What Are Nociceptors?

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There are thousands of ways you can experience pain. From stubbing your toe to cutting yourself while slicing vegetables, various degrees of pain are part of the human experience. No matter the "why" behind these pain experiences, they all share similar mechanisms in how the sensation is detected and transmitted—and this is where nociceptors come into play.

Nociceptors, often referred to as your "pain receptors," play a pivotal role in how you feel and react to pain.

Definition of Nociceptors

Nociceptors are free nerve endings located all over the body, including the skin, muscles, joints, bones, and internal organs. 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.

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, 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 muscle 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.

Due to the difference in transmission speed between the A and C fibers, the pain signals from the A fibers reach the spinal cord before that from the C fibers. 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 a stubbed toe 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.

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