Lecture exam #2 Flashcards

Question

Describe the process of wound healing

Answer 1

1. Cut blood vessels initiate bleeding into the wound. The blood brings clotting proteins, numerous leukocytes, and antibodies 2. A blood clot forms, temporarily patching the edges of the wound together and acting as a barrier to prevent the entry of pathogens into the body. Internal to the clot, macrophages and neutrophils clean the wound of cellular debris. 3. The cut blood vessels regenerate and grow in the wound. A soft mass deep in the wound becomes granulation tissue, which is a vascular connective tissue that initially forms in a healing wound. Macrophages within the wound begin to remove the clotted blood. Fibroblasts produce new collagen fibers in the region. 4. Epithelial regeneration of the epidermis occurs due to division of epithelial cells at the edge of the wound. These new epithelial cells migrate over the wound, moving internally to the now superficial remains of the clot (the scab). The connective tissue is replaced by fibrosis.

Answer 2

∙ Most common type of skin cancer ∙ Least dangerous type, as it seldom metastasizes (i.e., spreads to other locations within the body) ∙ Originates in stratum basale ∙ First appears as small, shiny elevation that enlarges and develops central depression with pearly edge ∙ Usually occurs on face ∙ Treated by surgical removal of lesion

Answer 3

∙ Arises from keratinocytes of stratum spinosum ∙ Lesions usually appear on scalp, ears, lower lip, or dorsum of hand. ∙ Early lesions are raised, reddened, scaly; later lesions form concave ulcers with elevated edges. ∙ Treated by early detection and surgical removal of lesion ∙ May metastasize to other parts of the body

Answer 4

∙ Most deadly type of skin cancer due to aggressive growth and metastasis ∙ Arises from melanocytes, usually in a preexisting mole ∙ Individuals at increased risk include those who have had severe sunburns, especially as children. ∙ Characterized by change in mole diameter, color, shape of border, and symmetry ∙ Survival rate improved by early detection and surgical removal of lesion ∙ Advanced cases (metastasis of disease) are difficult to cure and are treated with chemotherapy, interferon therapy, and radiation therapy. A = Asymmetry: One-half of a mole or birthmark does not match the other. B = Border: Edges are notched, irregular, blurred, or ragged. C = Color: Color is not uniform; differing shades (usually brown or black and sometimes patches of white, blue, or red) may be seen. D = Diameter: Affected area is larger than 6 mm (about 1/4 inch) or is growing larger. E = Evolving: Change in the size, shape, or color of a mole or a change in symptoms, such as how a mole feels (how itchy or tender it feels) or what happens on the surface of a mole (especially bleeding)

Answer 5

Our skeletal system includes the bones of the skeleton as well as cartilage, ligaments, and other connective tissues that stabilize or connect the bones.

Answer 6

Bones of the skeleton are the primary organs of the skeletal system. They form the rigid framework of the body and perform other functions, described shortly. Two types of bone connective tissue are present in most of the bones of the body: compact bone and spongy bone. Compact bone (also called dense or cortical bone) is a relatively rigid connective bone tissue that appears white, smooth, and solid. It makes up approximately 80% of the total bone mass. Spongy bone (also called cancellous or trabecular bone) is located internal to compact bone, appears porous, and makes up approximately 20% of the total bone mass.

Answer 7

Cartilage is a semirigid connective tissue that is more flexible than bone. Mature cartilage is avascular (lacks a blood supply). ∙ Hyaline cartilage attaches ribs to the sternum (costal cartilage), covers the ends of some bones (articular cartilage), and is the cartilage within growth plates (epiphyseal plates). Hyaline cartilage also provides a model during development for the formation of the fetal skeleton. ∙ Fibrocartilage is a weight-bearing cartilage that withstands compression. It forms the intervertebral discs, the pubic symphysis (cartilage between bones of the pelvis), and the cartilage pads of the knee joints (menisci).

Answer 8

Bones perform several basic functions: support and protection, levers for movement, hemopoiesis, and storage of mineral and energy reserves.

Answer 9

Long bones are greater in length than width. These bones have an elongated, cylindrical shaft (diaphysis). This is the most common bone shape. Long bones are found in the upper limbs (namely, the arm, forearm, palm, and fingers) and lower limbs (thigh, leg, sole of the foot, and toes). Long bones vary in size. The small bones in the fingers and toes are long bones, as are the larger tibia and fibula of the lower limb.

Answer 10

One of the principal gross features of a long bone is its shaft, which is called the diaphysis. The elongated, usually cylindrical diaphysis provides for the leverage and major weight support of a long bone. Extending internally from the compact bone along the length of the diaphysis are spicules (thin, needlelike structures) of spongy bone. The hollow, cylindrical space within the diaphysis is called the medullary (marrow) cavity. In children, this cavity contains red bone marrow, which later is replaced by yellow bone marrow in adults. An expanded, knobby region called the epiphysis is at each end of a long bone. A proximal epiphysis is the end of the bone closest to the body trunk, and a distal epiphysis is the end farthest from the trunk. An epiphysis is composed of an outer, thin layer of compact bone and an inner, more extensive region of spongy bone. Spongy bone within the epiphysis resists stress that is applied from many directions. Covering the joint surface of an epiphysis is a thin layer of hyaline cartilage called the articular cartilage. This cartilage helps reduce friction and absorb shock in movable joints. The metaphysis is the region in a mature bone sandwiched between the diaphysis and the epiphysis. This region contains the epiphyseal plate (or growth plate) in a growing bone. It is a thin layer of hyaline cartilage that provides for the continued lengthwise growth of the bone. The remnant of the epiphyseal plate in adults is a thin, defined area of compact bone called the epiphyseal line.

