Chapter 15: Pain, Temperature, Sleep, and Sensory Function
Pain
Definition:
Pain is an unpleasant but protective phenomenon uniquely experienced by each individual.
It cannot be defined, identified, or measured by an observer.
McCaffery's definition: "Pain is whatever the experiencing person says it is, existing whenever he says it does."
Neuroanatomy of Pain
Processing Harmful Stimuli
Nociception:
The processing of harmful stimuli through a normally functioning nervous system.
Nociceptors
Free nerve endings located in:
Skin, muscles, joints, arteries, and viscera.
Types of fibers:
A-delta myelinated fibers:
Responsible for localized fast pain sensations.
Unmyelinated C fibers:
Responsible for poorly localized dull pain sensations.
Phases of Nociception
Transduction: Activation of nociceptors.
Transmission: Conduction of signals to the dorsal horn and ascending in the spinal cord.
Perception: Conscious awareness of pain, including:
Sensory-discriminative system
Affective-motivational system
Cognitive-evaluative system
Pain concepts: Pain threshold, perceptual dominance, and pain tolerance.
Modulation: The increase or decrease in transmission before, during, or after perception.
Neurotransmitters of Pain Modulation
Modulating Pain
Pain modulation occurs in the periphery, spinal cord, and brain, typically triggered by tissue injury and/or inflammation.
Excitatory Neurotransmitters
Examples:
Glutamate
Aspartate
Substance P
Calcitonin
Inhibitory Neurotransmitters
Examples:
GABA
Glycine
Serotonin
Norepinephrine
Endogenous Opioids
Morphine-like neuropeptides that bind to specific opioid receptors inhibiting pain impulses in the periphery, spinal cord, and brain.
Key characteristics:
Responsible for sensations of well-being and modulation of various physical processes.
Types of Natural Opioids
Enkephalins:
Most prevalent natural opioids.
Include endorphins, dynorphins, and endomorphins, which have pharmacologic actions similar to morphine.
Endocannabinoids:
Synthesized from phospholipids and classified as eicosanoids.
Cannabis produces a resin containing cannabinoids which exhibit analgesic properties; however, they have psychoactive and addictive properties.
Pathways of Modulation
Descending Inhibitory or Facilitory Pathways
Involves:
Activating opioid receptors.
Inhibiting the release of excitatory neurotransmitters.
Releasing inhibitory neurotransmitters.
Stimulating inhibitory interneurons.
Stimulation of efferent pathways to inhibit pain signals.
Segmental Inhibition of Pain
Utilizes A-beta fibers to stimulate inhibitory interneurons, decreasing pain transmission.
Diffuse Noxious Inhibitory Control (DNIC)
Pain can be relieved when two noxious stimuli occur simultaneously from different sites.
Expectancy-related Cortical Activation
Associated with the placebo effect where cognitive expectations can induce physiological effects.
Acute Pain
Characteristics
A protective mechanism alerting individuals to conditions that are immediately harmful.
Generally transient, beginning suddenly and relieved once the pain stimulus is removed.
Types of Acute Pain
Somatic Pain:
Arises from skin, joints, and muscles; can range from sharp to dull pain.
Visceral Pain:
Arises from internal organs; often poorly localized with aching, gnawing, throbbing, or cramping qualities; can radiate or be referred.
Referred Pain:
Felt in a region distant from its point of origin, supplied by the same spinal segment as the site of origin; can be either acute or chronic.
Chronic Pain
Pain lasting longer than the expected healing time, typically defined as lasting at least 3 to 6 months.
Characteristics:
May be ongoing or intermittent, often associated with stress and leading to behavioral and psychological changes.
Persistent pain causes physiological adaptations.
Neuropathic Pain
Refers to dysfunction causing long-term changes in pain pathway structures and abnormal sensory processing.
Characteristics:
Amplification of pain without stimulation, often described as burning, shooting, shocklike, or tingling.
Classifications:
Peripheral Neuropathic Pain
Central Neuropathic Pain
Nociplastic Pain
Characterized by altered nociception without clear evidence of injury or threat.
