Chapter 18: The Senses

Types of Sensory Receptors Chemoreceptors respond to chemical substances, such as changes in pH, or the senses of taste and smell. Pain receptors are chemoreceptors that respond to chemicals from damaged tissues. Mechanoreceptors respond to mechanical forces. The senses of hearing and balance both involve mechanoreceptors.

Proprioceptors (mechanoreceptors) in tendons around joints make us aware of position; pressoreceptors in arteries detect blood pressure changes, and stretch receptors in lungs detect degree of inflation. Thermoreceptors respond to temperature changes; there are both warm receptors and cold receptors. Photoreceptors respond to light energy. Special photoreceptors called rods result in black-and-white vision, while cones detect color.

How Sensation Occurs Sensation occurs when nerve impulses reach the cerebral cortex. Perception is an interpretation of the meaning of sensations. The sensation that results depends on the part of the brain receiving the impulses. Receptors may integrate signals before sending nerve impulses. Sensory adaptation occurs when a stimulus continues but the receptor decreases its response. The functioning of our sensory receptors makes a significant contribution to homeostasis.

Sensation The stimulus received by a sensory receptor, which generates nerve impulses (action potentials). Nerve impulses are conducted to the CNS by sensory nerve fibers within the PNS, and only those impulses that reach the cerebral cortex result in sensation and perception.

These proprioceptors allow the muscles to maintain the proper length and tension, or muscle tone. The knee-jerk reflex involves muscle spindles. Signals to the CNS from muscle spindles help maintain balance and posture. Golgi tendon organs are proprioceptors with the opposite effect.

Muscle spindle When a muscle is stretched, a muscle spindle sends sensory nerve impulses to the spinal cord. Motor nerve impulses from the spinal cord result in muscle fiber contraction so that muscle tone maintained.

Cutaneous Receptors The dermis of the skin contains sensory receptors for touch, pressure, pain, and temperature (warmth and cold). Three types of cutaneous receptors are sensitive to fine touch: Meissner corpuscles are concentrated in finger tips, lips, tongue, nipples, and genital areas; Merkel discs are found where the epidermis meets the dermis; and

3) free nerve endings (root hair plexus) around hair follicles all detect touch. Three different types of pressure receptors are Pacinian corpuscles, Ruffini endings, and Krause end bulbs. Temperature receptors are simply free nerve endings in the epidermis; some are responsive to cold and others are responsive to warmth, although there are no structural differences between them. Pacinian corpuscles are onion-shaped sensory receptors that lie deep within the dermis. Ruffini endings and Krause end bulbs are encapsulated by sheaths of connective tissue and contain lacy networks of nerve fibers.

Sensory receptors in human skin The classical view is that each sensory receptor has the main function shown here. However, investigators report that matters are not so clear cut. For example, microscopic examination of the skin of the ear shows only free nerve endings (pain receptors), and yet the skin near the ear is sensitive to all sensations. Therefore, it appears that the receptors of the skin are somewhat, but not completely, specialized.

Pain Receptors Nociceptors are pain receptors on internal organs and may be sensitive to temperature, pressure, or chemicals. Referred pain occurs when stimulation of internal pain receptors is felt as pain from the skin. Referred pain most likely happens because of shared nerve pathways between the skin and internal organs.

Sense of Taste The taste buds located in papillae on the tongue contain taste cells that communicate with sensory nerve fibers. Microvilli on taste cells contain receptor proteins that match chemicals in food. The brain determines the taste according to a “weighted average” of incoming impulses from taste buds sensitive to either sweet, sour, salty, or bitter tastes.

Taste buds Papillae on the tongue contain taste buds that are sensitive to sweet, sour, salty, and bitter tastes in the region indicated. Enlargement of papillae. Taste buds occur along the walls of the papillae. Taste cells end in microvilli that bear receptor proteins for certain molecules. When molecules bind to the receptor proteins, nerve impulses are generated that go to the brain where the sensation of taste occurs.

Sense of Smell Olfactory cells (modified neurons) are located in epithelium in the roof of the nasal cavity. After molecules bind to receptor proteins on the varied cilia of olfactory cells, nerve impulses lead to olfactory areas of the cerebral cortex. The perceived odor is determined by the combination of olfactory cells stimulated. The effects of smell and taste combine. Olfactory bulbs have direct connections with the limbic systems and its centers for emotion and memory. Thus, odors often trigger vivid memories.

Olfactory cell location and anatomy a. The olfactory epithelium in humans is located high in the nasal cavity. b. Olfactory cells end in cilia that bear receptor proteins for specific color molecules. The cilia of each olfactory cell can bind to only one type of odor molecule (signified here by color). If a rose causes olfactory cells sensitive to “purple” and “green” odor molecules to be stimulated, then neurons designated purple and green in the olfactory bulb are activated. The primary olfactory area of the cerebral cortex interprets the pattern of neurons stimulated as the scent of a rose.

