Biology · Neural Control and Coordination · NEET
| Myelin sheath | Present; Schwann cells form it around the axon | Absent; Schwann cell encloses axon but makes no sheath |
| Nodes of Ranvier | Present (gaps between adjacent sheaths) | Absent |
| Speed of impulse | Fast (saltatory, jumps node to node) | Slow (continuous along membrane) |
| Where found (NCERT) | Spinal and cranial nerves | Autonomic (autonomous) and somatic neural systems |
Yes, they do. This is the most confusing point. NCERT says the unmyelinated nerve fibre is still enclosed by a Schwann cell, but that Schwann cell does NOT form a myelin sheath around the axon. So both types have Schwann cells; only the myelinated type has the Schwann cells wound into a myelin sheath. Do not write 'no Schwann cells' for unmyelinated fibres in the exam.
The myelinated axon is faster. The myelin sheath is a fatty insulating layer, so the impulse jumps from one node of Ranvier to the next (saltatory conduction) instead of moving smoothly along the whole membrane. The non-myelinated axon has no myelin and no nodes, so the impulse must travel along the entire membrane, which is slower.
No. Nodes of Ranvier are the gaps between two adjacent myelin sheaths. Since a non-myelinated axon has no myelin sheath, it has no nodes of Ranvier. Nodes and saltatory conduction belong only to myelinated fibres.
NCERT is specific: myelinated nerve fibres are found in the spinal and cranial nerves. Unmyelinated nerve fibres are commonly found in the autonomous (autonomic) and the somatic neural systems. NEET has directly tested these locations, so memorise them word for word.
No. The Schwann cell is the living glial cell; the myelin sheath is the fatty multi-layered covering that the Schwann cell wraps around the axon in myelinated fibres. One Schwann cell makes myelin for one segment of one axon in the PNS; oligodendrocytes do this job in the CNS.
Myelin sheath is produced by
Try the real previous-year questions from this chapter — each with the answer and a full solution.
In a myelinated axon the Schwann cells form a myelin sheath around the axon with gaps called nodes of Ranvier, giving fast (saltatory) conduction. In a non-myelinated axon a Schwann cell encloses the axon but forms no myelin sheath, so there are no nodes and conduction is slower.
Yes. NCERT states that myelinated nerve fibres are found in the spinal and cranial nerves. This is a commonly tested location fact for NEET.
Saltatory conduction means the impulse jumps between nodes of Ranvier. Only myelinated fibres have a myelin sheath and therefore nodes; non-myelinated fibres have neither, so the impulse moves continuously and slowly.
Yes, they conduct impulses, just more slowly than myelinated fibres. They are common in the autonomic and somatic neural systems where high speed is not always required.