Answer 11

A tough sheath called periosteum covers the outer surface of the bone except for the areas covered by articular cartilage. The periosteum consists of two layers. The outer, fibrous layer of dense irregular connective tissue protects the bone from surrounding structures, anchors blood vessels and nerves to the surface of the bone, and serves as an attachment site for ligaments and tendons. The inner, cellular layer includes osteoprogenitor cells, osteo- blasts, and osteoclasts. The periosteum is anchored to the bone by numerous collagen fibers called perforating fibers, or Sharpey's fibers, which run perpendicular to the diaphysis. The endosteum is an incomplete layer of cells that covers all internal surfaces of the bone within the medullary cavity. The endosteum, like the periosteum, contains osteoprogenitor cells, osteoblasts, and osteoclasts.

Answer 12

Bone marrow is the soft connective tissue of bone that includes both red bone marrow and yellow bone marrow. Red bone marrow (also called myeloid tissue) is hemopoietic (i.e., blood cell–forming) and contains reticular connective tissue, developing blood cells, and adipocytes The locations of red bone marrow differ between children and adults. In children, red bone marrow is located in the spongy bone of most of the bones of the body as well as the medullary cavity of long bones. Much of the red bone marrow changes as children mature into adults. Primarily within the medullary cavities of long bones and inner core of most epiphyses there is a progressive decrease in developing blood cells and an increase in adipocytes. This fatty-appearing substance is called yellow bone marrow. As a result, adults have red bone marrow only in selected portions of the axial skeleton, such as the flat bones of the skull, the vertebrae, the ribs, the sternum, and the ossa coxae (hip bones). Adults also have red bone mar- row in the proximal epiphyses of each humerus and femur.

Answer 13

Short, flat, and irregular bones differ in their gross anatomic structure from long bones. The external surface generally is composed of compact bone, the interior is composed entirely of spongy bone, and there is no medullary cavity. Observe the layer of spongy bone in between the roughly parallel segments of compact bone.

Answer 14

1. osteoprogenitor cells: stem cells derived from mesenchyme. divide by mitosis. another stem cell is produced along with a "committed cell" that matures into an osteoblast 2. osteoblasts: formed from osteoprogenitor cells. main function is to synthesize and secrete initial semisolid organic form of bone matrix called OSTEOID. these osteoids later calcify as a result of salt crystal deposition. this traps osteoblasts in the matrix they created and become osteocytes. 3. osteocytes: mature bone cells deriving from osteoblasts. function is to maintain bone matrix and detect mechanical stress on bone. 4. osteoclasts: large, multinuclear, phagocytic cells. function is to break down bone during a process called bone resorption.

Answer 15

The matrix of bone connective tissue has both organic and inorganic components. The organic component is osteoid, which is produced by osteoblasts. Osteoid is composed of both collagen and a semisolid ground substance of proteoglycans and glycoproteins that suspends and supports the collagen fibers. These organic components give bone tensile strength by resisting stretching and twisting, and contribute to its overall flexibility. The inorganic portion of the bone matrix is made up of salt crystals that are primarily calcium phosphate, Ca3(PO4)2. Calcium phosphate and calcium hydroxide, Ca(OH)2, interact to form crystals of hydroxyapatite, which is Ca10(PO4)6(OH)2. These crystals deposit around the long axis of collagen fibers in the extracellular matrix. The crystals harden the matrix and account for the rigidity or relative inflexibility of bone that provides its compressional strength. A loss of protein, or the presence of abnormal protein, results in brittle bones; insufficient calcium results in soft bones.

Answer 16

Bone formation begins when osteoblasts secrete osteoid. Calcification, or mineralization, subsequently occurs to the osteoid when hydroxyapatite crystals deposit in the bone matrix. Calcification is initiated when the concentration of calcium ions and phosphate ions reaches critical levels and precipitate out of solution, thus forming the hydroxyapatite crystals that deposit in and around the collagen fibers. The entire process of bone formation requires a number of substances, including vitamin D and vitamin C (which is required for collagen formation), as well as calcium and phosphate for calcification.

Answer 17

Bone resorption is a process whereby bone matrix is destroyed by substances released from osteoclasts into the extracellular space adjacent to the bone. Proteolytic enzymes released from lysosomes within the osteoclasts chemically digest the organic components (collagen fibers and proteoglycans) of the matrix, while hydrochloric acid (HCl) dissolves the mineral parts (calcium and phosphate) of the bone matrix. The liberated calcium and phosphate ions enter the blood. Bone resorption may occur when blood calcium levels are low.

Answer 18

Compact bone is composed of small, cylindrical structures called osteons, or Haversian systems. An osteon is the basic functional and structural unit of mature compact bone. Osteons are oriented parallel to the diaphysis of the long bone. Unlike compact bone, spongy bone contains no osteons Instead, its structure is an open lattice of narrow rods and plates of bone, called trabeculae. Bone marrow fills in between the trabeculae. Between adjacent lamellae are osteocytes resting in lacunae, with numerous canaliculi radiating from the lacunae. Nutrients reach the osteocytes by diffusion through cytoplasmic processes of the osteocytes, which extend within the canaliculi that open onto the surfaces of the trabeculae. Note that the trabeculae often form a meshwork of crisscrossing bars and plates of small bone pieces. This structure provides great resistance to stresses applied in many directions by distributing the stress throughout the entire framework.

Answer 19

Intramembranous ossification literally means “bone growth within a membrane.” It is so named because the thin layer of mesenchyme in these areas is sometimes referred to as a membrane. Intramembranous ossification also is called dermal ossification because the mesenchyme that is the source of these bones is in the area of the future dermis. Mesenchyme is an embryonic connective tissue that has mesenchymal cells and abundant ground substance. Intramembranous ossification produces the flat bones of the skull (e.g., frontal bone), some of the facial bones (e.g., zygomatic bone, maxilla), the mandible (lower jaw), and the central part of the clavicle (collarbone).

