Nociceptors

A nociceptor is a sensory nerve cell that responds to damaging or potentially damaging stimuli by sending signals to the spinal cord and brain. This process, called nociception, usually causes the sensation of pain in sentient beings.

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 styles of experiments to demonstrate that different types of stimulation to a nerve's receptive field led to different responses. Some intense stimuli trigger reflex withdrawal, autonomic responses and pain. The specific receptors for these intense stimuli were called nociceptors.

In mammals, nociceptors are sensory neurons that are found in any area of the body that can sense noxious stimuli either externally or internally. External examples are in tissues such as skin (cutaneous nociceptors), cornea and mucosa. Internal nociceptors are in a variety of organs, such as the muscle, joint, bladder, gut and continuing along the digestive tract. The cell bodies of these neurons are located in either the dorsal root ganglia or the trigeminal ganglia. The trigeminal ganglia are specialized nerves for the face, whereas the dorsal root ganglia associate with the rest of the body. The axons extend into the peripheral nervous system and terminate in branches to form receptive fields.

Nociceptors develop from neural crest stem cells. The neural crest is responsible for a large part of early development in vertebrates. More specifically it is responsible for development of the peripheral nervous system. The neural crest stem cells split off 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 neural crest, including embryonic nociceptors, express the TrkA which is a receptor to nerve growth factor (NGF). However, transcription factors that determine the type of nociceptor remain unclear.

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