#superior colliculus

The superior colliculi form the rostral two bumps (one on each side) on the dorsal aspect of the midbrain. The caudal two bumps are the inferior colliculi and together they (inferior and superior colliculi) comprise the TECTUM or roof of the midbrain. In contrast to the inferior colliculus, which is an AUDITORY structure, the superior colliculus is usually described as a VISUAL reflex center. It is a highly laminated (layered) structure. The top or dorsal-most three layers receive visual information primarily from two sources, i.e., the retina (retinocollicular) and the visual cortex (area 17; corticotectal). In contrast to the exclusively visual nature of the superficial layers, the intermediate and deep layers receive projections from many functionally different areas of the brain. These inputs are both “motor” and “sensory”. Since the latter category includes visual, auditory and somatosensory inputs, you can see that the superior colliculus is not exclusively related to visual function. Instead, it plays a role in helping orient the head and eyes to all types of sensory stimuli. One of the major efferent projections of the superior colliculus is to the CERVICAL SPINAL CORD. This TECTOSPINAL TRACT arises from cells within the intermediate and deep layers, crosses at midbrain levels and courses caudally through the midbrain, pons and medulla close to the MLF (we do not identify it in our sections). Upon reaching the spinal cord tectospinal axons course within the VENTRAL funiculus and terminate upon medially placed neurons within the cervical cord. This tract is important in reflex turning of the head in response to visual, auditory and somatosensory stimuli. For instance, a flash of light to your LEFT causes you to turn your head to the LEFT. This reflex would involve a projection from the retinae to the superficial layers of the RIGHT superior colliculus (retinocollicular), a short pathway from cells in the superficial layers to cells in the intermediate and deep layers and then the long CROSSED tectospinal axons to the LEFT side of the cervical spinal cord. Spinal cord neurons on the LEFT side then innervate muscles such as the splenius capitus and semispinalis capitus, which rotate your head to the LEFT. Cells within the intermediate and deep layers also are involved in the control of eye movements. We will not go into how the collicular neurons participate in such control, but you are already familiar with the PPRF, which is an area of the pons involved in the control of horizontal eye movements. For example, cells in the intermediate and deep layers of the LEFT superior colliculus project to the RIGHT PPRF (this pathway is not illustrated in the coursebook). You know the circuitry from here that moves both eyes to the RIGHT. If not, see that INFAMOUS POINT 13 for a review! A syndrome that you might see in the clinical literature involving the superior colliculus is called the dorsal midbrain or PARINAUD syndrome. This is usually caused by a tumor of the pineal gland that compresses the superior colliculi and results in a paralysis of UPWARD gaze. It is not clear if this deficit is due to involvement of ONLY the superior colliculi. A center for vertical eye movements lies just rostral to the superior colliculi and could be involved. For now, REMEMBER-PARINAUD SYNDROME===PARALYSIS OF UPWARD GAZE.