Pain receptors, called nociceptors, are nerve cells that transmit signals to the brain, interpreting them as pain. They respond to internal and external stimuli and release neurotransmitters. A-delta fibers transmit sharp pain, while C-sensory fibers transmit dull pain. Pain medications target second messengers and neurochemical mediators, but only offer temporary relief.
A pain receptor is a type of nerve cell that is primarily responsible for receiving and then transmitting stimulation signals from various nerve endings to the brain, which they typically interpret as pain. The receptors work by releasing chemicals called “neurotransmitters” that travel through your nerves, spine, and brain at very high rates. This whole process of pain transmission is called nociception and the pain receptors found in most body tissues are called nociceptors. Receptors are found in most parts of the body and respond to a wide variety of stimuli; they are prolific in humans as well as most animals and are the primary means by which pain is experienced.
Where are
Humans have an extensive nervous system, which forms the basis for most sensations and also for many muscle movements and coordination tasks. Pain is a sensation that is usually experienced in association with some sort of injury or trauma and typically occurs as a result of nerve signals and chemical relays that initiate with specific pain receptors. These exist on almost all nerve endings although they tend to be most prolific in the skin, muscles and joints; they are also common in connective tissues and internal organs.
In most cases they consist of only a few very distinctive cells, making them difficult to notice or observe without special equipment. Researchers know they exist, however, because of the dynamic ways they respond to stimulation and the pathways through which they transmit signals from even the furthest reaches of the body to the pain center in the brain. At the individual level, the receptors are therefore very small, but each one is part of a large and dynamic system of signals and exchanges.
How they are activated
The receptors are activated in response to some kind of stimulation, both internal and external. Pricking a finger with a pin is an example of external stimulation, while something like a twisted bowel or blocked intestine is internal. The receptors on the ends of the nerves closest to where this stimulation occurs are usually the ones responsible for cataloging and then sending the nerve into the main nervous system and eventually to the brain.
When tissue or other parts of the body are damaged, they typically release chemicals known as “second messengers.” Important second messengers include bradykinin, prostaglandins, histamine, serotonin, leukotrienes and potassium. When nerve endings sense these nearby, they usually activate their pain centers.
Differences in nerve type
The science behind how this all plays out can be somewhat complicated, but it often makes more sense when taken on a nervous basis. The peripheral nerve fibers that contain these types of receptors are afferent nerves. This means they send nerve impulses to the brain and spinal cord. There are two main types of afferent nociceptors in tissues: A-delta and C-sensory fibers.
A-delta fibers are myelinated nerves, which means they are covered in a smooth protective shield; as a result these nerves generally transmit pain impulses very rapidly. Pain receptors on A-delta fibers are activated in response to sharp, well-localized pain that requires an immediate response. This type of pain stimulus is sometimes referred to as “somatic pain” and usually involves damage to the skin or muscles.
In contrast, C-sensory pain fibers have receptors that are activated in response to dull, aching, or poorly localized pain stimuli. These pain fibers are unmyelinated, and as a result, nerve impulses are usually transmitted more slowly. C-sensory nerve fibers respond to so-called “visceral pain,” which is usually caused by damage to internal organs.
Travel pain signal
Once the second messenger stimulus is transmitted along the afferent nerves, it must pass through the dorsal horn of the spinal cord. This is called the “transmitting station” for pain signals and is where painful stimuli are transmitted to different parts of the brain. Some pain impulses are sent directly to the thalamus and brain stem for rapid response, while others are sent to the frontal cortex of the brain for further processing. It is in the frontal cortex that the conscious realization of pain takes place.
The brain’s response
The final stage in the pain transmission process is a response from the brain to tell the body how to react. These instructions are transmitted as impulses along efferent nerves, away from the brain. During pain transmission, many substances can be released in the brain and spinal cord that increase or decrease the perception of the painful stimulus. These are called neurochemical mediators and include endorphins, which are natural pain relievers, as well as serotonin and norepinephrine, which improve a person’s perception of pain.
Painkiller basics
Pain medications, sometimes called “painkillers,” usually work by targeting second messengers and neurochemical mediators. If a drug inhibits the release of second messengers, pain receptors will not be activated, the pain impulse will not reach the brain, and the person will not perceive pain from damaged tissue. The same thing happens if the brain’s response is delayed or neutralized. In most cases, medications can only offer temporary solutions and are usually unable to cure the underlying problem. All they do is prevent a person from feeling the pain associated with the injury or damage.
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