The McGill Physiology Virtual Lab

Compound Action Potential

Background > Nerve Anatomy

A typical nerve in a vertebrate like the frog consists of several thousand axons. For example, the vagus nerve in man consists of over 100,000 fibers.
The cell bodies of these axons are located either in the central nervous system (for motor fibres), or in the peripheral nervous system (e.g. dorsal root ganglia, for sensory fibres).
 The fibres may be large (15-25 Ám) and myelinated, or small (around 0.2 Ám) and unmyelinated. Regardless of the type or size of the fibres, they all have certain properties in common; e.g., they all conduct non-decremental action potentials.
 However, there are also certain differences, e.g. the conduction velocity of the fibres differs, depending on their size (diameter) and whether they are myelinated or non-myelinated.

Cross-section through an unmyelinated nerve.


Cross-section through a myelinated nerve (e.g. sciatic) showing individual nerve bundles each consisting of many fibres.

In general, small (less than 25mm), myelinated and unmyelinated fibres are a feature of vertebrates, while large (up to 500 mm or larger), unmyelinated fibres are a feature of invertebrates (e.g. the squid)
In vertebrates (including man), the whole nerve is encased in a connective tissue sheath called the Epineurium. The axons are further subgrouped into bundles called fasciculi (singular: fasciculus), each of which is also encased in a connective tissue sheath called the Perineurium. Lastly, each individual fibre is further surrounded by a delicate connective tissue sheath called the Endoneurium.


A higher magnification of several nerve fibre bundles showing individual axons as distinct circular structures.

 

The frog's sciatic consists of only a single bundle of fibres, surrounded by the perineum and loose epineurium

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