Nociceptor’s and Chronic Pain

NO·ci·cep·tor
/ˈnōsəˌseptər/

noun

  • 1. a sensory receptor for painful stimuli:” The heat from eating chilis produces “nociceptor firing”

Who discovered nociceptors?

  • NOciceptors were discovered by Charles Scott Sherrington in 1906. In earlier centuries, scientists believed that animals were like mechanical devices that transformed the energy of sensory stimuli into motor responses.

How many axons do nociceptors have?

  • Nociceptors have two different types of axons. The first is the A fiber axons. They are myelinated and can allow an action potential to travel at a rate of about 20 meters/second toward the CNS. The other type is the more slowly conducting C fiber axons. These only conduct at speeds of around 2 meters/second.

Nociceptors often referred to as your “pain receptors,” are free nerve endings located all over the body, including the skin, muscles, joints, bones, and internal organs. They play a pivotal role in how you feel and react to pain. The main purpose of a nociceptor is to respond to damage to the body by transmitting signals to the spinal cord and brain.

Looking at this in more detail, if you stub your toe, the nociceptors on your skin are activated, causing them to send a signal to the brain, via the peripheral nerves to the spinal cord. Pain resulting from any cause is messaged in this way.

Keep in mind, that these transmitted pain signals are complex, carrying information about both the location and intensity of the painful stimuli. That way your brain can fully process the pain and eventually send communication back to block further pain signals.

Classification of Nociceptors

There are different classes of nociceptors, which are based on which type of stimuli they respond to:2

    • Thermal: Thermal nociceptors respond to extreme hot or cold temperatures. For instance, if you touch a hot stove, nociceptors signaling pain are activated right away, sometimes before you’re even aware of what you’ve done.
    • Mechanical: Mechanical nociceptors respond to intense stretch or strain, like when you pull a hamstring or strain your Achilles tendon. The muscles or tendons are stretched beyond their ability, stimulating nociceptors and sending pain signals to the brain.
    • Chemical: Chemical nociceptors respond to chemicals released from tissue damage (for example, prostaglandins and substance P) or from external chemicals (for example, topical capsaicin).
    • Silent: Silent nociceptors must be first activated or “awakened” by tissue inflammation before responding to a mechanical, thermal, or chemical stimulus. Most visceral nociceptors (those located on organs inside the body) are silent nociceptors.
    • Polymodal: Polymodal nociceptors respond to mechanical, thermal, and chemical stimuli.
  • Mechano-thermal: Mechano-thermal nociceptors respond to both mechanical and thermal stimuli.

Transmission of Pain

In addition to the type of stimuli a nociceptor responds to, nociceptors are also classified by how fast they transmit pain signals. This speed of transmission is determined by the type of nerve fiber (called an axon) a nociceptor has. There are two main types of nerve fibers.

The first type is A fiber axon, which are fibers surrounded by a fatty, protective sheath called myelin. Myelin allows nerve signals (called action potentials) to travel rapidly. The second type is C fiber axons, which are not surrounded by myelin, and thus transmit action potentials at a slower speed.2

Due to the difference in transmission speed between the A and C fibers, the pain signals from the A fibers reach the spinal cord first. As a result, after an acute injury, a person experiences pain in two phases, one from the A fibers and one from the C fibers.3

Phases of Pain Perception

When an injury occurs (such as accidentally cutting your finger with a knife), the stimulated nociceptors activate the A fibers, causing a person to experience sharp, prickling pain. This is the first phase of pain, known as fast pain because it is not especially intense but comes right after the painful stimulus.

During the second phase of pain, the C fibers are activated, causing a person to experience an intense, burning pain that persists even after the stimulus has stopped.

The fact that burning pain is carried by the C fibers explains why upon touching a hot stove, there is a short delay before feeling the burn. Aching, sore pain is also carried by the C fibers and arises from organs within the body (for example, a sore muscle or stomachache).3

Discuss in-depth the research and science of massage therapy as it relates to Nociceptor’s, chronic pain, and therapeutic massage
ChatGPT

Massage therapy has been studied in the context of nociceptors, chronic pain, and therapeutic benefits. Nociceptors are sensory receptors that respond to noxious stimuli, signaling potential or actual tissue damage. Chronic pain, a complex condition often involving altered pain processing, inflammation, and neuroplastic changes, can be influenced by massage therapy. Here’s an in-depth discussion of the research and science related to massage therapy in the context of nociceptors and chronic pain:

1. Modulation of Nociceptor Activity:

  • Mechanical Stimulation: Massage therapy involves mechanical stimulation of tissues, which can modulate nociceptor activity. The pressure and movement applied during massage may influence pain signaling pathways.

