Neuroanatomy, Unmyelinated Nerve Fibers

Unmyelination vs. Myelination Axons could be myelinated (high, moderate, or thin) or unmyelinated fibers. Unmyelinated, also called type C, fibers include both nonpeptidergic (for mechanical sensitivity) and peptidergic (for heat/cold sensitivity) C-fiber axons. They lack the myelin envelope completely, with Schwann cells surrounding them forming the Remak fibers in bundles within peripheral nerves. One to four axons per Schwann cell unit-3 occurs in men. Because of this absence of myelin, there is no insulation causing impulse conduction spreading from one axon to another. However, this absence also provides the fibers with resistance to metabolic insults.Junction adhesion molecule 2 (JAM2) expression promotes local myelination inhibition. These contribute to the galectin-4 (gal-4), a galectin specifically sorted to axon membrane segments in a sulfatide-dependent process, forming the long, unmyelinated, discontinued segments along the axons, including the somatodendritic membrane. These are believed to be crucial for axonal plasticity and complex, higher-order processing in the brain. Location Unmyelinated fibers, being widely distributed, are found in both hairy and glabrous skin. Mechanoafferent C tactile fibers are found in hairy skin, associated with hair follicles. These are also present in the glabrous skin of the glans penis and glans clitoris.C fibers have greater innervation density compared to Type A-delta fibers. Torebjork and Hallin introduced the concept of the nociceptive field of microneurography in 1970. This process produced a marking technique to discriminate single unit impulses by unmyelinated nerve fibers, including putative itch afferents and sympathetic efferents, based on post-activation transmission delay of approximately several seconds vs. preceding impulses.