Sagittal Section of the Midbrain Showing Structures Related to Normal Eyelid Function

𝗢𝗿𝗶𝗴𝗶𝗻𝗮𝗹 𝗗𝗲𝘀𝗰𝗿𝗶𝗽𝘁𝗶𝗼𝗻: During a vertical saccade, the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) is activated, which excites the superior rectus (SR) and inferior oblique (IO) (IIIrd nerve) subnuclei. Additionally, the riMLF activates the nearby M-group. The M-group's primary excitatory output is to the CCN (central caudal nucleus), resulting levator palpebrae contraction and eyelid elevation. The opposite occurs during downgaze, resulting in levator relaxation. Eyelid retraction in midbrain disease may result from overstimulation of the M-group in an effort to overcome upgaze paresis, or could be related to loss of M-group inhibition by lesions involving the nearby interstitial nucleus of Cajal and/or nucleus of the posterior commissure. During a blink, the LPS abruptly ceases firing and the orbicularis oculi (OO), which is innervated by the facial nerve, briefly contracts. The coordination of LPS and OO is mediated by the superior colliculus (SC). The SC projects to the supraoculomotor area directly overlying the CCN as well as to the facial nuclei and is inhibited by the pars reticulata of the substantia nigra (SNr). This explains why in parkinsonism, due to increased activity of the SNr (which normally inhibits the SC), there is greater inhibition of the SC and reduced spontaneous blinking. 𝗡𝗲𝘂𝗿𝗼-𝗼𝗽𝗵𝘁𝗵𝗮𝗹𝗺𝗼𝗹𝗼𝗴𝘆 𝗮𝗻𝗱 𝗡𝗲𝘂𝗿𝗼-𝗼𝘁𝗼𝗹𝗼𝗴𝘆 𝗧𝗲𝘅𝘁𝗯𝗼𝗼𝗸 𝗟𝗲𝗴𝗲𝗻𝗱: Midbrain structures relevant to normal eyelid function