1) Review the neuroanatomical basis of vision, hearing, smell, taste, and vestibular system function
2) Develop a foundation for the discussion of sensory dysfunction associated with neurologic conditions
3) Examine common testing methods to identify sensory system integrity
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.
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). The cornea makes up the most central portion of the sclera, over the iris, and refracts light toward the lens as it enters the pupil. The eye and, more specifically, the retina are nourished by the underlying choroid, which is a highly vascular tissue that provides critical nutrients to maintain eye health. 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 that is comprised of muscles that control the pupils’ 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. Both the muscles of the iris and those for the pupil are controlled by the autonomic nervous system via CN III: sympathetic activity dilates the pupil; parasympathetic activity constricts the pupil and contracts the ciliary muscle for lens control (see Box 6-1). Although the greatest amount of light refraction occurs at the cornea, the lens is critical for finely focusing light on the fovea. In the center of the eye, between the lens and retina, there’s a gelatinous substance, known as the vitreous humor that serves to maintain the shape of the eye. The retina is a layered structure at the rear of the eye that contains the photosensitive receptors of the eye and the transmission neurons of ...