Copyright © 2016 by Elsevier Inc. All rights reserved.

Learning Objectives Lesson 10.1: Senses
Classify the sense organs as general or special and explain the basic differences between the two groups. Discuss how a stimulus is converted into a sensation. Discuss the general sense organs and their functions. Describe the structure of the eye and the functions of its components. Copyright © 2016 by Elsevier Inc. All rights reserved.

Learning Objectives Lesson 10.1: Senses (Cont.)
Discuss the anatomy of the ear and its sensory function in hearing and equilibrium. Describe the anatomy of the tongue and its sensory function in taste. Describe the anatomy of the nasal cavity and its sensory function in smell. Discuss how senses are integrated. Copyright © 2016 by Elsevier Inc. All rights reserved.

Classification of Senses
General senses Often exist as individual cells or receptor units Widely distributed throughout the body Special senses Large and complex organs Localized grouping of specialized receptors The senses are often classified as either general senses or special senses. The general senses are those detected by rather simple, microscopic receptors. Examples of special senses include smell, taste, vision, hearing, and equilibrium. Copyright © 2016 by Elsevier Inc. All rights reserved.

Classification of Senses (Cont.)
Sensory receptor types Classification by presence or absence of covering capsule Encapsulated Unencapsulated (“free” or “naked”) Individual receptor cells are often identified structurally according to whether they are encapsulated or unencapsulated. Copyright © 2016 by Elsevier Inc. All rights reserved.

Classification of Senses (Cont.)
Classification by type of stimuli required to activate receptors Photoreceptors (light) Chemoreceptors (chemicals) Pain receptors (injury) Thermoreceptors (temperature change) Mechanoreceptors (movement or deforming of capsule) Proprioceptors (position of body parts or changes in muscle length or tension) Sensory receptor cells are classified functionally by the types (or modes) of stimuli that activate them. Copyright © 2016 by Elsevier Inc. All rights reserved.

Sensory Pathways All sense organs have common functional characteristics All are able to detect a particular stimulus A stimulus is converted into a nerve impulse A nerve impulse is perceived as a sensation in the central nervous system The sensory pathway for the general senses typically involves conduction of action potentials generated in the receptors through the spinal cord to the thalamus. The sensory pathways for the special senses are varied, but they also ultimately end in specific sensory areas of the cerebral cortex. Copyright © 2016 by Elsevier Inc. All rights reserved.

General Senses Distribution is widespread; single-cell receptors are common Examples (Figure 10-1, Table 10-1) Free nerve endings Pain Temperature Crude touch Tactile (Meissner) corpuscles Fine touch Vibration Bulbous (Ruffini) corpuscles Touch Pressure Microscopic general sense organ receptors are found in almost every part of the body; most are concentrated in the skin. Copyright © 2016 by Elsevier Inc. All rights reserved.

General Senses (Cont.) Examples Lamellar (Pacini) corpuscles Pressure Vibration Bulboid corpuscles (Krause end bulbs) Touch Golgi tendon receptors Important proprioceptors Muscle spindles The ability to distinguish one touch stimulus from two is called two-point discrimination. Copyright © 2016 by Elsevier Inc. All rights reserved.

General Sense Receptors
This section of skin shows the placement of some receptors described in Table 10-1. Copyright © 2016 by Elsevier Inc. All rights reserved.

General Sense Organs The first part of Table 10-1 shows the type of general sense organ, its main location, and the involved general senses for free nerve endings. Copyright © 2016 by Elsevier Inc. All rights reserved.

General Sense Organs (Cont.)
The second part of Table 10-1 shows the type of general sense organ, its main location, and the involved general senses for encapsulated nerve endings. Copyright © 2016 by Elsevier Inc. All rights reserved.

Special Senses: Vision
Structure and function of the eye (Figure 10-2) Sclera Tough outer coat; “white” of eye; cornea is transparent part of sclera over iris Choroid Pigmented vascular layer prevents scattering of light; front part of this layer made of ciliary muscle and iris, the colored part of the eye; the pupil is the hole in the center of the iris; contraction of iris muscle dilates or constricts pupil Retina (Figure 10-4) Innermost layer of the eye; contains rods (receptors for night vision) and cones (receptors for day vision and color vision) Vision detects the color and intensity of light in our external environment. Copyright © 2016 by Elsevier Inc. All rights reserved.

Special Senses: Vision (Cont.)
Conjunctiva Mucous membrane covering the front surface of the sclera and also lines the eyelid Kept moist by tears found in the lacrimal gland Lens Transparent body behind the pupil Focuses light rays on the retina When you look at a person’s eye, you see only a small part of the whole eye. Copyright © 2016 by Elsevier Inc. All rights reserved.

Special Sense Organs Table 10-2 identifies the type of receptor cells in the special sense organs that are stimulated by specific types of stimuli. Copyright © 2016 by Elsevier Inc. All rights reserved.

