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10/11/12 Why do we blink? • 
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Maintenance of Tear Film Removal of foreign bodies ProtecAon from looming objects Visual-­‐motor coordinaAon: –  Blinks can help break fixaAon, facilitaAng saccades. –  Some individuals with oculomotor apraxia (paralysis of voluntary gaze control) will use blinks to help iniAate saccades. Movement of the eyes during a blink • The two eyes move mostly conjugately during a blink • Eyes tend to rotate towards primary posiAon during a blink 1 10/11/12 Lid Dynamics •  Latency –  as low as 25ms for corneal irritaAon or sudden loud sounds. –  Blinks to flashes of light have a longer latency (~50ms); perhaps includes a corAcal pathway •  Dura,on –  The typical eye blink lasts about 200-­‐400msec, with full pupil coverage for perhaps half that Ame •  Velocity –  Peak Velocity on the order of saccadic peak velociAes –  A full blink has a higher peak velocity than a parAal one, resulAng in a Main Sequence for lid closure. Blink main-­‐sequence 2 10/11/12 Types of Eye-­‐Blinks •  Reflex Blinks –  These are made in response to a specific sAmulus; flash of light, sudden loud sound, corneal irritaAon, looming object. –  occur involuntarily and o\en unconsciously. –  can be condiAoned, so that if a warning sound is presented before a puff of air, a\er some Ame it will elicit a reflex blink on its own. •  Voluntary Blinks –  These are made on command, at will. They are usually larger in amplitude and last longer than reflex blinks. •  Spontaneous Blinks –  These are made without any specific sAmulus or effort. Even if the cornea is anestheAzed, blinks conAnue at about the normal rate, indicaAng that spontaneous blinks are not responses to dry eyes or other irritants. –  Typical spontaneous rate is about 15 -­‐ 30 blinks per minute. If each lasts 1/4 second, then the eyes are closed for around 6% -­‐ 12% of the waking day. –  Blink rate varies greatly across individuals and across task. Infants have very low blink rates (about 2 per minute), and it increases to adult levels gradually through life. Vision during a blink •  The eyelids typically reduce light on reAna by two log units, for a duraAon of about 250msec, and all paaern is completely abolished. •  Despite this, we seldom noAce our spontaneous eye blinks due to a combinaAon of visual masking effects and visual suppression effects. •  Experiments show that both the amount of dimming and the duraAon of dimming are reduced in our percepAon during blinks, so the gap is "closed in." •  There is speculaAon that the same mechanism contributes to visual suppression during saccades and blinks, but this has not been confirmed experimentally. 3 10/11/12 Muscles controlling eyelids – Levator Palpebrae (LP) •  ElevaAon of the upper lid is accomplished by acAon of the levator muscle primarily. The levator is innervated by the IIIrd nerve and is closely associated with the superior rectus muscle. •  Muller's Muscle and the frontalis muscle play a minor role. •  In primary gaze, the levator must maintain constant tone to keep the lid elevated. •  On extreme downgaze, the levator acAvity and the superior rectus acAvity both cease almost enArely. Muscles controlling eyelids – Orbicularis Oculi •  This is the antagonist muscle to the LP •  AcAve closure of the eye involves the Orbicularis Oculi muscle, which surrounds the palpebral fissure and closes it off with a sphincter acAon. 4 10/11/12 CoordinaAon of eyelid muscles •  Blinking –  During an eyeblink, the levator is inhibited completely, followed by acAvaAon of the Orbicularis Oculi. –  At the end of the blink, the Orbicularis shuts down and the levator resumes its former tone. CoordinaAon of eyelid muscles •  Winking –  When one eye is closed only, as in a wink, the levator muscles remain acAve and the Orbicularis muscle in contracted on just one side, closing the eye against the force of the levator. –  Thus, we generally can't voluntarily relax one levator, but we can voluntarily contract one Orbicularis. •  Relaxed –  In a relaxed state, both muscles have low acAvity and the lids close almost enArely. This underscores the importance of levator tone in keeping the lids raised. 