Tissue Types and Functions PDF

VetBooks.ir CHAPTER 2 Tissue Types and Functions Objectives Upon completion of this chapter, you should be able to: Describe the properties, locations, func...

VetBooks.ir CHAPTER 2 Tissue Types and Functions Objectives Upon completion of this chapter, you should be able to: Describe the properties, locations, functions, and varieties Describe the properties, locations, functions, and varieties of epithelial tissues. of nerve tissues. Describe the properties, locations, functions, and varieties Link knowledge of tissues to clinical practice. of connective tissues. Describe the properties, locations, functions, and varieties of muscle tissues. Key Terms tissue basement membrane myofiber peripheral nervous system organs integument rigor mortis neurons displaced abomasum keratin porcine stress syndrome tying up, or Monday- foot-and-mouth disease tendons hypocalcemia morning disease (FMD) ligaments sweeny Horner’s syndrome epithelial tissues adipose tissue central nervous system Introduction In the previous chapter, learners examined the cell, tissues are then arranged into organs. Mammals have the basic unit of life. Cells develop specialized struc- four basic tissue types: epithelial, connective, muscle, ture and function. A collection of cells, organized for and nerve. a particular function, is called a tissue. Collections of 18 Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  20 Unit 1 Comparative Anatomy and Physiology VetBooks.ir A Day in the Life continued to know about nerve tissue to use anesthetic to avoid Knowledge of tissues affects my everyday life as causing pain to the cow. Moreover, my surgical incision a veterinarian. In the previously described surgery, I passed through epithelial, connective, and muscle had to know what tissues I would encounter, along with tissue. Each of these tissue layers required special their properties and functions. For example, I needed handling procedures. EPITHELIAL TISSUES The senses of smell and hearing are also aided by spe- cialized epithelial tissue. Objective Every epithelial lining has an underlying connec- Describe the Properties, Locations, Functions, and Varieties tive tissue layer. A basement membrane is a collection of Epithelial Tissues of fibers that ties the epithelial layer to the underlying connective tissue. The epithelium itself has no direct Epithelial tissues are collections of cells packed to- blood supply. Therefore, the connective tissue not only gether in sheets. The sheets line the body’s surface and provides support for the epithelium but also supplies openings. These tissues also cover all the openings of nutrients and removes wastes. The blood supply within the intestinal, reproductive, and urinary tracts. In addi- the connective tissue allows these functions. Nutrients tion, they line tubes in the body, such as blood vessels supplied by the blood diffuse through the basement and the heart. membrane into the epithelium. Likewise, waste prod- Epithelial tissues perform multiple functions. Skin, ucts created by cell metabolism are removed via the an important epithelium, offers defense in many forms. underlying blood supply. Skin protects the body from trauma, the sun’s ultravio- Epithelial tissues are classified based on the shape let light, extremes of temperature, drying, and bacterial of the cells and the number of layers (Figure 2–2). The invasion. The cells lining the respiratory, intestinal, uri- epithelial tissue may be categorized as follows: (1) simple, nary, and reproductive tracts also provide protection. with one cell layer; (2) stratified, with multiple layers; Specialized cells in the respiratory tract have cilia on and (3) transitional, with multiple layers (the shape of their surface. These cilia, tiny motile filaments on the the cells can change). Descriptive terms also identify the surface of the cell, are able to move particles from the shape of the cells: (1) squamous (very flat), (2) cuboidal large airways. For example, dust or bacteria is moved (cube shaped), and (3) columnar (more tall than wide). upward so it can be coughed from the airway. The two most appropriate terms are then combined Epithelial tissues produce a variety of secretions. to describe the epithelium. Simple squamous epithelium Tears and saliva help to moisten and protect the epithe- contains a single layer of flat cells. These cells can be so lium. In the airways, mucus secretions help to trap the thin that the nucleus forms a bulge in the surface of the particles mentioned in the previous paragraph. Also, cell. Simple squamous epithelium is found where there urine and sweat are forms of epithelial excretion. In ad- is need for exchange across the border. Blood vessels dition, the mammary system is lined with epithelial cells are lined with simple squamous epithelium, allowing that secrete milk. Dairy cows have very well-developed for transfer of fluids, nutrients, gases, and wastes. This mammary systems that produce large volumes of milk. smooth flat epithelium offers little resistance to the flow Epithelial cells can absorb materials in a highly of blood through the vessels. Likewise, the small air selective manner. Cells lining the intestines, lungs, spaces of the respiratory system have a similar lining, and kidneys all take in materials from the surrounding which provides for the exchange of oxygen and carbon fluids. Cells lining the blood vessels provide points of dioxide. Simple squamous epithelium is quite fragile exchange for materials from the blood and extracellular and only exists in protected areas. fluid (ECF). Simple cuboidal epithelium consists of a single Along with keeping substances from entering the layer of cells that are almost square when observed body, the epithelial cells conserve materials within the from the side. This type of epithelium is often asso- body. These cells help to prevent excessive loss of fluid ciated with secretion or absorption. Simple cuboidal and nutrients from the ECF. epithelium is found in many exocrine and endocrine Specialized cells within the epithelial tissues pro- glands, including the thyroid, pancreas, and salivary vide sensory input. The retina in the eye is a very spe- glands. Many of the tubules of the kidney are also lined cialized epithelial layer that is essential in transmitting with simple cuboidal epithelium. Typically, it is found the visual input to the central nervous system. The in tubules that are only responsible for transport. tongue has receptors for taste, touch, and temperature, Endocrine glands secrete hormones that are distrib- providing sensory information about the environment. uted to other regions of the body via the bloodstream. Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  Chapter 2 Tissue Types and Functions 21 VetBooks.ir Photomicrograph courtesy William J. Bacha, PhD, and Photomicrograph courtesy William J. Bacha, PhD, and A D Linda M. Bacha, MS, VMD. Linda M. Bacha, MS, VMD. Photomicrograph courtesy William J. Bacha, PhD, and Photomicrograph courtesy William J. Bacha, PhD, and B E Linda M. Bacha, MS, VMD. Linda M. Bacha, MS, VMD. Photomicrograph courtesy William J. Bacha, PhD, and FIGURE 2–2 Types of epithelial cells. A. Simple squamous epithelium C covering the outer layer of the esophagus in a pig (note the very flat, dark blue nuclei of the cells). B. Simple cuboidal epithelium lining the Linda M. Bacha, MS, VMD. openings within the thyroid gland of a cat. C. Simple columnar epithelium lining the uterus of a dog. D. Stratified squamous epithelium from the lip of a sheep. This sample was taken on the mucous membrane side of the lip. This epithelium has a large number of layers. E. Transitional epithelium from a duct within the kidney of a pig. The cells become much flatter, allowing the duct to distend. Hormones regulate the function of other organs. and the intestines. In locations such as the intestinal tract, Endocrine glands release the product directly into the the simple columnar epithelium has a surface covered bloodstream without having internal ducts or tubules. with microvilli. Microvilli or brush border are tiny finger- The thyroid, adrenal, and pituitary glands are all exam- like projections of the cell membrane that tremendously ples of endocrine glands. increase the surface area of the cell and, in turn, the ef- f Exocrine glands have ducts that transport their ficiency of absorption. Stratified cuboidal and columnar secretions to impact more local areas. Therefore, the epithelium also exist in certain glands and ducts. secretions produced do not enter the circulation. Skin provides the classic example of stratified squa- Examples of exocrine glands are sweat, mammary, and mous epithelium, with many layers of very flat cells salivary glands. (Figure 2–3). All these cells originate from a basal layer Simple columnar epithelium contains a single layer of cells that appear more cuboidal. The basal layer cells of cells that are taller than they are wide. The nuclei of continually divide and move outward and undergo columnar cells are aligned near the base of all these cells. changes. The cells begin to lose cytoplasm and the This type of epithelium (often associated with secretion nucleus, becoming much flatter. The cells in the outer or absorption) can be found in many glands, the stomach, layers have an increased amount of keratin, a structural Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  22 Unit 1 Comparative Anatomy and Physiology VetBooks.ir Sweat Pore Hair Shaft Sensory Nerve Ending for Touch Epidermis Photomicrograph courtesy William J. Bacha, PhD, and Linda M. Bacha, MS, VMD. Dermis Arrector Pili Muscle Sebaceous (Oil) Gland Subcutaneous Hair Follicle Fatty Tissue (Hypodermis) © 2017 Cengage Learning® Nerve Fiber Vein Vein Artery Nerve Artery Sweat Gland A B FIGURE 2–3 A. The structure of skin. The skin is composed of the epidermis and dermis. B. Photomicrograph of the specialized skin found in the foot pad of a cat. protein. These cells are described as cornified. The for the synthesis of vitamin D, which is essential in outermost layer is covered in dead squamous cells that calcium control. Moreover, the pigment in skin helps to are constantly being shed. Stratified squamous epithe- protect the body from damaging ultraviolet radiation. lium is found in regions of mechanical stress. Other Clinically, this is important in animals with little skin regions of stratified squamous epithelium are found in pigment. Gray horses have a much higher incidence of the mouth, esophagus, vagina, and rectum. The thick- malignant melanoma than darkly pigmented horses. ness of the layer depends on the stresses in the area. With time, the accumulated sun exposure causes highly In human terms, the skin on the sole of the foot has a malignant melanoma tumors to develop in the skin. As thick layer, adding protection, relative to an area such a sensory organ, skin detects pain, pressure, and temper- as the back. Specific regions facing repeated friction or ature, thus adding further means of protection. Further- trauma will become even thicker, developing a region more, skin’s flexibility allows for movement. called a callus. For example a person doing physical Clinically, the skin provides the first visible im- labor will develop calluses on the palms of the hand. pression of an animal’s health. Many diseases are evi- Transitional epithelium remains specific to the uri- dent by the appearance of the skin. For example, fleas nary tract. This epithelium possesses the ability to commonly cause hair loss and skin sores. Many other stretch. In its relaxed state, the epithelium appears to parasitic, nutritional, and endocrine diseases affect the have