What am I talking about? Nociceptors, which are the nerves that send pain signals from a distal site to the brain, and the stuff that sets them off (noxious stimuli). First, let's look at what nociceptive pain is:
"Nociceptive pain is caused when special nerve endings—called nociceptors—are irritated. Nociceptive pain is the type of pain you feel when you burn yourself, twist your ankle, or stub your toe. It is a dull or sharp aching pain, and it can be mild to severe. This type of pain can usually be controlled. Nociceptive pain can be a temporary condition, such as when you have a sprained ankle, but it can also be a chronic condition. Cancer pain and arthritis pain are common types of chronic nociceptive pain. Nociceptive pain usually responds well to pain medications, anti-inflammatory agents, or other drug therapies. It usually does not respond well to neurostimulation." http://www.poweroveryourpain.com/understand/chronic/paintypes
"Tissue damage or injury initiates signals that are transferred through peripheral nerves to the brain via the spinal cord. Pain signals are modulated throughout the pathways. This is how we become aware that something is hurting." http://www.medicinenet.com/pain_management/page2.htm
"Acute or nociceptive pain is part of a rapid warning relay instructing the motor neurons of the central nervous system to minimize detected physical harm. It is mediated by nocicepters, on A-δ and C fibers. These nociceptors are free nerve endings that terminate just below the skin, in tendons, joints, and in body organs. They serve to detect cutaneous pain, somatic pain and visceral pain. Nociception can be associated with nerve damage caused by trauma, diseases such as diabetes, shingles, irritable bowel syndrome, late-stage cancer or the toxic effects of chemotherapy. It typically responds well to treatment with opioids and NSAIDs
. One of the challenges for researchers and clinicians alike is that chronic pain may involve a mix of both inflammatory and neuropathic components. In inflammatory nocicpetive pain, inflammation may cause damage to the neurons and produce neuropathic pain. Likewise, neuronal injury may cause an inflammatory reaction (neurogenic inflammation) that contributes to inflammatory pain."
http://www.mdbiosciences.com/Portals/42723/docs/Nociception-Neuropathic-Inflammatory%20Pain.pdf
[For your own FYI, when you're trying to distinguish the type of pain:
"Neuropathic pain is caused by a malfunction of the nervous system due to an injury or an illness. Neuropathic pain can be a sharp, intense, shocking, or shooting pain. It is also very stubborn in that it does not usually respond as well as nociceptive pain to standard pain therapies, such as over-the-counter pain medications (for example, aspirin or ibuprofen) and prescription pain medications. Unlike nociceptive pain, neuropathic pain can often be managed by neurostimulation."
http://www.poweroveryourpain.com/understand/chronic/paintypes ]
"Noxious stimuli and responses
There are three categories of noxious stimuli:
- mechanical (pressure, swelling, abscess, incision, tumour growth);
- thermal (burn, scald);
- chemical (excitatory neurotransmitter, toxic substance, ischaemia, infection).
The cause of stimulation may be internal, such as pressure exerted by a tumour or external, for example, a burn. This noxious stimulation causes a release of chemical mediators from the damaged cells including:
- prostaglandin;
- bradykinin;
- serotonin;
- substance P;
- potassium;
- histamine.
These chemical mediators activate and/or sensitise the nociceptors to the noxious stimuli."
http://www.nursingtimes.net/nursing-practice/clinical-zones/pain-management/anatomy-and-physiology-of-pain/1860931.article
"
Endometriotic implants cause a local inflammatory reaction which irritates nerve endings and sends noxious stimuli along the nerve pathways to the spinal cord and into the central nervous system (CNS) where they are interpreted as burning, dull, achy sensations or as sharp, stabbing, or crampy pains. The local inflammatory reaction is mediated by the increased production of substances, such as a variety of cytokines and prostaglandins, originating from the endometriotic implants and cells of the immune system. These substances also stimulate development of scarring and nodules around the endometriotic implants which may compress peripheral nerves compounding pain symptoms with signs of peripheral neuropathy. Pain symptoms are usually elicited when the nodules are compressed during pelvic examination or sexual intercourse. Endometriotic (chocolate) cysts may compress on other pelvic organs, causing pain and pressure during urination or bowel movements. If there is bleeding from the endometriotic lesions, as it frequently happens during the menstrual period, a woman may notice blood in the urine or stools or in secretions from other organs affected by endometriosis, e.g. blood in the sputum with endometriosis of the lungs.
Because of increased systemic cytokine and prostaglandin production by the circulating immune cells, some women with endometriosis may experience generalized symptoms such as low-grade fever; crampy, generalized aches and pains; and nausea, vomiting, and diarrhea usually around the time of the menstrual period."
http://www.endometriosisinstitute.com/endometriosis/management-of-pelvic-pain
The main event of this feature:
Primary Afferent
Nociceptors and Visceral Pain
Victor Chaban
Charles R. Drew University of
Medicine and Science and University of California, Los
Angeles, USA
“Several
lines of evidence indicated that there is a close relationship between nerve
fiber density and endometriosis-associated pain. There is a significant
increase in nerve fiber density in women with endometriosis who reported pelvic
pain, suggesting these nerve fibers may play an important role in the
mechanisms of pain generation.
