Sensory Input and Motor Output
Chapter
28
Outline
	28.1	The Senses
	•	Chemoreceptors are almost universally found in animals for sensing chemical substances in food, liquids, and air.495
	•	Human taste buds and olfactory cells are chemoreceptors that respond to chemicals in food and air, respectively.495
	•	The inner ear of humans contains mechanoreceptors for hearing and for the sense of balance.496
	•	When we hear, hair cells in the cochlea of the inner ear respond to pressure waves.497
	•	Arthropods have a compound eye, while humans and squids have a camera-type eye.499
	•	In the human eye, the rods work in minimal light and detect motion; the cones require bright light and detect color.500
	•	Cutaneous receptors and proprioceptors send information to the primary sensory area of the brain.501
	28.2	The Motor Systems 
	•	Muscles and bones have several functions in addition to supporting the body and making body parts move.502
	•	The endoskeleton of humans is divided into the axial and appendicular skeletons.503
	•	Bone is a living tissue that stores calcium and fat and produces blood cells.504 
	•	Muscles shorten when they contract because myosin filaments pull on actin filaments in sarcomeres.505 
	•	Contracting muscles cause antagonistic movements at synovial joints.506 
When you think of the importance of calcium in nutrition, you most likely think about bone. Bone serves as a depository for calcium so that there is always a reserve in the body, and as an added benefit, storing calcium makes bone tissue strong. However, calcium plays other important roles in the body as well. 
One of the most essential functions of calcium involves muscle contraction. No muscle in the body (including the heart and the diaphragm) can contract without the presence of calcium. If you were dependent solely on obtaining calcium from your daily diet and you ran out of calcium, your heart and diaphragm wouldn’t contract. Then, the inability to circulate your blood and to breathe would cause you to die. Luckily, our bones do stockpile calcium, so when we don’t get enough in our diet, we get it from the reserve supply in our bones. The downside is that in order to tap into the reserve of calcium in our bones, bone tissue must be degraded. This is why obtaining enough calcium in the diet is critical. When there is an imbalance between the amount of calcium taken from bones and the amount of calcium deposited in bones, conditions such as osteoporosis often result. In osteoporosis, the bones are fragile, and a broken hip, spine, or wrist often occurs. This is why we hear so much about including plenty of calcium in the diet.
The functions of bones and muscle are closely related to each other. In this chapter, in addition to learning about these motor output systems, you will  learn about sensory input systems.
28.1		The Senses
All living things respond to stimuli. Stimuli are environmental signals that tell us about the external environment or the internal environment. In a previous chapter, we learned that plants often respond to external stimuli, such as light, by changing their growth pattern. An animal’s response often results in motion. Complex animals rely on sensory receptors to provide information to the central nervous system (brain and spinal cord), which integrates sensory input before directing a motor response (Fig. 28.1). 
Sense organs, as a rule, are specialized to receive one kind of stimulus. The eyes ordinarily respond to light, ears to sound waves, pressure receptors to pressure, and chemoreceptors to chemical molecules. Sensory receptors transform the stimulus into nerve impulses that reach a particular section of the cerebral cortex. Those from the eye reach the visual areas, and those from the ears reach the auditory areas. The brain, not the sensory receptor, is responsible for sensation and perception, and each part of the brain interprets impulses in only one way. For example, if by accident the photoreceptors of the eye are stimulated by pressure and not light, the brain causes us to see “stars” or other visual patterns. 
Chemical Senses
The fundamental functions of sensory receptors include helping animals to stay safe, find food, and find mates. Chemoreceptors give us the ability to detect chemicals in the environment, which is believed to be our most primitive sense. Chemoreceptors occur almost universally in animals. For example, they are present all over the body of planarians (flatworms), but concentrated on the auricles at the sides of the head. Male moths have receptors for a sex attractant on their antennae. The receptors on the antennae of the male silkworm moth are so sensitive that only 40 out of 40,000 receptor proteins need to be activated in order for the male to respond to a chemical released by the female (Fig. 28.2a). Other insects, such as the housefly, have chemoreceptors largely on their feet—a fly tastes with its feet instead of its mouth. In mammals, the receptors for taste are located in the mouth, and the receptors for smell are located in the nose.
Taste and Smell
In humans, taste buds, located primarily on the tongue, contain taste cells, and the nose contains olfactory cells (Fig. 28.2b,c). Receptors for chemicals are located on the microvilli of taste cells and on the cilia of olfactory cells. When molecules bind to these receptors, nerve impulses are generated in sensory nerve fibers that go to the brain. When they reach the appropriate cortical areas, they are interpreted as taste and smell, respectively.
There are at least four primary types of tastes (bitter, sour, salty, and sweet), and taste buds for each are located throughout the tongue, but may be concentrated in particular regions. A particular food can stimulate more than one of these types of taste buds. In this way, the response of taste buds can -result in a range of sweet, sour, salty, and bitter tastes. The brain appears to survey the overall pattern of incoming sensory impulses and to take a “weighted average” of their taste messages as the perceived taste. Similarly, an odor contains many odor molecules, which activate a characteristic combination of receptors. When this complex information is communicated to the cerebral cortex, we know we have smelled a rose—or an onion!
An important role of taste and also smell is to trigger reflexes that start the digestive juices flowing. A revolting or repulsive substance in the mouth can initiate the gag reflex or even vomiting. Smell is even more important to our survival. Smells associated with danger, such as smoke, can trigger the fight-or-flight reflex. Unpleasant smells can cause us to sneeze or choke. 
Have you ever noticed that a certain aroma vividly brings to mind a certain person or place? A person’s -perfume may remind you of someone else, or the smell of boxwood may remind you of your grandfather’s farm. The olfactory bulbs have direct connections with the limbic system and its centers for emotions and memory. One investigator showed that when subjects smelled an orange while viewing a painting, they not only remembered the painting when asked about it later, but they also had many deep feelings about it.
Hearing and Balance
The human ear has two sensory functions: hearing and balance (equilibrium). The sensory receptors for both of these consist of hair cells with long microvilli called stereocilia. These microvilli, unlike those of taste cells, are sensitive to mechanical stimulation. Therefore, they are termed mechano-receptors.