Answer 20

1. Ossification centers form within thickened regions of mesenchyme beginning at the eighth week of development. Some cells in the thickened, condensed mesenchyme divide, and the committed cells that are formed then differentiate into osteoprogenitor cells. Some osteoprogenitor cells become osteoblasts and begin to secrete osteoid. Multiple ossification centers develop within the thickened mesenchyme as the number of osteoblasts increases. 2. Osteoid undergoes calcification. Osteoid formation is quickly followed by calcification, as calcium salts are deposited onto the osteoid and then they crystallize (solidify). When calcification entraps osteoblasts within lacunae in the matrix, the entrapped cells become osteocytes. 3. Woven bone and its surrounding periosteum form. 4. Lamellar bone replaces woven bone, as compact and spongy bone form.

Answer 21

endochondral ossification begins with hyaline cartilage and produces most bones of skeleton including upper, lower limbs, pelvis, vertebrae, ends of clavicle.

Answer 22

1. A hyaline cartilage model of bone forms. 2. Bone first replaces hyaline cartilage in the diaphysis. 3. Next, bone replaces hyaline cartilage in the epiphyses. 4. Eventually, bone replaces hyaline cartilage everywhere, except the epiphyseal plates and articular cartilage. 5. By a person’s late 20s, all epiphyseal plates typically have ossified, and lengthwise bone growth is complete.

Answer 23

Bone growth occurs in length through interstitial growth within the epiphyseal plate and in width through appositional growth at the periosteum.

Answer 24

1. Zone of resting cartilage. This zone is farthest from the medullary cavity of the diaphysis and nearest the epiphysis. It is composed of small chondrocytes distributed throughout the cartilage matrix. It resembles mature and healthy hyaline cartilage. This region secures the epiphysis to the epiphyseal plate. 2. Zone of proliferating cartilage. Chondrocytes in this zone undergo rapid mitotic cell division, enlarge slightly, and become aligned like a stack of coins into longitudinal columns of flattened lacunae. These columns are parallel to the diaphysis. 3. Zone of hypertrophic cartilage. Chondrocytes cease dividing and begin to hypertrophy (enlarge in size) in this zone. The walls of the lacunae become thin because the chondrocytes resorb matrix as they hypertrophy. 4. Zone of calcified cartilage. This zone usually is composed of two or three layers of chondrocytes. Minerals are deposited in the matrix between the columns of lacunae; this calcification destroys the chondrocytes and makes the matrix appear opaque. 5. Zone of ossification. The walls break down between lacunae in the columns, forming longitudinal channels. These spaces are invaded by capillaries and osteoprogenitor cells from the medullary cavity. New matrix of bone is deposited on the remaining calcified cartilage matrix.

Answer 25

In this process, osteoblasts in the inner cellular layer of the periosteum produce and deposit bone matrix within layers parallel to the surface, called external circumferential lamellae. These lamellae are analogous to tree rings: As they increase in number, the structure increases in diameter. Thus, the bone becomes wider as new bone is laid down at its periphery. As this new bone is being laid down, osteoclasts along the medullary cavity resorb bone matrix, creating an expanding medullary cavity. The combined effects of bone growth at the periphery and bone resorption within the medullary cavity transform an infant bone into a larger version called an adult bone. Appositional growth continues throughout an individual’s lifetime.

Answer 26

The continual deposition of new bone tissue by osteoblasts and resorption of bone by osteoclasts are called bone remodeling. Mechanical stress occurs in the form of weight-bearing movement and exercise, and it is required for normal bone remodeling. Stress is detected by osteocytes and communicated to osteoblasts. Osteoblasts increase the synthesis of osteoid, and this is followed by deposition of mineral salts. Bone strength increases over a period of time in response to mechanical stress. Mechanical stresses that significantly affect bone result from skeletal muscle contraction and gravitational forces.

Answer 27

Growth hormone: Stimulates liver to produce the hormone IGF, which causes cartilage proliferation at epiphyseal plate and resulting bone elongation Thyroid hormone: Stimulates bone growth by stimulating metabolic rate of osteoblasts Calcitonin: Promotes calcium deposition in bone and inhibits osteoclast activity Calcitriol: Stimulates absorption of calcium ions from the small intestine into the blood Parathyroid hormone: Increases blood calcium levels by encouraging bone resorption by osteoclasts Sex hormones (estrogen and testosterone): Stimulate osteoblasts; promote epiphyseal plate growth and closure Glucocorticoids: Increase bone loss and, in children, impair bone growth when there are chronically high levels of glucocorticoids Serotonin: Inhibits osteoprogenitor cells from differentiating into osteoblasts when there are chronically high levels of serotonin

Answer 28

1. Ultraviolet light converts the precursor molecule in keratinocytes of the skin (7-dehydrocholesterol, a modified cholesterol molecule) to vitamin D3 (cholecalciferol), which is released into the blood. (Vitamin D3 also is absorbed from the small intestine into the blood from the diet.) 2. Vitamin D3 circulates throughout the blood. As it passes through the blood vessels of the liver, it is converted by liver enzymes to calcidiol by the addition of a hydroxyl group (—OH). Both steps 1 and 2 occur continuously with limited regulation. 3. Calcidiol circulates in the blood: As it passes through blood vessels of the kidney, it is converted to calcitriol by kidney enzymes (when another —OH group is added). Calcitriol is the active form of vitamin D3. The presence of parathyroid hormone increases the rate of this final enzymatic step in the kidney. Thus, greater amounts of calcitriol are formed when parathyroid hormone is present.

Answer 29

Parathyroid hormone (PTH) is secreted and released by the parathyroid glands in response to reduced blood calcium levels. The final enzymatic step converting calcidiol to calcitriol in the kidney occurs more readily in the presence of PTH. PTH and calcitriol interact with selected major organs as follows: ∙ Bone. PTH and calcitriol act synergistically (their combined effect is greater than the sum of their individual effects) to increase the release of calcium from the bone into the blood, by increasing osteoclast activity. ∙ Kidneys. PTH and calcitriol act synergistically to stimulate the kidneys to excrete less calcium in the urine (and thus retain more calcium in the blood). This occurs by increasing calcium reabsorption in the tubules in the kidneys

Answer 30

Stimulus: Low blood calcium levels Receptor: Parathyroid glands detect low blood calcium levels Control Center: Parathyroid glands release parathyroid hormone. Effector: Bone: PTH and calcitriol act synergistically to increase activity of osteoclasts. Kidney: PTH and calcitriol act synergistically to decrease calcium excreted in urine. Small Intestine: Calcitriol increases absorption of calcium from small intestine Blood calcium levels rise and return to a normal homeostatic range.

