Problem 161 - Objective Biology for NEET

Question

Assertion: Unmyelinated fibres are without myelin sheath. Reason: Schwann cells are absent in unmyelinated fibres.

Step-by-Step Solution

Answer

The assertion 'Unmyelinated fibres are without myelin sheath' is true, but the reason 'Schwann cells are absent in unmyelinated fibres' is false since Schwann cells are present in both unmyelinated and myelinated fibres. Hence, these two are not correctly connected.

Step 1: Understand the Assertion

The assertion states 'Unmyelinated fibres are without myelin sheath'. The myelin sheath is a layer guarding some nerve cells which speeds up the nerve impulses. Unmyelinated fibers are those nerve fibers that lack this myelin sheath. Thus, the assertion is correct.

Step 2: Understand the Reason

The reason states 'Schwann cells are absent in unmyelinated fibres'. Schwann cells are a variety of glial cell that provide the myelin sheath around the axons of neurons in the peripheral nervous system. While myelinated fibers employ Schwann cells to form the myelin sheath, unmyelinated fibers too use Schwann cells. Nevertheless, in unmyelinated fibers, Schwann cells envelop several axons at once without forming a myelin sheath and instead promote other support functions. Thus, the reason is incorrect because Schwann cells are not absent in unmyelinated fibers.

Step 3: Connect the Assertion and Reason

Now we will examine if the assertion and reason are connected. Based on the given data and understanding from step 1 and step 2, it is observed that while the assertion 'Unmyelinated fibres are without myelin sheath' is correct, the reason 'Schwann cells are absent in unmyelinated fibres' is not correct. Therefore, the assertion and reason are not correctly connected.

Key Concepts

Unmyelinated fibres

Unmyelinated fibres are nerve fibres that do not have a myelin sheath surrounding them. This lack of a myelin layer affects how these fibres conduct nerve impulses. Without a myelin sheath, nerve signals move slower as compared to their myelinated counterparts. In the nervous system, some nerve cells are permanently unmyelinated, while in others, the myelin may be removed during certain conditions.

These fibres can be found in both the peripheral and central nervous systems. In the peripheral nervous system, unmyelinated fibres are often bundled together and surrounded by a single Schwann cell, but they do not get the insulating layer that myelinated axons do. Understanding unmyelinated fibres is crucial as they perform significant roles despite their slower conduction speed.

These fibres are still functional and provide important sensory and motor functions. For example, they are involved in transmitting pain signals, which generally do not require rapid transmission speeds as compared to other nerve signals.

Myelin sheath

The myelin sheath is a protective, fatty layer that surrounds the axons of many nerve cells. This sheath plays a crucial role in speeding up the transmission of electrical impulses along the nerve cells. Myelin acts like an insulator, allowing nerve cell signals to jump across from one point to another, in a process called saltatory conduction.

Without myelin, nerve signals would slow down considerably, and this can affect everything from reflexes to coordination. The myelin sheath is composed of lipids and proteins and is essential in maintaining the health and functionality of neurons. Damage to the myelin sheath can result in neurological disorders such as multiple sclerosis, where this insulating layer is damaged, leading to disrupted nerve impulses.

While not all nerve fibres have a myelin sheath, those that do are extremely efficient at transmitting electric signals quickly and with precision. Myelinated and unmyelinated fibres together allow the nervous system to perform its wide array of functions efficiently.

Schwann cells

Schwann cells are specialized cells within the peripheral nervous system, known for their role in creating and maintaining the myelin sheath around nerve fibres. These cells wrap around the axons in a jelly-roll fashion to create the myelin sheath, enabling faster transmission of nerve impulses.

A single Schwann cell can insulate a segment of a myelinated fibre, creating a gap called a Node of Ranvier between segments. These nodes are crucial for rapid signal transmission as they allow impulses to "jump" from node to node, increasing conduction speed.

In contrast, Schwann cells also surround unmyelinated fibres, but do not form the layer of myelin. Instead, they play a supporting role by grouping together multiple axons, providing basic structural and metabolic support. This dual role showcases the versatility of Schwann cells in the nervous system, highlighting their importance beyond just myelin production.