The similarity of the sensory receptors for balance and hearing and their presence in the same organ suggest an evolutionary relationship between them. In fact, the sense organs of the mammalian ear may have evolved from a type of sense organ in fishes. 
Hearing 
Most invertebrates cannot hear. Some arthropods, including insects, do have sound receptors, but they are quite simple. In insects, the ear consists of a pair of air pockets, each enclosed by a membrane, called the tympanic membrane, that passes sound vibrations to sensory receptors. The human ear has a tympanic membrane also, but it is between the outer and middle ear (Fig. 28.3a). The outer ear collects sound waves that cause the tympanic membrane to move back and forth (vibrate) ever so slightly. Three tiny bones in the middle ear (the ossicles) amplify the sound about 20 times as it moves from one to the other. The last of the ossicles strikes an oval membrane, causing it to vibrate, and in this way, the pressure is passed to a fluid within the hearing portion of the inner ear called the cochlea (Fig. 28.3b). The term cochlea means snail shell. Specifically, the sensory receptors of hearing are located in the cochlear canal of the cochlea. The sensory receptors for hearing are hair cells whose stereocilia are embedded in a gelatinous membrane. Collectively, they are called the spiral organ (organ of Corti) (Fig. 28.3c,d). 
The outer ear and middle ear, which collect and amplify sound waves, are filled with air. The auditory tube functions to relieve pressure in the middle ear. But the inner ear is filled with fluid; therefore, fluid pressure waves actually stimulate the spiral organ. When the last ossicle strikes an oval membrane, pressure waves cause the hair cells to move up and down, and the stereocilia of the hair cells embedded in the gelatinous membrane bend. The hair cells of the spiral organ synapse with the cochlear nerve, and when their stereocilia bend, nerve impulses begin in the cochlear nerve and travel to the brain stem. When these impulses reach the auditory areas of the cerebral cortex, they are interpreted as sound.
Each part of the spiral organ is sensitive to different wave frequencies, or pitch. Near the tip, the spiral organ responds to low pitches, such as a tuba, and near the base, it responds to higher pitches, such as a bell or a whistle. The nerve fibers from each region along the length of the spiral organ lead to slightly different areas in the brain. The pitch sensation we experience depends upon which region of the spiral organ vibrates and which area of the brain is stimulated.
Volume is a function of the amplitude of sound waves. Loud noises cause the spiral organ to vibrate to a greater extent. The resulting increased stimulation is interpreted by the brain as volume. It is believed that the brain interprets the tone of a sound based on the distribution of the hair cells stimulated.
Hearing LossEspecially when we are young, the middle ear is subject to infections that can lead to hearing impairment. Today, it is quite common for youngsters to have “tubes” put into the tympanic membrane to allow the middle ear to drain in an effort to prevent this type of hearing loss. The mobility of ossicles decreases with age, and if bone grows over the stapes, the only remedy is implantation of an artificial stapes that can move. 
Deafness due to middle ear damage is called conduction deafness. Deafness due to spiral organ damage is called nerve deafness. In today’s society, noise pollution is common, and even city traffic can be loud enough to damage the stereocilia of hair cells (Fig. 28.4). It stands to reason, then, that frequently attending rock concerts, constantly playing a stereo loudly, or using earphones at high volume can also damage hearing. The first hint of danger can be temporary hearing loss, a “full” feeling in the ears, muffled hearing, or tinnitus (ringing in the ears). If exposure to noise is unavoidable, specially designed noise-reduction earmuffs are available, and it is also possible to purchase earplugs made from compressible, spongelike material at the drugstore or sporting-goods store. These earplugs are not the same as those worn for swimming, and they should not be worn interchangeably. Finally, you should be aware that some medicines may damage the ability to hear. Anyone taking anticancer drugs, such as cisplatin, and certain antibiotics, such as streptomycin, should be especially careful to protect the ears from loud noises. 
Balance
Humans have two senses of balance (equilibrium): rotational and gravitational. We are able to detect the rotational and/or angular movement of the head as well as the straight-line movement of the head with respect to gravity. 
Rotational equilibrium involves the semicircular canals (see Fig. 28.3b). In the base of each canal, hair cells have stereocilia embedded within a gelatinous membrane (Fig. 28.5a). Because there are three semicircular canals, each responds to head movement in a different plane of space. As fluid within a semicircular canal flows over and displaces the gelatinous membrane, the stereocilia of the hair cells bend, and the pattern of impulses carried to the central nervous system (CNS) changes. These data, usually supplemented by vision, tell the brain how the head is moving. Vertigo is dizziness and a sensation of rotation. It is possible to simulate a feeling of vertigo by spinning rapidly and stopping suddenly. Now the person feels like the room is spinning because of sudden stimulation of stereocilia in the semicircular canals. 
Gravitational equilibrium refers to the position of the head with relation to gravity. It depends on the utricle and -saccule, two membranous sacs located in the inner ear (see Fig. 28.3b). Both of these sacs also contain hair cells with stereocilia in a gelatinous membrane (Fig. 28.5b). Calcium carbonate (CaCO3) granules (the otoliths) rest on this membrane. When the head moves forward or back, up or down, the otoliths are displaced, and the membrane moves, bending the stereo-cilia of the hair cells. This movement alters the frequency of nerve impulses to the CNS. These data, usually supplemented by vision, tell the brain the direction of the movement of the head.
Similar Receptors in Other Animals
Gravitational equilibrium organs, called statocysts, are found in several types of invertebrates, from cnidarians to molluscs to crustaceans. These organs give information only about the position of the head; they are not involved in the sensation of movement (Fig. 28.6a). When the head stops moving, a small particle called a statolith stimulates the cilia of the closest hair cells, and these cilia generate impulses, indicating the position of the head. 
