Physiology of Pain, Itch and Temperature

Introduction

Pain, itch, and temperature are essential protective sensations that help detect tissue injury and environmental hazards. Their neural pathways not only generate conscious perception but also trigger autonomic, emotional, and endogenous pain-modulating responses that support survival and maintain physiological homeostasis.

Pain

General Concepts

• Pain is an essential protective sensation that warns the body of actual or potential tissue injury.
• It is often the earliest clinical indicator of disease or tissue damage.
• Pain initiates protective reflexes and behavioral responses that help minimize further injury.
• The perception of pain is influenced by sensory, emotional, cognitive, and psychological factors.
• Pain is commonly described as throbbing, burning, stabbing, aching, or sharp, although its quality varies among individuals.

Definition

Pain is defined as an unpleasant sensory and emotional experience associated with actual or potential tissue damage.

Nociceptive Stimuli

• A nociceptive stimulus is any potentially harmful stimulus capable of activating pain receptors.
• The sensory perception produced by such stimulation is known as nociception.
• Different tissues respond to different types of nociceptive stimuli.
• In the skin, pain is produced by mechanical injury, excessive heat, extreme cold, chemical irritation, or tissue damage.
• In the gastrointestinal tract, pain commonly results from inflammation, distension, smooth muscle spasm, or traction on the mesentery.
• In skeletal muscle, pain may occur because of ischemia, hemorrhage, inflammation, necrosis, trauma, or injection of irritating substances.
• In the heart, myocardial ischemia and inflammation are major causes of pain.
• In joints, pain arises from inflammation of the synovial membrane, ligament injury, joint capsule stretching, or chemical irritation.
• In blood vessels, pain may result from inflammation, vascular obstruction, needle puncture, or excessive arterial distension, such as during migraine.
• The brain parenchyma itself is insensitive to pain.
• Pain associated with intracranial disorders usually originates from the meninges, cerebral arteries, or other pain-sensitive intracranial structures.
• In peripheral nerves, pain commonly results from nerve compression, inflammation, or injury to the nerve sheath.
• Recognition of the tissue source and mechanism of pain is essential for accurate diagnosis and appropriate clinical management.

Types of Pain

Fast Pain vs Slow Pain

• Fast pain is the initial sharp, well-localized pain felt immediately after a harmful stimulus.
• It is transmitted by A delta fibers, which are thin, myelinated fibers with rapid conduction.
• Slow pain follows fast pain and is dull, aching, burning, or poorly localized.
• It is transmitted by C fibers, which are unmyelinated and conduct impulses more slowly.
• Fast and slow pain often occur sequentially after the same painful stimulus.

Superficial Pain vs Deep Pain

• Superficial pain originates from the skin and subcutaneous tissues.
• It is usually sharp, well localized, and predominantly transmitted by A delta fibers.
• Deep pain arises from muscles, bones, joints, ligaments, and connective tissues.
• It is generally dull, aching, poorly localized, and mainly transmitted by C fibers.

Somatic Pain vs Visceral Pain

• Somatic pain originates from the skin, muscles, bones, joints, or connective tissues.
• It is usually well localized and easier to identify.
• Visceral pain arises from internal organs.
• It is often diffuse, poorly localized, and may be associated with referred pain.

Peripheral Pain vs Central Pain

• Peripheral pain results from activation or injury of peripheral nociceptors or peripheral nerves.
• Neuropathic pain caused by peripheral nerve damage is a common example.
• Central pain develops due to lesions affecting the spinal cord or brain.
• It may occur after spinal cord injury, stroke, or other disorders involving central sensory pathways.

Physiologic Pain vs Pathologic Pain

• Acute pain is a protective response to tissue injury and usually resolves with healing or appropriate treatment.
• Chronic pain persists beyond normal tissue healing and often has significant physical and psychological effects.
• Neuropathic pain results from injury or disease affecting the somatosensory nervous system, including diabetic neuropathy or nerve ischemia.
• Inflammatory pain develops from tissue inflammation and is commonly associated with hyperalgesia and allodynia.