Anatomy of the Eye The eye has three layers. The sclera is the outer layer seen as the white of the eye and includes the transparent bulge in the front of the eye called the cornea. The choroid is the middle, darkly pigmented layer that absorbs stray light rays; it also becomes the iris that regulates the size of the pupil. Table 18.2 (page 350) contains a list of the parts of the eye and their functions.

Behind the iris, the choroid thickens and forms the ciliary body. The ciliary body contains the ciliary muscle, which controls the shape of the lens for near and far vision. The lens divides the eye into two compartments: the anterior compartment (containing aqueous humor) and the posterior compartment (containing vitreous humor). When a person has glaucoma,

Rod cells and cone cells are located in the retina that forms the inner layer. The retina lines the back half of the eye and has cone cells densely packed in one area called the fovea centralis. Sensory fibers from the retina form the optic nerve leading to the brain.

Anatomy of the human eye Notice that the sclera, the outer layer of the eye, becomes the cornea and that the choroid, the middle layer, is continuous with the ciliary body and the iris. The retina, the inner layer, contains the photoreceptors for vision; the fovea centralis is the region where vision is most acute.

Focusing The cornea and the lens focus light rays on the retina. To see a close object, the ciliary muscles change the lens shape to provide visual accommodation. After age 40, the lens is less able to accommodate and near vision is less acute. Cataracts occur when the lens becomes opaque; sun exposure might be a factor in developing cataracts.

Focusing a. Light rays from each point on an object are bent by the cornea and the lens in such a way that an inverted and reversed image of the object forms on the retina. b. When focusing on a distant object, the lens is flat because the ciliary muscle is relaxed and the suspensory ligament is taut. c. When focusing on a near object, the lens accommodates; it becomes rounded because the ciliary muscle contracts, causing the suspensory ligament to relax.

Photoreceptors Both rod cells and cone cells have an outer segment with membranous disks containing embedded pigments. Rods contain a deep purple pigment called rhodopsin that is composed of retinal (made from vitamin A) and the protein opsin. Rods are numerous and provide peripheral vision, perception of motion, and vision in dim light at night.

When a rod absorbs light, rhodosin splits into opsin and retinal, leading to a cascade of reactions and the closing of rod membrane ion channels. Inhibitory neurotransmitters are no longer released from the rod. Breakdown of rhodopsin in rods thus initiates nerve impulses. Cones have three different pigments (red, green and blue) made from retinal and opsin; opsin varies between the three.

Photoreceptors in the eye The outer segment of rods and cones contains stacks of membranous disks, which contain visual pigments. In rods, the membrane of each disk contains rhodopsin, a complex molecule containing the protein opsin and the pigment retinal. When rhodopsin absorbs light energy, it splits, releasing opsin, which sets in motion a cascade of reactions that ends when ion channels in the plasma membrane close.

Integration of Visual Signals in the Retina The retina has three layers of neurons: rods and cones are near the retina, bipolar cells are in the middle, and the innermost layer contains ganglion cells that carry impulses to the optic nerve. The rod and cones synapse with the bipolar cells, which in turn synapse with ganglion cells that initiate nerve impulses.

As signals pass from one layer to the next, integration occurs because each layer contains fewer cells than the previous layer. However each cone connects directly to one ganglion cell, while a hundred rods may synapse with only one ganglion cell. It is likely that much processing occurs in the retina before impulses are sent to the brain. There are no rods and cones where the optic nerve exits the retina; this is the blind spot. The fact that individual cone cells synapse directly with a single ganglion cell explains why cones, especially in the fovea, provides a sharper, more detailed image of an object.

Structure and function of the retina The retina is the inner layer of the eyeball. Rod cells and cone cells located at the back of the retina synapse with bipolar cells, which synapse with ganglion cells. Integration of signals occurs at these synapses; therefore, much processing occurs in bipolar and ganglion cells. Further, notice that many rod cells share one bipolar cell, but cone cells do not. Certain cone cells synapse with only one ganglion cell. Cone cells, in general, distinguish more detail than do rod cells.

Integration of Visual Signals in the Brain The visual pathway begins with the retina and passes through the thalamus before reaching the cerebral cortex. The visual pathway and the visual cortex split the visual field apart, but the visual association areas rebuild it so we correctly perceive the entire visual field. At the optic chiasma, fibers from the right half of each retina converge and continue together on the right optic tract, and fibers from the left half of each retina converge and continue of the left optic tract.

