Lacrimation, reflex

The ocular surface and the tear-secreting glands of the eye are now known to function as an integrated unit. This unit refreshes the tear supply and clears used tears. An autonomic neural reflex loop stimulates secretion of tear fluid and proteins by the lacrimal glands. The sensitivity of the ocular surface decreases as aqueous tear production and tear clearance decreases. This results in a decrease in sensory-stimulated reflex tearing which exacerbates dry eye.29,30 Over time, wearing contact lenses also desensitizes the cornea by constant stimulation.12... 

In mice ethyl acetate at 2 000 ppm for 20 minutes produced acute neurobehavioral effects including changes in posture, decreased arousal, increased tonic/clonic movements, disturbances in gait, and delayed righting reflexes. Some handling-induced convulsions and slight lacrimation were also observed." ... 

FIGURE 23.2 Lacrimal flow and drainage. (1) The lacrimal gland produces tears that are distributed by blinking. (2) At normal rates of flow, tears are cleared by nasolacrimal drainage. (3) Sudden increases produced by instillation or reflex tearing overspill the lower fornix. 

The surface tension of tear fluid at the eye temperature has been measured as 43.6 to 46.6 rriNm-1 for normal eyes and 49.6 mNm"1 for patients with dry eye. The instillation of a solution containing drugs or adjuvants that lower the surface tension may dismpt the outmost lipid of the tear film into numerous oily droplets, which become solubilized. The protective effect of the oily film against evaporation of the tear film aqueous layer disappears and diy spots will be formed. The dry spots are painful and irritant and elicit reflex blinks to eliminate the material. This irritation does not always occur immediately after the instillation. In many cases it appears 30 minutes to 1 hour following the application and is dependent on the substance and on its concentration. The tear film is destabilized when the surface tension of the instilled solution is much lower than the surface tension of the lacrimal fluid. 

Chronic dry eye is the result of an underlying cytokine and receptor-mediated inflammatory process that affects the ocular surface and lacrimal gland, leading to decreased tear production or altered tear film contents. Hormonal, anti-inflammatory, or immunomodulatory agents may be able to suppress the inflammation and normalize the neural reflex between the ocular smrfece and lacrimal glands. 

After the instillation of most topical anesthetics, many patients report a heaviness of the eyelids that frequently lasts for several minutes after the return of corneal sensation. Conjunctival hyperemia and mild lacrimation sometimes occur after the application of most topical anesthetics. Rarely, the reflex action associated with discomfort may cause the fellow eye to become hyperemic... 

Figure 24-1 Schematic view of the lacrimal system.The lacrimal gland supplies aqueous (reflex) secretions. Arrows indicate the pathway that tears follow to drainage, beginning at the punctum. The area enclosed by dashed lines represents the drainage apparatus. (Adapted from BoteUio SY Tears and the lacrimal gland. Sci Am 1964 211 78-85. Copyright 1964 by Scientific American, Inc. AU rights reserved.)...

The seventh cranial nerve is responsible for eyelid closure during the blink reflex. Partial or complete disturbance of cranial nerve VII can interrupt these impulses, resulting in incomplete lid closure. Loss of muscular tone can also lead to ectropion, disruption of the lacrimal pump, and ultimately impaired tear drainage. 

Moderate to severe corneal abrasions usually are accompanied by other ocular signs. Diffnse or focal conjunctival injection is present depending on the size and location of the abrasion. Eyelid edema is common when profuse reflex lacrimation occurs. If the lesion has been present for at least 12 to 24 hours, a secondary traumatic anterior... 

Marked deviation of pH and tonicity from that of tears provokes reflex lacrimation, diluting the drug and accelerating its elution from the eye. H) ertonic solutions may be further diluted by fluid drawn from subconjunctival tissues. Animals will tolerate a pH range of 3.5-10.5 and tonicity from 0.5% to 2% sodium chloride concentration equivalents (Miller 1992). 

PCSI materials used in the context of atmospherically dispersed materials are sometimes classified as stemutators if their main action is on the upper respiratory tract, and lacrimators if the principal action is on the eye. For most currently used PCSIs, this is not a useful classification descriptor, since both effects are frequently present. Thus, exposure to an airborne PCSI RCA will cause effects in skin, eye and respiratory tract. When dispersed in solution, the effects are generally limited to the area of contact, and the reflexes elicited are a function of the afferent nerve involvement. In general, PCSI effects appear within seconds of contact, and subside within 10-60 min, depending on exposure concentration and site affected. 

Typical ocular symptoms associated with exposure to OC aerosol exposure include lacrimation, conjunctival inflammation, redness, severe burning pain, swelling, and blepharospasm. The application of capsaicin to the eye causes neurogenic inflammation and unresponsiveness to chemical and mechanical stimuli. Topical application of capsaicin eliminates the blink reflex for up to 5 days following dosing. Systemic administration of capsaicin is associated with trigeminal nerve fiber degeneration in the cornea. In humans, exposure to OC can cause loss of the blink reflex. 

HUMAN HEALTH RISKS Acute Risks irritation of eyes, skin and respiratory tract burning in larynx reflex cough sneezing allergic respiratory reactions lacrimation skin bums and blisters bums in digestive tract nosebleeds headaches double vision conjunctivitis photophobia Chronic Risks dermatitis chronic bronchitis upper respiratory tract infection pulmonary edema. 

ACUTE HEALTH RISKS irritation of eyes, skin and respiratory system lacrimation (discharge of tears) blurred vision corneal damage dyspnea (breathing difficulty) first, second, and third degree bums contraction of pupils loss of reflexes agitation coma. 

ACUTE HEALTH RISKS irritation of eyes, nose and throat irritation of the upper respiratory tract burning in the larynx reflex cough lacrimation headaches nosebleeds impairment of vision double vision allergic reactions. 

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