The lateral line system of fishes utilizes sense organs similar to those we have been studying in the human inner ear (Fig. 28.6b). In bony fishes, the system consists of sense organs located within a canal that has openings to the outside. As you might expect, the sense organ is a collection of hair cells with cilia embedded in a gelatinous membrane. Water currents and pressure waves from nearby objects cause the membrane and the cilia of the hair cells to bend. Thereafter the hair cells initiate nerve impulses that go to the brain. Fishes use these data, not for hearing or balance, but to locate other fish, including predators, prey, and mates.
Vision
Sensory receptors that are sensitive to light are called photoreceptors. In planarians, eyespots allow these animals to determine the direction of light only. Other photoreceptors form actual images. Image-forming eyes provide information about an object and how far away it is. Such detailed information is invaluable to an animal. 
Among invertebrates, arthropods have compound eyes composed of many independent visual units, each of which possesses a lens to focus light rays on photoreceptors (Fig. 28.7a). The image that results from all the stimulated visual units is crude because the small size of compound eyes limits the number of visual units, which still might number as many as 28,000. 
Vertebrates (including humans) and certain molluscs, such as the squid and the octopus, have a camera-type eye (Fig. 28.7b). A single lens focuses light on photoreceptors, which number in the millions and are closely packed together within a retina. Since molluscs and vertebrates are not closely related, the camera-type eye evolved independently in each group.
Insects have color vision, but they make use of a slightly shorter range of the electromagnetic spectrum compared to humans. However, they can see the longest of the ultraviolet rays, and this enables them to be especially sensitive to the reproductive parts of flowers, which have particular ultraviolet patterns. Some fishes, all reptiles, and most birds are believed to have color vision, but among mammals, only humans and other primates have expansive color -vision. It would seem, then, that this trait was adaptive for a diurnal habit (active during the day), which accounts for its retention in only a few mammals. 
The Human Eye
When looking straight ahead, each of our eyes views the same object from a slightly different angle. This slight displacement of the images permits binocular vision, the ability to perceive three-dimensional images and to sense depth. Like the human ear, the human eye has numerous parts, many of which are involved in preparing the stimulus for the sensory receptors. In the case of the ear, sound wave energy is magnified before it reaches the sensory receptors. In the case of the eye, light rays are brought to a focus on the photoreceptors located within the retina. In Figure 28.8, the cornea and especially the lens are involved in focusing light rays on the photoreceptors. The iris, the colored part of the eye, regulates the amount of light that enters the eye by way of the pupil. The retina generates nerve impulses that are sent to the visual part of the cerebral cortex, and from this information, the brain forms an image of the object.
The shape of the lens is controlled by the ciliary muscles. When we view a distant object, the lens remains relatively flat, but when we view a near object, the lens rounds up. With normal aging, the lens loses its ability to accommodate for near objects; therefore, many people need reading glasses when they reach middle age. Aging, or possibly exposure to the sun, also makes the lens subject to cataracts; the lens becomes opaque, and therefore incapable of transmitting light rays. Currently, surgery is the only viable treatment for cataracts. First, a surgeon opens the eye near the rim of the cornea. The enzyme zonulysin may be used to digest away the ligaments holding the lens in place. Most surgeons then use a cryoprobe, which freezes the lens for easy removal. An intraocular lens attached to the iris can then be implanted so that the patient does not need to wear thick glasses or contact lenses.
Photoreceptors of the Eye
Figure 28.9 illustrates the structure of the photoreceptors in the human eye, which are called rods and cones. Both types of photoreceptors contain a visual pigment similar to that found in all types of eyes throughout the animal kingdom. The visual pigment in rods is a deep-purple pigment called rhodopsin. Rhodopsin is a complex molecule made up of the protein opsin and a light-absorbing molecule, called retinal, which is a derivative of vita--min A. When a rod absorbs light, rhodopsin splits into opsin and retinal, leading to a cascade of reactions that ends in the generation of nerve impulses. Rods are very sensitive to light, and therefore are suited to night vision. (Because carrots are rich in vitamin A, it is true that eating carrots can improve your night vision.) Rod cells are plentiful throughout the entire retina; therefore, they also provide us with peripheral vision and perception of motion.
The cones, on the other hand, are located primarily in a part of the retina called the fovea. Cones are activated by bright light; they allow us to detect the fine detail and the color of an object. Color vision depends on three different kinds of cones, which contain pigments called the B (blue), G (green), and R (red) pigments. Each pigment is made up of retinal and opsin, but a slight difference in the opsin structure of each accounts for their specific absorption patterns. Various combinations of cones are believed to be stimulated by in-between shades of color.
RetinaThe retina has three layers of cells and light has to penetrate through the first two layers to reach the photoreceptors (Fig. 28.10). The intermediate cells of the middle layer process and relay visual information from the photoreceptors to the ganglion cells that have axons forming the optic nerve. The sensitivity of cones versus rods is mirrored by how directly they connect to ganglion cells. Information from several hundred rods may converge on a single ganglion cell, while cones show very little convergence. As signals pass through the layers of the retina, integration occurs. Integration improves the overall contrast and quality of information sent to the brain which uses the information to form an image of the object.
No rods and cones occur where the optic nerve exits the retina. Therefore, no vision is possible in this area. You can prove this to yourself by putting a dot to the right of center on a piece of paper. Close your left eye, then use your right hand to move the paper slowly toward your right eye while you look straight ahead. The dot will disappear at one point—this point is your blind spot.
Seeing in the Dark
Some nocturnal animals rely on vision, but others use sonar (sound waves) to find their way in the dark. Bats flying in a dark room easily avoid obstacles in their path because they echolocate, like submarines. When searching for food, they emit ultrasonic sound (above the range humans can hear) in chirps that bounce back off their prey. They are able to determine the distance to dinner by timing the echo’s return—a long delay means the prey is far away.  
A bat-inspired sonar walking stick is being perfected to help visually impaired people sense their surroundings. It too emits ultrasonic chirps and picks up the echoes from nearby objects. Buttons on the cane’s handle vibrate gently to warn a user to dodge a low ceiling or to sidestep objects blocking the path. A fast, strong signal means an obstacle is close by. 
