Classification of nerve fibers

Factors determining conduction velocity

The ability of a nerve to rapidly transmit excitation depends on two key structural features. First, it is the fiber diameter: the thicker the fiber, the faster the signal propagates along it. Second, the presence of a myelin sheath is critically important. Myelinated fibers conduct impulses significantly faster than unmyelinated ones.

It is interesting to observe the behavior of the compound action potential (AP) of an entire nerve. If we record the AP close to the stimulation site, it will have a simple sinusoidal shape. This is because excitation starts synchronously across all structures. However, as the signal propagates further away from the stimulation site, the shape of the action potential becomes complex and multi-peaked. The reason for this change is that the nerve comprises fiber groups with different conduction velocities. Signals travel along them at different speeds, causing the impulses to become "stretched" in time and disrupting the initial synchrony.

Group A: the fastest and thickest (myelinated)

According to the Erlanger-Gasser classification, group A includes exclusively myelinated fibers. They are subdivided into four subcategories, each performing a specific function:

• Aα (alpha): Absolute record-holders in velocity (about 100 m/s) and thickness (diameter about 10 µm). Functionally, these are efferent somatic pathways running from the CNS to skeletal muscles, providing motor responses. They also form afferent pathways from muscle proprioceptors.
• Aβ (beta): Perform an afferent function. Their main task is to collect information from touch receptors located in the skin and transmit these signals to the central nervous system.
• Aγ (gamma): Efferent structures that extend to intrafusal fibers (the so-called muscle spindles). They play a crucial role in the continuous regulation of muscle tone.
• Aδ (delta): Predominantly afferent somatic fibers. These are the fibers that transmit signals from cutaneous thermoreceptors and nociceptors. In clinical practice, they are notable for providing the primary epicritic pain component—the fast, sharp, and very clearly localized pain felt in the first second after an injury.

Groups B and C: autonomic and slow

These fiber groups are closely related to the function of the autonomic nervous system (ANS) and are characterized by lower conduction velocities.

Group B is represented by thin yet myelinated fibers. In the human body, they function as preganglionic fibers of the autonomic nervous system, delivering impulses to autonomic ganglia.

Group C consists of the thinnest (about 1 µm) and slowest fibers (conduction velocity of only about 1 m/s). Their main distinguishing feature is that they are unmyelinated. In the autonomic nervous system, they function as postganglionic fibers. Additionally, type C fibers act as afferent pathways transmitting information from internal organs to the CNS. The clinical significance of this group is immense: they form the secondary protopathic pain component. This is the slow, dull, diffuse, exhausting, and heavily emotionally charged pain that replaces the sharp pain.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanism of saltatory conduction of excitation
• Structure of the myelin sheath and nodes of Ranvier
• Neurophysiology of proprioception (muscle spindles)
• Differences between the somatic and autonomic nervous systems
• Theories of pain perception (nociception)

Other topics in the section "Excitation Conduction and Synaptic Transmission"

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