2. Effects on Central Sensitization:

  • Central Nervous System Changes: Chronic pain often involves central sensitization, where the central nervous system becomes more responsive to stimuli. Massage therapy has been suggested to contribute to the modulation of central sensitization, potentially reducing the perception of pain.

3. Release of Endogenous Opioids:

  • Analgesic Effects: Massage has been associated with the release of endogenous opioids, such as endorphins. These substances have analgesic properties and can contribute to pain relief by interacting with the body’s natural pain modulation pathways.

4. Inflammation and Immune Response:

  • Reduction in Inflammatory Markers: Some studies suggest that massage therapy may reduce pro-inflammatory cytokines and increase anti-inflammatory markers. By modulating the inflammatory response, massage can potentially alleviate pain associated with inflammation.

5. Neurotransmitter Changes:

  • Serotonin and Dopamine: Massage therapy has been linked to changes in neurotransmitter levels, including increased serotonin and dopamine. These neurochemical changes may contribute to mood improvement and pain reduction.

6. Impact on Autonomic Nervous System:

  • Parasympathetic Activation: Massage therapy is associated with the activation of the parasympathetic nervous system, promoting relaxation and reducing sympathetic (fight-or-flight) activity. This autonomic modulation may contribute to pain relief and improved well-being.

7. Specific Techniques for Chronic Pain:

  • Myofascial Release: Techniques like myofascial release aim to address fascial restrictions, potentially impacting chronic pain conditions where myofascial dysfunction is present.
  • Trigger Point Therapy: Targeting trigger points, or hyperirritable nodules in muscles, is another technique employed in massage therapy for chronic pain management.

8. Research on Chronic Pain Conditions:

  • Fibromyalgia: Studies have explored the efficacy of massage therapy, including its impact on nociception, in conditions like fibromyalgia. Results suggest that massage may lead to improvements in pain, sleep, and quality of life.
  • Low Back Pain: Massage therapy has been studied extensively for its effectiveness in managing chronic low back pain, with research indicating positive outcomes in terms of pain reduction and functional improvement.

9. Patient-Reported Outcomes:

  • Pain Perception: Numerous studies have shown that individuals receiving massage therapy report reductions in pain intensity and improved pain-related outcomes.
  • Quality of Life: Patients often report improved quality of life, reduced anxiety, and better mood following massage therapy sessions.

10. Considerations for Individualized Treatment:

  • Tailoring to Patient Needs: Effective massage therapy for chronic pain often involves tailoring treatments to individual patient needs, considering factors like pain intensity, pain location, and patient preferences.

11. Integration with Multidisciplinary Approaches:

  • Complementary Approach: Massage therapy is frequently integrated into multidisciplinary pain management approaches, collaborating with other healthcare professionals to address the complex nature of chronic pain.

12. Educating Patients:

  • Self-Care Strategies: Massage therapists often educate patients on self-care strategies, such as stretching and relaxation exercises, to empower individuals in managing chronic pain between sessions.

In conclusion, the research and science of massage therapy as it relates to nociceptors and chronic pain highlight the multifaceted mechanisms through which massage exerts its therapeutic effects. The modulation of nociceptor activity, impact on central sensitization, release of endogenous opioids, and changes in neurotransmitters collectively contribute to the analgesic and anti-inflammatory effects of massage therapy. While more research is needed to further elucidate these mechanisms and optimize treatment protocols, existing evidence supports the inclusion of massage therapy in the comprehensive management of chronic pain conditions.

 

From Wikipedia, the free encyclopedia
Nociceptor

Four types of sensory neurons and their receptor cells. Nociceptors shown as free nerve endings type A
Identifiers
MeSH D009619
Anatomical terminology

nociceptor (from Latin nocere ‘to harm or hurt’; lit. ’pain receptor’) is a sensory neuron that responds to damaging or potentially damaging stimuli by sending “possible threat” signals[1][2][3] to the spinal cord and the brain. The brain creates the sensation of pain to direct attention to the body part, so the threat can be mitigated; this process is called nociception.

History

Nociceptors were discovered by Charles Scott Sherrington in 1906. In earlier centuries, scientists believed that animals were like mechanical devices that transformed the energy of sensory stimuli into motor responses. Sherrington used many different experiments to demonstrate that different types of stimulation to an afferent nerve fiber‘s receptive field led to different responses. Some intense stimuli trigger reflex withdrawal, certain autonomic responses, and pain. The specific receptors for these intense stimuli were called nociceptors.[4]

Location

In mammals, nociceptors are found in any area of the body that can sense noxious stimuli. External nociceptors are found in tissue such as the skin (cutaneous nociceptors), the corneas, and the mucosa. Internal nociceptors are found in a variety of organs, such as the muscles, the joints, the bladder, the visceral organs, and the digestive tract. The cell bodies of these neurons are located in either the dorsal root ganglia or the trigeminal ganglia.[5] The trigeminal ganglia are specialized nerves for the face, whereas the dorsal root ganglia are associated with the rest of the body. The axons extend into the peripheral nervous system and terminate in branches to form receptive fields.