Cutaneous Receptors and Proprioceptors
Cutaneous receptors are located in the skin, and proprioceptors are located in the muscles and joints. These sensory receptors are connected to the primary sensory area of the cerebral cortex where each part of the body is represented. Here, sensory input is received from the skin, muscles, and joints in each part of the body (Fig. 28.11). Thus, these receptors are important to our well-being.
Cutaneous Receptors
Skin, the outermost covering of our body, contains numerous sensory receptors that help us respond to changes in our environment, be aware of dangers, and communicate with others. The sensory receptors in skin are for touch, pressure, pain, and temperature. 
Skin has two regions, called the epidermis and the dermis (Fig. 28.12). The epidermis is packed with cells that become keratinized as they rise to the surface. Among these cells are free nerve endings responsive to cold or to warmth. Cold receptors are far more numerous than warmth receptors, but there are no known structural differences between the two. Also in the epidermis are pain receptors sensitive to extremes in temperature or pressure and to chemicals released by damaged tissues. Sometimes, stimulation of internal receptors is felt as pain in the skin. This is called referred pain. For example, pain from the heart may be felt in the left shoulder and arm. This effect most likely happens when nerve impulses from the pain receptors of internal organs travel to the spinal cord and synapse with neurons also receiving impulses from the skin. 
The dermis of the skin contains sensory receptors for pressure and touch (Fig. 28.12). The Pacinian corpuscle is an onion-shaped pressure receptor that lies deep inside the dermis. Several other cutaneous receptors detect touch. A free nerve ending, called a root hair plexus, winds around the base of a hair follicle and produces nerve impulses if the hair is touched. Touch receptors are concentrated in parts of the body essential for sexual stimulation: the fingertips, the palms, the lips, the tongue, the nipples, the penis, and the clitoris. 
Proprioceptors
Proprioceptors help the body maintain equilibrium and posture, despite the force of gravity always acting upon the skeleton and muscles. A muscle spindle consists of sensory nerve endings wrapped around a few muscle cells within a connective tissue sheath. Golgi tendon organs and other sensory receptors are located in the joints. The rapidity of nerve impulses by proprioceptors is proportional to the stretching of the organs they occupy. A motor response results in contraction of muscle fibers adjoining the proprioceptor.
28.2		The Motor Systems
In this section, we consider the motor systems of humans—the muscles and the bones—as forming a single system, namely the musculoskeletal system (Fig. 28.13). Combining the two seems appropriate because in many cases the functions of muscles and bones overlap:
Both skeletal muscles and bones support the body and make movement of body parts possible.  
Both skeletal muscles and bones protect internal organs. Skeletal muscles pad the bones that protect the heart and lungs, the brain, and the spinal cord. 
Both muscles and bones aid the functioning of other systems. Without the movement of the rib cage, breathing would not occur. As an aid to digestion, the jaws have sockets for teeth; skeletal muscles move the jaws so food can be chewed, and smooth muscle moves food along the digestive tract (see Fig. 24.5b). Red bone marrow supplies the red blood cells that carry oxygen, and the pumping of cardiac muscle in the wall of the heart moves the blood to the tissues, where exchanges with tissue fluid occur.
In addition to these shared functions, the muscles and bones have individual functions:
Skeletal muscle contraction assists movement of blood in the veins and lymphatic vessels (see Fig. 23.8). Without the return of lymph to the cardiovascular system and blood to the heart, circulation could not continue.
Skeletal muscles help maintain a constant body temperature (see Fig. 28.17).  Skeletal muscle contraction causes ATP to break down, releasing heat that is distributed about the body.
Bones store fat and calcium. Fat is stored in yellow bone marrow (see Fig. 28.15), and the extracellular matrix of bone contains calcium. Calcium ions play a major role in muscle contraction and nerve conduction. 
The Human Skeleton
The endoskeleton of vertebrates can be contrasted with the exoskeleton of arthropods. Both skeletons are jointed, which has helped these two groups of animals successfully live on land. The endoskeleton of humans is composed of bone, which is living material and capable of growth. Like an exoskeleton, an endoskeleton protects vital internal organs, but unlike an exoskeleton, it need not limit the space available for internal organs because it grows as the animal grows. The soft tissues that surround an endo-skeleton protect it, and injuries to soft tissues are apt to be easier to repair than is the skeleton itself. 
The exoskeleton of arthropods is composed of chitin, a strong, flexible, nitrogenous polysaccharide. Besides providing protection against wear and tear and against enemies, an exo-skeleton also prevents drying out. Although an arthropod exo-skeleton provides support for muscle contractions, it does not grow with the animal, and arthropods molt to rid themselves of an exoskeleton that has become too small (Fig. 28.14a). This process makes them vulnerable to predators. 
One other type of skeleton is seen in the animal kingdom. In animals, such as worms, that lack a hard skeleton, a fluid-filled internal cavity can act as a hydrostatic skeleton (Fig. 28.14b). A hydrostatic skeleton offers support and resistance to the contraction of muscles so that the animal can move. As an analogy, consider that a garden hose stiffens when filled with water, and that a water-filled balloon changes shape when squeezed at one end. Similarly, an animal with a hydrostatic skeleton can change shape and perform a variety of movements.
Axial and Appendicular Skeletons 
The 206 bones of the human skeleton are arranged into an axial skeleton and an appendicular skeleton. 
Axial SkeletonThe labels for the bones of the axial skeleton are in color in Figure 28.13. The skull consists of the cranium, which protects the brain, and the facial bones. The most prominent of the facial bones are the lower and upper jaws, the cheekbones, and the nasal bones. The vertebral column (spine) extends from the skull to the sacrum (tailbone). It consists of a series of vertebrae separated by pads of fibrocartilage called the intervertebral disks. On occasion, disks can slip or even rupture. A damaged disk pressing against the spinal cord or spinal nerves causes pain. Removal of the disk may be required. 
The rib cage, composed of the ribs and sternum (breastbone), demonstrates how the skeleton can be protective and flexible at the same time. The rib cage protects the heart and lungs but moves when we breathe. 
Appendicular SkeletonThe appendicular skeleton contains the bones of two girdles and their attached limbs. The pectoral (shoulder) girdle and upper limbs are specialized for flexibility; the pelvic (hip) girdle and lower limbs are specialized for strength. The pelvic girdle also protects internal organs. 