Answer 31

Aging affects bone connective tissue in two ways. First, the tensile strength of bone decreases due to a reduced rate of protein synthesis by osteoblasts. Consequently, the relative amount of inorganic minerals in the bone matrix increases (due to decreased matrix protein), and the bones of the skeleton become brittle and susceptible to fracture. Second, bone loses calcium and other minerals (demineralization). The bones of the skeleton become thinner and weaker, resulting in insufficient ossification, a condition called osteopenia. Aging causes all people to become slightly osteopenic. This reduction in bone mass may begin as early as 35–40 years of age, when osteoblast activity declines, while osteoclast activity continues at previous levels. Different parts of the skeleton are affected unequally. Vertebrae, jaw bones, and epiphyses lose large amounts of mass, resulting in reduced height, loss of teeth, and fragile limbs. osteoporosis: a condition characterized by reduction in bone mass sufficient to compromise normal function

Answer 32

1. A fracture hematoma forms. A bone fracture tears blood vessels inside the bone and within the periosteum, causing bleeding. This bleeding results in a fracture hematoma that forms from the clotted blood. 2. A fibrocartilaginous (soft) callus forms. Regenerated blood capillaries infiltrate the fracture hematoma. First, the fracture hematoma is reorganized into an actively growing connective tissue called a procallus. Fibroblasts within the procallus produce collagen fibers that help connect the broken ends of the bones. Chondroblasts in the newly growing connective tissue form a dense regular connective tissue associated with the cartilage. Eventually, the procallus becomes a fibrocartilaginous (soft) callus. The fibrocartilaginous callus stage lasts at least 3 weeks. 3. A hard (bony) callus forms. Within a week after the injury, osteoprogenitor cells in areas adjacent to the fibrocartilaginous callus become osteoblasts and produce trabeculae of primary bone. The fibrocartilaginous callus is then replaced by this bone, which forms a hard (bony) callus. The trabeculae of the hard callus continue to grow and thicken for several months. 4. The bone is remodeled. Remodeling is the final phase of fracture repair. The hard callus persists for at least 3 to 4 months as osteoclasts remove excess bony material from both exterior and interior surfaces. Compact bone replaces primary bone.

Answer 33

Body movement. Contraction of your skeletal muscles generates large body movements, such as those of walking, and the smaller, more precise body movements such as picking up an object. It is also responsible for the highly developed movements involved in communicating that occur when speaking, writing, and changing facial expressions; the movements associated with breathing; and those involved in the voluntary phase of swallowing. ∙ Maintenance of posture. Contraction of specific skeletal muscles stabilizes your trunk, pelvis, legs, neck, and head to keep you erect. These postural muscles contract continuously when you are awake to keep you from collapsing. ∙ Protection and support. Skeletal muscle is arranged in layers within the walls of the abdominal cavity and the floor of the pelvic cavity. These layers of muscle protect the internal organs and support their normal position within the abdominopelvic cavity. ∙ Regulating elimination of materials. Circular muscle bands, called sphincters contract and relax to regulate passage of material. These skeletal muscle sphincters at the orifices of the gastrointestinal and urinary tracts allow you to voluntarily control the expulsion of feces and urine, respectively ∙ Heat production. Energy is required for muscle tissue contraction, and heat is always produced by this energy use. Thus, muscles are like small furnaces that continuously generate heat and function to help maintain your normal body temperature. You shiver when you are cold because involuntary skeletal muscle contraction gives off heat. Likewise, you sweat during exercise to release the additional heat produced by your working muscles

Answer 34

Excitability is the ability of a cell to respond to a stimulus (e.g., chemical, stretch). The stimulus causes a local change in the resting membrane potential by triggering the movement of ions across the plasma membrane of the excitable cell. A skeletal muscle cell responds when its receptors bind neurotransmitter (acetylcholine), which is released from a motor neuron. Conductivity involves an electrical signal that is propagated along the plasma membrane as voltage-gated channels open sequentially during an action potential. These electrical signals functionally connect the plasma membrane of the muscle cell (where stimulation occurs) to the interior of the muscle cell. Contractility is exhibited when contractile proteins within skeletal muscle cells slide past one another. Contractility is what enables muscle cells to cause body movement and to perform the other functions of muscles. Extensibility is the lengthening of a muscle cell. This lengthening is possible because the contractile proteins slide past one another to decrease their degree of overlap. Muscle’s extensibility is exhibited when we stretch our muscles, such as before exercising. Elasticity is the ability of a muscle cell to return to its original length following either shortening or lengthening of the muscle. Elasticity of muscle cells is dependent upon the release of tension in the springlike connectin protein associated with contractile proteins

Answer 35

The epimysium is a layer of dense irregular connective tissue that surrounds the whole skeletal muscle. This fibrous tissue ensheathes the entire skeletal muscle to protect and support it like a tough leather sleeve. The perimysium is a layer of dense irregular connective around each fascicle. These tough, fibrous connective tissue sleeves also provide protection and support, but to each bundle of muscle fibers. The endomysium is composed of areolar connective tissue that surrounds each muscle fiber. These more delicate coverings function to electrically insulate the muscle fibers.

Answer 36

A tendon is a thick, cordlike structure composed of dense regular connective tissue, whereas an aponeurosis is a thin, flattened sheet of dense regular connective tissue. Both tendons and aponeuroses attach a muscle either to a skeletal component (bone or ligament) or to fascia. Imagine the typical scenario as skeletal muscle fibers contract, pulling on the connective tissue sheaths, with the force transferred to a tendon that moves a bone.