Example:
Fibromyalgia:
Chronic muscle, bone, and joint pain with increased sensitivity to sensory input.
Lacks biomarkers and is likely multifactorial.
Symptoms include tender points.
Temperature Regulation
Control Mechanisms
Body temperature is controlled through:
Heat production
Heat conservation
Heat loss
Temperature regulation varies based on:
Location, activity, environment, circadian rhythm, and gender.
Mediating Structures
Hypothalamus: Primary temperature control center, mediated by the endocrine system.
Peripheral Thermoreceptors:
Located in skin, liver, and skeletal muscle.
Central Thermoreceptors:
Located in hypothalamus, spinal cord, and viscera.
Mechanisms for Heat Management
Heat Production
Generated via:
Chemical reactions of metabolism.
Skeletal muscle tone and contraction.
Chemical thermogenesis.
Heat Conservation
Achieved through:
Vasoconstriction.
Shivering.
Voluntary mechanisms (e.g., wearing warm clothes).
Heat Loss
Mechanisms include:
Radiation, conduction, convection.
Vasodilation.
Evaporation.
Decreased muscle tone.
Increased respiration rates.
Voluntary measures and adaptation to warmer climates.
Special Considerations
Infants
Produce sufficient heat but cannot conserve it effectively due to:
Small body size and higher body surface-to-weight ratio.
Thin subcutaneous layer.
Inability to shiver.
Lower sweating rate.
Higher peripheral blood flow in heat.
Older Adults
Respond poorly to temperature extremes due to:
Slow blood circulation.
Skin changes.
Decreased heat-producing activities.
Reduced shivering, sweating responses, metabolic rates, vasoconstrictive responses, and perception of heat and cold.
Fever
Definition
Fever represents a temporary elevation of the hypothalamic thermostat to a higher level, resulting in increased heat production and conservation.
Influenced by:
Exogenous Pyrogens: External agents that induce fever.
Endogenous Pyrogens: Internally produced substances leading to fever.
Antipyretic Cytokines: Molecules that help lower fever.
The individual typically first feels cold, then warm after the fever breaks.
Associated conditions:
Febrile seizures.
Fever of unknown origin (FOU): Characterized by high fever lasting longer than 3 weeks without a diagnosis.
Benefits of Fever
Helps the body respond to infections.
Kills many microorganisms.
Decreases serum levels of minerals (iron, zinc, copper).
Promotes lysosomal breakdown and autodestruction of cells.
Increases lymphocytic transformation and phagocyte motility.
Augments antiviral interferon production and phagocytosis.
Hyperthermia
Definition
Characterized by an elevation of body temperature without an increase in the hypothalamic set point.
Can lead to nerve damage, coagulation of cell proteins, and potentially death.
May be associated with conditions such as stroke or head trauma.
Types of Hyperthermia
Therapeutic Hyperthermia:
Local, regional, or whole-body increase in temperature used for destroying microorganisms or tumor cells.
Accidental Hyperthermia:
Heat Cramps: Spasmodic cramps in abdomen/extremities due to sodium loss after sweating.
Heat Exhaustion: Profound vasodilation and excessive sweating due to prolonged exposure to high core or environmental temperatures.
Heat Stroke: Heat loss mechanisms fail at core temperatures >40° C (104° F), can be lethal.
Malignant Hyperthermia: Uncoordinated muscle contractions and increased lactic acid; associated with a genetic muscular disorder.
Hypothermia
Definition
Body temperature <35° C (95° F).
Effects
Causes depression of nervous and respiratory systems.
Leads to vasoconstriction, changes in microcirculation and coagulation, and tissue ischemia.
In severe cases, ice crystals form within cells, causing rupture and cell death.
Mechanisms
Tissue Hypothermia:
Slows the rate of cellular metabolism.
Increases blood viscosity, facilitating coagulation and stimulating vasoconstriction.
Therapeutic Hypothermia:
Utilized to slow metabolism and preserve ischemic tissue during procedures (e.g., surgery, limb reimplantation).