A clavicle (collarbone) and a scapula (shoulder blade) make up the shoulder girdle, but the humerus of the arm articulates only with the scapula. The joint is stabilized by tendons and ligaments that form a rotator cuff. Vigorous circular movements of the arm can lead to rotator cuff injuries. Two bones (the radius and ulna) contribute to the easy twisting motion of the forearm. The flexible hand contains bones of the wrist, palm, and five fingers. 
The pelvic girdle contains two massive coxal bones that form a bowl, called the pelvis, and articulate with the longest and strongest bones of the body, the femurs (thighbones). The kneecap protects the knee. The tibia of the leg is the shinbone. The fibula is the more slender bone in the leg. Each foot contains bones of the ankle, instep, and five toes.
Structure of a Bone 
When a long bone is split open, as in Figure 28.15, a cavity is revealed that is bounded on the sides by compact bone. Compact bone contains many osteons (Haversian systems) where bone cells called osteocytes lie in tiny chambers arranged in concentric circles around central canals. The cells are separated by an extracellular matrix that contains mineral deposits, primarily calcium and phosphorous salts. Two other cell types are constantly at work in bones. Osteoblasts deposit bone, and osteoclasts secrete enzymes that digest the matrix of bone and release calcium into the bloodstream. When a person has osteoporosis (weak bones subject to fracture), osteoclasts are working harder than osteoblasts. Intake of high levels of dietary calcium, especially when a person is young and more active, encourages denser bones and lessens the chance of getting osteoporosis later in life. 
A long bone has spongy bone at each end. Spongy bone has numerous bony bars and plates separated by irregular spaces. The spaces are often filled with red bone marrow, a specialized tissue that produces red blood cells. The cavity of a long bone is filled with yellow bone marrow and stores fat. Beyond the spongy bone are a thin shell of compact bone and a layer of hyaline cartilage, which is important to healthy joints. 
Skeletal Muscle Structure and Physiology
The three types of muscle—smooth, cardiac, and skeletal—have different structures (see Fig. 22.7). Smooth muscles contain sheets of long, spindle-shaped cells, each with a single nucleus. Cardiac cells are striated, each with a single nucleus. Cardiac muscle cells contain branched chains of cells that interconnect, forming a lattice network. Skeletal muscle cells, called muscle fibers, are elongated and run the length of a skeletal muscle. Skeletal muscle fibers arise during development when several cells fuse, resulting in one long, multinucleated cell. Skeletal muscle contraction has been extensively studied. 
Skeletal Muscle Contraction
When skeletal muscle fibers contract, they shorten. Let’s look at details of the process, beginning with the motor axon (Fig. 28.16). When nerve impulses travel down a motor axon and arrive at an axon terminal, synaptic vesicles release ACh (acetylcholine) into a synaptic cleft. ACh quickly diffuses across the cleft and binds to -receptors in the plasma membrane of a muscle fiber, called the sarcolemma. The sarcolemma generates impulses that travel along its T tubules to the endoplasmic reticulum, which is called the sarcoplasmic reticulum in muscle fibers. The release of calcium from calcium storage sites causes muscle fibers to contract. 
The contractile portions of a muscle fiber are many parallel, threadlike myofibrils (Fig. 28.16). An electron microscope shows that myofibrils (and therefore skeletal muscle fibers) are striated because of the placement of protein filaments within contractile units called sarcomeres. A sarcomere extends between two dark lines called the Z lines. Sarco-meres contain thick filaments made up of the protein myosin and thin filaments made up of the protein actin. As a muscle fiber contracts, the sarcomeres within the myo-fibrils shorten because actin (thin) filaments slide past the myosin (thick) filaments and -approach one another. The movement of actin filaments in relation to myosin filaments is called the sliding filament model of muscle contraction. During the sliding process, the sarcomere shortens, even though the filaments themselves remain the same length. 
Why the Filaments SlideThe thick filament is a bundle of myosin molecules, each having a globular head with the capability of attaching to the actin filament when calcium (Ca2) is present. First, myosin binds to and hydrolyzes ATP. Thus, energized myosin heads attach to an actin filament. The release of ADP  P causes myosin to shift its position and pull the actin filaments to the center of the sarcomere. The action is similar to the movement of your hand when you flex your forearm (Fig. 28.17).
In the presence of another ATP, a myosin head detaches from actin. Now the heads attach at a location further along the actin filament. The cycle occurs again and again, and the actin filaments move nearer and nearer the center of the sarcomere each time the cycle is repeated. Contraction continues until nerve impulses cease and calcium ions are returned to their storage sites. The membranes of the sarcoplasmic reticulum contain active transport proteins that pump calcium ions back into the calcium storage sites, and the muscle relaxes.
Muscles Move Bones at Joints
All of our movements, from those of graceful and agile ballet dancers to those of aggressive and skillful football players, occur because muscles are attached to bones by tendons that span joints (Fig. 28.18). Because muscles shorten when they contract, they have to work in antagonistic pairs. If one muscle of an antagonistic pair flexes the joint and raises the limb, the other extends the joint and straightens the limb. Figure 28.19 illustrates this principle with regard to the movement of the forearm at the elbow joint. 
Figure 28.20b illustrates the anatomy of a freely movable synovial joint—that is, a joint having a cavity filled with synovial fluid, a lubricant for the joint. Ligaments connect bone to bone and support or strengthen a joint. The joint contains menisci (sing., meniscus), C-shaped pieces of hyaline cartilage between the bones. These give added stability and act as shock absorbers. Fluid-filled sacs called bursae (sing., bursa) ease friction between bare areas of bone and overlapping muscles, or between skin and tendons. 
The ball-and-socket joints at the hips and shoulders allow movement in all planes, even rotational movement (Fig. 28.20c). The elbow and knee joints are synovial joints called hinge joints because, like a hinged door, they largely permit movement in one direction only (Fig. 28.20d). 
