Lacrimal Gland and Tears PDF

Lacrimal gland and tears: 2 Next Thursday’s OP1206 lecture is now at 4.00 pm on Tuesday Nervous involvement in tears Tears can be secreted as a reflex response to a number of stimuli: Thermal stimuli Irritation of the cornea or conjunctiva Peppery foods Bright light Coughing Yawning Vomiting Emot...

Lacrimal gland and tears: 2 Next Thursday’s OP1206 lecture is now at 4.00 pm on Tuesday Nervous involvement in tears Tears can be secreted as a reflex response to a number of stimuli: Thermal stimuli Irritation of the cornea or conjunctiva Peppery foods Bright light Coughing Yawning Vomiting Emotional upset Tears produced above resting level - more aqueous – less protein content Lacrimal cells – under influence of hypothalamus The hypothalamus involved in the control of emotions and sexual activity Sympathetic nervous system (red) activates what is termed the fight or flight response. Involved in production of adrenaline and control of the cardiovascular system Parasympathetic (blue) rest and digest system, conserves energy - slows heart rate, increases intestinal and gland activity, and relaxes sphincter muscles in the gut The trigeminal ganglion is a sensory ganglion of the trigeminal nerve - responsible for sensation in the face The pons relays sensory information between the cerebellum and cerebrum; aids in relaying other messages in the brain; controls arousal, and regulates respiration Lacrimal cells – under influence of hypothalamus – hence neuronal pathways involved in emotional crying. Release of emotional pain New research - Tears from 'emotional crying' carry toxins not normally found in the tear film created to simply moisten the eye. Reflex excess lacrimation (irritation of cornea or conjunctiva) via trigeminal pathway (responsible for sensation in the face) Innervation by parasympathetic fibers and sympathetic fibers. Parasympathetic – secretion Sympathetic mainly controlling blood supply to lacrimal gland Stimulated Secretion by the Lacrimal Gland ACh – acetylcholine: neurotransmitter G proteins – messenger proteins, activated by ACh, find and activate enzyme systems involved in production of secretory products from the cell What makes us blink? We blink because: Evaporation of tear film – hypertonicity (High salt) Hypertonicity draws water from epithelial cells Epithelial cells then draw water from stroma Hypertonicity - slight sting that elicits blink to restore isotonicity The drainage system Drainage – driven by pressure gradients hypothesis 1 Before blink System Open (Canaliculi filled with  1.5l fluid)no presuure gradient in system 2 Closing phase Puncta meet & close Canaliculi & sac – compressed (+ Pressue) Fluid moves into duct 2 Compression of sac Horner,s muscle (part of Obicularis oculi) contracts during a blink Squeezes the lacrimal sac Reopening Sac & Canaliculi re-expand Negative pressue in sac End Puncta reopen Fluid flows into puncta & canaliculi Bunny Rabbits How do we assess the condition of the tear film? Paper tabs inserted into lower lids and removed after 5 mins Less than 10mm wetting is abnormal Schirmer test shows: Topical proparacaine hydrochloride anaesthesia reduces mean tear flow rate by 40% - prevents tearing due to the irritation from the paper There are no statistically significant differences in tear production between men and women Tear production does not drop significantly with age (although some texts say the opposite!) Tear Breakup Time (BUT) Measure of the length of time the eye can be kept open before the tear film ruptures spontaneously. Eye treated with fluorescein and tear film scanned with cobalt blue light. End point recorded (in seconds) at first appearance of dry spot. Tear Breakup Time (BUT) Problems: Uneven mixing of fluorescein Unknown fluorescein concentration Lag time between appearance of a dry spot and its discovery by the observer BUT toposcope. Patient’s head stabilized and eye fixed on target. Image of reflected grid evaluated by examiner. Tear BUT is seen as a distortion in the otherwise regular grid. Rose Bengal Stain Advantages: 1. stains dry epithelium while cells are still in situ 2. stains corneal filaments bright pink 3. stains lipid-contaminated mucous strands keratoconjunctivitis What can go wrong? Dry Eyes: Symptoms: mild burning and grittiness caused by corneal drying. Signs: reduced tear meniscus on lower eyelid; lack-lustre cornea; fluorescein and rose bengal staining of inferior cornea and conjunctiva; reduced wetting on Schirmer’s test. Causes: abnormalities in any of the tear film layers. Watery Eyes: Symptoms: excess production of fluid from the lacrimal gland. Causes: reflex watering due to inflammatory conditions such as foreign body, iritis, acute glaucoma, injury etc. Lacrimal gland disorders Dacryoadenitis Symptoms: Swelling and tenderness of the lateral aspect of the upper eyelid Signs: Inflammation of lacrimal gland. Dacryoadenitis: Symptoms: Swelling and tenderness of the lateral aspect of the upper eyelid Signs: Inflammation of lacrimal gland. Causes: Acute infections; chronic infections;mumps etc.