A neuron’s axon is a long projection that transmits electrical impulses, known as action potentials, away from the nerve cell body. This article focuses on a specific type: the central axon. These structures are found exclusively within the central nervous system (CNS) and are integral to its functions, from processing thoughts and controlling movement to registering sensations.
What is a Central Axon?
A central axon is part of a neuron whose cell body and axon are both located entirely within the central nervous system (CNS), forming the networks that constitute white matter. These axons relay signals between different regions of the brain and along the spinal cord. Axons vary significantly in length; many are just a few millimeters long, while others traveling from the brain down the spinal cord can be over a meter.
The structure of a central axon begins at the axon hillock, where electrical impulses are generated. From there, the axon extends, filled with cytoplasm (axoplasm) and enclosed by a membrane (axolemma). The axoplasm transports molecules, such as proteins and neurotransmitters, from the cell body down to the axon terminals. At its end, the axon branches into terminals that form synapses with other neurons, allowing the signal to be passed to the next cell.
Signal Conduction in Central Axons
The efficiency of signal transmission in central axons is enhanced by myelin, a fatty substance that encases the axon in segments, forming a myelin sheath. In the CNS, these sheaths are produced by glial cells called oligodendrocytes. Myelin acts as an electrical insulator, preventing the leakage of ions across the axon’s membrane and allowing for a highly efficient method of signal propagation.
The myelin sheath is not continuous; it is interrupted by small gaps called the nodes of Ranvier, where the action potential is regenerated. The electrical signal “jumps” from one node to the next, a process called saltatory conduction. This method is significantly faster and more metabolically efficient than conduction along an unmyelinated axon. The high concentration of ion channels at the nodes ensures the signal is regenerated at each gap, maintaining its strength over long distances.
Central vs. Peripheral Axons
While central and peripheral axons both transmit signals, they have important structural differences. A primary distinction is the cells that produce their myelin sheaths. In the CNS, oligodendrocytes are responsible for myelination, while in the peripheral nervous system (PNS), Schwann cells perform this role. An individual oligodendrocyte can myelinate multiple axons, while a single Schwann cell only myelinates one segment of a single peripheral axon.
A significant difference between these axon types is their capacity for regeneration after injury. Peripheral axons have an ability to regrow if damaged, but central axons have a very limited ability to regenerate. This is why injuries to the brain and spinal cord often result in permanent functional loss. This failure to repair is due to the CNS environment, where astrocytes form a glial scar that acts as a physical barrier, and oligodendrocytes release molecules that inhibit axon growth.
Consequences of Central Axon Damage
The limited regenerative capacity of central axons means that damage can lead to severe and lasting neurological deficits. Because these axons are the communication pathways in the CNS, the specific consequences depend on the location and extent of the damage. For instance, damage to axons in the spinal cord is a primary cause of paralysis and loss of sensation associated with spinal cord injuries.
Certain neurological diseases are characterized by damage to central axons or their myelin sheaths. In multiple sclerosis (MS), the immune system attacks the myelin produced by oligodendrocytes. This demyelination disrupts the flow of nerve signals, leading to symptoms like vision problems, muscle weakness, and coordination difficulties. Traumatic brain injury (TBI) often involves diffuse axonal injury, where trauma damages axons throughout the brain, and the resulting functional impairments are often permanent.