Accidental Hypothermia:
Results from cold water immersion, prolonged cold exposure, or impaired thermoregulation.
Trauma and Temperature
Central Nervous System (CNS) Trauma
CNS trauma can lead to central fever (neurogenic fever) due to:
Inflammation.
Increased intracranial pressure (ICP).
Intracranial bleeding.
This type of fever does not induce sweating and is resistant to antipyretic therapies.
Other trauma affecting temperature includes:
Accidental injuries.
Hemorrhagic shock.
Major surgeries.
Thermal burns.
Sleep
Overview
Sleep is an active, multiphase process essential for restorative functions and memory consolidation.
The Hypothalamus serves as the major sleep center, promoting wakefulness and REM sleep through hypocretins (orexins).
Phases of Sleep
Rapid Eye Movement (REM) Sleep:
Accounts for 20% to 25% of total sleep time, occurring roughly every 90 minutes after 1 to 2 hours of non-REM sleep.
Known as “paradoxical sleep” due to EEG patterns resembling those of an awake state.
Features include increased parasympathetic activity, variable sympathetic activity, muscle relaxation, variable thermoregulation, altered heart rates, blood pressure, respiration, and memorable dreams.
Non-Rapid Eye Movement (NREM) Sleep:
Comprises 75% to 80% of sleep time.
Characterized by decreased sympathetic tone and increased parasympathetic activity, leading to reduced metabolic rate, temperature, heart rate, respiration, blood pressure, and muscle tone.
Sleep Disorders
Common Dyssomnias
Insomnia
Obstructive Sleep Apnea Syndrome (OSAS):
Risk factors include obesity, age, etc.
Characterized by multiple apneic episodes, potentially leading to untreated complications such as polycythemia, hypertension, stroke, heart failure, dysrhythmias, liver congestion, and edema.
Hypersomnia
Narcolepsy
Circadian Rhythm Sleep-Wake Disorders:
Advanced Sleep Phase Disorders
Delayed Sleep Phase Disorder
Shift Work Sleep Disorder
Jet Lag Disorder
Common Parasomnias
Sleepwalking (somnambulism)
Night terrors
Rearranging furniture during sleep
Eating food while asleep
Sleep sex (sexsomnia)
Violent behaviors during sleep
Restless leg syndrome
Special Senses
Overview
Vision: Involves the eyes.
Hearing: Involves the ears.
Olfaction (smell): Engaged by cranial nerves I and V.
Taste (gustation): Engaged by cranial nerves VII and IX.
The Eye
Structure
Sclera:
Thick, white outermost layer that becomes transparent at the cornea.
Choroid:
Deeply pigmented middle layer, includes the iris and pupil.
Retina:
Innermost layer containing rods and cones.
Visual Processing
Nerve impulses pass through optic nerves to the optic chiasm.
Light enters the eye and focuses on the retina via the lens, a flexible biconcave structure.
Accommodation: Ability of the lens to change shape for focusing on different distances.
Anterior chamber filled with aqueous humor and vitreous chamber filled with vitreous humor.
Visual Dysfunction
Alterations in Ocular Movements
Strabismus: One eye deviates leading to:
Amblyopia: Reduced vision in affected eye.
Diplopia: Double vision.
Nystagmus: Involuntary rhythmic movement of eyes with types including:
Pendular Nystagmus: Regular back and forth movement.
Jerk Nystagmus: One phase of eye movement is faster than the other.
Paralysis of individual extraocular muscles can lead to:
Limited abduction, abnormal eyelid closure, ptosis, diplopia.
Alterations in Visual Acuity
Visual Acuity: The ability to see sharp details; affected by:
Amblyopia: Reduced or dimmed vision.
Scotoma: Blind spot in vision.
Cataracts: Cloudy area in lens.
Papilledema: Edema of the optic nerve.
Dark Adaptation: Influences visual performance in low-light conditions.
Glaucoma: High intraocular pressure.
Retinal Detachment: Separation of the retina from its underlying tissue.
Age-related Macular Degeneration: A progressive disease affecting the retina.
