Biology · Neural Control and Coordination · NEET
Both, but in different places. In the peripheral nervous system (spinal and cranial nerves) Schwann cells wrap around the axon and form the myelin sheath. In the central nervous system (brain and spinal cord) the myelin is made by oligodendrocytes. NCERT names Schwann cells directly, so for NEET remember: Schwann cells = PNS myelin, oligodendrocytes = CNS myelin.
The gap between two adjacent myelin sheaths is called the node of Ranvier. At these nodes the axon membrane is exposed (not covered by myelin). These nodes are the reason the nerve impulse can jump from node to node, which is the basis of fast saltatory conduction.
Only the axon is covered by the myelin sheath, not the whole neuron. The cell body (cyton) and dendrites are not myelinated. This is why myelin questions always talk about 'nerve fibres' or 'axons', not the cell body.
The myelin sheath itself is a fatty insulating layer, but it is formed by living Schwann cells (or oligodendrocytes in the CNS). The Schwann cell is a living neuroglial cell that wraps its membrane around the axon many times, and this wrapped membrane forms the myelin. So the maker is living; the sheath is its lipid-rich product.
No. There are two types of axons: myelinated and non-myelinated. Myelinated fibres (found in spinal and cranial nerves) have a myelin sheath with nodes of Ranvier. Non-myelinated fibres are still enclosed by a Schwann cell, but that Schwann cell does NOT form a myelin sheath; these are common in the autonomic and somatic neural systems.
Myelin sheath is produced by
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The myelin sheath is a fatty layer that insulates the axon. It stops the impulse from leaking out along the whole axon and lets the impulse jump from one node of Ranvier to the next, which greatly speeds up conduction.
Nodes of Ranvier are the small gaps between two adjacent myelin sheaths along an axon, where the axon membrane is exposed. The nerve impulse jumps from node to node, giving fast saltatory conduction.
According to NCERT, myelinated nerve fibres are found in the spinal and cranial nerves. Non-myelinated fibres are common in the autonomic and somatic neural systems.
Yes. A non-myelinated fibre is still enclosed by a Schwann cell, but that Schwann cell does not form a myelin sheath around the axon. So Schwann cell present, myelin absent.
NEET repeatedly asks who makes myelin (Schwann cells / oligodendrocytes), what the gaps are called (nodes of Ranvier), and why myelinated fibres are faster (saltatory conduction). These are direct, high-yield one-mark facts straight from NCERT.