Receptors and Peripheral Fibers

Types of Nociceptors

• Nociceptors are specialized sensory receptors that detect potentially harmful mechanical, thermal, or chemical stimuli.
• They are formed by free nerve endings distributed throughout most tissues of the body.
• Nociceptors are abundant in the skin, muscles, joints, blood vessels, and visceral organs.
• The brain parenchyma lacks nociceptors, although the meninges and cerebral blood vessels are richly supplied with pain-sensitive nerve endings.
• Activation of nociceptors initiates protective reflexes and conscious perception of pain.
• Based on their physiological properties, nociceptors are classified into A delta mechanical nociceptors, polymodal C fiber nociceptors, and other specialized nociceptors.

Aδ Mechanical Nociceptors

• These receptors are the peripheral endings of A delta fibers, which are thin, myelinated sensory fibers.
• They respond primarily to intense mechanical stimuli, such as pinprick or tissue injury.
• They show little or no response to chemical or thermal stimuli.
• A delta fibers have diameters of approximately 1–5 micrometers and conduct impulses at 5–30 meters per second.
• They transmit fast pain, which is sharp, well localized, and of short duration.

Polymodal C Fiber Nociceptors

• These receptors are the endings of C fibers, which are small, unmyelinated sensory fibers.
• They respond to intense mechanical, thermal, and chemical stimuli.
• C fibers are activated by temperatures above approximately 45 degrees Celsius and by inflammatory chemical mediators released during tissue injury.
• They have diameters of approximately 0.2–1.5 micrometers and conduct impulses at 0.5–2 meters per second.
• These fibers transmit slow pain, which is dull, burning, aching, and poorly localized.

Other Nociceptors

• Additional nociceptors detect extreme heat and cold.
• Some A delta and C fibers respond selectively to temperatures below 5 degrees Celsius or above 45 degrees Celsius.
• Different receptor proteins enable detection of specific thermal stimuli.

Vanilloid Receptors

• Transient receptor potential vanilloid 1 is an ion channel expressed mainly on C fiber endings.
• It is activated by temperatures above 43 degrees Celsius, tissue acidity, and capsaicin, the pungent compound found in chili peppers.
• Another related receptor, transient receptor potential vanilloid 2, responds to temperatures above 50 degrees Celsius but is not activated by capsaicin.
• The presence of multiple receptor types allows nociceptors to detect a wide range of harmful thermal and chemical stimuli with high sensitivity.

TRP Family of Ion Channels

• Transient receptor potential ion channels are membrane proteins that detect thermal, chemical, and mechanical stimuli involved in pain perception.
• Important members include transient receptor potential melastatin 8, transient receptor potential vanilloid 1, and transient receptor potential vanilloid 2.
• These channels generate receptor potentials by allowing the movement of cations across the cell membrane.
• Transient receptor potential vanilloid 1 contains a binding site for phosphatidylinositol 4,5-bisphosphate, which regulates channel sensitivity.
• Reduced binding of phosphatidylinositol 4,5-bisphosphate increases the responsiveness of this receptor to painful thermal stimuli.
• Activation of cutaneous nociceptors produces depolarization, initiating action potentials in sensory nerve fibers.
• Depolarization primarily results from increased sodium and calcium influx through ion channels, together with reduced potassium efflux.
• Cooling-sensitive receptors are activated mainly through calcium-dependent mechanisms.

Spinal Cord Termination of Pain Fibers

• The cell bodies of pain-sensitive neurons are located in the dorsal root ganglia.
• Their central processes enter the spinal cord through the dorsal roots.
• Pain fibers supplying the face terminate in the spinal trigeminal nucleus, the functional equivalent of the dorsal horn.
• Within the dorsal root, pain fibers occupy the lateral portion.
• A delta fibers terminate mainly in laminae I and V of the dorsal horn.
• C fibers terminate predominantly in lamina II, also known as the substantia gelatinosa.
• Some fine pain fibers ascend or descend briefly in the tract of Lissauer before entering the dorsal horn.
• Second-order neurons then cross the midline and ascend in the anterolateral system to the thalamus.

Specificity of Nociceptors

• Several theories have been proposed to explain pain perception.
• The specificity theory states that pain is mediated by dedicated receptors and specialized neural pathways.
• The overstimulation theory proposes that excessive stimulation of different sensory receptors can produce pain.
• The pattern theory suggests that pain depends on the pattern of neural activity rather than activation of specific receptors.
• Current experimental evidence strongly supports the specificity theory, indicating that specialized nociceptors and distinct ascending pathways are primarily responsible for pain perception.

