What Are Nociceptors?

woman icing painful knee
PhotoAlto/Odilon Dimiertty Images/Getty Images

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, 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:

  • 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 axons, 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.

    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.

    Phases of Pain Perception

    When an injury occurs (such 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).

    A Word From Verywell

    In the end, while experiencing pain is actually a healthy, adaptive human process (our body's way of telling us something is wrong), pain can also be inaccurate. For instance, while banging your knee on your desk may hurt badly at the moment, it likely didn't cause lasting damage. This is why taking a pain medication, such as a nonsteroidal anti-inflammatory (NSAID), to block nociceptor activation is sensible when the "pain alert" is being addressed.

    Was this page helpful?
    Article Sources