Alterations in Accommodation
Accommodation: The lens's ability to change shape for focusing.
Issues arise from changes in the oculomotor nerve, leading to manifestations such as diplopia, blurred vision, or headaches.
Presbyopia: Loss of accommodation associated with aging.
Alterations in Refraction
Refractive Errors: Common visual problems caused by irregularities affecting:
Corneal curvature
Focusing power of the lens
Length of the eye
Types:
Myopia: Nearsightedness.
Hyperopia: Farsightedness.
Astigmatism: Unequal curvature of the cornea, may coexist with other errors.
Alterations in Color Vision
Caused by age-related changes (yellowing of lens) or genetic factors (color blindness, predominantly red-green color blindness) and acquired conditions from diseases.
Neurologic Disorders Affecting Vision
Vision may be disrupted along the visual pathway at various points, leading to:
Blindness in the entire visual field.
Half of the visual field (hemianopia).
External Eye Disorders
Blepharitis: Inflammation of the eyelid.
Hordeolum (stye): Infection of oil glands at the eyelid margin.
Chalazion: Blockage of oil gland leading to cyst formation.
Entropion: Inward turning of the eyelid.
Conjunctivitis: Inflammation of the conjunctiva, types include:
Acute Bacterial Conjunctivitis (pinkeye).
Viral Conjunctivitis.
Allergic Conjunctivitis.
Trachoma (chlamydial conjunctivitis).
Keratitis: Inflammation of the cornea.
The Ear
Structure
External Ear:
Functions in hearing, composed of the pinna and external auditory canal.
Tympanic Membrane: Separates external from middle ear.
Middle Ear:
Functions in hearing, composed of the tympanic cavity containing the malleus, incus, and stapes.
Connected to the thorax via the eustachian tube.
Inner Ear:
Involves hearing and equilibrium, composed of osseous labyrinths filled with perilymph, including cochlea, vestibule, and semicircular canals.
Organ of Corti: Contains hair cells sensitive to sound.
Equilibrium Receptors: Include crista ampullaris and maculae.
Auditory Dysfunction
Types
Conductive Hearing Loss:
Impairment of sound conduction from the outer ear to inner ear.
Sensorineural Hearing Loss:
Impairment of the organ of Corti or its central connections, often resulting in presbycusis (age-related hearing loss).
Mixed Hearing Loss:
Combination of conductive and sensorineural losses.
Functional Hearing Loss:
Individual appears not to hear and does not respond to auditory cues.
Ménière Disease:
Episodic disorder involving excessive pressure in the labyrinth leading to symptoms like vertigo, tinnitus, and hearing loss.
Ear Infections
Otitis Externa:
Infection of the outer ear, generally caused by bacteria, commonly known as swimmer’s ear resulting from moisture retention.
Otitis Media:
Includes acute otitis media and otitis media with effusion, primarily affecting the middle ear.
Olfaction and Taste
Overview
Olfaction (Smell):
Function primarily of cranial nerve I and partially of V.
Involves olfactory stimulants.
Gustation (Taste):
Function via cranial nerves VII and IX involving taste sensations.
Olfactory and Taste Dysfunctions
Types
Hyposmia: Impaired sense of smell.
Anosmia: Complete loss of sense of smell.
Olfactory Hallucinations: Smelling non-existent odors.
Parosmia: Abnormal sense of smell.
Hypogeusia: Decreased sense of taste.
Ageusia: Absence of taste.
Dysgeusia: Perversion in the sense of taste.
Somatosensory Function
Touch
Receptors present in skin:
Meissner Corpuscles
Pacinian Corpuscles
Merkel Disks
Ruffini Endings
Tactile Discrimination: Tends to decrease with advanced age.
Proprioception
Refers to the awareness of the position of body and its parts, dependent on the inner ear as well as receptors in joints and ligaments.
Progressive loss with age contributes to an increased risk of falls and injuries.
Dysfunctions in Somatosensory Function
May manifest as:
Vestibular Nystagmus
Vertigo
Ménière Disease
Peripheral Neuropathies