There is a special type of pain nerve fibre found around the human body, with the vestibule in the vulva containing many – the C fibre. These fibres may be linked with the specific type of pain associated with vulvodynia, particularly neuroproliferative vulvodynia.
Vulvodynia, chronic vulvar or vestibular pain, may be caused due to an overactivity, oversensitivity, or proliferation of C fibre nerves.
C fibres, which are unmyelinated (uncoated, like a bare wire) conduct energy from between 0.5 and 2.0 metres per second. This is in contrast to myelinated (coated, like a power lead that has a rubber or plastic coating) A-delta pain fibres, which conduct between 5 to 30 metres per second.
To put this into context, our very sensitive touch nerves conduct between 35 and 120 metres per second. The faster it can conduct per second, the faster a nerve fibre is.
C fibres respond to broad pain stimulus, like heat or cold (thermal), getting knocked or pinched (mechanical), or metabolic. C fibres cause a slow, long-lasting, burning type of pain, which is precisely the type of pain felt in vulvodynia.
The way these nerves work is that they take a while to build up their pain release, with the excitation building with prolonged stimulus. The A-delta fibre, by comparison, responds to stimuli quickly – sharp, bright pain. These nerve fibres are respectively slow and fast to respond to what is happening, for very good reasons.
This is why we have different types of nerve fibres in different places – where we need to feel a burn quickly, our A-deltas act, and where we just need to back off what we’re doing, the C fibres kick into gear.
C fibres and inflammation
When a C fibre produces pain, it also causes the production of substance P in local tissues. This dilates blood vessels and releases histamine, fulfilling the process of inflammation with more pain and swelling, with the local reaction to deal with whatever is happening.
Why capsaicin may be suggested for vulvodynia symptoms
We have receptors for capsaicin, found in hot peppers, in C fibres, which is why capsaicin cream has been studied (largely unsuccessfully) for vulvodynia. Capsaicin causes a severe burning sensation.