Chapter 48: Somatic Sensations: II. Pain, Headache, and Thermal Sensations

Written By: Razaullah Khattak (DPT Medical Professional)

Student Source Reference: Guyton and Hall Textbook of Medical Physiology

Chapter: 48

To read more check out Chapter 31:

https://mednotesbyrazaullah.blogspot.com/2026/09/chapter-31-diuretics-kidney-diseases.html

Pain Physiology

Pain serves as a vital protective mechanism for the body by alerting an individual to ongoing or impending tissue damage. When tissues are injured, pain receptors send signals through the nervous system to trigger immediate protective reactions and prevent further physical harm. Without this sensation, routine pressure or unrecognized injury could lead to severe tissue destruction.

Understanding the physiological pathways, receptor types, and neurotransmitters involved in pain perception is essential in medical science. Physicians rely on the specific qualities, locations, and mechanisms of pain to accurately diagnose a wide range of clinical conditions.

Pain as a Protective Mechanism

• Pain signals ongoing tissue damage.
• It triggers immediate protective reactions.
• Ischemia causes subconscious weight shifts.
• Loss of pain causes skin breakdown.
• Classification: Fast Pain vs. Slow Pain
• Pain is divided into two major types.
• Fast pain occurs within 0.1 second.
• Fast pain is sharp, acute, or electric.
• Fast pain comes from mechanical or thermal stimuli.
• Fast pain is not felt in deep tissues.
• Slow pain begins after 1 second or more.
• Slow pain is burning, aching, or throbbing.
• Slow pain connects to tissue destruction.
• Slow pain occurs in skin and deep organs.

Pain Receptors and Stimulation

• Pain receptors are free nerve endings.
• Receptors are widespread in superficial skin layers.
• Internal pain receptors supply periosteum and arterial walls.
• Receptors supply joint surfaces and cranial membranes.
• Deep tissues have sparse pain receptors.
• Summation causes slow pain in deep tissues.
• Stimuli are mechanical, thermal, and chemical.
• Fast pain uses mechanical and thermal stimuli.
• Slow pain uses all three stimulus types.
• Chemical stimuli include bradykinin, histamine, and serotonin.
• Potassium ions and acids excite chemical pain.
• Acetylcholine and proteolytic enzymes excite pain.
• Prostaglandins and substance P enhance receptor sensitivity.
• Chemical substances stimulate slow, suffering pain.
• Pain receptors adapt very little or not at all.
• Continued stimulation increases receptor sensitivity.
• Increased sensitivity to pain is called hyperalgesia.
• Nonadapting receptors keep the body aware of damage.

Tissue Damage and Pain Perception

• Pain perception begins at skin heat above 45 °C.
• Heat damage starts at 45 °C.
• Heat pain correlates with rate of tissue damage.
• Heat pain does not correlate with total past damage.
• Infection and ischemia correlate with damage rate.
• Damaged tissue extracts cause intense pain.
• Bradykinin is a major agent causing injury pain.
• High potassium ion concentrations excite pain.
• Proteolytic enzymes increase nerve membrane permeability.
• Ischemia causes tissue pain within minutes.
• High metabolic rates speed up ischemic pain.
• Exercising ischemic muscle causes pain in 15–20 seconds.
• Resting ischemic muscle causes pain in 3–4 minutes.
• Anaerobic metabolism creates lactic acid buildup.
• Lactic acid stimulates pain nerve endings.
• Bradykinin and proteolytic enzymes form during ischemia.
• Muscle spasm stimulates mechanosensitive receptors.
• Muscle spasm compresses blood vessels, causing ischemia.
• Muscle spasm increases metabolism, worsening ischemia.

Peripheral Pain Pathways

• Free nerve endings use two transmission pathways.
• Fast-sharp pain travels via type Aδ fibers.
• Type Aδ fibers conduct at 6 to 30 m/sec.
• Slow-chronic pain travels via type C fibers.
• Type C fibers conduct at 0.5 to 2 m/sec.
• Sudden injury causes a double pain sensation.
• Fast pain prompts immediate withdrawal reactions.
• Slow pain increases over time to demand relief.
• Peripheral fibers terminate in spinal dorsal horns.

The Neospinothalamic Tract

• Neospinothalamic tract transmits fast Aδ pain.
• Fast fibers terminate in lamina I (lamina marginalis).
• Second-order neurons cross through the anterior commissure.
• Axons ascend in the anterolateral columns.
• Few fibers terminate in brain stem reticular areas.
• Most fibers terminate in the thalamic ventrobasal complex.
• Some fibers terminate in the posterior nuclear group.
• Signals project to somatosensory cortex and basal areas.
• Fast pain can be localized accurately.
• Tactile co-stimulation via dorsal columns improves localization.
• Fast pain alone localizes within 10 centimeters.
• Glutamate is the transmitter for type Aδ fibers.
• Glutamate action lasts for a few milliseconds.

