Question

While intravenous sodium is necessary to reverse the effects of hyponatremia, doing so too rapidly can cause a serious problem in the myelin sheath. What part of the neuron does myelin insulate? A. Nucleus B. Synapse C. Axon D. Node of Ranvier

Short answer

C. Axon

Step-by-step solution

Understand the Role of Myelin

Myelin is a fatty substance that wraps around certain nerve fibers. It acts as an insulator to speed up the transmission of electrical impulses along the nerve cells.

Identify Parts of a Neuron

A neuron is comprised of the cell body (which includes the nucleus), dendrites, the axon, and the synapse. Remember that the axon is a long, thin fiber that transmits signals away from the neuron's cell body.

Determine the Correct Answer Based on the Function of Myelin

Since myelin insulates to speed up transmission, it logically wraps around the part of the neuron where impulse transmission occurs. This part is the axon. The other options (Nucleus, Synapse, Node of Ranvier) do not fit this role as precisely.

Key concepts

Myelin Sheath

The myelin sheath is a fatty layer that wraps around the axons of neurons. It functions as an insulator, much like the plastic coating around electrical wires. This insulation is crucial because it vastly increases the speed at which electrical impulses travel along the nerve cells.

• **Protection:** The myelin sheath not only facilitates faster transmission of nerve impulses but also protects the axon from damage.
• **Speed:** Without the myelin sheath, nerve impulses would travel much slower, significantly affecting the nervous system's efficiency.

The myelin sheath is made up of multiple layers of myelin, which are produced by glial cells. In the central nervous system (CNS), these cells are called oligodendrocytes, while in the peripheral nervous system (PNS), they are known as Schwann cells.

When the myelin sheath is damaged or degraded, it can lead to serious neurological conditions like multiple sclerosis (MS), where the transmission of nerve impulses is greatly impaired.

Axon

The axon is a critical part of the neuron responsible for transmitting electrical signals away from the neuron's cell body to other neurons, muscles, or glands. Think of it as a communication highway in the nervous system.

• **Structure:** Axons are long, thin fibers that can vary in length from less than a millimeter to more than a meter in some cases.
• **Function:** Their principal role is to convey nerve impulses, known as action potentials, to different parts of the body.

The surface of the axon is covered with protein channels and pumps that help maintain its electrical charge. When a stimulus reaches the neuron, the axon transmits the impulse to the target cell, such as another neuron or a muscle cell, rapidly and efficiently.

Understanding the axon's role sheds light on why the myelin sheath is so important. Without this protective insulation, the axon's transmission capabilities would be severely reduced.

Nerve Impulse Transmission

Nerve impulse transmission is the process by which electrical signals are sent from one part of the neuron to another, or to other neurons, muscles, or glands. This process involves a series of crucial steps:

• **Initiation:** Nerve impulses begin when a neuron is stimulated by an external factor like pressure, heat, or a chemical signal.
• **Propagation:** The impulse travels down the axon in a wave-like manner, thanks to the opening and closing of ion channels in the neuron membrane.
• **Transmission:** Once the impulse reaches the end of the axon, neurotransmitters are released into the synapse (the gap between neurons), which then bind to receptors on the next neuron or target cell.

The myelin sheath plays an integral role in this process. It speeds up nerve impulse transmission by enabling the axon to conduct signals more efficiently and quickly through a process known as saltatory conduction. Here, the electrical impulse jumps from one node of Ranvier (gaps in the myelin sheath) to the next, significantly enhancing the speed of communication.

When myelin is damaged, this efficient transmission process is disrupted, leading to slower nerve signaling and potential neurological issues.