Sensory Organs Help (page 2)
Sensory organs are specialized extensions of the nervous system that contain sensory (afferent) neurons adapted to respond to specific stimuli and conduct nerve impulses to the brain. Sensory organs are very specific as to the stimuli to which they respond.
The senses of the body are classified as general senses or special senses. General senses include the cutaneous receptors (touch, pressure, heat, cold, and pain) within the skin that provide the sense of touch. Special senses are localized in complex receptor organs and have extensive neural pathways. The special senses are the senses of taste, smell, sight, hearing, and balance.
Receptors for the sense of taste (gustation) are located in taste buds on the surface of the tongue. The taste buds are associated with peglike projections of the tongue called lingual papillae (Figure 12-1). A few taste buds are also located in the mucous membranes of the palate and pharynx. A taste bud contains a cluster of 40 to 60 gustatory cells, each innervated by a sensory neuron, as well as many more supporting cells. The four primary taste sensation are sweet (evoked by sugars, glycols, and aldehydes); sour (evoked by H+, which is why all acids taste sour); bitter (evoked by alkaloids); and salty (evoked by anions of ionizable salts). Sensory innervation of the tongue and pharynx is by a branch of the facial nerve, CN VI, from the anterior 2/3 of the tongue, the glossopharyngeal nerve, CN IX, from the posterior 1/3 of the tongue, and the vagus nerve, CN X, from the pharyngeal region. Taste sensations are transmitted to the brain stem, then to the thalamus, and finally to the cerebral cortex, where taste perception occurs.
Receptors for the sense of smell (olfaction) are located in the nasal mucosa of the superior nasal concha. Like taste receptors, smell receptors are chemoreceptors, specialized neurons that respond to chemical stimuli and require a moist environment to function. The airborne chemicals become dissolved in the mucous layer lining the superolateral part of the nasal cavity. The olfactory nerve, CN I, transmits most impulses related to smell. Olfactory sensations are conveyed along each olfactory tract to the olfactory portions of the cerebral cortex where olfactory perception occurs.
Structure and Function of the Eye
Accessory structures of the eye either protect the eye or enable eye movement. These structures include the bony orbit, the eyebrow, the eyelids, the lacrimal apparatus (lacrimal glands that produce lacrimal fluid or tears, and the lacrimal canals and lacrimal sac, which drain the fluid into the nasal cavity), and the eye muscles (responsible for eye movements).
Structure of the Eye
The spherical eye is approximately 25 mm (1in.) in diameter. It consists of three tunics (layers), a lens, and two principal cavities (Figure 12-2).
Fibrous Tunic (outer layer)
The fibrous tunic has two parts. The sclera is composed of dense regular connective tissue that supports and protects the eye and is the attachment site for the extrinsic eye muscles. The transparent cornea forms the anterior surface of the eye. Its convex shape refracts incoming light rays.
Vascular Tunic (middle layer)
The vascular tunic has three parts. The choroid is a thin, highly vascular layer that supplies nutrients and oxygen to the eye and absorbs light, preventing it from being reflected. The ciliary body is the thickened anterior portion of the vascular tunic. It contains smooth muscle fibers that regulate the shape of the lens. The iris forms the most anterior portion of the vascular tunic and consists of pigment (that gives the eye color) and smooth muscle fibers arranged in a circular and radial pattern that regulate the diameter of the pupil, which is the opening in the center of the iris.
Internal Tunic (inner layer, or retina)
The receptor component of the eye contains two types of photoreceptors. Cones (approximately 7 million cones per eye) function at high light intensities and are responsible for daytime color vision and acuity (sharpness). Rods (approximately 100 million per eye) function at low light intensities and are responsible for night (black-and-white) vision. The retina also contains bipolar cells, which synapse with the rods and cones, and ganglion cells, which synapse with the bipolar cells. The axons of the ganglion cells course along the retina to the optic disc and form the optic nerve (CN II). The fovea centralis is a shallow pit at the back of the retina that contains only cones. It is the area of keenest vision. Surrounding the fovea centralis is the macula lutea, which also has an abundance of cones.
The lens is a transparent, biconvex structure composed of tightly arranged proteins. It is enclosed in a lens capsule and held in place by the suspensory ligament that attaches to the ciliary body. The lens focuses light rays for near and far vision.
Cavities of the Eye
The interior of the eye is separated by the lens into an anterior cavity and a posterior cavity (vitreous chamber). The anterior cavity is partially subdivided by the iris into an anterior and a posterior chamber. The anterior cavity contains a watery fluid called aqueous humor. The posterior cavity contains a transparent jellylike substance called vitreous humor.
The field of vision is what a person visually perceives. There are three visual fields, the macular field, the area of keenest vision, the binocular field, the portion viewed by both eyes, but not keenly focused on, and the monocular field, that area viewed by one eye and not shared by the other.
The neural pathway for vision consists of the light rays striking the photoreceptors in the retina, which causes the transmission of nerve impulses along the optic nerve to the optic chiasma. The optic tract, a continuation of optic nerve fibers from the optic chiasma, carries the impulses to the occipital cerebral lobes where vision occurs.
For an image to be focused on the retina, the more distant the object, the flatter must be the lens. Adjustments in lens shape, accomplished by the ciliary muscles in the ciliary body, are called accommodation. When these smooth muscles contract, the fibers within the suspensory ligament slacken, causing the lens to thicken and become more convex.