The Paleospinothalamic Tract

• Paleospinothalamic pathway transmits slow-chronic pain.
• Pathway mainly uses peripheral type C fibers.
• Type C fibers terminate in laminae II and III.
• Laminae II and III form the substantia gelatinosa.
• Neurons pass through short fibers to lamina V.
• Axons cross anterior commissure to opposite side.
• Axons ascend in the anterolateral pathway.
• Type C fibers release glutamate and substance P.
• Glutamate acts instantaneously for brief durations.
• Substance P releases slowly over seconds or minutes.
• Glutamate drives fast initial spinal transmission.
• Substance P drives lagging slow pain sensations.
• Only 1/10 to 1/4 of fibers reach the thalamus.
• Most fibers terminate in brain stem regions.
• Terminations include reticular nuclei of medulla, pons, mesencephalon.
• Terminations include tectal area of mesencephalon.
• Terminations include periaqueductal gray region.
• Lower brain regions process suffering pain qualities.
• Brain stem signals relay to intralaminar thalamic nuclei.
• Signals relay to ventrolateral thalamus and hypothalamus.
• Paleospinothalamic pathway gives poor pain localization.
• Slow pain localizes only to major body parts.
• Multisynaptic diffuse connections cause imprecise localization.

Brain Functions and Pain Perception

• Cortex removal does not stop pain perception.
• Brain stem and thalamus perceive conscious pain.
• Somatosensory cortex interprets pain quality.
• Cortical electrical stimulation causes mild pain.
• Reticular and intralaminar areas excite the brain.
• Slow pain signals activate the main arousal system.
• Severe pain prevents sleep due to arousal.
• Cordotomy interrupts anterolateral pathway for pain relief.
• Thoracic cordotomy relieves lower body pain temporarily.
• Cordotomy cuts opposite anterolateral quadrant.
• Upper body fibers may not cross immediately.
• Pain can return due to sensitized alternate pathways.
• Thalamic intralaminar cauterization relieves suffering pain.
• Cauterization preserves acute pain protection.

The Analgesia System

• The brain possesses an internal pain suppression system.
• Periaqueductal gray and periventricular areas send signals.
• Signals reach raphe magnus nucleus in lower pons.
• Signals reach nucleus reticularis paragigantocellularis in medulla.
• Second-order signals travel down dorsolateral columns.
• Signals enter pain inhibitory complex in dorsal horns.
• Periaqueductal gray stimulation suppresses strong pain signals.
• Hypothalamic periventricular nuclei excite periaqueductal gray.
• Medial forebrain bundle excites periaqueductal gray.
• Periventricular and periaqueductal neurons release enkephalin.
• Raphe magnus neurons secrete serotonin in dorsal horns.
• Serotonin stimulates local cord neurons to release enkephalin.
• Enkephalin causes presynaptic and postsynaptic inhibition.
• Inhibition blocks incoming Aδ and C pain fibers.
• System blocks incoming pain at spinal cord entry.
• System blocks local spinal withdrawal reflexes.

The Brain’s Opiate System

• Morphine injection into brain ventricles causes extreme analgesia.
• Opiates act on receptors in the analgesia system.
• Natural opiate-like substances derive from three protein precursors.
• Precursors are pro-opiomelanocortin, proenkephalin, and prodynorphin.
• Important opiates include β-endorphin, met-enkephalin, leu-enkephalin, dynorphin.
• Enkephalins exist in brain stem and spinal cord.
• β-endorphin exists in hypothalamus and pituitary gland.
• Dynorphin exists in same areas as enkephalins.
• Opiate activation suppresses peripheral pain signals.

Tactile and Electrical Pain Inhibition

• Stimulating Aβ tactile fibers depresses pain signals.
• Inhibition occurs via local lateral inhibition in cord.
• Rubbing skin near painful areas relieves pain.
• Liniments activate tactile fibers for pain relief.
• Acupuncture combines tactile inhibition and analgesia activation.
• Electrical stimulation on skin or spinal cord suppresses pain.
• Stereotaxic electrodes in thalamus allow personal pain control.
• Brief electrical stimulation gives hours of pain relief.

Principles of Referred Pain

• Referred pain is felt far from source tissue.
• Visceral pain refers to body surface areas.
• Visceral pain fibers synapse on same cord neurons.
• Second-order neurons receive skin and visceral inputs.
• Brain perceives visceral signals as skin pain.

Visceral Pain Characteristics

• Visceral pain aids in diagnosing internal disease.
• Viscera often lack non-pain sensory receptors.
• Localized visceral damage rarely causes severe pain.
• Cutting gut entirely causes minimal pain.
• Diffuse stimulation of visceral receptors causes severe pain.
• Diffuse ischemia causes extreme visceral pain.
• Visceral stimuli include ischemia, chemicals, and spasms.
• Visceral stimuli include distention and stretch.
• Visceral pain travels via small type C fibers.
• Visceral pain is chronic, aching, and suffering.
• Damaging GI chemicals cause severe peritoneal pain.
• Smooth muscle spasm causes mechanical pain receptor stimulation.
• Spasm causes ischemia by compressing blood vessels.
• Visceral spasm causes cramping pain cycles.

