Pain is a complex phenomenon composed of sensory experiences (time, space, intensity) and emotion, cognition, and motivation.
The thalamus, cortex, and postcentral gyrus perceive, describe, and localize pain. The reticular formation and limbic system control the emotional and affective response to pain.
The portions of the nervous system responsible for the sensation and perception of pain may be divided into three areas: (a) the afferent fibers, (b) the central nervous system, and (d) the efferent pathways.
The afferent system is composed of nociceptors, A and C fibers, the dorsal horn of the spinal column, and afferent neurons in the spinothalamic tract.
Efferent pathways from the ventromedial thalamus and periaqueductal gray are responsible for modulation or inhibition of afferent pain signals.
The specificity theory of pain proposes that the intensity of pain is directly related to the degree of associated tissue injury. According to the gate control theory, there are specialized cells within the substantia gelatinosa that act as a gate, opening and closing the afferent pathways to transmission of painful stimuli. The neuromatrix theory of pain proposes that chronic pain is related to multidimensional inputs triggered from the periphery or originating independently within the brain.
Modulators of pain include substances that stimulate pain receptors (i.e. prostaglandins, bradykinins, lymphokines, substance P, glutamate) and substances that suppress pain (i.e., endorphins, GABA, serotonin, norepinephrine).
Endorphins are endogenous opioids that attach to opiate receptors and inhibit transmission of pain impulses. Enkephalins and dynorphin are other opioid peptides. They are present in varying concentrations in the neurons of the brain, spinal cord, and gastrointestinal tract.
Clinical descriptions of pain include somatogenic pain (with a known physiologic cause) acute pain (signal to the person of a harmful stimulus), and chronic pain (persistence of pain of unknown cause or unusual response to therapy).
Acute pain may be (a) somatic (superficial), (b) visceral (internal), or (c) referred (present in an area distant from its origin). The area of referred pain is supplied by the same spinal segment as the actual site of pain.
Chronic pain is persistent pain lasting at least 3 to 6 months and may be related to inflammation or injury to the nervous system or chronic inflammation.
Neuropathic pain is increased sensitivity to painful stimuli and results from abnormal processing of pain information in the peripheral or central nervous system.
Psychologic, behavioral, and physiologic responses to chronic pain include depression, difficulty in sleeping, preoccupation with pain, lifestyle changes, and physiologic adaptation.
Pain threshold is the point at which pain is perceived.
Pain tolerance is the duration of time or the intensity of pain that an individual will endure before initiating overt pain response.
Newborns and young children have the anatomic and functional ability to perceive pain. Older individuals tend to have a slightly higher pain threshold, probably because of changes in the thickness of the skin and peripheral neuropathies. Women appear to be more sensitive to pain than are men in all age groups.
Temperature regulation is achieved through precise balancing of heat production, heat conservation, and heat loss. Body temperature is maintained in a range around 37° C (98.6° F).
Temperature regulation is mediated by the hypothalamus through thermoreceptors in the skin, hypothalamus, spinal cord, and abdominal organs.
Heat is produced through chemical reactions of metabolism, skeletal muscle contraction, chemical thermogenesis, and vasoconstriction.
Heat is lost through radiation, conduction, convection, vasodilation, and decreased muscle tone, evaporation of sweat, increased respiration, and voluntary mechanisms. Heat conservation is accomplished through vasoconstriction and voluntary mechanisms.
Infants do not conserve heat very well because of their greater body surface/mass ratio and decreased subcutaneous fat. Elderly persons have poor responses to environmental temperature extremes as a result of slowed blood circulation, structural and functional changes in skin, and overall decrease in heat-producing activities.
Fever is triggered by the release of pyrogens from bacteria, leukocytes and other cells involved in the immune response. Fever is both a normal immunologic mechanism and symptom of a disease.
Fever involves the “resetting of the hypothalamic thermostat” to a higher level. When the fever breaks the set point returns to normal.
Fever production aids responses to infectious processes. Higher temperatures kill many microorganisms and decrease serum levels of iron, zinc, and copper that are needed for bacterial replication.
Hyperthermia (marked warming of core temperature) can produce nerve damage, coagulation of cell proteins, and death. Forms of accidental hyperthermia include heat cramps, heat exhaustion, heat stroke, and malignant hyperthermia. Heat stroke and malignant hyperthermia are potentially lethal.
Hyperthermia (marked cooking of core temperature) slows the rate of chemical reaction (tissue metabolism), increases the viscosity of the blood, slows blood flow through the microcirculation, facilitates blood coagulation, and stimulates profound vasoconstriction. Hypothermia may be accidental or therapeutic.
Sleep may be divided into REM and mom-REM stages, each of which has its own series of stages. While asleep, an individual progresses through REM and non-REM (slow wave) sleep in a predictable cycle.
