Chapter 6: The Special Senses

OBJECTIVES

In this chapter, we will review the organization and function of the special senses, focusing on vision, vestibular, hearing, taste, and proprioception. These special senses, like somatosensation, each have specific receptors that translate external stimuli to neural coding, neural projections that convey these neural impulses centrally, and brain networks that decode the sensation. This review should help the reader with later chapters when these systems are disrupted by neurologic injury or disease.

VISION

The Eye

The visual system, of course, starts with the eye. The eye is uniquely shaped to allow perception of our environment; its rounded shape allows the projection of almost 180 degrees of horizontal vision without head or eye movement and nearly 130 degrees of vertical vision. The eye is protected from the environment by a tough external membrane, the sclera, which is opaque (white); interestingly, the sclera is continuous with the dura mater (the protective covering of the brain). A clear mucous membrane, the conjunctiva, covers the sclera. The cornea is transparent and makes up the most central portion of the sclera, covering the iris and refracting light toward the lens as it enters the pupil. The eye and, more specifically, the retina are nourished by the choroid underlying the sclera, which is a highly vascular tissue that secretes a thin liquid, called aqueous humor, which provides critical nutrients to maintain eye health and helps maintain pressure in the eye. Light is projected by the cornea, through the pupil (the central opening of the eye) onto the lens, which in turn, focuses it onto the retina (Figure 6-1). The pupil is surrounded by the iris, which is comprised of muscles that control the pupil’s size: the sphincter pupillae muscle constricts the pupil in bright light, and the dilator pupillae muscle dilates the pupil when light is low. Although the pupil determines the amount of light that enters the eye, it does not play a critical role in focusing the light. This is done by the cornea and the lens. The lens sits behind the pupil and is controlled by the ciliary muscles, located within the ciliary body, which surrounds the lens. Both the muscles of the iris and those for the pupil are controlled by the autonomic nervous system: sympathetic activity, originating in the superior ganglion, dilates the pupil; parasympathetic activity via CN III constricts the pupil and contracts the ciliary muscle to thicken the lens to allow focus on near objects (see Box 6-1). Although the greatest amount of light refraction occurs at the cornea, the lens is critical for finely focusing light on ...