Cytology · MU Pleven

Topic 42 Nerve fibres and their sheaths. Peripheral nerves

Covers nerve fibres as axons wrapped in glial sheaths, contrasting myelinated and unmyelinated fibres, with the myelin and neurolemmal sheaths and their Schwann cell origin. It then describes myelination timing, the layered connective tissue of peripheral nerves, and a three-group classification of peripheral fibres by conduction speed and diameter. The note closes with spinal nerve roots and how cranial nerves differ from spinal ones.

Inside: 7 sections · 1 labelled picture · 16 flashcards · 5 min

Topic 42 of 59 in the MU Pleven syllabus for Cytology (Medicine, Year 1). The free AceMed note covers it in 7 sections, about 754 words (5 min of reading), with 16 flashcards.

In this note

  1. Topic 42 Nerve fibres and their sheaths. Peripheral nerves
  2. Nerve fibre
  3. Types of nerve fibres
  4. Myelination
  5. Nerve organisation
  6. Peripheral nerves
  7. Spinal nerves and spinal nerve roots

Nerve fibre

  • Nerve fibre is a thread-like extension of a neuron and is formed by the axon and its covering
  • Nerve fibres are axons covered by sheaths of glial cells specialised to facilitate the axonal function
  • Larger axons are covered by a myelin sheath and are called myelinated fibres
  • Thinner axons do not have a myelin sheath and are called non-myelinated fibres
  • However, all the fibres, regardless of if they’re myelinated or not, have a neurolemmal sheath, which is uniformly absent in the tracts
  • In the peripheral nerves, both myelin and neurolemmal sheaths are derived from Schwann cells
  • Sheath derived from cells of ectodermal origin
  • Oligodendrocyte – CNS (in central nervous system, axons are sheathed by oligodendrocytes)
  • Schwann cell – PNS (in peripheral nerve fibres, axons are sheathed by Schwann cells)

Types of nerve fibres

  • Unmyelinated fibres
  • Both in the CNS and PNS
  • Lack nodes of Ranvier
  • Slower conduction velocity 0.5–2 m/sec
  • Smaller axons of the CNS, peripheral postganglionic autonomic fibres and olfactory nerves are non-myelinated
  • Consist of a group of small axons that have invaginated separately into a single Schwann cell
  • Myelinated fibres
  • Both in the CNS and PNS
  • Nodes of Ranvier present
  • Faster conduction velocity 4–120 m/sec
  • Form the bulk of somatic nerves

Consist of:

Axis cylinder – forms the central core of the fibre. Consists of axoplasm and axolemma

Myelin sheath derived from Schwann cells – surrounds the axis cylinder; interrupted at regular intervals called nodes of Ranvier

Neurolemmal sheath (sheath of Schwann) – surrounds the myelin sheath

Neurolemmal sheath is needed for regeneration of a damaged nerve. Tracts don’t regenerate because they lack this

Endoneurium – a delicate CT sheath which surrounds the neurolemmal sheath

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