Protective mechanisms in the eye

1. OPTOM FASLU MUHAMMED

2.  eyelid movements are mediated by the
orbicularis oculi (OO) and levator palpebrae
superioris (LPS) muscles, as well as by the
superior tarsal muscle (ST) OO and LPS are
striated muscles
 the superior tarsal muscle is a smooth muscle
(sympathetic innervation via α1 receptors)

3. 1.maintaining ocular opening
 tonic activation of LPS and ST;
inactivation OO
2 gentle opening/closing, adjustment to
changes in globe position
 activation/inactivation of LPS;
inactivation OO
3.blinking, firm closure of eyes
 OO activation; inhibition of LPS

4.  corneal lubrication
 eye protection
 visual information processing

5. 1.spontaneous blinking:
 is precisely conjugated, periodic,
symmetrical, brief and occurs in the absence
of external stimuli or internal effort
 show a wide variation in rate (typically 10-20
blinks/minute in adults; lower in children)
 originates in premotor brainstem structures
that are highly influenced by dopaminergic
activity
 decreased in parkinson’s disease, and increased
in schizophrenia and Huntington's disease, for
example

6. 2.the blink reflex:
 can be initiated by either touch to the
cornea (afferents in the trigeminal nerve) or
by bright light/rapidly approaching objects
(afferents in the optic nerve)
 is faster than spontaneous blinking

7.  the tear film that covers the suface of the
eye has 3 layers:
1.lipid secred by oil glands in the eyelids
2.aqueous-based solution from lacrimal gland
(contains lysozyme and other enzymes that
provide protection against infection)
3.mucous from the conjunctiva

8.  the composition of the tear layer varies
depending upon the stimulus and with age:
1.emotional tears contain more hormones,
such as prolactin, ACTH and enkephalin
2.basal tear production decreases with age

9. tear flow occurs via evaporation and drainage through the
nasolacrimal ducts into the nasal cavity
•parasympathetic stimulation produces epiphora (overflow
of tears) by:
1.increasing tear production by the lacrimal gland
2.decreasing outflow by facilitating closure of the
lacrimal duct passage

10. •epiphora can be induced by:
1.stimulation of the cornea (cranial nerve V)
which produces reflex tears
2.strong emotional responses (mediated by
the limbic system, especially the
hypothalamus) which produce psychic tears
(crying or weeping)
-strong parasympathetic stimulation is
accompanied by other symptoms, like
reddening of the face and convulsive
breathing