Development

Nociceptors develop from neural-crest stem cells. The neural crest is responsible for a large part of early development in vertebrates. It is specifically responsible for the development of the peripheral nervous system (PNS). The neural crest stem cells split from the neural tube as it closes, and nociceptors grow from the dorsal part of this neural crest tissue. They form late during neurogenesis. Earlier forming cells from this region can become non-pain sensing receptors, either proprioceptors or low-threshold mechanoreceptors. All neurons derived from the neural crest, including embryonic nociceptors, express the tropomyosin receptor kinase A (TrkA), which is a receptor to nerve growth factor (NGF). However, transcription factors that determine the type of nociceptor remain unclear.[6] Following sensory neurogenesis, differentiation occurs, and two types of nociceptors are formed. They are classified as either peptidergic or nonpeptidergic nociceptors, each of which expresses a distinct repertoire of ion channels and receptors. Their specializations allow the receptors to innervate different central and peripheral targets. This differentiation occurs in both perinatal and postnatal periods. The nonpeptidergic nociceptors switch off the TrkA and begin expressing RET proto-oncogene, which is a transmembrane signaling component that allows the expression of glial cell line-derived neurotrophic factor (GDNF). This transition is assisted by runt-related transcription factor 1 (RUNX1) which is vital in the development of nonpeptidergic nociceptors. On the contrary, the peptidergic nociceptors continue to use TrkA, and they express a completely different type of growth factor. There currently is a lot of research about the differences between nociceptors.[6]

Types and functions

The peripheral terminal of the mature nociceptor is where the noxious stimuli are detected and transduced into electrical energy.[7] When the electrical energy reaches a threshold value, an action potential is induced and driven towards the central nervous system (CNS). This leads to the train of events that allows for the conscious awareness of pain. The sensory specificity of nociceptors is established by the high threshold only to particular features of stimuli. Only when the high threshold has been reached by either chemical, thermal, or mechanical environments are the nociceptors triggered. The majority of nociceptors are classified by which of the environmental modalities they respond to. Some nociceptors respond to more than one of these modalities and are consequently designated polymodal. Other nociceptors respond to none of these modalities (although they may respond to stimulation under conditions of inflammation) and are referred to as sleeping or silent. Nociceptors have two different types of axons. The first are the Aδ fiber axons. They are myelinated and can allow an action potential to travel at a rate of about 20 meters/second towards the CNS. The other type is the more slowly conducting C fiber axons. These only conduct at speeds of around 2 meters/second.[8] This is due to the light or non-myelination of the axon. As a result, pain comes in two phases. The first phase is mediated by the fast-conducting Aδ fibers and the second part due to (Polymodal) C fibers. The pain associated with the Aδ fibers can be associated to an initial extremely sharp pain. The second phase is a more prolonged and slightly less intense feeling of pain as a result of the acute damage. If there is massive or prolonged input to a C fiber, there is a progressive build up in the spinal cord dorsal horn; this phenomenon is similar to tetanus in muscles but is called wind-up. If wind-up occurs there is a probability of increased sensitivity to pain.[9]

Thermal[

Thermal nociceptors are activated by noxious heat or cold at various temperatures. There are specific nociceptor transducers that are responsible for how and if the specific nerve ending responds to the thermal stimulus. The first to be discovered was TRPV1, and it has a threshold that coincides with the heat pain temperature of 43 °C. Other temperature in the warm–to–hot range is mediated by more than one TRP channel. Each of these channels expresses a particular C-terminal domain that corresponds to the warm–hot sensitivity. The interactions between all these channels and how the temperature level is determined to be above the pain threshold are unknown at this time. The cool stimuli are sensed by TRPM8 channels. Its C-terminal domain differs from the heat-sensitive TRPs. Although this channel corresponds to cool stimuli, it is still unknown whether it also contributes in the detection of intense cold. An interesting finding related to cold stimuli is that tactile sensibility and motor function deteriorate while pain perception persists.

