potential, is sent from the neuron's cell body towards the axon. Along the surface of the axon membrane are channels that allow ions like sodium and potassium to enter or exit the neuron. When the signal from the cell body reaches the nearest sodium channels, these channels open and sodium ions flood the inside of that segment of the axon.

This change in concentration of ions also changes the membrane voltage, which leads to ion channels in the next segment of the axon opening. At the same time, the previous segment returns to its resting potential. These steps repeat along the length of the axon until the signal reaches the end of axon, called the axon terminal.

At the axon terminal, neurotransmitters stored within the axon terminal are released. The neurotransmitters then cross a small gap, called a synapse, and are received by the dendrites of the adjacent neuron.

Different neurotransmitters deliver different chemical messages.

A single axon can have many different branches, each with different neurotransmitters. Depending on which branches are fired, multiple messages can be delivered simultaneously and rapidly between neurons.

Causes of Axons Injury

Axons can be damaged directly and indirectly. When this happens, certain physiological or neurological functions can be impaired, either temporarily or permanently. The impairment may be localized to one part of the body or involve the body as a whole.

The causes of axonal injury can be broadly categorized as follows:

Brain and Spinal Cord Trauma

Head and spinal cord trauma can cause the compression or severing of axonal connections, referred to as traumatic axonal injury (TAI) of the brain or spinal cord. When the brain is affected, a coma and other neurological problems can occur. When the spinal cord is affected, weakness, chronic pain, paralysis, or urinary or fecal incontinence can occur.

Axonal Degeneration

Age can cause axons to gradually deteriorate. The same can occur with neurodegenerative diseases like Alzheimer’s disease, Huntington’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis (ALS), which tend to be aging-related.

Axonal Demyelination

Diseases like multiple sclerosis (MS) cause the progressive destruction of the myelin sheath, resulting in the loss of motor function as axons "misfire." The loss of myelin is referred to as demyelination.

Diabetes and certain medications also cause demyelination, resulting in disruptive pain, burning, or tingling sensations known as neuropathy.

Metabolic Encephalopathy

Metabolic encephalopathies are conditions that cause chemical imbalances in the brain that lead to axonal inflammation and injury.

These include:

• Acute metabolic encephalopathy: Caused by a deficiency of vitamins, oxygen, or glucose
• Toxic metabolic encephalopathy: Caused by toxic substances or the accumulation of toxins from failing organs (such as kidney failure or liver failure)

Brain Infections

Certain brain infections can cause axonal damage, either acute (sudden and severe) or chronic (persistent or recurrent). These include conditions like cerebral malaria which can lead to seizures, coma, or death, and AIDS dementia complex, a complication of advanced untreated HIV infection.

Ischemic Injury

This occurs when decreased blood flow to the brain starves neurons and axons of the oxygen and nutrients they need to survive. Ischemic stroke is an example of this, often causing irreparable brain damage.

Subcortical ischemic vascular dementia (SIVD) is a form of dementia associated with atherosclerosis in which the narrowing of the arteries causes progressive damage to axons in the deeper issues of the brain (called the white matter).

Summary

An axon is a thin fiber that connects neurons (nerve cells) to that they can communicate. Neurons communicate via electrical impulses that trigger the release of "chemical messengers" called neurotransmitters. Axons also transmit electrical impulses from muscle and gland cells to the brain.

Axons can be damaged by trauma, inflammation, infection, or reduced blood flow to the brain. Damage can also be caused to the protective coating of axons, called the myelin sheath, due to older age, disease, or toxic substances.