Types of nerve fibers, their sheaths and endings

Introduction

Nerve fibres are basically axons together with their supporting cells and connective tissue layers. They differ in diameter, in amount of myelin, and in the speed at which they conduct impulses. Understanding these differences makes it easier to recognise nerves in histology slides and to link their structure with their function in the body.

Nerves also have several protective sheaths and a number of specialised endings, which allow them to sense pressure, pain, touch or to control muscles. Below is a simple overview.

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Types of Nerve Fibres

A Fibres

A fibres are the fastest and thickest fibres in the body. They are heavily myelinated and conduct impulses quickly thanks to saltatory conduction. There are four subgroups:

• Aα fibres: largest and fastest; go to skeletal muscle (motor) and also carry information from muscle spindles and Golgi tendon organs.
• Aβ fibres: used for touch, vibration and pressure.
• Aγ fibres: supply intrafusal fibres inside muscle spindles, helping regulate muscle tone.
• Aδ fibres: carry fast, sharp pain and cold sensation.

These fibres appear as large, strongly myelinated axons in cross-sections.

B Fibres

B fibres are thinly myelinated, medium-speed fibres. They mainly belong to the autonomic nervous system, especially preganglionic fibres. They are smaller than A fibres and somewhat slower because the myelin sheath is thinner.

C Fibres

C fibres are unmyelinated and the slowest fibres. They carry dull pain, warm sensations, itch and many autonomic signals. Since they lack myelin, several C fibres can sit inside the same Schwann cell, forming what Junqueira calls Remak bundles. Under the microscope they look like very small axons without myelin rings.

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Sheaths of Nerve Fibres

Nerves are wrapped in three layers, like a cable with multiple coverings.

Endoneurium

This is the innermost layer, surrounding individual axons (both myelinated and unmyelinated). It contains reticular fibres (type III collagen) and small capillaries. In histology you won’t see it easily but it forms a delicate support net around each fibre.

Perineurium

The perineurium wraps groups of axons into fascicles. It is made of several layers of specialised perineurial cells that have tight junctions, forming the important blood–nerve barrier.

It looks like a clear, more solid border around each fascicle and gives nerves their quite tough structure.

Epineurium

The outermost layer, made of dense irregular connective tissue. It bundles all fascicles together into a whole nerve. It also contains blood vessels that supply the nerve. In gross anatomy it is the thickest layer and in histology it appears as a thickerh connective tissue ring.

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Types of Nerve Endings

Free Nerve Endings

These have no special covering, just the bare terminal of the axon. They are found even in the epidermis, which is something Junqueira stresses a lot.

Functions: pain (Aδ and C fibres), temperature, and crude touch.

They look like unmyelinated fibres branching toward the surface.

Encapsulated Nerve Endings

These endings have a capsule that modifies how the mechanical stimulus is received.

• Meissner corpuscles: located in dermal papillae, especially in fingertips and lips. They detect light touch and low-frequency vibration. They have a stack-like appearance.
• Pacinian corpuscles: deep in dermis or hypodermis, very large and onion-like. They sense deep pressure and high-frequency vibration.
• Ruffini endings: respond to skin stretch. They are elongated and less sharply defined.
• Merkel discs: consist of specialised epidermal cells with axon terminals. They detect steady pressure, shape and texture.

Sensory Endings in Muscle

Important in movement and proprioception.

• Muscle spindles: encapsulated structures lying among skeletal muscle fibres. They detect stretch and contain both sensory and motor supply (Aα and Aγ fibres).
• Golgi tendon organs: found at muscle–tendon junction. They detect tension and help protect the muscle from excessive force.

Citation

Morphologia, E. O. O. J. (2019). Junqueira’s Basic Histology : Text & Atlas (15th ed.), 2018. Morphologia, 13(2), 101–104. https://doi.org/10.26641/1997-9665.2019.3.101-104

slides from Ph.D. Lucia Fraser Lantová