Dermatomal Distribution of Pain Fibers

• Pain fibers are distributed according to the dermatomal arrangement of spinal nerves, which assists in identifying the level of neurological lesions.
• Cutaneous pain follows a well-defined segmental pattern corresponding to individual spinal nerve roots.
• Pain from deep structures also shows segmental organization but does not always match the cutaneous dermatome precisely.
• Thoracic organs are mainly supplied by thoracic spinal segments T1 to T4.
• Upper abdominal viscera primarily receive sensory fibers through T6 to T8 segments.
• Lower abdominal and pelvic viscera commonly transmit pain through T10 to L2 spinal segments.

Localization

• Sharp pain is accurately localized because it preserves dermatomal and somatotopic organization.
• Dull pain is less precisely localized owing to greater divergence of ascending pain pathways.
• In general, pain is localized less accurately than other sensory modalities because nociceptors are relatively sparse and their central pathways show extensive branching.
• Visceral pain is especially poorly localized due to the lower density of pain receptors and limited cortical representation of internal organs.
• Knowledge of dermatomal pain distribution is valuable in the clinical diagnosis of neurological and visceral disorders.

Pain Pathways

• Pain pathways transmit nociceptive impulses to higher centers mainly through the lateral spinothalamic tract of the anterolateral system.
• Two major ascending pain pathways are recognized: the paleospinothalamic pathway and the neospinothalamic pathway.

Paleospinothalamic Pathway

• The paleospinothalamic pathway is the evolutionarily older pathway.
• It primarily conveys slow pain, which is dull, aching, burning, and poorly localized.
• This pathway mainly carries impulses through C fibers from deep somatic tissues and visceral organs.

First Order Neurons

• First-order neurons have their cell bodies in the dorsal root ganglia.
• Their central processes enter the spinal cord through the dorsal roots.
• Most fibers terminate in lamina II of the dorsal horn, also known as the substantia gelatinosa.

Second Order Neurons

• Second-order neurons arise from the dorsal horn and cross the midline through the anterior white commissure.
• Their axons ascend in the contralateral lateral spinothalamic tract.
• Within the brainstem, these fibers give numerous collateral branches to several important nuclei.
• Projections to the reticular formation contribute to the spinoreticulothalamic pathway.
• Additional fibers terminate in the periaqueductal gray, parabrachial nuclei, and other midbrain regions.
• Some fibers also project to the hypothalamus, forming the spinohypothalamic pathway.

Third Order Neurons

• Most fibers terminate in the midline and intralaminar nuclei of the thalamus.
• Third-order neurons project diffusely to the cerebral cortex, including sensory association areas and the limbic cortex.

Special Features

• Diffuse cortical projections make slow pain poorly localized.
• Connections with the reticular formation promote arousal and maintain wakefulness during painful stimuli.
• Projections to the limbic system generate the emotional and motivational aspects of pain.
• Connections with the hypothalamus produce autonomic responses, including changes in heart rate, blood pressure, and sweating.
• Thus, the paleospinothalamic pathway integrates the sensory, emotional, autonomic, and behavioral responses associated with persistent pain.

Neospinothalamic Pathway

• The neospinothalamic pathway is the principal pathway for transmitting fast pain.
• It is highly developed in humans and other primates.
• This pathway mainly carries impulses through A delta fibers, which conduct rapidly.
• Fast pain is sharp, well localized, and enables quick recognition of tissue injury.

First Order Neurons

• First-order neurons have their cell bodies in the dorsal root ganglia.
• Their central processes enter the spinal cord and terminate mainly in laminae I and V of the dorsal horn.
• Primary nociceptive terminals release glutamate as the principal excitatory neurotransmitter.
• Substance P and other neuropeptides also contribute to pain transmission, particularly during intense or persistent stimulation.

Second Order Neurons

• Second-order neurons cross the midline through the anterior white commissure within the same spinal segment.
• Their axons ascend in the contralateral lateral spinothalamic tract.
• The tract maintains somatotopic organization, with sacral fibers located laterally and cervical fibers positioned medially.
• Most fibers terminate in the ventral posterolateral nucleus of the thalamus.
• A smaller number of collateral fibers project to the periaqueductal gray, reticular formation, and hypothalamus, contributing to pain modulation and autonomic responses.
• This pathway allows rapid and accurate localization of painful stimuli before conscious perception in the primary somatosensory cortex.