Cells of the Retina Figure 10-4: Photoreceptors called rods and cones (notice their shapes) detect changes in light and relay the information to bipolar neurons. The bipolar cells, in turn, conduct the information to ganglion cells. The information leaves the eye by way of the optic nerve. Copyright © 2016 by Elsevier Inc. All rights reserved.

Special Senses: Vision (Cont.)
Structure and function of the eye Eye fluids Aqueous humor: In anterior chamber in front of the lens Vitreous humor: In posterior chamber behind the lens Visual pathway Innermost layer of retina contains rods and cones Impulse travels from the rods and cones through the bipolar and ganglionic layers of retina (Figure 10-4) Nerve impulse leaves the eye through the optic nerve; the point of exit is free of receptors and is therefore called a blind spot Visual interpretation occurs in the visual cortex of the cerebrum The “blind spot” is known as the optic disk. Visual interpretation of the nervous impulses generated by light striking the retina results in “seeing.” Copyright © 2016 by Elsevier Inc. All rights reserved.

Special Senses: Hearing and Equilibrium
The ear functions in hearing and in equilibrium and balance Receptors called mechanoreceptors Structure and function of the ear (Figure 10-6) External ear Auricle (pinna) External auditory canal Curving canal 2.5 cm (1 inch) in length Contains ceruminous glands Ends at the tympanic membrane The ear also functions as the sense organ of balance. The stimulation (“trigger”) that activates receptors involved with hearing and equilibrium is mechanical. Copyright © 2016 by Elsevier Inc. All rights reserved.

Special Senses: Hearing and Equilibrium (Cont.)
Structure and function of the ear Middle ear Houses ear ossicles Malleus Incus Stapes Ends in the oval window The auditory (eustachian) tube connects the middle ear to the throat Inflammation called otitis media The middle ear is a tiny and very thin epithelium-lined cavity hollowed out of the temporal bone. Copyright © 2016 by Elsevier Inc. All rights reserved.

Special Senses: Hearing and Equilibrium (Cont.)
Structure and function of the ear Inner ear (Figure 10-8) Bony labyrinth filled with perilymph Subdivided into the vestibule, semicircular canals, and cochlea Membranous labyrinth filled with endolymph The receptors for balance in the semicircular canals are called cristae ampullares Specialized hair cells on the organ of Corti respond when bent by the movement of surrounding endolymph set in motion by sound waves The inner ear consists of three spaces in the temporal bone. Copyright © 2016 by Elsevier Inc. All rights reserved.

Special Senses: Taste Receptors are chemoreceptors called taste buds Two cranial nerves (VII and IX) carry gustatory impulses Most pathologists list four kinds of “primary” taste sensations: Sweet, sour, bitter, and salty Metallic and umami (meaty) tastes are also unique and may soon be added to the list of “primary” taste sensations Nasal congestion interferes with stimulation of olfactory receptors and thereby dulls taste sensations Our sense of taste (gustation) allows us to chemically analyze food before we bite or swallow it. Taste buds are made of supporting cells and chemoreceptors called gustatory cells. Copyright © 2016 by Elsevier Inc. All rights reserved.

Special Senses: Taste (Cont.)
Gustatory and olfactory senses work together to permit creation of many other taste sensations Although a few taste buds are located in the lining of the mouth and on the soft palate, most are located on the sides of much larger and differing shaped bumps scattered across the tongue called papillae. Copyright © 2016 by Elsevier Inc. All rights reserved.

The Tongue Figure 10-12, A: Dorsal surface of tongue showing circumvallate papillae. Figure 10-12, B: Section through a papilla with taste buds on the side. Figure 10-12, C: Enlarged view of a section through a taste bud. Copyright © 2016 by Elsevier Inc. All rights reserved.

Special Senses: Smell Receptors for fibers of olfactory or cranial nerves lie in olfactory mucosa of nasal cavity Olfactory receptors are extremely sensitive but easily adapt (become fatigued) Odor-causing chemicals initiate a nervous signal that is interpreted as a specific odor by the brain The sense of smell (olfaction) helps us detect certain chemicals in our environment. The location of the olfactory receptors is somewhat hidden. Copyright © 2016 by Elsevier Inc. All rights reserved.

Olfactory Structures Figure shows olfactory structures and how sensory information is conducted along the nerves in the olfactory bulb and olfactory tract to sensory processing centers in the brain. Copyright © 2016 by Elsevier Inc. All rights reserved.

Integration of Senses Sensations are all perceived in the brain, not at individual receptors scattered throughout body Severe nasal congestion can interfere with the stimulation of olfactory receptors by food odors in the mouth, which leads to dull flavor sensations Smell sensations are often powerful triggers of memory Some sensory information is processed and perceived subconsciously. Senses eventually start to fail us as we age. Copyright © 2016 by Elsevier Inc. All rights reserved.

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