Sensory Organs Help
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.
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