Answer 37

Deep fascia, also called visceral or muscular fascia, is an additional, expansive sheet of dense irregular connective tissue that is external to the epimysium. Deep fascia separates individual muscles; binds together muscles with similar functions; contains nerves, blood vessels, and lymph vessels; and fills spaces between muscles. The deep fascia is internal or deep to a layer called the superficial fascia (or subcutaneous layer). The superficial fascia is composed of areolar connective tissue and adipose connective tissue that separates muscle from skin

Answer 38

The plasma membrane of a skeletal muscle fiber is called the sarcolemma. Deep invaginations of the sarcolemma, called T-tubules or transverse tubules, extend into the skeletal muscle fiber as a network of narrow, membranous tubules to the sarcoplasmic reticulum, which is the endoplasmic reticulum (ER) of the muscle. Located within the membrane of both the sarcolemma (along its length) and the T-tubules are voltage-gated channels. The sarcoplasmic reticulum is an internal membrane complex that is similar to the smooth endoplasmic reticulum of other cells. Segments of the sarcoplasmic reticulum (SR) fit around the myofibril like a sleeve of membrane netting. At either end of individual sections of the sarcoplasmic reticulum are blind sacs called terminal cisternae, which are much like the hem of a sleeve. Terminal cisternae serve as the reservoirs for calcium ions (Ca2+) and are immediately adjacent to each T-tubule. Together, two terminal cisternae and a centrally located T-tubule form a structure called a triad. Within the triad, the T-tubule membrane contains voltage-sensitive Ca2+ channels (dihydropyridine receptors), which are responsive to electrical signals

Answer 39

Myofilaments are contractile proteins that are bundled within myofibrils. A myofilament is not as long as a myofibril; rather, it takes many successive units of myofilaments to extend the entire length of the myofibril. Myofibril bundles contain two types of myofilaments: thick filaments and thin filaments Approximately 80% of the volume of a skeletal muscle fiber is composed of long, cylindrical structures termed myofibrils. A skeletal muscle fiber contains hundreds to thousands of myofibrils. Each myofibril extends the entire length of the skeletal muscle fiber. Note that each myofibril is composed of bundles of contractile pro- teins called myofilaments and is enclosed in portions of the sarcoplas- mic reticulum

Answer 40

Thick Filaments Thick filaments (or thick myofilaments) are assembled from bundles of 200 to 500 myosin protein molecules. Each myosin protein consists of two strands; each strand has a globular head and an elongated tail. The myosin head contains a binding site for actin of the thin filaments. The head also has a catalytic ATPase site where adenosine triphosphate (ATP) attaches and is split into adenosine diphosphate (ADP) and phosphate (Pi). (It is because the head of myosin functions as an ATPase enzyme that myosin is often referred to more specifically as myosin ATPase.) The tails of two strands of a myosin molecule are intertwined. Each myosin molecule composing a thick filament is oriented so that its tails point toward the center of the thick filaments and its heads point toward the ends of the thick filaments. Thin Filaments or thin myofilaments are approximately half of the diameter of thick filaments. Thin filaments are primarily composed of two strands of actin protein twisted around each other to form a helical shape. In each strand of actin, many (about 300 to 400) small, spherical molecules (G, or globular, actin) are connected to form a fibrous strand (F, or filamentous, actin). F-actin resembles two beaded necklaces that are twisted and intertwined together, with G-actin as the individual beads. Each G-actin molecule has a significant feature called a myosin binding site. The myosin head attaches to the myosin binding site of actin during muscle contraction

Answer 41

Each G-actin molecule has a signifi- cant feature called a myosin binding site. The myosin head attaches to the myosin binding site of actin during muscle contraction. Tropomyosin and troponin are regulatory proteins associated with thin filaments. Together they form the troponin-tropomyosin complex. Tropomyosin is a short, thin, twisted filament that is a “stringlike” protein. Consecutive tropomyosin molecules cover small regions of the actin strands, including the myosin binding sites in a non-contracting muscle. Troponin is a globular, or “ball-like,” protein attached to tropomyosin. Troponin contains the binding site for Ca2+

Answer 42

Connectin also called titin, is a “cablelike” protein that extends from the Z discs to the M line through the core of each thick filament . It stabilizes the position of the thick filament and maintains thick filament alignment within a sarcomere. Additionally, portions of the connectin molecules are coiled and “springlike” so that during sarcomere shortening they are compressed to produce passive tension. This passive tension is then released to return the sarcomere to its normal resting length. Thus, connectin contributes to skeletal muscle fiber elasticity. Dystrophin is part of a protein complex that anchors myofibrils that are adjacent to the sarcolemma to proteins within the sarcolemma. These proteins of the sarcolemma also extend to the connective tissue of the endomysium that encloses the muscle fiber. Thus, dystrophin links internal myofilament proteins of a muscle fiber to external proteins. The genetic disorder of muscular dystrophy is caused by abnormal structure, or amounts, of dystrophin protein

Answer 43

Myofilaments within myofibrils are arranged in repeating, microscopic, cylindrical units called sarcomeres. The number of sarcomeres varies with the length of the myofibril within the skeletal muscle fiber. Each sarcomere is composed of overlapping thick filaments and thin filaments. Z discs are composed of specialized proteins that are positioned perpendicular to the myofilaments and serve as anchors for the thin filaments. Although the Z disc appears as a flat disc when the myofibril is viewed from its end, only the edge of the disc is visible in a side view, and it sometimes looks like a zigzagged line. The thick filaments and thin filaments overlap within a sarcomere, forming the following regions: I bands extend from both directions of a Z disc and are bisected by the Z disc. These end regions contain only thin filaments. At maximal muscle shortening, the thin filaments are pulled parallel along the thick filaments, causing the I bands to disappear. The A band is the central region of a sarcomere that contains the entire thick filament. Thin filaments partially overlap the thick filament on each end of an A band. The A band appears dark when viewed with a microscope. The A band does not change in length during muscle contraction. ∙ The H zone (also called the H band) is the most central portion of the A band in a resting sarcomere. This region does not have thin filament overlap; only thick filaments are present. During maximal muscle shortening, this zone disappears when the thin filaments are pulled past thick filaments. ∙ The M line is a thin transverse protein meshwork structure in the center of the H zone. It serves as an attachment site for the thick filaments and keeps the thick filaments aligned during contraction and relaxation events.