REM sleep is controlled by mechanisms in the pons and mesencephalon. Non-REM sleep accounts for 75% to 80% of sleep time and is controlled by release of inhibitory signals from the hypothalamus.
The sleep patterns of the newborn and young child vary from those of the adult in total sleep time, cycle length, and percentage of time spent in each sleep cycle. Elderly persons experience a total decrease in sleep time.
The restorative, reparative, and growth processes occur during slow wave (non-REM) sleep. Sleep deprivation can cause profound changes in personality and functioning.
Sleep disorders include (a) dyssomnias (disorders of initiating sleep [i.e., insomnia, sleep disordered breathing, hypersomnia, or disorders of the sleep-wake schedule]) and (b) parasomnias (i.e., sleepwalking or night terrors).
The Special Senses
The wall of the eye has three layers: sclera, choroid, and retina. The retina contains millions of baroreceptors known as rods and cones that receive light through the lens and then convey signals to the optic nerve and subsequently to the visual cortex of the brain.
The eye is filled with vitreous and aqueous humor, which prevent it from collapsing.
The eyelids, conjunctiva, and lacrimal apparatus protect the eye. Infections are the most common disorders; they include blepharitis, conjunctivitis, chalazion, and hordeolum.
Structural eye changes caused by aging result in decreased visual acuity.
The major alterations in ocular movement include strabismus, nystagmus, and paralysis of the extra-ocular muscles.
Alterations in visual acuity can be caused by ambylopia, scotoma, cataracts, papilledema, macular degeneration, and glaucoma.
Alterations in accommodation develop with increased intraocular pressure, inflammation, and disease of the oculomotor nerve. Presbyopia is loss of accommodation caused by loss of elasticity of the lens with aging.
Alteration sin refraction, including myopia, hyperopia, and astigmatism, are the most common visual disorders.
Trauma or disease of the optic nerve pathways, or optic radiation, can cause blindness in the visual fields. Homonymous hemianopsia is caused by damage of one optic tract.
Conjunctivitis can be acute or chronic, bacterial, viral or allergic. Redness, edema, pain, and lacrimation are common symptoms. Chlamydia conjunctivitis is the leading cause of blindness in the world and is associated with poor sanitary conditions.
Keratitis is a bacterial or viral infection of the cornea that can lead to corneal ulceration. Photophobia, pain, and tearing are common symptoms.
The ear is composed of external, middle, and inner structures. The external structures are the pinna, auditory canal, and tympanic membrane. The tympanic cavity (containing three bones: the malleus, the incus, and the stapes), oval window, eustachian tube, and fluid compose the middle ear and transmit sound vibrations to the inner ear.
The inner ear includes the bony and membranous labyrinths that transmit sound waves through the cochlea to the division of the eighth cranial nerve. The semicircular canals and vestibule help maintain balance through the equilibrium receptors.
Approximately one third of all people older than 65 years have hearing loss.
Hearing loss can be classified as conductive, sensorineural, mixed, or functional.
Conductive hearing loss occurs when sound waves cannot be conducted through the middle ear.
Sensorineural hearing loss develops with impairment of the organ of Corti or its central connections.
A combination of conductive and sensorineural loss is a mixed hearing loss.
Loss of hearing with no known organic cause is a functional hearing loss.
Meniere disease is a disorder of the middle ear that affects hearing and balance.
Otitis externa is an infection of the outer ear associated with prolonged exposure to moisture.
Otitis media is an infection of the middle ear that is common in children. Accumulation of fluid (effusion) behind the tympanic membrane is a common finding.
The perception of flavor is altered if olfaction or taste dysfunctions occur. Sensitivity to odor and taste decreases with aging.
Hyposmia is a decrease in the sense of smell, and anosmia is the complete loss of the sense of smell. Inflammation of the nasal mucosa and trauma or tumors of the olfactory nerve lead to a diminished sense of smell.
Hypogeusia is a decrease in taste sensation, and ageusia is the absence of the sense of taste. Loss of taste buds or trauma to the facial or glossopharyngeal nerves decreases taste sensation.
Tactile sensation is a function of receptors present in the skin (pacinian corpuscles), and the sensory response is conducted to the brain through the dorsal column and anterior spinothalamic tract.
Alterations in touch can result from disruption of skin receptors, sensory transmission, or central nervous system perception.
Proprioception is the position and location of the body and its parts. Proprioceptors are located in the inner ear, joints, and ligaments. Proprioceptive stimuli are necessary for balance, coordinated movement, and grading of muscular contraction.
Disorders of proprioception can occur at any level of the nervous system and result in impaired balance and lack of coordinated movement.