at least six or seven layers of cells. As the urinary appearance of the skin. bladder fills, the epithelium stretches, allowing storage As described earlier, the skin consists of the epi- of urine. With the bladder full, the epithelium seems dermis and the underlying dermis. The epidermis is to be only a few cells thick. In addition to its ability to a stratified squamous epithelium. The cells originate stretch, transitional epithelium prevents exchange of from the basal layer and move outward. The outermost fluid between the urine and the underlying tissue. cells of the epidermis are dead and continually shed The integument, or skin, performs many important from the surface. A basement membrane separates the functions. As generally discussed with epithelium, skin epidermis from the underlying dermis. The dermis is offers a two-way barrier over the body. Skin keeps dam- a connective tissue layer that contains blood vessels, aging agents out of the body and fluids and nutrients in nerves, and glands. Another layer of connective tissue, the body. Specialization of the skin (hair, fur, and sweat called the hypodermis, then supports the skin, epider- glands) helps animals to maintain a stable body tem- mis with dermis. (Note the relationship with the term perature. During sun exposure, the skin is responsible hypodermic needle, which allows injection to layers  Chapter 2 Tissue Types and Functions 23 beneath the skin.) The hypodermis has a large percent- The arrector pili muscle is attached to the connective VetBooks.ir age of fat, which provides a layer of insulation from tissue around the hair follicle. When contracted, this extremes in external temperatures. This layer of fat muscle makes the hair stand upright. In cold weather, also functions as a shock absorber. Connective tissues this process is used to improve the insulation effect are discussed more thoroughly later in this chapter. of hair. Dogs also use this as a signal of aggression or Connective tissues are mentioned at this point to aid fear. Many dogs make the hair stand on their backs in the understanding of their close connection with the when showing aggression. Approach these dogs with epithelium. caution! The foot pads or digital pads of dogs and cats are Claws and hooves are regions of modified extremely thickened and hardened specialized areas of epidermis. Keratin, a specialized protein, is deposited the skin. This thickened skin resists physical trauma. A in the cells, giving the typical hardness and durability. pad of fat found under this skin acts as a cushion for The claws of dogs and cats surround the last bone of the foot. the toe. A rich blood supply surrounds the bone and Hair serves as another modification of the epider- subsequently nourishes the claw. Hooves have a sim- mis. Hair provides insulation, protection, and sensa- ilar anatomy. A section of the equine foot shows how tion. The hair originates from a follicle in the dermis the last bone of the foot is actually suspended within (Figure 2–4). The shaft of the hair is made of epithelial the hoof (Figure 2–5). The laminar corium has a rich cells, much like the outer layers of the epidermis. blood supply and connective tissue that supports and Growth occurs as cells are attached to the base of the nourishes the hoof wall. Growth of the hoof starts hair. Compound follicles exist where multiple hairs from the coronary band, where additional cells are exit one opening in the skin. Each hair has its own deposited. follicle. Typically, there is one long primary hair with Horns have a similar structure to claws and hooves. multiple finer and shorter secondary hairs in one com- The center of the horn, a bone, extends from the skull. pound follicle. Shedding is a process in which hairs are This bone communicates with the sinus of the skull. lost during a cycle of growth, loss, and replacement. When a cow is dehorned in a manner designed to Shedding is influenced by genetics and environmental prevent regrowth, an opening to the sinus is created. temperatures. Breeds of dogs have significant variation If not removed, horns continue to grow throughout in the amount of hair shed. the life of the animal. Blood in the surrounding tissue The benefit of insulation occurs because air is feeds the horn. The horn material, called keratinized trapped among the hairs. In addition, dark coat col- epithelium, is similar to a hoof. The keratin makes the ors help absorption of heat on exposure to sunshine. epithelium hard and durable. Hair Pore of Sweat Gland Epidermis Photomicrograph courtesy William J. Bacha, PhD, and Linda M. Bacha, MS, VMD. Sebaceous or Oil Gland Dermis Sweat or Sudoriferous Gland © 2017 Cengage Learning® Root of Hair Subcutaneous Tissue A B FIGURE 2–4 A. Hair follicles and surrounding structures. B. Photomicrograph of a region of thin skin in a pig, showing a section through a hair follicle and a nearby sweat gland. Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  24 Unit 1 Comparative Anatomy and Physiology Laminar Corium VetBooks.ir Hoof Wall Coronary Band Coronary Band Laminar Corium B A Hoof Wall © 2017 Cengage Learning® © 2017 Cengage Learning® Coffin Bone Coffin Bone FIGURE 2–5 A. The structure of the equine foot. The laminar corium suspends the bone of the toe (coffin bone) within the hoof and nourishes the hoof wall. Growth of the hoof originates from the coronary band. B. Photograph shows the internal structure of a cow’s foot. CONNECTIVE TISSUES Cartilage lacks blood vessels and is nourished by the surrounding fluid. This feature limits the thickness of car- Objective tilage and the speed at which it can heal. Hyaline cartilage Describe the Properties, Locations, Functions, and Varieties provides a durable contact surface between the bones of a of Connective Tissues moveable joint. In addition, hyaline cartilage is found in the rings