Nociceptor

Overview
A nociceptor is a sensory receptor that sends signals that cause the perception of pain in response to potentially damaging stimulus. Nociceptors are the nerve endings responsible for nociception, one of the two types of persistent pain (the other, neuropathic pain, occurs when nerves in the central or peripheral nervous system are not functioning properly). When they are activated, nociceptors can trigger a reflex.

Location
Nociceptors are sensory neurons that are found in external tissues such as skin, cornea and mucosa as well as in internal organs, such as the muscle, joint, bladder and gut. The cell bodies of these neurons are located in either the dorsal root ganglia or the trigeminal ganglia.

Types and functions
There are several types of nociceptors and they are classified according to the stimulus modalities to which they respond: i.e. thermal, mechanical or chemical. Some nociceptors respond to more than one of these modalities and are consequently designated polymodal. Other nociceptors respond to none of these modalities (although they may respond to stimulation under conditions of inflammation) and have thereby earned the more poetic title of sleeping or silent nociceptors (Kandel et al, 2000). Thermal nociceptors are activated by noxious heat or cold, temperatures above 45°C and below 5°C (Kandel et al, 2000). Mechanical nociceptors respond to excess pressure or mechanical deformation. Polymodal nociceptors respond to damaging stimuli of a chemical, thermal, or mechanical nature (Kandel et al, 2000). Nociceptors may have either Aδ fiber axons or more slowly conducting C fiber axons. Thus, pain often comes in two phases, the first mediated by the fast-conducting Aδ fibers and the second part due to C fibers. Silent nociceptors do not usually fire action potentials, though they are much more likely to do so in the presence of inflammation or damaging chemicals (Kandel et al, 2000). Together these nociceptors allow the organism to feel pain in response to damaging pressure, excessive heat, excessive cold and a range of chemicals, the majority of which are damaging to the tissue surrounding the nociceptor.

Pathway
Afferent nociceptive fibers (those that send information to, rather than from the brain) travel back to the spinal cord where they form synapses in its dorsal horn. The cells in the dorsal horn are divided into physiologically distinct layers called laminae. Different fiber types form synapses in different layers. Aδ fibers form synapses in laminae I and V, C fibers connect with neurons in lamina II, Aβ fibers connect with lamina I, III, & V. Information is then sent from the spinal cord to the thalamus and the cerebral cortex in the brain.