The nervous system is a complex network responsible for everything from our thoughts to our movements, built upon long-distance connections formed by nerve cells, or neurons. These connections serve as the brain’s communication highways. Each neuron sends out a long, cable-like extension called an axon. An axonal projection is the connection formed when an axon extends to a distant target, creating the physical wiring that allows information to travel throughout the brain and body. These projections can be very long, with some stretching from the spinal cord to the muscles in the foot.
The Blueprint for Brain Wiring
The brain’s wiring forms during development in a highly organized process. Each growing axon is piloted by a structure at its tip called a growth cone. This structure extends and retracts finger-like filopodia to probe its immediate surroundings. The growth cone’s journey is directed by a system of molecular cues in the environment.
This navigation, known as axon guidance, relies on chemical cues that tell the growth cone where to go. Some of these chemicals are chemoattractants, which draw the axon toward them, while others are chemorepellents, which push it away. Prominent guidance molecules include netrins, which often act as attractants, and semaphorins and slits, which typically function as repellents. The surface of the growth cone has specific protein receptors that detect these chemical signals.
When a receptor on the growth cone binds to a guidance molecule, it triggers internal changes. This process alters the axon’s cytoskeleton, causing the growth cone to steer itself. By integrating numerous signals from its environment, the growth cone navigates complex pathways to reach its correct target cell. This process is repeated by billions of neurons, establishing the circuits that underpin nervous system function.
Sending Long-Distance Messages
Once an axonal projection is established, it transmits information rapidly over long distances using electrical signals called action potentials. An action potential is a wave of electrical charge that travels down the axon from the neuron’s cell body to its end, similar to a spark racing down a fuse.
When the action potential reaches the axon terminal, it arrives at a junction called a synapse, where the electrical message is converted into a chemical one. The signal triggers the release of neurotransmitters from the axon terminal into the gap between cells. These neurotransmitters then bind to receptors on the target cell, passing the message along.
To make this transmission faster, many axons are wrapped in a fatty substance called a myelin sheath. This sheath acts as an insulator and has periodic gaps called nodes of Ranvier. The electrical signal “jumps” from one node to the next in a process called saltatory conduction. This greatly increases the message’s speed, allowing for the swift communication the nervous system requires.
Consequences of Disconnection
The nervous system’s function depends on the integrity of its axonal projections. When these connections are compromised, the results can be severe. Physical trauma, such as a spinal cord injury, can sever axons. This disconnects the brain from parts of the body, leading to a loss of motor control and sensation below the injury site. A traumatic brain injury can also cause widespread damage by stretching and tearing axons, a condition known as diffuse axonal injury.
Neurodegenerative diseases also damage these connections. In multiple sclerosis (MS), the immune system attacks and destroys the myelin sheath that insulates axons. This demyelination impairs the axon’s ability to conduct signals, leading to neurological symptoms that affect movement, sensation, and cognition.
In conditions like Alzheimer’s disease, there is a progressive loss of synapses, the connection points between neurons. This is followed by the degeneration of the axons themselves. As these connections are dismantled, the neural circuits for memory and other cognitive functions degrade, leading to the functional decline characteristic of the disease.
Mapping the Neural Highways
Scientists have developed methods to map the brain’s network of axonal projections, providing insight into how different brain regions are connected. A foundational method is tract tracing. In this approach, a chemical tracer, such as a fluorescent dye, is injected into a specific brain area. Neurons absorb the dye, which is then transported along their axons to their distant terminals, revealing the full path of the projection.
While powerful, this classic technique is invasive. Modern methods allow for mapping these connections in living humans non-invasively. A prominent example is Diffusion Tensor Imaging (DTI), an advanced magnetic resonance imaging (MRI) technique that tracks the movement of water molecules in the brain.
In the brain’s white matter, water molecule movement is constrained by axon bundles, as water diffuses more easily along an axon’s length than across it. DTI measures this directional diffusion in thousands of small volumes, or voxels. Computer algorithms then follow the primary direction of water movement from voxel to voxel to reconstruct the pathways of major axon bundles, creating detailed wiring diagrams of the living brain.