Third Order Neurons

• Third-order neurons originate from the ventral posterolateral nucleus of the thalamus and project to the primary somatosensory cortex in the postcentral gyrus.
• The pathway maintains precise somatotopic organization from the thalamus to the cerebral cortex.
• Neurons in the sensory cortex are arranged into modality-specific functional columns.

Special Features

• This organized projection enables accurate localization of fast pain.
• The neospinothalamic pathway mediates the sensory-discriminative aspects of pain, including identification of the site, intensity, and quality of noxious stimuli.
• It allows rapid conscious perception and precise interpretation of painful sensory input.

Deep Pain

• Deep pain originates from nociceptors located in muscles, tendons, bones, periosteum, ligaments, and other deep somatic tissues.
• It is typically dull, aching, poorly localized, and longer lasting than superficial pain.
• Deep tissues contain relatively fewer nociceptors and fewer A delta fibers, reducing the precision of pain localization.
• Pain from deep structures is transmitted predominantly by C fibers.
• Deep pain may be accompanied by autonomic responses, such as sweating, nausea, or changes in heart rate.
• Painful stimuli often trigger reflex contraction of nearby skeletal muscles.
• Sustained muscle contraction can reduce local blood flow, causing ischemia that further intensifies and prolongs the pain.

Visceral Pain

• Visceral pain originates from internal organs and differs significantly from pain arising in skin, muscles, or joints.
• It is commonly described as dull, aching, cramping, or burning, although severe visceral injury may produce sharp pain.
• Visceral pain is usually diffuse, poorly localized, and frequently accompanied by autonomic manifestations.
• It often radiates or presents as referred pain in distant somatic regions.

Receptors

• Visceral pain is detected by nociceptors that are structurally similar to those found in somatic tissues.
• Internal organs contain relatively few nociceptors compared with the skin.
• The low density of pain receptors contributes to poor localization of visceral pain.
• As a result, patients often cannot accurately identify the exact site of pain.

Stimulus

• Distension of hollow organs is the most common stimulus for visceral pain.
• Smooth muscle spasm can also activate visceral nociceptors.
• Ischemia, inflammation, and chemical irritation are important causes of visceral pain.
• Stretching of the organ capsule or surrounding connective tissue also produces pain in many solid organs.

Afferent Fibers

• Visceral pain fibers travel to the central nervous system mainly through autonomic nerves.
• Their cell bodies are located in the dorsal root ganglia or corresponding cranial sensory ganglia.
• Sensory fibers from the esophagus, stomach, small intestine, and proximal colon travel predominantly in the vagus nerve.
• These fibers relay mainly to the nucleus of the solitary tract in the medulla.
• Pain fibers from many thoracic and abdominal organs also accompany sympathetic nerves.
• Pelvic visceral pain is transmitted mainly through sacral visceral afferent pathways.
• Sensory fibers from the face and orbit are conveyed by the trigeminal nerve.

Ascending Pathway

• Second-order neurons ascend mainly through the lateral spinothalamic tract.
• They terminate in the thalamus before projecting to the cerebral cortex.
• Cortical representation of visceral sensation is relatively diffuse and overlaps with somatic sensory areas.

Special Characteristics of Visceral Pain

• The limited number of nociceptors makes visceral pain poorly localized.
• Distension is generally a stronger stimulus than cutting or burning.
• Inflammation lowers the activation threshold of visceral nociceptors, increasing pain sensitivity.
• Visceral pain has a prominent emotional component because of extensive connections with the limbic system.
• It is frequently associated with autonomic responses such as sweating, nausea, vomiting, pallor, and changes in heart rate or blood pressure.
• Abdominal visceral pain may produce reflex contraction of the abdominal wall muscles, known as muscle guarding, particularly when the parietal peritoneum is inflamed.
• Referred pain is a characteristic feature because visceral and somatic sensory fibers converge on common spinal neurons.

