The Science of Nociceptors: How the Body Reacts to Pain

Can understanding how nociceptors function and their role in processing pain signals help individuals who are managing injuries and/or living with chronic pain conditions?

Nociceptors

Nociceptors are nerve endings that detect harmful stimuli, such as extreme temperatures, pressure, and chemicals, and signal pain. They are the body’s first defense against potentially damaging environmental inputs.

• Nociceptors are in the skin, muscles, joints, bones, internal organs, deep tissues, and cornea.
• They detect harmful stimuli and convert them into electrical signals.
• These signals are sent to the brain’s higher centers.
• The brain interprets the signals as pain, which prompts the body to avoid the harmful stimulus.

Nociceptors, often called pain receptors, are free nerve endings all over the body. They play a pivotal role in how the body feels and reacts 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. (Purves D, Augustine GJ, Fitzpatrick D, et al., editors. 2001) If you bang your foot, the nociceptors on the skin are activated, sending a signal to the brain via the peripheral nerves to the spinal cord. Pain resulting from any cause is transmitted this way. Pain signals are complex, carrying information about the stimuli’s location and intensity. This causes the brain to fully process the pain and send communication back to block further pain signals.

Classification

There are different classes of nociceptors, which are based on which type of stimuli they respond to (University of Texas McGovern Medical School, 2020)

Thermal

• Thermal nociceptors respond to extreme hot or cold temperatures.
• For instance, when touching a hot stove, the nociceptors, which signal pain, are activated immediately, sometimes before you know what you’ve done.

Mechanical

• Mechanical nociceptors respond to intense stretching or strain, such as pulling a hamstring or straining a 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 external chemicals like topical capsaicin pain creams.

Silent

• Silent nociceptors must be first activated by tissue inflammation before responding to a mechanical, thermal, or chemical stimulus.
• Most visceral nociceptors are located on organs in the body.

Polymodal

• Polymodal nociceptors respond to mechanical, thermal, and chemical stimuli.

Mechano-thermal

• Mechano-thermal nociceptors respond to mechanical and thermal stimuli.

Pain Transmission

Nociceptors are also classified by how fast they transmit pain signals. Transmission speed is determined by the type of nerve fiber known as an axon a nociceptor has. There are two main types.

• The first type is A fiber axon, fibers surrounded by a fatty, protective sheath called myelin.
• Myelin allows nerve signals/action potentials to travel rapidly.
• The second type is C fiber axons, which are not surrounded by myelin and transmit slower. (University of Texas McGovern Medical School, 2020)

Because of the difference in transmission speed, the pain signals from the A fibers reach the spinal cord first. As a result, after an acute injury, an individual experiences pain in two phases, one from the A fibers and one from the C fibers. (Ngassapa D. N. 1996)

Pain Perception Phases

When an injury occurs, the stimulated nociceptors activate the A fibers, causing a person to experience sharp, prickling pain.

1. This is the first phase of pain, known as fast pain, because it is not especially intense but comes right after the stimulus.
2. During the second phase of pain, the C fibers are activated, causing an intense, burning pain that persists even after the stimulus has stopped.
3. The fact that the C fibers carry burning pain explains why there is a short delay before feeling the sensation.
4. The C fibers also carry aching, sore pain caused by organs within the body, such as a sore muscle or stomachache. (Ngassapa D. N. 1996)

Injury Medical Chiropractic and Functional Medicine Clinic

Injury Medical Chiropractic and Functional Medicine Clinic works with primary healthcare providers and specialists to build optimal health and wellness solutions. We focus on what works for you to relieve pain, restore function, prevent injury, and help mitigate issues through adjustments that help the body realign itself. They can also work with other medical professionals to integrate a treatment plan to resolve musculoskeletal problems.

From Injury To Recovery With Chiropractic Care

References

Purves D, A. G., Fitzpatrick D, et al., editors. (2001). Nociceptors. In Neuroscience. 2nd edition. (2nd ed.). Sunderland (MA): Sinauer Associates. https://www.ncbi.nlm.nih.gov/books/NBK10965/

University of Texas McGovern Medical School. (2020). Chapter 6: Pain Principles. https://nba.uth.tmc.edu/neuroscience/m/s2/chapter06.html

Ngassapa D. N. (1996). Comparison of functional characteristics of intradental A- and C-nerve fibres in dental pain. East African medical journal, 73(3), 207–209.