sensiory philosophy

The Sense of Sight (Vision)

To study the anatomy of the eye, view the model available in lab. The clear covering over the front of the eye is the cornea, through which light enters the eye (Figure 4-1). Behind the cornea is the colored iris, which is capable of constricting or relaxing to regulate the size of the pupil (the central opening of the iris) and, thus, the amount of light entering the eye. Immediately behind the iris is the lens. The chamber at the front of the eyeball, between the lens and the cornea, is the aqueous humor, a clear fluid-filled space that provides nutrients and determines eye pressure. Behind the lens is a large chamber known as the vitreous humor, a transparent gel-like tissue that provides structure and fills the posterior of the eye. Ciliary muscles, originating from the ciliary body, are attached to the margin of the lens and change its shape to focus images onto the retina at the back of the eye. The retina is composed of a pigmented layer and a neural layer. The neural layer of the retina contains the photosensitive cells, the rods and cones, which are stimulated by light. The fovea is a depression in the retina in which rods and cones are most densely packed. When you look directly at an object, the image is centered on the fovea. From the retina, the visual information is transmitted to the brain by the optic nerve. The region of the retina where the optic nerve leaves the eye, called the optic disc, is filled with nerve fibers and blood vessels. It contains no photosensitive cells and perceives no visual images. In addition to the retina, the thick protective covering making up the back of the eye consists of the choroid (a layer of vascular tissue) and the sclera (a layer of tough connective tissue).

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Figure 4-1. The anatomy of the human eye.

The Sense of Sound (Audition)

Look at the model of the human ear as you read this description. The ear is a complex sensory apparatus. It not only perceives sounds but also contains structures important for balance and the perception of equilibrium. It is composed of three chambers: the outer ear (the portion of the ear in which you stick a Q-tip), the middle ear, and the inner ear. The outer and middle ear cavities are filled with air, while the inner ear is fluid-filled. Sound waves (pressure waves through the air) are collected by the pinnae and directed into the ear through the external auditory canal (Figure 4-2), causing the tympanic membrane, or eardrum, to vibrate. This vibration is transmitted through three very small bones in the middle ear (the malleus, incus, and stapes) that amplify the vibration. The sound vibrations pass through a small opening in the cochlea, the oval window, which is covered by the stapes. Within the cochlea in the inner ear, ciliated cells on the tips of fibers of the auditory nerve are stimulated by sound waves of varying frequency. These sensations are perceived by the auditory portions of the brain as sounds.

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Figure 4-2. The anatomy of the human ear.

Associated with the cochlea in the inner ear is a series of three small tubes, the semicircular canals. Small granules called otoliths move freely within the fluid in the canal system. When you turn your head, the motion causes these otoliths to move and this motion is perceived by ciliated cells in the sacculus and utriculus. These are responsible for the perception of balance and equilibrium.

Pressure within the middle ear cavity is regulated by the eustachian tube, which communicates directly with the pharynx, an area of the throat. When you “pop your ears” in an airplane you are actually letting air in or out of your middle ear through the eustachian tube.

The Sense of Touch (Mechanoreception)

Mechanoreception refers to the detection of a variety of physical forces that are involved in many sensory systems. Here we focus on cutaneous sensations, i.e., detecting physical sensations on the skin. The skin provides multiple modalities of cutaneous sensation traditionally recognized as temperature, pain, pressure, and touch (but can also include texture, itch, and tickle). In the epidermis, i.e., the outer layer of skin, free nerve endings detect heat, cold, and pain (Figure 4-3). Also in the epidermis, Merkel’s discs provide information about objects continuously touching the skin whereas Meissner’s corpuscles provide information about changes in objects touching the skin. Deeper in the skin, in the dermis, Ruffini endings provide information about vibrating stimuli of low frequencies (e.g., detecting if an object is slipping from a grip) whereas Pacinian corpuscles provide information about high-frequency alternating stimuli (e.g., detecting a rough surface). Sensory information from these various receptors is transmitted via nerves to the central nervous system, where the stimulus and its location are perceived.

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Figure 4-3. Anatomy of the skin.

Reflexes

Reflexes are unconscious responses to specific stimuli. They are involuntary and, frequently, do not involve the brain. Simple reflexes may involve as few as three neurons, forming a reflex arc (Figure 4-4). A reflex arc consists of a receptor neuron (for instance, a pain receptor in the tip of your finger), a sensory neuron, which carries the impulse to the brain or spinal cord, and a motor neuron, which carries the impulse to the effector, such as a muscle. You already investigated one reflex, the pupillary reflex, above.