Neurons are the basic communication units of the nervous system, enabling the brain and body to interact. These specialized cells process and transmit information through electrical and chemical signals. A key component of a neuron is the axon, a long projection that sends these signals over distances. Understanding the axon’s structure and function is essential to comprehending how the nervous system operates.
The Axon’s Anatomy
An axon is an elongated projection extending from the neuron’s cell body, originating from a region called the axon hillock. This hillock integrates signals before they are sent down the axon. Axons vary in length, from fractions of a millimeter to over a meter, connecting distant parts of the nervous system. At its end, the axon branches into multiple axon terminals.
These terminals are where the axon communicates with other neurons or target cells, forming synapses. Many axons are enveloped by a fatty, insulating layer called the myelin sheath. This sheath is not continuous but is interrupted at regular intervals by small gaps called Nodes of Ranvier. Within the axon, the cytoplasm contains structures like microtubules that transport substances along the axon.
How Axons Transmit Signals
Axons transmit information through electrical impulses called action potentials. An action potential is a rapid, brief change in the electrical potential across the axon’s membrane. This process begins when the membrane potential at the axon hillock reaches a threshold, triggering a rapid influx of positively charged sodium ions into the axon. This influx causes the inside of the membrane to become temporarily positive, a phase known as depolarization.
Following depolarization, sodium channels in the membrane close, and potassium channels open, allowing positively charged potassium ions to flow out of the axon. This outflow restores the negative charge inside the membrane, a process called repolarization. The action potential then propagates along the axon, as depolarization in one segment triggers the opening of ion channels in the adjacent segment. This propagation adheres to the “all-or-none” principle, meaning an action potential either fires completely or not at all once the threshold is met, maintaining its strength as it travels.
The Speed of Signal Transmission
The speed at which electrical signals travel along an axon, or conduction velocity, is influenced by several factors. The presence of a myelin sheath significantly increases this speed. In myelinated axons, the action potential “jumps” from one Node of Ranvier to the next, a process called saltatory conduction. This jumping mechanism is faster than continuous conduction, as the signal only needs to be regenerated at the unmyelinated nodes.
Unmyelinated axons, lacking this insulating sheath, conduct signals more slowly and continuously along their entire length. Here, depolarization and repolarization occur sequentially along every point of the membrane. The diameter of the axon also plays a role in conduction velocity; larger diameter axons conduct signals faster due to less internal resistance to ion flow. For instance, some myelinated axons can transmit signals at speeds up to 120 meters per second, while unmyelinated axons might transmit at less than 1 meter per second.
Axons and Neurological Health
Healthy axons are essential for the nervous system’s function, enabling communication between the body and the brain. Damage or degeneration of axons can disrupt this communication, leading to neurological impairments. When axons are compromised, electrical signal transmission can be slowed, distorted, or halted. This disruption can manifest as problems with sensation, movement coordination, and cognitive functions.
Multiple sclerosis (MS) is an example where axon health is compromised. In MS, the immune system attacks and damages the myelin sheath surrounding axons in the central nervous system. This demyelination impairs signal conduction, leading to symptoms like muscle weakness, numbness, vision problems, and fatigue. Peripheral neuropathies involve damage to axons in the peripheral nervous system, often resulting in pain, tingling, and muscle weakness in the affected limbs.