that support the shape of the trachea and within Tendons, fat, blood, cartilage, and bone are all exam- the growth plates of bones in immature animals. The ples of connective tissue. These tissues share the com- densely packed fibers and ground substance produce a mon features of specialized cells embedded in large very resistant tissue but one that is more flexible than bone. amounts of extracellular material. This extracellular Elastic cartilage has more elastic fibers than hyaline material, the matrix, is produced and deposited by cartilage, making it more flexible. Elastic cartilage is the connective tissue cells. The matrix includes both found in regions where repeated movement occurs. extracellular fiber and a ground substance. The ap- Examples include the earflap and the epiglottis, which pearance of the connective tissue ranges from fibrous protects the opening to the larynx. to smooth, on the basis of the amount of each in the Bone has functions similar to those of cartilage, but tissue. Connective tissues have a number of functions the matrix is mineralized. This mineral gives bone its (Figure 2–6). As the name implies, connective tissues characteristic hardness. Bones give the body its shape, connect one organ or tissue to another. Tendons serve allow for movement, and protect internal organs. In to connect muscles to bones. The matrix in tendons is contrast to cartilage, bone does have blood vessels. The mainly composed of a protein called collagen. Collagen vessels provide nourishment for the bone cells. is arranged in bundles of fibers, which provide great Connective tissues support organs and hold tissues strength. Ligaments, another type of fibrous connective together. For example, connective tissue holds muscles tissue, connect bones to bones. In addition to collagen, together and attaches the skin to underlying tissues. ligaments have another protein called elastin. Collagen This type of tissue quite obviously appeared in the provides strength, whereas elastin provides the ability DA surgery discussed at the beginning of the chapter. to stretch and return. The collagen fibers in both ten- After incising the skin, the connective tissue layers dons and ligaments are arranged in a tightly packed were encountered. Adipose tissue (fat) was also found parallel manner. This type of connective tissue is de- deposited in this layer. Adipose tissue, another form scribed as dense regular connective tissue. of connective tissue, consists of cells filled with lipid. Connective tissues provide both support and pro- Adipose tissue is also found between muscles, behind tection. Bone and cartilage are two of the supporting the eye, within bone marrow, and in the abdomen. The connective tissues. (These topics are covered in detail in number of fat cells does not change, but the amount Chapter 3.) Just as in other connective tissues, bone and of stored lipid does vary with the nutritional status of cartilage are cellular tissues with a large amount of matrix. the animal. Fat tissue has a large blood supply which Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  Chapter 2 Tissue Types and Functions 25 VetBooks.ir Photomicrograph courtesy William J. Bacha, PhD, and Photomicrograph courtesy William J. Bacha, PhD, and A C Linda M. Bacha, MS, VMD. Linda M. Bacha, MS, VMD. Photomicrograph courtesy William J. Bacha, PhD, and Photomicrograph courtesy William J. Bacha, PhD, and B Linda M. Bacha, MS, VMD. D Linda M. Bacha, MS, VMD. FIGURE 2–6 Types of connective tissues. A. Mesentery of a cat. The mesentery is a loosely arranged connective tissue attached to abdominal organs. This shows collagen fibers (pink) and elastic fibers (thin dark fibers). B. Densely packed elastic fibers found within a ligament of a sheep. C. Adipose tissue. The actual lipid content of the cells is no longer present. Also visible are capillaries with red blood cells. D. Cartilage found in a pig, showing the large cartilage cells, some elastic fibers, and the lightly stained matrix. allows the stored fat to be readily accessed as an energy reserve. The fat underlying the skin provides an effec- tive insulation against extremes in temperature. Blood is considered a special type of connective tis- sue. The cells are suspended in a large volume of liquid matrix. The liquid portion of the blood is called plasma. Three formed elements are found in blood. Red blood © 2017 Cengage Learning® cells transport oxygen and carbon dioxide. White blood cells fight infection. Platelets aid the blood in clotting. Further details on blood can be found in Chapter 4. MUSCLE TISSUES Tendon Gastrocnemius Muscle Objective FIGURE 2–7 Photograph of the gastrocnemius tendon in a cadaver dog. Describe the Properties, Locations, Functions, and Varieties This is better known as the Achilles’ tendon. Note how the connective of Muscle Tissues tissue surrounding the muscle blend into the tendon. Muscles allow mammals to move. Three muscle types the gastrointestinal tract, urinary bladder, and blood exist in mammals: skeletal, smooth, and cardiac. vessels. The third type, cardiac muscle, is found in the Skeletal muscle attaches to the skeleton and allows heart. This type is an involuntary muscle as well. Invol- motion. The animal controls the movement of skeletal untary muscle functions without the conscious thought muscle with nerve signals from the nervous system. of the animal. These muscles continue to work at all The animal can control which muscles it will move. times, even while the animal sleeps. Smooth muscle (involuntary muscle) is located Skeletal muscle is a striated voluntary muscle in many of the hollow organs of the body, including (Figure 2–7). The description of striation comes from Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  26 Unit 1 Comparative Anatomy and Physiology VetBooks.ir Nucleus Photomicrograph courtesy