5 10/11/12 Neural Control of Levator Palpabrae •  The Levator muscle is controlled from the Central Caudal Nucleus via the IIIrd Cranial Nerve. –  The CCN is a midline structure and very near the area of the OMN which controls the SR muscle on both sides. –  Although individual levator neurons project to only one eyelid, they are co-­‐localized in the CCN and therefore bilateral control of eyelids is achieved. •  Supranuclear control of the Levator is not well mapped, but regions have been found in cortex which elicit only lid elevaAon when sAmulated. Neural Control of Orbicularis Oculi •  The Orbicularis muscle is controlled from the ipsilateral facial nucleus in the brainstem. •  This receives primarily contralateral projecAons from the Motor Cortex for voluntary control of blinks and eye closing. •  It also gets inputs from the basal ganglia and other subcorAcal structures for control of spontaneous and reflex blinking, and for emoAonal expressions of the face. •  Because the voluntary and involuntary pathways are disAnct, one can show deficits which affect voluntary blinks but not reflex blinks, and vice versa. 6 10/11/12 Disorders of Lid Control – Insufficient Opening •  Ptosis –  Failure to maintain elevaAon of the lids is referred to as a ptosis. –  Because lid elevaAon is controlled bilaterally at the supranuclear level, unilateral ptosis implies a problem at the neuromuscular juncAon or at the level of the IIIrd nerve. Mild ptosis also accompanies ocular sympatheAc paralysis, such as Horner’s syndrome, and this can be unilateral. –  A corAcal deficit almost always produces a bilateral ptosis. •  Pseudoptosis –  Some condiAons mimic ptosis without there being an actual deficit in Levator funcAon. –  Hypotropia will include a lid component, such that the depressed eye will have a depressed lid as well. This simply reflects the normal yoking between eye posiAon and lid. –  Sagging skin in the upper lids can give the lids a droopy appearance and may obscure vision. –  Abnormally small or retracted globes will also give the appearance of ptosis. Disorders of Lid Control – Excessive opening •  Collier’s Sign is an excessive retracAon of the lids on aaempted upgaze, and usually is caused by damage to the dorsal mesencephalon, where verAcal gaze centers are located. The lids follow gaze normally on downgaze, but on aaempted upgaze the lids lead the eye considerably. •  Lid Lag occurs when the eyelids fail to lower as gaze is lowered, or do so very sluggishly. This occurs with “extrapyramidal syndromes”; diseases affecAng the basal ganglia and other supranuclear structures that normally inhibit the levator during downgaze. •  Lid retracAon may also accompany horizontal gaze in abducens palsy, as the increased effort to alter gaze shows up in increased lid elevaAon. •  Lid retracAon in the eye opposite to a ptosis is due to the common innervaAon to the lids. Increased effort to raise the ptoAc lid results in retracAon of the normal lid. Covering the ptoAc eye usually makes the retracAon disappear. This is another example of Hering’s Law of equal innervaAon. 7 10/11/12 Disorders of Lid Control – Blepharospasm (Excessive closure) •  Repeated, forceful involuntary closure of the eyes. In extreme cases, the paAent becomes funcAonally blind. •  Ocular Blepharospasm is due to ocular disease which irritates the cornea, such as dry eyes, uveiAs, keraAAs. •  EssenAal Blepharospasm is idiopathic –  It strikes women more than men, usually in late middle age. •  Oculomotor Apraxia is someAmes accompanied by excessive blinking, but specifically in relaAon to saccades. Disorders of Lid Control – Insufficient eyelid closure •  CorAcal lesions in the frontal lobes can lead to loss of voluntary control over acAve closure of the eyes. Early signs o\en are a loss of ability to wink. Reflex blinks and emoAonal responses (e.g. smiling) will be intact. •  Diseases affecAng the basal ganglia may also produce eye closure insufficiency. O\en the rate of spontaneous blinking will be reduced, as in Parkinsonism. When the rate drops to just once per minute there may be corneal drying as well. •  Diseases affecAng the facial nerve or nucleus will usually involve other facial muscles besides the Orbicularis Oculi, and may be bilateral or unilateral. 8