Referred Pain

• Referred pain is the perception of pain at a site distant from its actual origin.
• It commonly occurs when pain arising from a visceral organ is perceived in a somatic structure.
• The site of referred pain usually does not overlie the affected organ.
• Instead, pain is felt in skin or muscles supplied by the same or closely related spinal segments as the diseased organ.
• Referred pain is an important clinical clue for identifying the source of visceral disease.
• In acute myocardial infarction, pain commonly radiates to the left shoulder, medial aspect of the left arm, neck, or jaw.
• In acute cholecystitis, pain may radiate to the right shoulder because of irritation of the diaphragm.
• Diaphragmatic irritation stimulates the phrenic nerve, which arises from cervical spinal segments that also supply the shoulder region.

Dermatomal Theory

• This theory proposes that referred pain occurs because the affected organ and the painful skin region share a common embryological segmental origin.
• Sensory impulses from both structures enter the spinal cord through the same spinal segments.
• Consequently, the brain interprets visceral pain as arising from the corresponding somatic dermatome.

Convergence Theory

• The convergence theory is the most widely accepted explanation for referred pain.
• Visceral and somatic sensory fibers converge on the same second-order neurons within the spinal cord.
• These common neurons transmit impulses through identical ascending pathways to the brain.
• Because somatic pain is encountered more frequently than visceral pain, the brain misinterprets visceral signals as originating from the corresponding somatic region.
• This mechanism explains the predictable pattern of referred pain in many visceral disorders.

Facilitation Theory

• According to the facilitation theory, visceral afferent activity increases the excitability of spinal neurons that also receive somatic sensory input.
• As a result, even mild stimulation of the corresponding somatic area may produce pain.
• Persistent visceral input lowers the activation threshold of these shared neurons, enhancing pain perception in the related dermatome.
• Referred pain reflects the close anatomical and functional relationship between visceral and somatic sensory pathways and has significant value in clinical diagnosis.

Experience Theory

• The experience theory suggests that previous painful experiences can influence the perceived location of referred pain.
• The brain may interpret visceral pain as arising from a site where pain was experienced earlier.
• For example, abdominal visceral pain may be perceived at the site of a previous surgical scar instead of its usual referred location.
• This theory highlights the influence of prior sensory experiences on pain perception.

Mechanism of Pain Recognition and Perception

Pain Recognition

• Pain recognition begins when tissue injury activates specialized nociceptors.
• Damaged cells release proteolytic enzymes and inflammatory mediators into the surrounding tissue.
• These substances stimulate or sensitize nociceptors, increasing the generation of pain impulses.
• Important pain mediators include prostaglandins, histamine, serotonin, bradykinin, leukotrienes, and extracellular potassium ions.
• These mediators lower the activation threshold of nociceptors, making them more responsive to painful stimuli.
• Activated C fibers release substance P from their peripheral terminals.
• Substance P enhances pain transmission and contributes to neurogenic inflammation by promoting vasodilation and increased vascular permeability.
• Persistent release of inflammatory mediators produces peripheral sensitization, resulting in increased pain sensitivity.

Modulation of Pain Perception

• Pain perception is influenced by physiological, psychological, and environmental factors.
• Inflammation lowers the pain threshold, causing normally mild stimuli to become painful.
• Local anesthetics reduce pain by blocking nerve impulse conduction.
• Centrally acting analgesics increase the pain threshold by modifying pain processing within the central nervous system.
• Placebo responses may also reduce pain through activation of endogenous pain-control mechanisms.
• Distraction decreases pain awareness by shifting attention away from the painful stimulus.
• Strong emotions and descending neural pathways can suppress pain transmission in selected situations.
• Conscious perception of pain occurs after nociceptive impulses reach the thalamus and cerebral cortex.
• The thalamus serves as the principal sensory relay for pain signals.
• The primary somatosensory cortex determines the location, intensity, and quality of painful stimuli, allowing accurate conscious interpretation.

Endogenous Pain Control Mechanisms

• The body possesses endogenous pain control mechanisms that reduce pain transmission and limit excessive nociceptive activity.
• Two major systems participate in endogenous analgesia: the descending pain-modulating system and the endogenous opioid system.
• The descending pain-modulating system suppresses pain signals before they reach higher brain centers.