Joint DisordersSprains occur when ligaments and tendons are overstretched at a joint. For example, a sprained ankle can result if you turn your ankle too much. Overuse of a joint may cause inflammation of a bursa, called bursitis. Tennis elbow is a form of bursitis. Cartilage injuries, often called torn cartilage, involve the tearing of knee menisci. Because fragments of menisci can interfere with joint movements, most physicians believe they should be removed. Today, arthroscopic surgery is possible to remove cartilage fragments or to repair ligaments or cartilage. A small instrument bearing a tiny lens and light source is inserted into a joint, as are the surgical instruments. Fluid is then added to distend the joint and allow visualization of its structure. Usually, the surgery is displayed on a monitor so that the whole operating team can see the operation. Arthroscopy is much less traumatic than surgically opening the knee with long incisions. The benefits of arthroscopy include small incisions, faster healing, a more rapid recovery, and less scarring. Because arthroscopic surgical procedures are often performed on an outpatient basis, the patient is able to return home on the same day. 
Rheumatoid arthritis, discussed on page 465, is not as common as osteoarthritis (OA), which is the deterioration of an overworked joint. Constant compression and abrasion continually damage articular cartilage, and eventually it softens, cracks, and wears away entirely in some areas. As the disease progresses, the exposed bone thickens and forms spurs that cause the bone ends to enlarge and restrict joint movement. Weight loss can ease arthritis. Taking off 3 pounds can reduce the load on a hip or knee joint by 9 to 15 pounds. A sensible exercise program helps build up muscles, which stabilize joints. Low-impact activities, such as biking and swimming, are best. 
Today, replacement of damaged joints with a prosthesis (artificial substitute) is often possible. Some people have found glucosamine-chondroitin supplements beneficial as an alternative to joint replacement. Glucosamine, an amino sugar, is thought to promote the formation and repair of cartilage. Chondroitin, a carbohydrate, is a cartilage component that is thought to promote water retention and elasticity and to inhibit enzymes that break down cartilage. Both compounds are naturally produced by the body. 
ExerciseExercise improves muscular strength, muscular endurance, and flexibility. It improves cardiorespiratory endurance and may lower the blood cholesterol level. People who exercise are less likely to develop various types of cancer. Exercise promotes the activity of osteoblasts, and therefore helps prevent osteoporosis (see page 504). It helps prevent weight gain, not only because of increased activity, but also because as muscle mass increases, the body is less likely to accumulate fat. Exercise relieves depression and enhances the mood. A sensible exercise program provides all these benefits without the detriments of a too-strenuous program. 
The Chapter in Review
Summary
28.1		The Senses
All living things respond to stimuli. In animals, stimuli generate nerve impulses that often go to a CNS, which integrates the information before initiating motor responses:
Chemical Senses
Chemoreception is found universally in animals and is therefore believed to be the most primitive sense. Human taste buds and olfactory cells are chemoreceptors.
	•	Taste buds have microvilli with receptors that bind to chemicals in food.
	•	Olfactory cells have cilia with receptors that bind to odor molecules.
The Senses of Hearing and Balance
Mammals have an ear that may have evolved from the lateral line of fishes.
The sensory receptors for hearing are hair cells with stereocilia that respond to pressure waves. 
	•	Hair cells respond to stimuli that have been received by the outer ear and amplified by the ossicles in the middle ear.
	•	Hair cells are found in the spiral organ and are located in the cochlear canal of the cochlea. The spiral organ generates nerve impulses that travel to the brain.
The sensory receptors for balance (equilibrium) are also hair cells with stereo-cilia.
	•	Hair cells in the base of the semicircular canals provide rotational equilibrium.
	•	Hair cells in the utricle and saccule provide gravitational equilibrium.
The Sense of Vision
Arthropods have a compound eye; squids and humans have a camera-type eye. In humans, the photoreceptors:
	•	Respond to light that has been focused by the cornea and lens.
	•	Consist of two types, rods and cones. In rods, rhodopsin splits into opsin and retinal.
	•	Communicate with the next layer of cells in the retina. Integration occurs in the three layers of the retina before nerve impulses go to the brain.
Cutaneous Receptors and Proprioceptors
These receptors communicate with the primary sensory area of the brain. They consist of receptors for hot, cold, pain, touch, and pressure (cutaneous) and stretching (proprioceptors). 
28.2  		The Motor Systems
Together, the muscles and bones:
	•	Support the body and allow parts to move.
	•	Help protect internal organs. 
	•	Assist the functioning of other systems.
In addition:
	•	Skeletal muscle contraction assists movement of blood in cardiovascular veins and lymphatic vessels.
	•	Skeletal muscles provide heat that warms the body.
	•	Bones are storage areas for calcium and phosphorous salts, as well as sites for blood cell formation.
The Skeleton
Arthropods have an exoskeleton, and molting is needed to replace it as the animal grows. Humans have an endoskeleton that grows with them. Worms have a hydrostatic skeleton.
The human skeleton is divided into two parts:
	•	The axial skeleton is made up of the skull, vertebral column, sternum, and ribs.
	•	The appendicular skeleton is composed of the girdles and their appendages.
Bone contains:
	•	Compact bone (site of calcium storage) and spongy bone (site of red bone marrow).
	•	A cavity (site of fat storage).
Skeletal Muscle 
Skeletal muscle is one of the three types of muscles:
	•	Smooth muscle is composed of spindle-shaped cells that form a sheet.
	•	Cardiac muscle has striated cells that form a lattice network.
	•	Skeletal muscle has tubular cells (fibers) that run the length of the muscle.
In a skeletal muscle cell:
	•	Myofibrils, myosin filaments, and actin filaments are arranged in a sarcomere.
	•	Myosin filaments pull actin filaments, and sarcomeres shorten.
Skeletal muscles:
	•	Move bones at joints. 
	•	Work in antagonistic pairs.
	•	Have freely movable synovial joints that are subject to a number of disorders, such as sprains, bursitis, and torn ligaments and cartilage.
Thinking Scientifically
	1.	The two leading causes of blindness are age-related macular degeneration and diabetic retinopathy. Both are characterized by the development of an abnormally high number of blood vessels (angiogenesis) in the retina. Why do you suppose angiogenesis impairs vision? Progress in cancer research has led to new strategies for the treatment of these eye diseases. Do you see a connection between the causes of blindness and cancer?