Mechanical

Mechanical nociceptors respond to excess pressure or mechanical deformation. They also respond to incisions that break the skin’s surface. The reaction to the stimulus is processed as pain by the cortex, just like chemical and thermal responses. These mechanical nociceptors frequently have polymodal characteristics. So it is possible that some of the transducers for thermal stimuli are the same for mechanical stimuli. The same is true for chemical stimuli since TRPA1 appears to detect both mechanical and chemical changes. Some mechanical stimuli can cause the release of intermediate chemicals, such as ATP, which can be detected by P2 purinergic receptors, or nerve growth factor, which can be detected by Tropomyosin receptor kinase A (TrkA).[10]

Chemical

Chemical nociceptors have TRP channels that respond to a wide variety of spices. The one that sees the most response and is very widely tested is capsaicin. Other chemical stimulants are environmental irritants like acrolein, a World War I chemical weapon, and a component of cigarette smoke. Apart from these external stimulants, chemical nociceptors have the capacity to detect endogenous ligands, and certain fatty acid amines that arise from changes in internal tissues. Like in thermal nociceptors, TRPV1 can detect chemicals like capsaicin and spider toxins and acids.[6][10] Acid-sensing ion channels (ASIC) also detect acidity.[10]

Sleeping/silent

Although each nociceptor can have a variety of possible threshold levels, some do not respond at all to chemical, thermal, or mechanical stimuli unless injury actually has occurred. These are typically referred to as silent or sleeping nociceptors since their response comes only on the onset of inflammation to the surrounding tissue.[5]

Polymodal

Many neurons perform only a single function; therefore, neurons that perform these functions in combination are given the classification “polymodal.”[11]

Pathway

Ascending

Afferent nociceptive fibers (those that send information to, rather than from the brain) travel back to the spinal cord where they form synapses in its dorsal horn. This nociceptive fiber (located in the periphery) is a first-order neuron. The cells in the dorsal horn are divided into physiologically distinct layers called laminae. Different fiber types form synapses in different layers, and use either glutamate or substance P as the neurotransmitter. Aδ fibers form synapses in laminae I and V, C fibers connect with neurons in lamina II, and Aβ fibers connect with lamina I, III, & V.[5] After reaching the specific lamina within the spinal cord, the first-order nociceptive project to second-order neurons that cross the midline at the anterior white commissure. The second-order neurons then send their information via two pathways to the thalamus: the dorsal column medial-lemniscal system and the anterolateral system. The former is reserved more for regular non-painful sensations, while the latter is reserved for pain sensations. Upon reaching the thalamus, the information is processed in the ventral posterior nucleus and sent to the cerebral cortex in the brain via fibers in the posterior limb of the internal capsule.

Descending

As there is an ascending pathway to the brain that initiates the conscious realization of pain, there also is a descending pathway that modulates pain sensation. The brain can request the release of specific hormones or chemicals that can have analgesic effects which can reduce or inhibit pain sensation. The area of the brain that stimulates the release of these hormones is the hypothalamus.[12] This effect of descending inhibition can be shown by electrically stimulating the periaqueductal grey area of the midbrain or the periventricular nucleus. They both in turn project to other areas involved in pain regulation, such as the nucleus raphe magnus which also receives similar afferents from the nucleus reticularis paragigantocellularis (NPG). In turn, the nucleus raphe magnus projects to the substantia gelatinosa region of the dorsal horn and mediates the sensation of spinothalamic inputs. This is done first by the nucleus raphe magnus sending serotoninergic neurons to neurons in the dorsal cord, which in turn secrete enkephalin to the interneurons that carry pain perception.[13] Enkephalin functions by binding opioid receptors to cause inhibition of the post-synaptic neuron, thus inhibiting pain.[10] The periaqueductal grey also contains opioid receptors which explains one of the mechanisms by which opioids such as morphine and diacetylmorphine exhibit an analgesic effect.

Sensitivity

Nociceptor neuron sensitivity is modulated by a large variety of mediators in the extracellular space.[14] Peripheral sensitization represents a form of functional plasticity of the nociceptor. The nociceptor can change from being simply a noxious stimulus detector to a detector of non-noxious stimuli. The result is that low-intensity stimuli from regular activity initiates a painful sensation. This is commonly known as hyperalgesia. Inflammation is one common cause that results in the sensitization of nociceptors. Normally hyperalgesia ceases when inflammation goes down, however, sometimes genetic defects and/or repeated injury can result in allodynia: a completely non-noxious stimulus like light touch causes extreme pain. Allodynia can also be caused when a nociceptor is damaged in the peripheral nerves. This can result in deafferentation, which means the development of different central processes from the surviving afferent nerve. In this situation, surviving dorsal root axons of the nociceptors can make contact with the spinal cord, thus changing the normal input.[9]

Other animals

Nociception has been documented in non-mammalian animals, including fish[15] and a wide range of invertebrates, including leeches,[16] nematode worms,[17] sea slugs,[18] and larval fruit flies.[19] Although these neurons may have pathways and relationships to the central nervous system that are different from those of mammalian nociceptors, nociceptive neurons in non-mammals often fire in response to similar stimuli as mammals, such as high temperature (40 degrees C or more), low pHcapsaicin, and tissue damage.

Terminology

Due to a historical misunderstanding of pain, nociceptors are also inappropriately referred to as pain receptors. Although all pain is real, psychological factors can strongly influence subjective intensity[20] and the threshold of pain for each person.

 

 

 

 

 

 

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