Descending Pain Modulating Systems

• A major control center is the periaqueductal gray of the midbrain.
• The periaqueductal gray receives input from the frontal cortex, hypothalamus, and ascending pain pathways.
• It projects to the nucleus raphe magnus and adjacent reticular nuclei in the medulla.
• Neurons from the nucleus raphe magnus descend to the spinal cord through the raphespinal pathway.
• These descending fibers are predominantly serotonergic.
• They terminate mainly in laminae I, II, and V of the dorsal horn.
• Descending fibers activate inhibitory interneurons within the dorsal horn.
• These interneurons release enkephalins, which suppress transmission of pain signals.
• Enkephalins inhibit the release of substance P from primary nociceptive afferents.
• This presynaptic inhibition reduces activation of second-order neurons and decreases ascending pain transmission.
• As a result, fewer pain impulses reach the thalamus and cerebral cortex.

Physiological Significance

• Persistent activation of ascending pain pathways stimulates the periaqueductal gray, initiating descending inhibition.
• This mechanism provides an intrinsic negative feedback system that limits excessive pain perception.
• Endogenous pain inhibition is particularly important during chronic pain, where it reduces the intensity of persistent nociceptive input.
• The descending analgesic system therefore contributes to natural pain control while preserving the protective function of pain.

Descending Ceruleospinal Norepinephrinergic Pathway

• The ceruleospinal norepinephrinergic pathway originates from the locus coeruleus in the dorsolateral pons.
• These descending fibers project to the dorsal horn and inhibit nociceptive transmission from primary afferent neurons, reducing ascending pain signals independently of the periaqueductal gray.

Endogenous Opioid Systems

• The endogenous opioid system is a natural pain-control mechanism that suppresses nociceptive transmission within the central nervous system.
• Opioid receptors are present at synapses of pain pathways in the spinal cord and brain.
• Three major opioid receptor types are mu, delta, and kappa, all of which belong to the G protein-coupled receptor family.
• Morphine produces potent analgesia mainly by activating mu receptors.
• Opioids inhibit pain transmission at both presynaptic and postsynaptic sites.
• Presynaptically, they reduce the release of substance P and glutamate from A delta and C fiber terminals.
• Postsynaptically, they increase potassium conductance, causing neuronal hyperpolarization and reducing excitability.
• Naloxone reverses opioid effects by competitively blocking opioid receptors.
• The body synthesizes several endogenous opioid peptides, collectively known as endorphins.
• Important endogenous opioids include enkephalins, beta-endorphin, and dynorphins.
• Opioid receptors are highly concentrated in the periaqueductal gray, nucleus raphe magnus, and dorsal horn of the spinal cord.
• Activation of these receptors produces powerful analgesia by enhancing descending pain inhibition.
• Acupuncture and acupressure may reduce pain partly by promoting the release of endogenous opioid peptides.
• Prolonged use of opioid medications can lead to tolerance, physical dependence, and addiction, limiting their long-term therapeutic use.

Gate Control Theory of Pain

• The gate control theory proposes that pain transmission is modulated within the dorsal horn of the spinal cord.
• Activation of large myelinated touch fibers stimulates inhibitory interneurons that suppress transmission from A delta and C fibers.
• This spinal gating mechanism reduces the number of pain impulses reaching the brain.
• Techniques such as rubbing the affected area, transcutaneous electrical nerve stimulation, and acupuncture may relieve pain partly through this mechanism and endogenous opioid release.

Stress-induced Analgesia

• Stress-induced analgesia temporarily reduces pain perception by activating descending pain-inhibitory pathways through both endogenous opioid and non-opioid mechanisms.

Specific Pain Syndromes

Terminologies

• Hyperalgesia is an increased sensitivity to painful stimuli caused by a lowered pain threshold.
• It commonly develops in inflamed tissues because inflammatory mediators sensitize nociceptors.
• Hypoalgesia is a reduced sensitivity to painful stimuli due to an increased pain threshold.
• Analgesia is the complete absence of pain perception while other sensory modalities may remain intact.
• Hyperpathia is an exaggerated and often delayed response to painful stimulation.
• It may also be associated with excessive responses to other sensory stimuli.
• Allodynia is the perception of pain in response to a stimulus that is normally non-painful, such as light touch.
• Careful assessment of pain should include its onset, character, location, radiation, intensity, duration, aggravating factors, relieving factors, and associated physiological or behavioral changes.
• These features help determine the underlying cause and guide appropriate management.

