Axon | How Do Your Brain Cells Talk to Each Other?

The axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body. Think of it as the primary transmission line of the nervous system. Every thought, sensation, and action is dependent on the proper functioning of these structures. The process begins at the axon hillock, a specialized part of the cell body that integrates incoming signals. If the total signal strength is sufficient, it triggers an electrical signal known as an action potential, which then travels down the entire length of the axon. The axon's structure is optimized for this rapid transmission. While some axons are very short, others can be remarkably long, such as those that extend from the spinal cord to the muscles in the feet. At its end, the axon branches into smaller extensions called axon terminals. These terminals are responsible for transmitting the signal to other cells, completing the communication circuit. Therefore, the axon functions as a critical conduit, ensuring that information is relayed accurately and efficiently from one part of the body to another.

The myelin sheath is a fatty, insulating layer that surrounds many axons. This sheath is not continuous; it is segmented by periodic gaps called the nodes of Ranvier. The primary function of myelin is to dramatically increase the speed at which electrical signals, or action potentials, travel along the axon. It does this through a process called saltatory conduction, where the nerve impulse effectively 'jumps' from one node to the next, bypassing the myelinated sections. This is significantly faster than the continuous conduction that occurs in unmyelinated axons. In the central nervous system (the brain and spinal cord), myelin is produced by specialized cells called oligodendrocytes, while in the peripheral nervous system (the nerves outside the brain and spinal cord), it is produced by Schwann cells. This insulation is crucial for high-speed communication required for complex motor functions and rapid sensory processing.

An action potential is the fundamental electrical signal that travels along an axon. It is a brief, all-or-nothing event, meaning it either fires at full strength or not at all. This impulse is generated by the rapid movement of charged particles, specifically sodium (Na+) and potassium (K+) ions, across the axon's membrane. When a neuron is stimulated past a certain threshold, channels in the membrane open, allowing a flood of positively charged sodium ions to rush into the axon. This influx rapidly reverses the electrical charge across the membrane, creating the spike of the action potential. Immediately after, potassium ion channels open to allow positive charges to exit, repolarizing the membrane and restoring the resting state. This wave of depolarization and repolarization travels down the axon to its terminal.

When an action potential reaches the axon terminal, it does not simply jump to the next neuron. Instead, the signal is converted from an electrical form to a chemical one. The space between the axon terminal of one neuron and the dendrite of the next is called the synapse. The arrival of the action potential at the terminal triggers the release of chemical messengers called neurotransmitters, such as dopamine or serotonin, into this synaptic gap. These neurotransmitter molecules travel across the synapse and bind to specific receptor proteins on the membrane of the receiving neuron. This binding opens ion channels on the postsynaptic neuron, initiating a new electrical signal and thus passing the message along. This synaptic transmission is a fundamental process for all neural communication.

Axonal integrity is essential for a healthy nervous system. In demyelinating diseases like Multiple Sclerosis (MS), the body's own immune system mistakenly attacks and destroys the myelin sheath that insulates axons in the central nervous system. This process, known as demyelination, disrupts the flow of nerve impulses. The loss of myelin slows down or completely blocks the transmission of action potentials along the affected axons. Consequently, the communication between the brain and other parts of the body is impaired. This can lead to a wide variety of neurological symptoms, including muscle weakness, difficulty with coordination and balance, sensory problems like numbness or tingling, and cognitive impairments. In the later stages of MS, the axon itself can become damaged, leading to permanent neurological disability. This highlights the critical dependence of neural function on both the axon and its protective myelin sheath.