William J. Bacha, PhD, © 2017 Cengage Learning® A and Linda M. Bacha, MS, VMD. Striated Muscle Cells Centrally Located Nucleu Nucleus Photomicrograph courtesy William J. Bacha, PhD, B © 2017 Cengage Learning® Striations and Linda M. Bacha, MS, VMD. Alkaline Nucleus Photomicrograph courtesy William J. Bacha, PhD, C and Linda M. Bacha, MS, VMD. © 2017 Cengage Learning® FIGURE 2–8 The three types of muscle tissues. A. Skeletal muscle from the tongue of a cat. The photomicrograph shows cells cut along their length and in cross section. B. Cardiac muscle from the heart of a goat. Note the intercalated disks that connect the cells. These structures allow the cells to act together in an organized contraction. C. Smooth muscle from the colon of a horse. Note that smooth muscle lacks the striations found in cardiac and skeletal muscle. its appearance under the microscope. A muscle The ability of a muscle fiber to contract comes from consists of thousands of muscle fibers, or muscle a very complicated system. Within the fiber, a highly cells. An entire muscle cell is called a myofiber. This organized system of myofilaments exists (Figure 2–8A). descriptive term originated because of the very long, Two proteins, actin (thin filament) and myosin (thick thin, fiberlike appearance of the cell. Myofibers have filament), make up these filaments. These units are several nuclei and a large number of mitochondria. organized along the entire length of the cell. During Also, myofibers organize in parallel rows. They are contraction, the actin and myosin filaments slide along separated by connective tissue that includes blood each other. The filaments have small bridges between vessels and nerves. them that bind and release as they slide (Figure 2–9). Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  Chapter 2 Tissue Types and Functions 27 Rigor mortis (muscle stiffness) occurs after death VetBooks.ir Muscle because there is no supply of energy to pump calcium back into the endoplasmic reticulum. Without the en- ergy, muscles cannot relax and remain stiff. Porcine stress syndrome (PSS) is a genetically transmitted disease in pigs in which calcium is not transported back into the endoplasmic reticulum. Therefore the muscles do not relax normally. Bouts of PSS typically occur when pigs are stressed from heat or transporta- tion. Extreme muscling and leanness or shaking can Bundle of Muscle Fibers generally identify pigs with a predisposition to this Nuclei condition. In severe cases, the pig may die. These pigs enter rigor mortis rapidly, which is expected based on the lack of calcium transport. Obviously calcium plays an essential role in muscle activity. Dairy cows Single Muscle Fiber (Cell) can develop a lack of calcium around calving time. At this point the cow dramatically increases the use of calcium for milk production. So much calcium can be excreted in the milk that calcium levels in blood and muscles become too low. This is technically called hypocalcemia but is commonly referred to as milk fever. A cow with milk fever becomes weak and unable to rise. Veterinarians treat milk fever with a calcium solution placed directly into the bloodstream. As the calcium transfers to the muscles, the cow re- sponds. Within a few minutes after treatment, the affected cow often stands. Dogs are also susceptible to low blood calcium lev- Actin Myosin els following parturition. In dogs the term eclampsia is used to describe this condition, which typically occurs in small-breed dogs in the first four weeks of lactation. © 2017 Cengage Learning® Initially the signs may be mild, including restlessness, anorexia, and vomiting. The signs quickly progress to muscle twitching (fasciculation), ataxia (lack of co- ordination), stiffness, and elevated body temperature in response to the extreme muscle activity. If calcium FIGURE 2–9 Muscle structure. A muscle is a collection of muscle fibers or levels drop lower the dog will progress to seizures and cells. Each muscle fiber is a multinucleated cell. Actin and myosin fila- even death. Once again there is rapid improvement in ments are arranged in an organized manner to allow contraction. response to intravenous calcium solutions. A nerve cell stimulates more than one muscle fiber. A motor unit is the collection of the nerve cell and all The contraction of a muscle fiber begins with stim- the muscle fibers it stimulates. Once stimulated a mus- ulation from a nerve cell. At the neuromuscular junc- cle cell contracts completely. Variation in the strength tion the nerve cell releases acetylcholine, resulting of a contraction is not within one cell, but within a in depolarization of the cell membrane. This change number of cells that are recruited. In other words, the in voltage opens channels to allow a rapid flow of more cells recruited, the stronger the contraction that sodium ions across the membrane. The ion flow is results. When fine control of movement is necessary, propagated along the length of the muscle cell. The the number of muscle fibers involved per nerve cell impulse stimulates the release of calcium that is stored remains small. This is seen in the muscles of the eye in the endoplasmic reticulum. This flow of calcium ions and larynx. Muscles engaged for gross movement, causes the filaments to slide across each other. Energy such as the upper leg, have a large number of fibers for is required for this entire process. A large number of each nerve cell. Muscles, a collection of a large number mitochondria are present to supply the needed energy. of myofibers, can partially contract. The more motor During relaxation, the cell actively transports calcium units used, the more completely the muscle contracts; back into the endoplasmic reticulum. This process also conversely, the fewer motor units used, the less the requires energy. muscle contracts. Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  28 Unit 1 Comparative Anatomy and Physiology The number of muscle fibers in a muscle stays contraction for prolonged periods without tiring. By VetBooks.ir basically constant. However, muscles do get larger in keeping this muscle tone, the opening within an organ response to their usage. This occurs because individual can be kept at the same diameter. Blood vessels may fibers add more myofilaments to become larger. When also be maintained at the same diameter for long peri- a muscle is not used, it decreases in size. This can oc- ods. The autonomic nervous system controls the action cur from inactivity. A house pet that gets little exercise of smooth muscle. will experience a decrease in muscle size. Having a limb immobilized in a cast also causes the limb mus- NERVE TISSUES cles to shrink. After the cast is removed, these same muscles increase in size as the limb is exercised. If the Objective nerve supply to a muscle is damaged, the muscle also shrinks. An example of this can occur in draft horses Describe the Properties, Locations, Functions, and Varieties used for pulling. A nerve in front of the shoulder can of Nerve Tissues be damaged from pulling the harness. When this nerve Nerves allow communication among areas of the body is damaged, the muscles on top of the shoulder blade by receiving and transmitting electrical signals. Nerve shrink. This condition is called sweeny. tissue is found in the brain and spinal cord, as well as Cardiac muscle is also striated in appearance but is in the peripheral nerves. Together the brain and spinal involuntary in action. (Figure 2–8B). The appearance of cord are called the central nervous system. In addition, the cardiac myofilaments is very similar to that of skel- nerves extend from these areas to other locations. The etal muscle. Moreover, the mechanism that allows for peripheral nervous system includes all the nerves out- cardiac muscle contraction is identical to that in skele- side the brain and spinal cord. tal muscle. The myofibers (muscle cells) are branched Nerve tissue contains cells called neurons in cardiac muscle and contain even more mitochondria (Figure 2–10), which can be very large. The body of than skeletal muscle. the neuron houses the nucleus and many other organ- Cardiac muscle cells have the unique ability to ini- elles. The axon, a hairlike extension from the cell body, tiate their own contraction. No nerve cell stimulation is carries the nerve impulse. The axon may end on other necessary for a contraction to begin. Specialized pace- neurons or other tissues, such as muscles. The axons maker cells are responsible for establishing the rate of from many neurons are bundled together to form a contraction. However, nerve cells are present to influ- nerve. The axon can be very long. In a horse, for ex- ence the rate of contraction. The autonomic nervous ample, some axons may be more than 2 m long. The system can increase or decrease the rate. As long as neuron has other extensions called dendrites. An axon oxygen and glucose are provided to the cardiac muscle often ends on the dendrite of another neuron. This site cells, the heart continues to beat. of connection is called a synapse. When stimulated, the For the heart to effectively serve as a pump, the dendrite begins the nerve impulse. cells must contract in a very organized manner. The The nerve impulse occurs as a flow of ions passes cardiac muscle cells have a specialized connection through the cell membrane. In a resting nerve cell, between them. When one cell contracts, the electrical sodium ions are actively transported into the extracel- signal immediately passes to the next cell through this lular fluid. At the same time, potassium is pumped into junction. This allows for a chamber of the heart to func- the cytoplasm. Once stimulated, the ions flow rapidly tion as one unit. across the membrane. Stimulation at one point then Smooth muscle gets its name because it lacks the moves down the axon in a rapid progression. After- striated appearance of skeletal muscle (Figure 2–8C). ward, the neuron prepares for the next impulse. Using Smooth muscle contains actin and myosin filaments, microelectrodes, the nerve impulse can be measured as but not in the same arrangement as skeletal muscle. an electrical event. Each myofiber is a spindle-shaped cell, tapered at each There are three basic types of neurons: end. In addition, each cell has one nucleus. Smooth muscle is arranged in sheets around hol- 1. Sensory neurons: These neurons have receptors low openings such as those in the gastrointestinal tract. that are stimulated in response to changes in the Contraction of this sheet of muscle may make the open- animal’s environment. Once stimulated, the nerve ing smaller. In blood vessels, this occurrence is called signal transmits through the neuron back to the constriction. In an organ such as the esophagus, the central nervous system. There are many types of contraction aids in propelling food toward the stom- receptors in the body. Table 2–1 lists the common ach. This organized contraction that propels the food is receptor types. The signals from these neurons called peristalsis. are then transmitted to motor neurons or inter- Smooth muscle contracts more slowly than skeletal neurons. These neurons give feedback on changes muscle. Moreover, the cells are able to maintain occurring outside and within the animal. Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  Chapter 2 Tissue Types and Functions 29 2. Interneurons: These neurons are found within VetBooks.ir Dendrites the central nervous system. Sensory neurons or interneurons stimulate them. There are a tremen- dous number of connections within the brain and spinal cord. The connections among all these neu- rons provide the pathways that allow the central nervous system to control the animal’s activities. Nucleus 3. Motor neurons: These neurons begin in the central nervous system and extend to a muscle or gland. Neuron Soma Axon When stimulated by the motor neuron, an action (Cell Body) occurs. For example, the muscle contracts or the Beads of Neuron Myelin (Nerve Cell) gland releases its secretion. A sensory neuron or A an interneuron stimulates the motor neuron. CLINICAL PRACTICE Objective Link Knowledge of Tissues to Clinical Practice © 2017 Cengage Learning® Certain infectious diseases may infect specific tissue types. For example, foot-and-mouth disease selectively attacks epithelial tissue. This disease attracted inter- national media attention when an outbreak occurred in the United Kingdom. This highly infectious dis- ease spreads very rapidly. It commonly infects cattle, sheep, goats, and swine. FMD was last diagnosed in the United States in 1929 but has been present in other countries since then. The prior outbreak in Europe raised the concern that FMD might recur in the United Photomicrograph courtesy William J. Bacha, PhD, States. B Although it is usually not fatal, FMD, a viral in- fection, causes very serious signs in affected animals. and Linda M. Bacha, MS, VMD. The epithelium in the mouth and tongue develops blisters. These blisters cause great pain, making the an- imal drool and quit eating. The epithelium around the hooves is also commonly involved, making movement painful as well. Other epithelial linings, such as those deeper in the intestinal tract, can also be infected. FIGURE 2–10 A. The structures of a neuron. B. Photomicrograph of a This disease spreads rapidly by contact with in- motor neuron from the spinal cord of a cow. fected animals. Humans can also transport the virus (for instance, on clothing and shoes or in the respira- Table 2–1 Receptor Types tory tract), which makes its spread more difficult to Light control. Fortunately for my client, her friends from Germany were not likely to be a source of introduc- Heat ing the disease into her farm. This tendency for rapid spread is the major concern of potential FMD reintro- Touch/pressure duction into the United States. Hearing In the United Kingdom, infected and exposed animals had to be destroyed in an attempt to stop Balance the disease. No treatment is available, although most © 2017 Cengage Learning® Taste animals eventually recover. Vaccines are available to control the spread of the disease but are not currently Smell being used in the United States. Distinguishing between a natural infection and a vaccinated animal through a Internal chemical receptor (results in feeling thirst) blood test can be difficult. Because the United States Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  30 Unit 1 Comparative Anatomy and Physiology has been free of FMD for such a long time, no animals The cause of this disease process is very complex. VetBooks.ir would test positive for this disease. If vaccination were The muscle tissue requires a great deal of energy and started, it would be more difficult to use screening oxygen for the work involved. High metabolism also tests to monitor the incidence of the disease, because produces waste products that must be cleared from these tests may show positive in vaccinated animals. the cells. With the right combination of rest and diet, However, if FMD were to occur in the United States, followed by intense exercise, normal balance is altered. vaccination might be used to stop its spread. Note that The buildup of waste products results in damage to the FMD is not related to hand, foot, and mouth disease, muscle and results in tying up syndrome. which commonly affects small children and is known Nerves often provide signals to several muscles in to spread rapidly in preschools and day care centers. one area. Damage to a nerve can be detected on the basis A common problem encountered in companion of the group of observable signs. Horner’s syndrome re- animal medicine occurs when a larger dog attacks sults from damage to a nerve in the autonomic nervous a small dog. In Remmy’s case the groundhog hap- system. This nerve comes through the neck and the pened to be the culprit. In these situations, the skin base of the skull to control several eye functions. When receives lacerations, which need repair. In addition to damage occurs, several signs are observed, including the lacerations, the skin may pull loose from the un- the following: (1) The pupil is constricted; (2) the up- derlying connective tissue. The preceding discussion per eyelid droops; (3) the third eyelid protrudes; and of tissues emphasized the significance of the connective (4) the eye is sunken in the socket. When these signs tissue holding the skin to the underlying muscle. In a are present, the animal has Horner’s syndrome. These traumatic injury that causes the skin to pull loose, this signs help to identify what nerve is damaged. Knowing bond has been destroyed. this, the veterinarian can direct attention to diagnosing If the torn skin is pulled back together without the underlying cause. further repair of the connective tissue, dead space is The integument protects the body from the envi- created. Dead space, a pocket under the skin, allows ronment. With long-term exposure to damaging condi- tissue fluid to accumulate. This buildup of fluid often tions, the integument itself can become diseased. Foot prevents complete healing. To avoid this problem, the rot in cattle and sheep provides an excellent example. dead space may be closed with an additional layer of Persistent exposure to wet and dirty conditions works sutures. Placing a drain into the dead space can also aid to soften the skin and hoof. Combining the soft skin in repair. This drain provides an opening that allows with trauma allows bacteria to invade the skin be- any fluid to drain. Both