Causalgia

• Causalgia, also called complex regional pain syndrome type II, is a persistent burning pain that develops after peripheral nerve injury.
• The pain is continuous and is frequently accompanied by hyperalgesia and allodynia.
• Symptoms may persist even after apparent recovery of the injured nerve.
• Abnormal interaction between injured sensory fibers and the sympathetic nervous system contributes to pain persistence.
• Sympathetic nerve fibers may establish abnormal connections with sensory neurons in the dorsal root ganglia, increasing pain transmission.
• Pain may improve after sympathetic blockade or other treatments that reduce sympathetic activity.
• Early recognition and multidisciplinary management are important to prevent chronic disability and improve functional recovery.

Inflammatory Pain

• Inflammatory pain is the most common type of pain and develops following tissue injury or infection.
• Inflammatory mediators, including cytokines, prostaglandins, bradykinin, and substance P, sensitize nociceptors.
• This lowers the pain threshold, causing normally mild painful stimuli to produce exaggerated pain responses.
• Inflammation therefore commonly results in hyperalgesia.

Neuropathic Pain

• Neuropathic pain arises from injury or disease affecting the somatosensory nervous system.
• Common causes include diabetic neuropathy, peripheral nerve injury, polyneuropathies, nerve root compression, and spinal cord injury.
• Patients frequently experience burning, shooting, or electric shock-like pain.
• Neuropathic pain is often associated with hyperalgesia, allodynia, hyperesthesia, and hyperpathia.
• Conventional analgesics are often less effective than medications targeting neuropathic pain.
• Selected patients may benefit from epidural injections or other interventional procedures when conservative treatment fails.

Tic Douloureux

• Trigeminal neuralgia, also called tic douloureux, is characterized by sudden, severe, stabbing facial pain lasting for seconds to minutes.
• The condition commonly results from compression of the trigeminal nerve root by a blood vessel.
• Pain is often triggered by light facial stimulation, chewing, speaking, or brushing the teeth.
• Early diagnosis and appropriate treatment can significantly improve quality of life.

Thalamic Syndrome

• Thalamic syndrome is characterized by severe, persistent, and distressing central pain that occurs spontaneously or after minor sensory stimulation.
• The pain arises from lesions involving the posterior thalamic nuclei, usually following ischemic or hemorrhagic stroke.
• Pain occurs despite the absence of ongoing peripheral tissue injury.
• Management may require medications, neuromodulation, or selected surgical procedures in refractory cases.

Toothache

• Teeth are richly supplied with nociceptors carried by the maxillary and mandibular divisions of the trigeminal nerve.
• Pain fibers include A delta, A beta, and C fibers.
• The dental pulp contains abundant sensory nerve endings, whereas enamel and cementum lack nerve fibers.
• Exposure or inflammation of the dental pulp causes severe pain triggered by heat, cold, pressure, or mechanical stimulation.

Itch

Receptors and Pathway

Definition

• Itch is an unpleasant sensory experience that creates an urge to scratch.
• It is a distinct sensory modality and differs from the pleasant sensation of tickle.
• Itch can be triggered by mechanical or chemical stimulation of the skin.

Receptors

• Specialized unmyelinated C fiber endings act as the primary receptors for itch.
• These fibers are functionally different from those transmitting pain.
• Histamine is the principal chemical mediator responsible for many forms of itching.
• Other mediators, including kinins, can also induce itch.
• Elevated bile salts in the blood commonly produce generalized itching in hepatobiliary disorders.

Pathway

Itch impulses are transmitted through the lateral spinothalamic tract to the thalamus and subsequently to the cerebral cortex for conscious perception.

Temperature

Receptors and Pathway

Receptors

• Temperature receptors are specialized free nerve endings that detect changes in environmental and body temperature.
• Cold receptors respond most effectively to temperatures between approximately 10 and 40 degrees Celsius.
• Warm receptors are activated over a range of approximately 30 to 50 degrees Celsius.
• Temperature sensation is mediated by specialized transient receptor potential ion channels.
• Transient receptor potential melastatin 8 responds to moderate cold.
• Transient receptor potential vanilloid 1 is activated by noxious heat.
• Transient receptor potential vanilloid 2 responds to higher temperatures than transient receptor potential vanilloid 1.

Pathway

• Cold sensations are transmitted mainly by A delta and C fibers, whereas warmth is conveyed predominantly by C fibers.
• Temperature impulses ascend through the lateral spinothalamic tract to the thalamus and primary somatosensory cortex for conscious perception.