	2.	Human genome researchers have found a family of approximately 80 genes that encode receptors for bitter-tasting compounds. The genes encode proteins made in taste receptor cells of the tongue. Typically, many types of taste receptors are expressed per cell on the tongue. However, in the olfactory system, each cell expresses only one of the 1,000 olfactory receptor genes. Different cells express different genes. How do you suppose this difference affects our ability to taste versus smell different chemicals? Why might the two systems have evolved such different patterns of gene expression?
Testing Yourself
Choose the best answer for each question.
	1.	The most primitive sense is probably the ability to detect
	a. light.
	b. sound.
	c. chemicals.
	d. pressure.
	2.	Label the parts of the ear in the following illustration.
	3.	Loud noises generally lead to hearing loss due to damage to the
	a. outer ear.
	b. middle ear.
	c. inner ear.
	4.	The function of the lateral line system in fishes is to
	a. detect sound.
	b. locate other fish.
	c. maintain gravitational equilibrium.
	d. maintain rotational equilibrium.
	5.	The human eye focuses by
	a. changing the thickness of the lens.
	b. changing the shape of the lens.
	c. opening and closing the pupil.
	d. rotating the lens.
	6.	A blind spot occurs where the
	a. iris meets the pupil.
	b. retina meets the lens.
	c. optic nerve meets the retina.
	d. cornea meets the retina.
	7.	Label the parts of the eye in the following illustration.
	8.	Cold receptors differ from warmth receptors in
	a. abundance.
	b. shape.
	c. structure.
	d. All of these are correct.
	9.	Unlike an exoskeleton, an endoskeleton
	a. grows with the animal.
	b. is composed of chitin.
	c. is jointed.
	d. protects internal organs.
	10.	A component of the appendicular skeleton is the
	a. rib cage.
	b. skull.
	c. femur.
	d. vertebral column.
For questions 11-–14, identify the type of muscle in the key that matches the description. Some answers may be used more than once. Some questions may have more than one answer.
Key:
	a. smooth
	b. cardiac
	c. skeletal
	11.	Composed of cells with one nucleus each.
	12.	Composed of fibers that result when many cells fuse.
	13.	Composed of a lattice network of branched cells.
	14.	Striated.
	15.	The deterioration of a joint over time can cause
	a. rheumatoid arthritis.
	b. a sprain.
	c. bursitis.
	d. tendonitis.
Bioethical Issue
A lesbian couple, both of whom are deaf, sought out a deaf sperm donor in order to have a deaf child. The women considered deafness to be a cultural identity rather than a disability. They felt that they would be better parents to a deaf child because they understood the “culture” of deafness. Should this couple have been allowed to choose a father based on his ability to contribute genes for deafness? In a situation like this, whose rights should prevail, the parents’ rights to choose their mate or the child’s rights to have a chance for normal hearing? Is deafness a cultural identity or a disability?
Understanding The Terms
actin505
appendicular skeleton503
axial skeleton503
ball-and-socket joints506
blind spot500
bursa506
camera-type eye499
clavicle503
compact bone504
compound eye499
conduction deafness497
cone500
cornea499
coxal bones503
cranium503
cutaneous receptor501
endoskeleton502
exoskeleton502
facial bones503
femurs503
fibula503
fovea500
Golgi tendon501
gravitational equilibrium498
hinge joint506
hip (pelvic) girdle503
humerus503
hydrostatic skeleton503
intervertebral disks503
iris499
lateral line498
lens499
ligaments506
meniscus506
muscle spindle501
myofibril505
myosin505
nerve deafness497
osteoblast504
osteoclast504
osteon504
osteoporosis504
photoreceptor499
proprioceptor501
pupil499
radius503
red bone marrow504
retina499
rhodopsin500
rib cage503
rod500
rotational equilibrium498
rotator cuff503
saccule498
sacrum503
sarcomere505
scapula503
shoulder (pectoral) girdle503
skull503
sliding filament model505
spiral organ496
spongy bone504
statocyst498
sternum503
synovial joint506
taste buds495
tibia503
ulna503
utricle498
vertebral column503
Match the terms to these definitions:
a.	_______________	Organ that contains the sensory receptors for hearing.
b.	_______________	Gravitational equilibrium organs found in several types of invertebrates.
c.	_______________	Region of the eye that regulates the amount of light that enters.
d.	_______________	Portion of the retina that contains cones.
e.	_______________	Portion of the axial skeleton that extends from the skull to the sacrum.
f.	_______________ 	Cell type that deposits bone tissue.
g.	_______________	Contractile portion of muscle tissue.
h.	_______________	Thick filaments in a sarcomere.
i.	_______________	Joint with a cavity filled with lubricant.
j.	_______________	Hyaline cartilage between bones.
Calcium is essential for muscle contraction.
Humans have about 10,000 taste buds.
Check Your Progress
	1.	Why is the term “taste cells” a misnomer?
	2.	Explain why a certain odor can often evoke a strong memory of a person or place.
Answers:1. We don’t taste with taste cells. After chemicals bind to the receptors of taste cells, nerve impulses go to the brain, which interprets the nerve impulses as tastes.2. The olfactory system is directly tied to the centers for emotion and memory in the limbic system, so an odor can help us recall an emotion or memory.
Figure 28.2Chemoreceptors.
Chemoreceptors are common in the animal kingdom. a. Male moths have receptors on their antennae for a sex attractant released by the female. b. Humans have receptors for taste on the microvilli of taste cells and (c) receptors for smell on the cilia of olfactory cells. 
Figure 28.1Sensory input and motor output.
After detecting a stimulus, sensory receptors initiate nerve impulses within the PNS. These impulses give the central nervous system (CNS) information about the external and internal environment. The CNS integrates all incoming information and then initiates a motor response to the stimulus. 
Check Your Progress
	1.	
Describe the evidence for an evolutionary relationship between the sensory receptors for balance and hearing.
