Question

Unmyelinated axons conduct action potentials without decrement, but when myelinated axons lose myelin in demyelinating diseases such as multiple sclerosis, conduction of action potentials is blocked. Why?

Short answer

The myelin sheath in the myelinated axons facilitates fast and efficient conduction of action potentials. When this myelin is lost in demyelinating diseases like multiple sclerosis, the conduction of these action potentials is blocked or significantly slowed. Unmyelinated axons, on the other hand, conduct action potentials slower but without decrement because they do not rely on myelin for signal transmission.

Step-by-step solution

Understanding the Role of Myelin

Myelin is a substance that covers some nerve fibers, including the axons in neurons, to protect and insulate them. This insulation speeds up the transmission of electrical signals (action potentials) along the axon, thus allowing for efficient communication between different neurons.

Explaining the Consequences of Demyelination

Demyelinating diseases, such as Multiple Sclerosis, cause damage to the myelin sheath. When the myelin sheath is damaged, the conduction of action potentials is disrupted. The signals can't travel as quickly or efficiently, which impairs the communication between neurons.

Comparing Myelinated and Unmyelinated Axons

Unmyelinated axons do not have a myelin sheath to speed up the conduction of action potentials. Therefore, they conduct action potentials at a much slower rate. However, the conduction is consistent (without decrement). On the other hand, myelinated axons can conduct action potentials more rapidly due to the insulating effect of myelin, but when they lose the myelin sheath due to conditions like multiple sclerosis, the action potentials can become blocked or slowed.

Key concepts

Myelin sheath

The myelin sheath is a critical component of the nervous system that wraps around the axons of many neurons. This protective layer is composed of fats and proteins, creating a "blanket" that insulates the nerve fibers. By insulating the axons, the myelin sheath enhances the speed at which electrical signals, known as action potentials, travel along a neuron.
Myelin influences not only the velocity but also the efficiency of neural communication:

• Increased speed: Myelin enables impulses to jump quickly from one node (Node of Ranvier) to the next, a process called saltatory conduction.
• Energy efficiency: Less energy is needed for neural transmission because the ion exchanges that generate action potentials occur only at the nodes.

Without effective myelination, communication between neurons becomes slow and unreliable, which can severely affect bodily functions.

Action potential conduction

Action potential conduction is the process through which neurons transmit signals along their axons. These signals are crucial for communication within the nervous system. When an action potential is initiated, a rapid change in electrical charge travels along the axon's membrane as sodium ions flow in and potassium ions flow out.
In myelinated axons, conduction is enhanced through saltatory conduction. Here, action potentials "hop" from one node to the next, significantly boosting speed—a necessary feature for complex and fast responses in the body.
In contrast, unmyelinated axons transmit action potentials more slowly and continuously because the charge must travel the entire length of the axon membrane, without jumping. Despite the slower speed, these axons conduct signals reliably.

• Myelinated axons: Fast, efficient, and energy-saving signal transmission.
• Unmyelinated axons: Slower but consistent transmission over long distances.

The difference in conduction speed between these axons can have significant implications for functioning if disrupted.

Multiple sclerosis

Multiple Sclerosis (MS) is a debilitating condition that affects the central nervous system. It is characterized by the progressive loss of the myelin sheath surrounding the nerve fibers. This demyelination hinders normal action potential conduction, leading to a range of neurological symptoms.
Symptoms can vary widely among individuals, often including:

• Fatigue and difficulty walking.
• Muscle weakness and spasms.
• Visual disturbances.
• Pain and numbness.

As the disease progresses, demyelinated regions (plaques) form along the nerve fibers, further disrupting the transmission of nerve impulses.
Despite ongoing research, the exact cause of the autoimmune attack on myelin in MS is still not fully understood. However, therapies exist that can help manage symptoms and slow disease progression by modulating the immune system or promoting remyelination efforts.