Intro arrow 8. Oculomotor System
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7. Eye Movements
8. Oculomotor System
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8. Oculomotor System

From perception to action, the Oculomotor System is the control system that coordinates the 12 muscles that move the 2 eyes. The main job of this system is Foveating: control the position of our eyes so that the gaze-image falls on the center of the Fovea. The Cerebral Cortex chooses significant objects in the environment as targets for eye movements. Cortical signals are relayed to motor circuits in the brain stem by the Superior Colliculus. The cortical and collicular signals of desired eye movements are transformed in the Brainstem into signals appropriate for each Extraocular Muscle.

 EXTRAOCULAR MUSCLES: The eye movements that direct gaze are due to 6 muscles, or 3 pairs, both members of a pair rotate the eye about the same axis. The neural signal send to each muscle has two components, one related to eye position and the other to eye velocity. Velocity and position signals are generated by different neural systems, which converge in the motor neuron.

 CRIANAL NERVES (CN): Are the nerves that emerge from the Brainstem instead of the Spinal Cord and transport signals to our muscles. Three of them are involved at innervating the left and right eye's muscles:


CN III - Oculomotor: Eyelid and eyeball movement (4 muscles):

Originates from the Ventral surface of the Brainstem, from motor neurons in the Oculomotor (somatomotor) and Edinger-Westphal (visceral motor) Nuclei in the Brainstem.


CN IV - Trochlear: Turns eye downward and laterally (1 muscle):

Originates from the Ventral part of the Brainstem in the Trochlear Nucleus, and it is purely a motor nerve, the only cranial nerve to exit the brain Dorsally. Axons from this nucleus pass Dorsally, the only Cranial Nerve to do so.


CN VI - Abducens: Turns eye laterally (1 muscle):

Originates from the ipsilateral Abducens Nucleus between the Caudal Pons beneath the floor of the fourth ventricle and the Medulla (the pontomedullary junction).

 Vestibular Receptor Organs: the five vestibular re ceptor organs in the inner ear complement each other in function. The semicircular canals (horizontal, anterior, and posterior) transduce rotational head movements (angular accelerations). The otolith organs (utricle and saccule) respond to translational head movements (linear accelerations) or to the orientation of the head relative to gravity. Each semicircular canal and otolith organ is spatially aligned to be most sensi tive to movements in specific planes in three-dimen sional space.In humans, the horizontal semicircular canal and the utricle both lie in a plane that is slightly tilted anterodorsally relative to the naso-occipital plane. When a person walks or runs, the head is normally declined (pitched downward) by approximately 30 degrees, so that the line of sight is directed a few meters in front of the feet. This orientation causes the plane of the horizontal canal and utricle to be parallel with the earth horizontal and perpendicular to gravity. The anterior and posterior semicircular canals and the sac cule are arranged vertically in the head, orthogonal to the horizontal semicircular canal and utricle. The two vertical canals in each ear are positioned orthogonal to each other, whereas the plane of the anterior canal on one side of the head is coplanar with the plane of the contralateral posterior canal.


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