Anatomy & Physiology

Anatomy & Physiology Study Guide • The olfactory system can distinguish thousands of chemical stimuli. The CNS interprets smells by the pattern of receptor activity. • The olfactory receptor population displays a substantial turnover. The quantity of olfactory receptors decreases with age. Gustation, the sense of taste, involves taste receptors responding to chemical stimuli: • Taste (gustatory) receptors are clumped in taste buds. • Taste buds are associated with epithelial projections (lingual papillae) on the dorsal surface of the tongue. • Each taste bud contains basal cells, which appear to be stem cells, and gustatory cells, which extend taste hairs through a narrow taste pore. • The number of taste buds declines with age. • The primary taste sensations are sweet, salt, sour, and bitter. Receptors also exist for umami and water. • Taste sensitivity exhibits significant individual differences, some of which are inherited. • The taste buds are monitored by cranial nerves that synapse within the solitary nucleus of the medulla oblongata. Postsynaptic neurons carry the nerve impulses on to the thalamus, where third- order neurons project to the primary sensory cortex. Internal eye structures contribute to vision, while accessory eye structures provide protection: • The accessory composition of the eye includes the eyelids (palpebrae), separated by the palpebral fissure, the eyelashes, and the tarsal glands. • An epithelium, called the conjunctiva, covers most of the exposed surface of the eye. The cornea is transparent. • The secretions of the lacrimal gland contain lysozyme. Tears collect in the lacrimal lake and reach the inferior meatus of the nose after they pass through the lacrimal puncta, the lacrimal canaliculi, the lacrimal sac, and the nasolacrimal duct. • The eye has three layers: an outer fibrous tunic, a middle vascular tunic, and an inner neural tunic. • The fibrous tunic consists of the sclera, the cornea, and the corneal limbus. • The vascular tunic, or uvea, is composed of the iris, the ciliary body, and the choroid. The iris contains muscle fibers that alter the diameter of the pupil. The ciliary body is made of the ciliary muscle and the ciliary processes, which adhere to the suspensory ligaments of the lens. • The neural tunic, or retina, consists of an outer pigmented part and an inner neural part; the latter contains visual receptors and associated neurons. • The retina contains two types of photoreceptors: rods and cones. • Cones are densely grouped in the fovea, at the center of the macula lutea. • The direct line to the CNS goes from the photoreceptors to bipolar cells, then to ganglion cells, and, finally, to the brain via the optic nerve. The axons of ganglion cells converge at the optic disc, known as the blind spot. Horizontal cells and amacrine cells modify the signals passed among other components of the retina.

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