Pain, temperature, and itch are related somatosensory modalities that share common transmission pathways. Pain serves as a critical warning system that protects the body from harm, while non-noxious temperature and itch detect stimuli that do not signal tissue damage. Depending on the type of stimulus, lightly myelinated Aδ fibers or unmyelinated C fibers transmit all three types of information to the CNS.

Pain

Receptors

The type of pain perceived depends on which receptor type is activated.

Pain sensation is in response to the activation of special branching, bare nerve endings called nociceptors. Activation of nociceptors usually occurs in response to tissue damage or the threat of damage. Nociceptors are located throughout the body in skin, muscles, and viscera, but nociceptors are sparse in most CNS tissue.

There are three types of nociceptors that are each activated by different types of harmful stimuli. Type A delta I fibers are most sensitive to mechanical stimulation, such as intense pressure or an incision on the skin. Type A delta II fibers are most sensitive to thermal stimulation and activate in extremely hot environments. Type C fibers tend to be polymodal or activated by a range of stimuli, including mechanical, chemical, and thermal.

The A delta and C fibers transmit information to the CNS at different speeds because of their different myelination levels. We can often perceive this difference in the form of “first pain” and “second pain”. Think about a time where you were injured; perhaps you hit your thumb with a hammer. Immediately, you might have felt an intense, sharp pain—first pain—but then a milder, burning or aching pain that continues longer—second pain. A delta fibers are responsible for first pain transmission, whereas C fibers are responsible for second pain transmission.

Transduction

Like the other sensory systems, specialized proteins in the cell membrane of nociceptors convert noxious stimuli into electrical potentials in the neurons. Piezo channels, believed to underlie mechanical pain transduction, are expressed in Aδ-I fibers and open in response to intense mechanical force such as pressure or an incision, allowing cation influx that depolarizes the nociceptor.

A family of ion channels called transient receptor potential (TRP) channels can be expressed in nociceptor nerve endings and have been shown to be activated by thermal and chemical stimuli. For example, TRPV1, a non-selective cation channel found in Aδ-II fibers, can be activated by temperatures above 43°C and can also be activated by a chemical in hot peppers called capsaicin. The molecular basis of noxious cold detection remains an active area of research and appears to involve multiple channel types. Recent work has identified GluK2, a receptor better known for detecting glutamate in the brain, as an important mediator of noxious cold sensing in dorsal root ganglion neurons, acting through G-protein signaling rather than direct channel gating.

Once TRPV1 or Piezo channels depolarize the nociceptor terminal to threshold, action potential propagation depends on voltage-gated sodium channels, the same general mechanism used throughout the nervous system. Nociceptors express several voltage-gated sodium channel types, but one, Nav1.7, is essential for pain signaling. A mutation that eliminates Nav1.7 function results in the inability to feel pain, even though the other sodium channels present in nociceptors remain functional. Local anesthetics such as lidocaine exploit this same general mechanism, blocking voltage-gated sodium channels to prevent action potential propagation and dull pain in the treated area.

Pathway to Brain

Spinal Cord Branching

Primary afferent pain fibers have their cell bodies located in the dorsal root ganglion, like the fibers of the mechanoreceptors responsible for touch. The axons of these first-order neurons enter the ipsilateral dorsal side of the spinal cord, and then they branch and travel up and down the spinal cord a couple of segments in a white matter region just posterior to the dorsal horn called Lissauer’s tract. All the branches terminate in the dorsal horn.

Spinothalamic Pathway

The nociceptor fibers make synaptic contact with second-order neurons in the dorsal horn of the spinal cord. These neurons immediately cross the midline, or decussate, and then ascend to the brain through the anterolateral aspect of the spinal cord and brainstem via the spinothalamic tract. The axons terminate in the ventral posterior lateral nucleus of the thalamus. The thalamic neurons then project to the primary somatosensory cortex located in the postcentral gyrus in the parietal lobe.

Trigeminothalamic Pathway

Nociceptors in the face and head send information to the brain primarily through cranial nerve V, the trigeminal nerve. The first-order neurons have their cell bodies in the trigeminal ganglion, located just outside of the brainstem. The fibers enter the brainstem and descend to the spinal trigeminal nucleus in the medulla, where they synapse on a second-order neuron. The second-order neurons cross the midline and project up to the ventral posterior medial nucleus of the thalamus. These neurons then send projections to the face region of the somatosensory cortex.

Sensitization

Pain sensitization occurs after injury and causes the feeling of pain in situations that would not normally cause pain. There are two types of pain sensitization. Hyperalgesia is increased pain in response to a stimulus that normally causes pain but less intensely, like having a second injury at or near a previously injured site. Allodynia is pain sensation in response to a stimulus that would normally not cause pain, like a light touch on a sunburn.

In the periphery, tissue damage causes pain signals to be sent to the CNS, as seen above, but the injured tissue also releases substances like prostaglandins, cytokines, and protons, which cause inflammation and begin the healing process. Nociceptors themselves also release substance P from their peripheral terminals, contributing further to local inflammation. Additionally, non-neuronal cell types such as mast cells and macrophages come to the injured site, releasing more inflammatory substances. These chemicals, particularly prostaglandins, however, can act on nociceptors and cause cellular changes that allow the receptors to be more sensitive to stimuli. This results in a decreased threshold for pain sensation, causing hyperalgesia.