One such
mechanism may be the convergence of nociceptive stimuli and estrogen input on
the primary afferent neurons which innervate viscera. Based on our results, it
is likely that estrogen receptors (ERs) expressed in primary afferent neurons
modulate nociceptive signaling. Our recent data suggest that estrogen acting on
primary afferent nociceptors modulates the response to proand anti-nociceptive
signals associated with the clinical presentation of functional disorders such
as endometriosis.
The mechanism
of endometriosis-induced nociceptive signaling is poorly understood and in some
cases pain can be exacerbated by co-morbidity with other chronic pelvic pain syndromes
such as irritable bowel syndrome, painful bladder syndrome, vulvodynia and fibromyalgia.
It has also been shown that ectopic implants develop sensory nerve supply both
in women and in animal models of endometriosis. Sensory input arriving from the
visceral organ to the spinal cord divergences at the level of primary sensory
neurons which further transmit considerable information from periphery to the
central nervous system.
Several
researchers have investigated the presence of nerve fibers in endometriotic
lesions in both human and animal study. Using different types of specific
immunohistochemical neuronal markers such as substance P (SP) and calcitonin
gene related peptide (CGRP) sensory nerve fibers markers) in human peritoneal
endometriotic lesions from women with visually and biopsy proven endometriosis,
investigators have demonstrated multiple, small unmyelinated nerve fibers are
present in peritoneal endometriotic lesions, and these peritoneal endometriotic
lesions contain both Aδ and C nerve fibers. Accumulating evidence has shown
these nerve fibers may play a critical role in pain production in patients with
endometriosis, and a close histological relationship has been identified
between these nerve fibers and endometriosis associated pain. Tulandi et al. (2001)
reported that the distance between endometriotic glands and nerve fibers in
endometriotic lesions from women with pain was closer than in women with no
pain.
Endometriosis
is an inflammatory disease, which is known to contain proinflammatory cytokines,
prostaglandins, and other neuroactive agents that could readily activate the
CGRP- and SP-positive C-fiber nociceptive afferents found in the endometriotic
lesions. When these sensory nerve fibers are stimulated by inflammatory
substances, neurotransmitters such as SP, CGRP can be secreted from sensory
nerve endings. SP and CGRP can contribute to the inflammatory response by
causing vasodilation, plasma extravasation and cellular infiltration by
interacting with endothelial cells, arterioles, mast cells, neutrophils and
immune cells. SP can also act on mast cells in the vicinity of sensory nerve
endings to evoke de-granulation and the release of TNF-, histamine, prostaglandin D2 (PGD2)
and leukotriene, providing a positive feedback. CGRP has a wide range of biological
activities, including sensory transmission, regulation of glandular secretion,
and inhibiting SP degradation by a specific endopeptidase to enhance SP
release, thereby amplifying the effects.
Based on our
preliminary results, it is likely that estrogen receptors (ERs) expressed in
primary afferent neurons modulate chemical signaling associated with
nociception. Nociception is a balance of pro- and anti-nociceptive inputs that
is subject to regulation depending on the normal state of the organism.
Sensitization of primary afferent neurons to stimulation may play a role in the
enhanced perception of visceral sensation and pain. Chest pain from coronary
heart disease, endometriosis, acute and recurrent/chronic pelvic pain in women
or abdominal are all visceral pain sensations that may result in part from
sensitization (Berkley et al. 2001; Mayer et al. 2001). Mechanisms of
peripheral sensitization may involve increased transduction that is secondary
to repeated stimulation or an increase in the excitability of the afferent
nerves by molecules that decrease the excitation threshold (Zimmermann 2001).
These
findings suggest that E2 may modulate sensory input at the primary afferent
level. E2 can alter gene transcription, resulting in pro-nociceptive (reducing -endorphin expression) or
anti-nociceptive (increasing enkephalin expression) changes of endogenous
opioid peptides , opioid receptors (Micevych and Sinchak 2001) and, by
increasing levels of CCK, an anti-nociceptive and anti-opioid molecule
(Micevych et al. 2002).
Our data
support the idea that E2 modulates nociceptive responses in pelvic pain syndromes
such as endometriosis, however, whether E2 is pro- or anti-nociceptive remains
unresolved….Our data clearly showed the new role of nociceptors in
pathophysiological aspects of chronic pelvic pain and potential way of designing
future therapies.”
"Neurotrophins (NTs), a family of neuronal growth factors, are overexpressed in endometriosis and encompass NGF, BDNF and NT-3 and NT-4/5. NT receptors, TrkA and p75NTR, and NT receptor-interacting proteins, MAGE and NDN, were also expressed. NTs and their receptors play a role in the development and maintenance of neural tissues in non-neuronal cell types such as endometriosis.
