IN PRECEDING CHAPTERS, WE LEARNED about the motor systems that control the movements of the body in space. In this and the next chapter, we consider the motor systems that control our gaze, balance, and posture as we move through the world around us. In examining these motor systems, we will focus on three biological challenges that these systems resolve: How do we visually explore our environment quickly and efficiently? How do we compensate for planned and unplanned movements of the head? How do we stay upright?
In this chapter, we describe the oculomotor system and how it uses visual information to guide eye movements. It is one of the simplest motor systems, requiring the coordination of only the 12 evolutionarily old muscles that move the two eyes. In humans and other primates, the primary objective of the oculomotor system is to control the position of the fovea, the central point in the retina that has the highest density of photoreceptors and thus the sharpest vision. The fovea is less than 1 mm in diameter and covers less than 1% of the visual field. When we want to examine an object, we must move its image onto the fovea (Chapter 22).
The Eye Is Moved by the Six Extraocular Muscles
Eye Movements Rotate the Eye in the Orbit
To a good approximation, the eye is a sphere that sits in a socket, the orbit. Eye movements are simply rotations of the eye in the orbit. The eye’s orientation can be defined by three axes of rotation—horizontal, vertical, and torsional—that intersect at the center of the eyeball, and eye movements are described as rotations around these axes. Horizontal and vertical eye movements change the line of sight by redirecting the fovea; torsional eye movements rotate the eye around the line of sight but do not change where the eyes are looking.
Horizontal rotation of the eye away from the nose is called abduction, and rotation toward the nose is adduction (Figure 35–1A). Vertical movements are referred to as elevation (upward rotation) and depression (downward rotation). Finally, torsional movements include intorsion (rotation of the top of the cornea toward the nose) and extorsion (rotation away from the nose).
The different actions of eye movements and the muscles that control them.
A. View of the left eye and the three dimensions of eye movement.
B. 1. Lateral view of the left eye with the orbital wall cut away. Each rectus muscle inserts in front of the equator of the globe so that contraction rotates the cornea toward the muscle. Conversely, the oblique muscles insert behind the equator, and contraction rotates the cornea away from the insertion. The superior oblique tendon passes through the trochlea, a bony pulley on the nasal side of the orbit, before ...