Answer 44

Skeletal muscle fibers have abundant mitochondria for aerobic cellular respiration. The fibers also contain glycogen stores for use as an immediate fuel molecule. Myoglobin is a molecule unique to muscle tissue. Myoglobin is a reddish, globular protein that is somewhat similar to hemoglobin. It binds oxygen when the muscle is at rest and releases it for use during muscular contraction. This additional source of oxygen provides the means to enhance aerobic cellular respiration and the production of ATP. Skeletal muscle fibers also contain another type of molecule called creatine phosphate. Creatine phosphate provides muscle fibers with a very rapid means of supplying ATP.

Answer 45

A single motor neuron and the skeletal muscle fibers it controls is called a motor unit. The number of skeletal muscle fibers a single motor neuron innervates—and thus the size of the motor unit—varies and can range from small motor units that have less than five muscle fibers to large motor units that have several thousand muscle fibers. The size of the motor unit determines the degree of control. There is an inverse relationship between the size of a motor unit and the degree of control.

Answer 46

The synaptic knob of a motor neuron is an expanded tip of an axon. Where the axon nears the sarcolemma of a muscle fiber, the synaptic knob enlarges and flattens to cover a relatively large surface area of the sarcolemma. The synaptic knob cytosol houses numerous synaptic vesicles (small membrane sacs) filled with molecules of the neurotransmitter acetylcholine. The motor end plate is a specialized region of the sarcolemma of a skeletal muscle fiber. It has numerous folds and indentations (junction folds) to increase the membrane surface area covered by the synaptic knob. The motor end plate has vast numbers of ACh receptors. These plasma membrane protein channels are chemically gated ion channels. Binding of ACh opens these channels, allowing Na+ entry into the muscle fiber and K+ to exit. ACh receptors are like doors; ACh is the only “key” to open these receptor doors. The synaptic cleft is an extremely narrow, fluid-filled space separating the synaptic knob and the motor end plate. The enzyme acetylcholinesterase (AChE) resides within the synaptic cleft and quickly breaks down ACh molecules following their release into the synaptic cleft.

Answer 47

One essential feature of skeletal muscle fibers is the electrical charge difference across the sarcolemma; the cytosol right inside the plasma membrane is relatively negative in comparison to the interstitial fluid outside of the cell. This electrical charge difference when the cell is at rest is called the resting membrane potential (RMP). Skeletal muscle fibers have an RMP of about –90 millivolts (mV). An RMP is established and maintained by both leak channels and Na+/K+ pumps. The primary function of the Na+/K+ pumps is to maintain the concentration gradients for Na+ (with more Na+ outside the cell) and K+ (with more K+ inside the cell). The acetylcholine receptors (chemically gated ion channels) within the motor end plate and the voltage-gated Na+ channels and voltage-gated K+ channels in the sarcolemma and T-tubules are closed, Ca2+ ions are stored within the terminal cisternae of the sar- coplasmic reticulum, and the contractile proteins (myofilaments) within the sarcomeres are in their relaxed position.

Answer 48

1. Ca2+ entry at synaptic knob A nerve signal is propagated down a motor axon and triggers the entry of Ca2+ into the synaptic knob. Ca2+ binds to proteins in synaptic vesicle membrane. 2. Release of ACh from synaptic knob Calcium binding triggers synaptic vesicles to merge with the synaptic knob plasma membrane and ACh is exocytosed into the synaptic cleft. 3. Binding of ACh to ACh receptor at motor end plate ACh diffuses across the fluid-filled synaptic cleft at the motor end plate to bind with ACh receptors.

Answer 49

Skeletal muscle physiology involves three major events: excitation, excitation-contraction coupling, and crossbridge cycling.

Answer 50

End plate potential (EPP) at motor end plate of muscle fiber: ACh binds to ACh receptors in motor end plate. Chemically gated ion channels stimulated to open. Na rapidly diffuses in and K* slowly diffuses out of muscle fiber. If sufficient Na* diffuses in to change RMP (-90 ml) to -65 mV; the threshold is reached, which is called an end plate potential (EPP). Action potential propagated along sarcolemma and T-tubule: EPP initiates an action potential (depolarization and repolarization). Depolarization: Voltage-gated Na* channels open; Na* moves in; sufficient Na* moves in to cause inside to become +30 mV (this reversal from negative (-65 V to +30 ml) is called depolarization. Depolarization is complete as the voltage-gated Na* channels close. Repolarization: Voltage-gated K* channels open after Na* channels; K* moves out; sufficient K* moves out to reestablish the RMP (-90 ml); this reversal from positive (+30 m) to the negative RMP (-90) mV is repolarization. Release of Ca2+ from the sarcoplasmic reticulum: Action potential reaches a triad. Voltage-sensitive Ca2 channels of T-tubules are stimulated, which triggers Ca2+ release channels of the sarcoplasmic reticulum (SR) to open. Ca2+ diffuses out of the SR through Ca2* release channels into the cvtosol of the muscle fiber.

Answer 51

Multiple repetitions of attach, pull, release, and reset lead to fully contracted sarcomere. 1. Ca2+ binding Ca2+ binds to troponin in muscle thin filaments, causing a conformational change in troponin. Troponin changes shape and the entire troponin-tropomyosin complex is moved—thus, tropomyosin no longer covers the myosin binding site on actin. 2. Crossbridge formation (“attach”) Myosin heads, which are in the “cocked” position, bind to the exposed myosin binding site on actin, forming a crossbridge between myosin and actin. 3. Power Stroke (pull) The myosin head swivels toward the center of the sarcomere, pulling along the attached thin filament. This motion is called a power stroke. ADP and Pi are released during this process. 4. Release of myosin head ATP binds to the ATP binding site on the myosin head, which causes the release of the myosin head from the binding site on actin. 5. Reset myosin head (“reset”) ATP is split into ADP and Pi by myosin ATPase. This provides the energy to reset the myosin head.