Optic chiasma Both eyes “see” the entire visual field. Because of the optic chiasma, data from the right half of each retina go to the right visual areas of the cerebral cortex, and data from the left half of the retina go to the left visual areas of the cerebral cortex. These data are then combined to allow us to see the entire visual field. Note that the visual pathway to the brain includes the thalamus, which has the ability to filter sensory stimuli.

Abnormalities of the Eye Color Blindness The most common abnormality is a lack of red and/or green cones. Distance Vision Nearsighted individuals (elongated eyeball) cannot see distant objects; this is corrected by a concave lens. Farsighted individuals (shortened eyeball) see distant objects well but not up close; this is corrected by a convex lens. Astigmatism occurs with an uneven cornea or lens. The shape of the eyeball determines the need for corrective lenses.

Common abnormalities of the eye In nearsightedness, the individual has a long eyeball, and rays focus in front of the retina when viewing distant objects, preventing them from being seen clearly. This condition is corrected using a concave lens. In farsightedness, individuals have an eyeball that is too short, and rays focus behind the retina when close objects are viewed. This condition is corrected using a convex lens. Astigmatism occurs when the surface of the cornea is uneven, and light rays do not focus evenly. An uneven corrective lens allows the patient to see objects clearly.

Anatomy of the Ear The ear is divided into three parts. The outer ear consists of the pinna and the auditory canal, which direct sound waves to the middle ear. The middle ear begins at the tympanic membrane (eardrum) and contains the ossicles: the malleus, incus, and stapes that amplify sound waves.

The malleus is attached to the tympanic membrane, and the stapes is attached to the oval window, which is covered by membrane. The inner ear contains semicircular canals and vestibule involved in equilibrium, and the cochlea for hearing.

Anatomy of the human ear In the middle ear, the malleus (hammer), the incus (anvil), and the stapes (stirrup) amplify sound waves. In the inner ear, the mechanoreceptors for equilibrium are in the semicircular canals and vestibule, and the mechanoreceptors for hearing are in the cochlea.

Process of Hearing Sound waves enter the auditory canal and vibrate the tympanic membrane. If the vibrations are strong enough, the outer and middle portions (ossicles) of the ear convey and amplify the sound waves about 20 times and vibrate against the oval window. These vibrations set up pressure waves within the fluid of the cochlea.

The cochlea contains the spiral organ consisting of hair cells on the basilar membrane whose stereocilia are embedded within the tectorial membrane. Vibrations within the cochlea cause the sterocilia to vibrate against the tectorial membrane, thus generating nerve impulses.

Different regions are sensitive to different frequencies or pitch. When the stereocilia of the hair cells bend, nerve impulses are generated in the cochlear nerve and are carried to the brain.

Mechanoreceptors for hearing The spiral organ (organ of Corti) is located within the cochlea. In the uncoiled cochlea, note that the spiral organ consists of hair cells resting on the basilar membrane, with the tectorial membrane above. Hearing occurs when pressure waves move from the vestibular canal to the tympanic canal, causing the basilar membrane to vibrate and the stereocilia (of a least a portion of the 20,000 hair cells) to bend within the tectorial membrane. Nerve impulses traveling in the cochlear nerve result in hearing.

Gravitational Equilibrium Stimulation of hair cells within the utricle and the saccule, two sacs located in the vestibule, by the slippage of calcium carbonate granules or otoliths, provide impulses that tell the brain the direction of movement of the head. The movement of the otoliths provides a sense of gravitational equilibrium.

Mechanoreceptors for equilibrium a. Rotational equilibrium. The ampullae of the semicircular canals contain hair cells with stereocilia embedded in a cupula. When the head rotates, the cupula is displaced, bending the stereocilia. Thereafter, nerve impulses travel in the vestibular nerve to the brain. b. Gravitational equilibrium. The utricle and the saccule contain hair cells with stereocilia embedded in an otolithic membrane. When the head bends, otoliths are displaced, causing the membrane to sag and the stereocilia to bend. The rapidity of nerve impulses in the vestibular nerve tells the brain how much the head has moved.

Proprioceptors in muscles and joints help the body maintain balance and posture. Cutaneous receptors in the skin respond to touch, pressure, pain, and temperature (both warmth and cold). In the mouth, the microvilli of taste cells have membrane protein receptors that respond to certain molecules.

Olfactory cells within the olfactory epithelium respond to molecules and result in a sense of smell. Photoreceptors for sight contain visual pigments, which respond to light rays. Some integration occurs in the retina of the eye before nerve impulses are sent to the brain.

Sensory receptors for hearing are hair cells in the cochlea of the inner ear that respond to pressure waves. Sensory receptors for balance are hair cells in the vestibule and semicircular canals of the inner ear that respond to the tilt of the head and to the movement of the body, respectively.