Nerve fibers contain unmyelinated sensory C, myelinated sensory Adelta and adrenergic nerve fibers that innervate abnormal cell growths. An increased release of proinflammatory cytokines from endometriotic lesions is responsible for the excessive sensory innervation and development of chronic pelvic pain. The preponderance of the inflammatory milieu and subsequent hyperinnervation might be involved in the pathophysiology of pain generation in women with endometriosis."
http://link.springer.com/article/10.1007/s00404-013-3049-8
"Calcium-binding proteins seem to be increased in endometriosis-associated nerve fibres and might play an important role in the chronic inflammatory condition and the pain pathogenesis of endometriosis."
http://onlinelibrary.wiley.com/doi/10.1002/j.1532-2149.2013.00323.x/abstract;jsessionid=A4A598F96BBAA715C3121D4E3C8056D0.f02t02?deniedAccessCustomisedMessage=&userIsAuthenticated=false
"While increased levels of leptin have been reported in patients with endometriosis, their contribution to endometriosis pain has not been explored. Using a rodent model of endometriosis we provide evidence for an estrogen-dependent contribution of leptin in endometriosis-induced pain....This sensitivity to leptin is dependent on estrogen levels."
http://www.sciencedirect.com/science/article/pii/S0306452213009482
"The exact aetiology of endometriosis is still not clear although a role for inflammation is increasingly accepted. We therefore investigated the inflammatory activity of eutopic tissue and that of the matching ectopic lesions from different locations by measuring the genetic expression of inflammatory chemokines and cytokines. The gene expression in matching eutopic and ectopic tissue was compared, as was the gene expression in lesions from different locations. A significantly higher mRNA expression of the chemokines ENA-78 and RANTES and the cytokines IL-6 and TNF
α was observed in endometriotic lesions of the rectovaginal septum (RVS) compared to that of matching eutopic tissue. Comparisons across lesion locations showed a significantly higher expression of IL-6 and TNF
α in the RVS compared to lesions from either the ovaries or the peritoneum. These results show that the production of some inflammatory chemokines and cytokines is significantly increased in the ectopic endometrial tissue compared to matching eutopic tissue. Furthermore, IL-6 and TNF
α are produced in significantly higher quantities in RVS lesions compared to other lesions."
http://www.hindawi.com/journals/mi/2013/450950/abs/
BONUS FEATURE:
We have talked before about the research into the connection between migraines and endo (
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3165129/ ,
http://www.ncbi.nlm.nih.gov/pubmed/22442736 ,
http://humrep.oxfordjournals.org/content/19/12/2927.full). Below is an article about the latest investigations into the cause of migraine pain and symptoms. Notice any similarities about the neurons and neuro chemicals?
"Based on research, the best understanding we
now have is that migraine arises from abnormally excitable neurons in the brain
and trigeminal nerve. What causes the neurons to be abnormally excitable?
Various things can do this, including low magnesium, abnormal calcium channels
on the surface of the neuron, mitochondrial abnormalities, or other inherited
brain chemical abnormalities. The newest things in the migraine story are the
glia—the support cells in the brain—which also appear to have a role in
transmitting pain, perhaps more so in chronic headache, although their story is
still being determined....While there is still some controversy over the "vascular" part of migraine, the
situation was recently summed up by Dr. Andrew Charles, UCLA migraine
researcher. Dr. Charles indicated that while it is clear that vascular changes
occur in migraine, it does not mean migraine is triggered by vascular processes,
and that the dilation of blood vessels is neither necessary nor sufficient for
causing migraine pain.
"According to existing trigeminovascular
theory, once the messages come from the activated cells in the trigeminal
nucleus in the brainstem, and travel to the trigeminal nerves that go to the
dural blood vessels on the brain's surface, it causes dilation. However, the
trigeminal activation also causes the release of brain chemicals called
neuropeptides (substance P, CGRP or calcitonin gene-related peptide, neurokinin
A, 5HT or serotonin, and noradrenalin).
"The release of these chemicals causes
inflammation, and what is called peripheral sensitization. This is most likely
what results in the throbbing pain most people experience. As the attack
progresses, something can occur called central sensitization.
When this occurs, it causes what is known as cutaneous allodynia. This means
that things that are usually just a normal touch are now felt as painful. Many
headache patients with allodynia cannot continue to wear
earrings, necklaces or neckties, or their glasses. Some find that they cannot
lie down on the side of the head pain, or report that "even their hair hurts."
Up to 80% of migraine sufferers are affected by some degree of cutaneous
allodynia, and it generally occurs in the late stages of a migraine attack when
the pain is severe. This is why it is important to treat early when the pain is
mild or moderate.
"When central sensitization becomes advanced,
it can involve areas beyond the head, and simple touch on the arms or shoulder
can be perceived as painful. For example, I am aware of one migraine sufferer
who is bothered by the seams in her clothing during such an attack. At this
stage of the migraine, migraine-specific medication is less likely to be
helpful, and studies have shown that while they will reduce the pain and relieve
the throbbing, they cannot abort the attack, and allodynic pain remains as well
as other migraine symptoms. In late-stage migraine, other medications may
be necessary in order to end the attack." http://www.migrainesurvival.com/understand-migraine-pathophysiology-allodynia