Nociceptors are sensory receptors of the peripheral nervous system. They are located at the end of nerve cells that originate in the dorsal root ganglion and trigeminal ganglion. Nociceptors are responsible for sending signals to the spinal cord and the brain when damaging stimuli is detected in the skin, mucous membranes, muscles, joints, and organs. They are also known as pain receptors because they produce the sensation of pain.
Pain can be caused by a number of influences, such as extreme temperatures, exposure to chemicals or physical force. The degree to which pain is felt depends on the sensitivity of the nociceptors. A certain level of stimulation must be detected by the receptors in order for a nerve impulse to be sent to the central nervous system.
There are several types of nociceptors. Some respond to a specific stimuli while others respond to several types of stimuli. Thermal nociceptors, for example, respond to hot and cold temperatures, and mechanical nociceptors respond to intense pressure. Silent nociceptors, on the other hand, become active when tissue becomes inflamed. Initially, they respond only to the chemicals released during inflammation. Once they are activated, they become sensitive to thermal and mechanical stresses as well. Polymodal nociceptors respond immediately to thermal, mechanical and chemical stresses.
When a pain receptor located in the face is stimulated, a signal is sent to the trigeminal ganglion, which is located in the brain. When a pain receptor in any other part of the body is stimulated, the signal goes to the dorsal root ganglion, located next to the spinal cord. The stimulus typically is some type of tissue damage, such as a burn or a cut.
After the pain receptor detects tissue damage, the receptor undergoes changes which alter the chemical environment. This changes the membrane potential, which is the voltage difference between the interior and exterior of the receptor, forming a receptor potential. This signal is sent through the axon to the synapse via several action potentials. Once the signal reaches the synapse, chemicals known as synaptic transmitters are released. The neighboring nerve cell carries the signal after interacting with the synaptic transmitters.
The speed at which the signal travels can be measured. An electrode and recording device may be used to introduce a voltage at the receptor and detect the resulting action potential at the cell body. By measuring the distance between the receptor and cell body, and the time it takes the action potential to arrive, the conduction speed can be determined. For thermal and mechanical pain receptors, the conduction speed typically is 7 to 89 miles per hour (about 11 to 143 kilometers per hour). For polymodal and silent pain receptors, the conduction speed usually is less than 7 miles per hour (about 11 kilometers per hour).