Answer 52

The following changes to the sarcomere occur in the contracted muscle: The H zone disappears, the I band narrows in width and may disappear, and the Z discs in each sarcomere move closer together. However, the thin and thick filaments do not shorten. A description of the repetitive movement of thin filaments sliding past thick filaments is called the sliding filament theory.

Answer 53

Nerve signals stop -ACh is no longer released and any left-over is hydrolyzed by AChE -Action Potentials and EPP cease -Ca2+ returned to terminal cisternae via Ca2+ pumps, remaining is stored in sarcoplasmic reticulum -Troponin returns to original shape with Ca2+ is removed, tropomyosin moves over myosin binding sites to prevent crossbridge formation

Answer 54

Through the natural elasticity of the muscle fiber, the muscle returns to its original relaxed position, a process facilitated by the release of passive tension that developed in connectin proteins that were compressed during contraction.

Answer 55

Available ATP and phosphate transfer to ADP: Oxygen not required/Limited ATP available/ATP produced from creatine P; also limited amounts Glycolysis: Oxygen not required/More rapid production of ATP (than aerobically)/Lesser amounts of ATP are produced (than aerobically)/Fuel: Glucose (typically) from glycogen breakdown and blood Aerobic cellular respiration: Oxygen required (aerobic)/Slower production of ATP (than glycolysis)/Greater amount of ATP are produced (than in glycolysis)/Fuel: Pyruvate (product of glycolysis), fatty acids, amino acids (with NH2 removed) The intensity and duration of an activity are important factors in energy utilization. For short sprints, the available ATP and the ATP made available through phosphate transfer are primarily used, whereas for longer runs, glycolysis is used initially but will be replaced by aerobic cellular respiration.

Answer 56

When an individual participates in exercise during which the demand for oxygen exceeds the avail- ability of oxygen, an oxygen debt is incurred. Oxygen debt is the amount of additional oxygen that is consumed following exercise to restore pre-exercise conditions. This additional oxygen is required primarily by ∙ Skeletal muscle fibers to replace oxygen on myoglobin molecules, replenish ATP and creatine phosphate, and replace glycogen stores Liver cells to convert lactate back to glucose through gluconeogenesis

Answer 57

Skeletal muscle fibers that compose a muscle are differentiated into three categories based on two criteria: (1) the type of contraction generated and (2) the primary means used for supplying ATP. Speed has traditionally been described based on whether the skeletal muscle fiber expresses the relatively slow or fast genetic variant of myosin ATPase, the enzyme that splits ATP. Those with a fast variant are called fast-twitch fibers, and those with the slow variant are called slow-twitch fibers. 2. The primary means used for supplying ATP: oxidative fibers & glycolytic fibers. The higher levels of ATP generated provide energy for oxidative fibers to continue contracting for extend- ed periods of time without tiring, or fatiguing—thus, these fibers are also called fatigue-resistant. Glycolytic fibers generally tire easily after a short time of sustained muscular activity—thus, these fibers are also called fatigable.

Answer 58

Slow oxidative fibers: contain slow myosin ATPase, slow contractions less power, dark red (large amounts of myoglobin and mitochondria). Fast oxidative fibers: fast and powerful contractions, appear light red due to aerobic respiration as supply for ATP. Fast glycolytic fibers: Largest in diameter, powerful and fast but only in short burst, ATP supply from glycolysis, appear white.

Answer 59

Slow Oxidative (SO) Fibers (Type I Fibers): Trunk and lower limb muscles/Endurance (e.g., maintaining posture, marathon running Fast Oxidative (FO) Fibers (Type IIa Fibers): Lower limb muscles/Medium duration, moderate movement (e.g., walking, biking) Fast Glycolytic (FG) Fibers (Type IIb Fibers): Short duration, intense movement (e.g., sprinting, lifting weights)/Upper limb muscles

Answer 60

A myogram of a single brief stimulation of a skeletal muscle results in a single contraction event called a muscle twitch, which is recorded using a myograph. The latent period is the elapsed time between stimulation of the muscle fiber and the generation of contractile force. The contraction period is the time during which there is an increase in muscle tension. The relaxation period is the time when there is a decrease in muscle tension. Skeletal muscle responds to one stimulus: muscle twitch Skeletal muscle responds to change in stimulus strength: motor unit recruitment Skeletal muscle responds to change in stimulus frequency

Answer 61

Increasing the intensity of stimulation (a) increases muscle tension, (b) causes a progressive increase in the number of motor units contracting, and (c) activates progressively larger motor units. This phenomenon is referred to as either recruitment or multiple motor unit summation.

Answer 62

Wave summation, incomplete tetany, and tetany are seen when the muscle is stimulated at varying frequencies that allow different degrees of relaxation. Stimulation can occur so rapidly (e.g., 20 to 50 stimuli per second) that complete relaxation of the skeletal muscle does not occur before the next stimulation event. The rapidly restimulated muscle displays a summation of contractile forces as the effect of each new wave is added to the previous wave. This effect is often called either wave summation because contraction waves are added together, or “summed,” or temporal summation because it depends upon increasing frequency (tempo, or timing) of stimulation. Further increases in stimulation frequency allow less time for relaxation between contraction cycles, and now incomplete tetany (the tension tracing continues to increase and the distance between waves decreases) is noted. Stimulation frequency is further increased (e.g., 40 to 50 stimuli per second) until ultimately the contractions of the skeletal muscle fiber “fuse” and form a continuous contraction that lacks any relaxation. This continuous contraction is called tetany (the tension tracing is a smooth line).