	2.	Explain how the ear can distinguish between sounds with different pitches.
Answers:1. Both consist of hair cells containing stereocilia, which are sensitive to mechanical stimulation.2. Each part of the spiral organ is sensitive to a different pitch. The nerve fibers in each part connect to and stimulate a different part of the brain.
Figure 28.4Effect of noise on hearing.
a. Normal hair cells in the spiral organ of a guinea pig. b. Spiral organ damaged by 24-hour exposure to a noise level equivalent to that of a heavy-metal rock concert. Damaged cells cannot be replaced, so hearing is permanently impaired. 
Figure 28.3The human ear.
a. The outer ear collects sound waves, the middle ear amplifies sound waves, and the cochlea contains the sensory receptors for hearing. b. The inner ear contains the cochlea, and also the semicircular canals and the vestibule. c. The sensory receptors for hearing are hair cells within the spiral organ. d. We hear when pressure waves within the canals of the cochlea cause the hair cells to vibrate and their stereocilia to bend.
Check Your Progress
	Contrast a compound eye with a camera-type eye.
Answer:A compound eye contains many lenses that focus light rays on photoreceptors, while a camera-type eye uses a single lens to focus light rays on the photoreceptors.
Figure 28.7Eyes.
a. A compound eye has several visual units. Each visual unit has a lens that focuses light onto photoreceptor cells. b. A camera-type eye has one lens that focuses light onto a retina containing many photoreceptors. 
Figure 28.8The human eye.
Light passes through the cornea and through the pupil (a hole in the iris), and is focused by the lens on the retina, which houses the photoreceptors. 
Figure 28.5Sense of balance in humans. 
a. Sensory receptors for rotational equilibrium. When the head rotates, the gelatinous membrane is displaced, bending the stereocilia. b. Sensory receptors for gravitational equilibrium. When the head bends, otoliths are displaced, causing the gelatinous membrane to sag and the stereocilia to bend. 
Figure 28.6Sensory receptors in other animals.
a. Invertebrates utilize statocysts to determine their position. When the statolith stops moving, cilia of the nearest hair cells are stimulated, telling the position of the head. b. The lateral line of fishes is not for hearing or balance. It is for knowing the location of other fishes. 
Figure 28.12Cutaneous receptors.
Numerous receptors are in the skin. Free nerve endings (yellow) in the epidermis detect pain, heat, and cold. Various touch and pressure receptors (red) are in the dermis. 
Figure 28.11Sensory input to primary sensory area of brain.
Sensory receptors in the skin, muscles, and joints send nerve impulses to the CNS. Sensation and perception occur when these reach the primary sensory area of the cerebral cortex. A motor response can be initiated by the spinal cord and cerebellum without the involvement of the cerebrum.
Figure 28.9Photoreceptors of the eye.
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 in nerve impulses.
Figure 28.10The retina.
The retina contains a layer of rods and cones, a layer of intermediate cells, and a layer of ganglion cells. Integration of signals occurs at the synapses between the layers, and much processing occurs before nerve impulses are sent to the brain. 
Check Your Progress
	Describe the functions of bones and muscles when they work together.
Answer:Bones and muscles support the body and allow body parts to move, protect internal organs, and help other systems to function.
Figure 28.14Types of skeletons.
a. Arthropods have an exoskeleton that must be shed as they grow. b. Worms have a hydrostatic skeleton, in which muscle contraction pushes against a fluid-filled internal cavity.
Figure 28.15Structure of bone.
A bone has a central cavity that is usually filled with yellow bone marrow. Both spongy bone and compact bone are living tissues composed of bone cells within a matrix that contains calcium. The spaces of spongy bone contain red bone marrow. 
Figure 28.13Musculoskeletal system.
Major muscles and bones of the body are labeled. The human skeleton contains bones that belong to the axial skeleton (red labels) and those that belong to the appendicular skeleton (black labels). 
Check Your Progress
	List the components of the appendicular skeleton.
Answer:The appendicular skeleton is composed of the shoulder girdle, the pelvic girdle, and their attached limbs.
Check Your Progress
	List the three types of muscle.
Answer:Smooth muscle, cardiac muscle, and skeletal muscle. 
Figure 28.17Why a muscle shortens when it 
contracts.
The presence of calcium (Ca2) sets in motion a chain of events (1–3) that causes myosin heads to attach to and pull an actin filament toward the center of a sarcomere. After binding to other ATP molecules, myosin heads will return to their resting position. Then, the chain of events (1–3) occurs again, except that the myosin heads reattach further along the actin filament. 
Figure 28.16Skeletal muscle fiber structure and function.
A muscle fiber contains many myofibrils divided into sarcomeres, which are contractile. When innervated by a motor neuron, the myofibrils contract, and the sarcomeres shorten because actin filaments slide past the myosin filaments. 
Check Your Progress
	1.	
Explain why a muscle shortens when it contracts.
	2.	Contrast a hinge joint with a ball-and-socket joint.
Answers: 1. The globular heads of myosin molecules attach to actin filaments and pull them toward the center of the sarcomere. Then ATP allows the heads to be released, and they reattach at a new location along the actin filament, resulting in contraction.2. A hinge joint, such as a knee or elbow, allows movement mainly in one direction. A ball-and-socket joint, such as a hip or shoulder, allows movement in all planes.
Figure 28.20Synovial joints.
a. The synovial joints of the human skeleton allow the body to be flexible and move with precision even when bearing a weight. b. Generalized synovial joint. Problems arise when menisci or ligaments are torn, bursae become inflamed, and articular cartilage wears away. c. The shoulder is a ball-and-socket joint that permits movement in three planes. d. The elbow is a hinge joint that permits movement in a single plane.
Figure 28.19Action of muscles.
When muscles contract, they shorten. Therefore, muscles only pull—they cannot push. This causes them to work in antagonistic pairs; each member of the pair pulls on a bone in the opposite direction. For example, (a) when the biceps contracts, the forearm flexes (raises), and (b) when the triceps contracts, the forearm extends (lowers).
Figure 28.18Muscles and bones.
Graceful movements are possible because tendons attach muscles to bones across a joint.