What are the Small Fibres?
Small fibres are small narrow diameter myelinated (Aδ) and unmyelinated (C) nerve fibres of the peripheral nervous system (figure 2). Somatosensory Aδ-fibres and C-fibres innervating skin pass through the dermis where they innervate cutaneous structures; both groups of fibres end as free nerve endings in the epidermis (the Aδ-fibres lose their myelin sheath as they cross the dermo-epidermal junction). Aδ-fibres are responsible for conveying cold input and nociceptive input. C-fibres convey innocuous warm sensations and possibly innocuous cold sensations, and noxious input from a variety of high threshold mechanical, thermal and chemical stimuli. Small fibres play an important role in the autonomic nervous system, because thin myelinated fibres contribute to preganglionic fibres and C-fibres contribute to postganglionic fibres, innervating structures such as sweat glands, blood vessels and the heart. Nerve growth factor (discovered in a series of groundbreaking experiments by the Nobel laureate Rita Levi-Montalcini) is a key target-derived factor for these neurones. Mutations in nerve growth factor or its high-affinity receptor NTRK1 result in degeneration of nociceptors and sympathetic neurones; this leads to hereditary sensory and autonomic neuropathy type 5 and 4 respectively. The severe disability caused by painless injuries illustrates the vital importance of the nociceptive system.
(A) Confocal image of a skin biopsy taken from the finger of a healthy subject illustrating different subtypes of sensory fibre: PGP 9.5 is used as an axonal marker (red) and myelin basic protein as a marker for myelin (green). There are numerous free nerve endings entering the epidermis (white arrows) and a single myelinated fibre is innervating a Meissner's corpuscle (yellow arrow). (B) Electron micrograph of the sural nerve. The blue arrow shows a large diameter myelinated fibre, the purple arrow marks a small diameter myelinated fibre and the red arrow marks a Remak bundle in which one non-myelinating Schwann cell associates with multiple unmyelinated C-fibres which are ensheathed in Schwann cell pockets.