Cytology · MU Sofia

66. Nerve fibers - types. Myelin sheath. Peripheral nerves.

Covers what a nerve fibre is, then sorts fibres into myelinated and unmyelinated types, with Group A, B and C classes and their conduction speeds and sensory roles. Next it describes how myelin forms in the peripheral and central nervous systems, its composition and function, and structural features such as nodes of Ranvier, Schmidt-Lanterman clefts and major dense lines. It ends with peripheral nerves, their endoneurium, perineurium and epineurium layers, and the functional classification into sensory, motor, autonomic and mixed nerves.

Inside: 5 sections · 4 labelled pictures · 16 flashcards · 5 min

Topic 66 of 127 in the MU Sofia syllabus for Cytology (Dental Medicine, Year 1). The free AceMed note covers it in 5 sections, about 756 words (5 min of reading), with 16 flashcards.

In this note

  1. Nerve Fibers - Types. Myelin Sheath. Peripheral Nerves.
  2. I. Nerve Fibers - General Description
  3. II. Types of Nerve Fibers (Based on Myelination)
  4. III. Myelin Sheath
  5. IV. Peripheral Nerves

I. Nerve Fibers - General Description

  • A nerve fiber is a collective term for an axon of a nerve cell, along with its myelin sheath, all bundled together.
  • In the Peripheral Nervous System (PNS), a nerve fiber also includes Schwann cells and the endoneurium (connective tissue around the myelin sheath of each myelinated nerve fiber).
  • Nerve fibers are found in both the Central Nervous System (CNS) and the PNS.
  • Their primary role is to facilitate axonal function and transmit nerve impulses.

II. Types of Nerve Fibers (Based on Myelination)

Nerve fibers are primarily classified by the presence or absence of a myelin sheath.

  1. 1. Myelinated Nerve Fibers
  • Characteristically found around thicker, large-diameter axons.
  • Designed for fast nerve conduction.
  • Impulse conduction is saltatory, meaning the nerve impulse "jumps" from one exposed axon region (Node of Ranvier) to the next, significantly increasing speed.
  • In the PNS, one Schwann cell typically myelinates a portion of a single axon.
  • In the CNS, oligodendrocytes myelinate parts of several axons.
  • Group A fibers in the PNS are large diameter, myelinated, and have the highest conduction velocity, responsible for both sense and movement.
  • Group B fibers are smaller diameter, myelinated, part of the autonomic nervous system, with relatively low conductivity.
  1. 2. Unmyelinated Nerve Fibers
  • Typically associated with smaller-diameter axons.
  • Conduction is much slower than in myelinated fibers as action potentials must be generated along the entire length of the axolemma.
  • Nodes of Ranvier are not seen along unmyelinated fibers.
  • In the PNS, a single Schwann cell can envelop portions of many small-diameter axons within simple folds of its plasma membrane, without forming the multiple spiral wrappings of a myelin sheath.
  • Group C fibers are unmyelinated, small diameter, and have low conduction velocity. They are specifically responsible for transmitting pain, temperature, touch, pressure, and itch sensations.

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