these methods keep the skin in tween the claws of the hoof. Dichelobacter nodosus and contact with the connective tissue. Over time the two Fusobacterium necrophorum are two bacteria commonly tissues heal together. found in the environment that colonize the skin and Horses may be affected by a disease called tying develop an infection, termed foot rot. This is a very up, or Monday-morning disease. This condition often painful condition that causes a distinct lameness. Typ- occurs on a Monday after a weekend of rest, when the ically the area between the claws of the hoof has an horse consumes a full diet. As the horse begins working open infection with an accompanying distinct odor. or exercising, it develops severe cramping. The cramps Animals with foot rot often lose weight because they are often so severe that damage occurs to the muscle are less likely to be walking and grazing or competing tissue. Products from the muscle leak from the cells for food at a feed bunk. Fortunately, the condition is and eventually are cleared from the bloodstream by responsive to topical treatment and antibiotics. Preven- the kidneys. These breakdown products may damage tion revolves around keeping the environment clean the kidneys. and dry. SUMMARY In Chapter 2, four tissue types were examined: epithe- cells embedded in vast amounts of extracellular ma- lial, connective, muscle, and nerve. Epithelial tissues terial. Moreover, three types of muscle tissue exist: line the body’s surfaces; openings, including the intesti- skeletal, cardiac, and smooth. Lastly, nerve tissues pro- nal, reproductive, and urinary tracts; and tubes, such as vide for communication within the body. These four the blood vessels and the heart. Connective tissues vary tissue types allow the body to function efficiently and in type but share the common feature of specialized effectively.  Chapter 2 Tissue Types and Functions 31 REVIEW QUESTIONS VetBooks.ir 1. Define any 10 of the following terms: 5. What type of tissue lines the body’s surface and openings? tissue organs 6. What type of epithelial tissue lines the urinary displaced abomasum tract? foot-and-mouth disease 7. What type of tissue is under attack in foot-and- epithelial tissues mouth disease? basement membrane integument 8. Name the hairlike extension from the nerve cell keratin body that carries the nerve impulse. tendons 9. After death, the body lacks energy to pump ligaments calcium back into the endoplasmic reticulum. adipose tissue Consequently, the body stiffens. Name this myofiber condition. rigor mortis porcine stress syndrome 10. Why do light-colored horses have a higher hypocalcemia incidence of melanoma than dark-colored horses? sweeny 11. Describe the shape of squamous cells. central nervous system 12. Differentiate between tendons and ligaments. peripheral nervous system neurons 13. List the three muscle types. tying up, or Monday-morning disease 14. List two involuntary muscle types. Horner’s syndrome 15. List three types of neurons. 2. True or False: Hair is epidermal tissue. 16. Describe the functions of the three different muscle 3. True or False: Kidney damage may occur in types: skeletal, smooth, and cardiac. Monday-morning disease. 4. Which of the four stomachs of a cow becomes displaced when a twisted stomach occurs? ACTIVITIES Materials needed for completion of activities: 2. Dissect a chicken drumstick and attempt to iden- tify and classify connective tissue as tendons or light microscope ligaments. slides and cover slips toothpicks 3. From a local butcher shop, secure cardiac muscle iodine (portion of the heart), smooth muscle (such as from cardiac, smooth, and skeletal muscle samples the wall of the gastrointestinal tract or the urinary dissecting kits bladder), and skeletal muscle samples. Cut each rulers sample. Compare and contrast muscle tissue types. chicken legs 4. Receptors are not evenly spaced throughout the paper plates body. The pressure receptors in the skin are more 1. Perform a simple cheek cell scrape. After securing common in sensitive areas, such as a finger, than a light microscope, toothpicks, slides, cover slips, on the back. Tape a toothpick to the end of a ruler, and iodine (or another suitable stain), prepare with the end sticking below the ruler. Hold a slides in the following manner: Scrape toothpick second toothpick close to the first one (Figure 2–11). across the inside of the cheek. Place the scraped Begin with the two toothpicks next to each other. cells onto a slide. Place a drop of iodine on the Touch the toothpicks to the skin of another student. cells, and cover with a cover slip. View under low The toothpicks should contact the skin at the same and high power. Try to identify the epithelial cells. time. The student being touched should not be Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  32 Unit 1 Comparative Anatomy and Physiology able to see the contact. Move the second toothpick VetBooks.ir farther apart until the student being touched can tell that there are two contact points. Repeat this experiment at different sites on the body, such as the finger and back. Is there a difference in the dis- tance detected between the two areas? If so, why? 5. The mission of the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS) is to protect the health and value of American agriculture and natural resources. Investigate the history, purpose, © 2017 Cengage Learning® and current programming of the agency. 6. Research the current status of foot-and-mouth dis- ease. Possible sources for information include the U.S. Department of Agriculture, state departments of agriculture, land grant university departments FIGURE 2–11 of animal or veterinary science, or the local Cooperative Extension Service. Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. 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