• Cramps occur in appendicitis, gastroenteritis, and gallstones.

• Overdistention stretches tissue and collapses blood vessels.
• Liver parenchyma and lung alveoli lack pain sensitivity.
• Liver capsule, bile ducts, and bronchi feel pain.

Parietal Pain and Transmission Pathways

• Disease spreading to parietal peritoneum causes sharp pain.
• Parietal surfaces receive extensive spinal nerve innervation.
• Incising parietal peritoneum produces severe pain.
• True visceral pain travels via autonomic nerve bundles.
• Visceral pain refers to embryonic origin dermatomes.
• Heart pain refers to neck, shoulder, and chest.
• Heart origin corresponds to C-3 to T-5 segments.
• Stomach pain refers to epigastrium (T-7 to T-9).
• Parietal pain travels directly into local spinal nerves.
• Parietal pain localizes directly over the painful area.
• Appendicitis causes dual pain transmission.
• Visceral appendix pain refers to umbilicus (T-10).
• Parietal appendix pain localizes to right lower quadrant.
• Clinical Abnormalities of Somatic Sensation
• Hyperalgesia means excessive sensitivity to pain.
• Primary hyperalgesia stems from hyperactive pain receptors.
• Sunburned skin is an example of primary hyperalgesia.
• Histamine and prostaglandins sensitize burned skin.
• Secondary hyperalgesia stems from central pathway facilitation.
• Herpes zoster infects dorsal root ganglion cells.
• Herpes causes severe pain in a single dermatome.
• Viral transport down axons causes a vesiculating rash.
• Tic douloureux causes facial lancinating electrical pain.
• Trigeminal or glossopharyngeal nerves drive tic douloureux.
• Mechanoreceptive trigger zones set off facial pain shocks.
• Surgical nerve sectioning relieves facial trigger pain.
• Brown-Séquard syndrome results from partial cord transection.
• Motor functions are lost ipsilaterally below transection.
• Spinothalamic pain and temperature loss occur contralaterally.
• Pain loss occurs 2 to 6 segments below level.
• Kinesthetic, position, and vibration loss occur ipsilaterally.
• Discrete touch discrimination is lost on transected side.
• Crude touch persists due to contralateral spinothalamic paths.

Mechanisms and Types of Headache

• Headaches represent referred pain from head structures.
• Brain tissue itself is insensitive to pain.
• Cortical sensory stimulation causes paresthesias, not pain.
• Tugging venous sinuses or dura causes severe headache.
• Stretching meningeal blood vessels causes headache pain.
• Middle meningeal artery is exceptionally pain sensitive.
• Supratentorial pain travels via fifth cranial nerve.
• Supratentorial headaches refer to front half of head.
• Subtentorial pain travels via CN IX, X, and C-2.
• Subtentorial headaches refer to occipital scalp areas.
• Meningitis causes intense headache across entire head.
• Low CSF pressure causes brain distortion and headache.
• Migraines may stem from abnormal vascular phenomena.
• Migraine prodromes include nausea and visual aurasa.
• Initial arterial vasospasm theoretically causes ischemic prodrome.
• Secondary vessel dilation and pulsation cause severe pain.
• Migraines show strong genetic predisposition (65–90%).
• Migraines affect women twice as often as men.
• Alcohol causes headache by directly irritating meninges.
• Muscle spasm from emotional tension causes scalp headache.
• Nasal sinus infection causes referred retro-orbital headache.
• Eye muscle strain causes tonic contraction and headache.
• Intense light exposure irritates conjunctivae and retina.

Thermal Receptors and Transmission

• Humans discriminate cold, cool, warm, and hot.
• Discrimination uses cold, warmth, and pain receptors.
• Extreme temperatures stimulate pain receptors.
• Cold spots outnumber warmth spots across skin.
• Warmth receptors are presumed free nerve endings.
• Warmth signals travel on type C fibers (0.4–2 m/sec).
• Cold receptors are specialized small Aδ nerve endings.
• Cold signals travel on Aδ fibers (20 m/sec).
• Cold fibers reach peak stimulation at 24 °C.
• Warmth fibers activate above 30 °C and fade at 49 °C.
• Heat-pain fibers activate around 45 °C.
• Extreme cold and heat produce similar painful sensations.
• Thermal receptors adapt rapidly to abrupt temperature changes.
• Thermal senses respond strongly to changing temperatures.
• Temperature alters intracellular chemical reaction rates.
• Reaction rates change twofold per 10 °C shift.
• Spatial summation allows detection of 0.01 °C body shifts.
• Small skin areas require larger temperature shifts.
• Thermal signals travel in tract of Lissauer.
• Fibers terminate in laminae I, II, and III.
• Axons cross to anterolateral tract and ascend.
• Signals terminate in reticular areas and thalamic ventrobasal complex.
• Cortical postcentral gyrus receives few thermal signals.

Disclaimer: This content is for educational purposes only and not a substitute for professional medical advice.