Allodynia can involve sensitization in the peripheral nervous system, the central nervous system, or both, depending on the underlying condition. Centrally, sustained nociceptor input causes NMDA receptor activation and increased excitability in dorsal horn neurons. As a result, input from low-threshold mechanoreceptors (A beta fibers), which normally signal touch, is processed as pain instead.

Modulation of Pain

Pain signals that enter the CNS can be modified.

Peripheral

Think of a time when you cut a finger or stubbed a toe. It is common after an injury like this to put pressure at or near the injured location—squeeze the hurt finger or toe. This touch stimulation can decrease the pain sensation felt because sensory mechanoreceptors send signals through inhibitory interneurons in the dorsal horn of the spinal cord to pain neurons that ascend to the brain. This effect is known as the gate control theory of pain.

One type of therapy that is believed to activate this process is transcutaneous electrical nerve stimulation (TENS). TENS units, which use small electrical impulses at the site of the pain, are often used to minimize both long- and short-term pain in joints and muscles.

Central

Descending regulation of pain also occurs. In this case, neurons from the medulla, which are innervated by the periaqueductal gray (PAG), descend and synapse in the dorsal horn of the spinal cord. The medulla neurons release either serotonin or norepinephrine onto enkephalin-releasing interneurons. These interneurons inhibit both the nociceptors and the second-order pain neurons that project to the brain. Electrical stimulation of the PAG results in widespread analgesia.

Non-Noxious Temperature

Not all temperature information is signaled by pain pathways. Innocuous warmth and coolness, the everyday sensation of a cool glass or a warm room, are detected by a separate population of thermoreceptors, distinct from the Aδ-II nociceptors that signal painful heat above 43°C. These thermoreceptors are free nerve endings that use their own TRP channels, tuned to a moderate, non-damaging temperature range.

Separate receptor populations, not separate intensities of the same receptor, distinguish non-noxious temperature from painful heat and cold.

Cool sensation is mediated primarily by TRPM8, a cation channel activated by temperatures roughly between 8°C and 28°C. TRPM8 is also activated by menthol, which is why menthol produces a cooling sensation independent of actual temperature change. Warm sensation in this non-noxious range is less precisely defined at the molecular level, but channels in the TRPV3 and TRPV4 families are thought to contribute, responding to temperatures in the moderate warm range below the noxious heat threshold of TRPV1.

Because warmth and coolness are judged relative to resting skin temperature (approximately 32°C) rather than core body temperature (37°C), a stimulus that is below core body temperature can still feel warm if it raises skin temperature above its adapted baseline.

Like pain signals, non-noxious temperature information travels centrally via thinly myelinated Aδ and unmyelinated C fibers, enters the dorsal horn, decussates, and ascends in the spinothalamic tract to the thalamus.

Itch

Itch, or pruriception, is signaled by a distinct population of unmyelinated C fibers called pruriceptors. Two separate signaling pathways produce the sensation, depending on the triggering stimulus.

Histaminergic itch occurs when histamine, released by mast cells during an allergic or inflammatory response such as an insect bite, binds H1 histamine receptors on pruriceptor terminals. Receptor activation triggers an intracellular signaling cascade that opens TRPV1 (the same heat- and capsaicin-gated channel described in the Pain section above), allowing cation influx and depolarization.

Non-histaminergic itch is triggered by a separate set of stimuli, including certain plant compounds and some medications and signals through Mas-related G-protein-coupled receptor (Mrgpr) family receptors rather than histamine receptors. Mrgpr activation opens the TRPA1 cation channel, driving depolarization. Because antihistamines block only the H1 receptor pathway, they relieve histaminergic itch but have little effect on non-histaminergic itch.

Pruriceptor axons enter the dorsal horn and ascend via the spinothalamic tract, following the same route as pain and temperature fibers. This shared entry point underlies the interaction between itch and pain; similar to the gate control theory of pain, activating nociceptors—such as through scratching—engages local inhibitory interneurons in the dorsal horn that suppress itch signal transmission, producing temporary relief. TRPM8, the same channel responsible for non-noxious cool sensation, also relieves itch when activated, which is why cooling agents such as menthol are used to treat itch clinically.

Conclusion

Pain, temperature, and itch are related somatosensory modalities that share a common transmission pathway to the brain, but each relies on a distinct set of receptors tuned to a specific range of stimuli. Nociceptors detect tissue-damaging stimuli and generate pain, a protective signal that warns the body of harm. Thermoreceptors detect non-noxious temperature, allowing the body to sense warmth and coolness separately from painful heat and cold. Pruriceptors detect itch-inducing stimuli through either histaminergic or non-histaminergic pathways, producing a sensation distinct from pain despite sharing overlapping fiber types and central circuitry. Together, these three modalities illustrate how the somatosensory system uses specialized receptor populations, rather than variations in a single receptor type, to distinguish between different categories of external stimuli.

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