Answer 63

Muscle tone is the resting tension in a muscle generated by involuntary nervous stimulation of the muscle. The resting muscle tone establishes constant tension on the muscle's tendon, thus stabilizing the position of the bones and joints.

Answer 64

Isometric contraction -Although tension is increased, it is insufficient to overcome resistance -muscle length stays the same Isotonic contraction -muscle tension overcomes resistance resulting in movement -length changes +concentric contraction- muscles shortens as it contracts +eccentric contraction- muscle lengthens as it contracts

Answer 65

The tension a muscle produces depends on its length at the time of stimulation If the skeletal muscle fiber is already contracted at the time of stimulation, it does not have the ability to shorten much more, and it exhibits a weak contraction. (b) A skeletal muscle fiber at its normal resting length is generally capable of exhibiting the strongest contraction because of optimal overlap of myofilaments. (c) If the skeletal muscle fiber is very stretched when stimulated, relatively little contraction may occur because myofilaments have minimal overlap. Production of the most forceful contraction is facilitated if the skeletal muscle (a) is composed primarily of fast, glycolytic skeletal muscle fibers, (b) contains large motor units, (c) is stimulated more frequently, and (d) is stimulated when it is at its resting length.

Answer 66

Muscle fatigue is the reduced ability or the inability of the skeletal muscle to produce muscle tension. The primary cause of muscle fatigue during excessive or sustained exercise is a decrease in glycogen stores. Other causes: excitation at the neuromuscular junction, excitation- contraction coupling, and crossbridge cycling

Answer 67

Endurance exercise -leads to better ATP production -more mitochindria Resistance exercise -hypertrophy -increases in size due to increases in synthesis of contractile proteins -increases glycogen reserves and mitochondria Lack of exercise -Atrophy: decrease in size to to lack of use -initially reversible, but becomes permanent if extreme

Answer 68

Loss of muscle mass -slow loss starts during mid 30s because of decrease activity -decreased size, power, and endurance of muscle -loss in fiber number and diameter -decreased oxygen storage capacity -decreased circulatory supply to muscles with exercise 2) Reduces capacity to recover from injury -decreased number of satellite cells (muscle stem cells) -Fibrosis: muscle mass often replaced by dense regular connective tissue which decreases flexibility

Answer 69

Skeletal muscle: generally attaches to the skeleton, voluntary movement of body, multinucleate Cardiac muscle: heart only, involuntary movement, two centrally located nuclei, Many mitochondria—use aerobic respiration Intercalated discs are unique to cardiac muscle; they are composed of desmosomes and gap junctions -both striated and contain sarcomeres

Answer 70

Blood vessels, bronchioles, intestine, ureters, kidney, bladder, uterus

Answer 71

Smooth Muscle Fibers (a) attached to bones or (some facial muscles) to skin (b) Single, very long, cylindrical, multinucleate cells with obvious striations (c) Epimysium, perimysium, and endomysium Skeletal Muscle Fibers (a) Unitary muscle in walls of hollow visceral organs (other than the heart) multi unit muscle in intrinsic eye muscles, airways, large arteries (b) Single, fusiform, uninucleate, no striations (c) Endomysium

Answer 72

Smooth muscle also contracts in response to being stretched. This physiologic response is called the myogenic response. The myogenic response occurs, for example, in smooth muscle in the walls of blood vessels, the stomach, and the urinary bladder. Its response, however, is not continuous if the stretch is prolonged. Instead, the smooth muscle exhibits what is called the stress-relaxation response. This occurs when smooth muscle is “stressed” by being stretched. It responds by contracting, but after a given period of time, it relaxes. For example, swallowed materials entering the stomach cause its wall to stretch, and the smooth muscle in the wall initially contracts. After a period of time it relaxes, allowing additional food to more easily enter the stomach. Smooth muscle is also stimulated to contract by various hormones, a decrease in pH, low oxygen concentration, increased carbon dioxide levels, certain drugs, and pacemaker cells.

Answer 73

Collect information Processes and evaluate information Initiate response to information

Answer 74

Central nervous system (CNS) Brain/Spinal cord Peripheral nervous system (PNS) Nerves/Ganglia

Answer 75

Sensory nervous system: detects stimuli and transmits information from receptors to the CNS. Somatic sensory: Sensory input that is consciously perceived from receptors (e.g., eyes, ears, and skin) Visceral sensory: Sensory input that is not consciously perceived from receptors of blood vessels and internal organs (e.g., heart) Motor nervous system: initiates and transmits information from the CNS to effectors. Somatic motor: Motor output that is consciously or voluntarily controlled; effector is skeletal muscle Autonomic motor: Motor output that is not consciously or is involuntarily controlled; effectors are cardiac muscle, smooth muscle, and glands

Answer 76

A nerve is ensheathed with a dense irregular connective tissue layer called the epineurium. Each fascicle (bundle of axons) is also wrapped with a dense irregular connective tissue layer called the perineurium. An axon is surrounded by an aerolar connective tissue layer called the endoneurium. In myelinated axons, neurolemmocytes are between the axon and endoneurium. An electron micrograph showing the partial structure of a nerve, including connective tissue coverings within a nerve. (c) A ganglion is a collection of cell bodies along the length of a nerve. Cranial nerves extend from the brain and spinal nerves extend from the spinal cord Functional classification of nerves is based upon the functional type of neuron (sensory neuron or motor neuron) a nerve contains. Sensory nerves contain only sensory neurons that relay information to the CNS, and motor nerves contain primarily motor neurons that relay information from the CNS. In contrast, mixed nerves contain both sensory and motor neurons.

Answer 77

Whereas a nerve is a bundle of axons within the peripheral nervous system, a ganglion is a cluster of neuron cell bodies within the peripheral nervous system

Answer 78

excitability: the stimulus causes a local change in the resting membrane potential in the excitable cell. conductivity: involves an electrical change that is quickly propagated along the plasma membrane as voltage gated channels open sequentially during an action potential. secretion: neurons release neurotransmitters in response to conductive activity. extreme longevity: neurons formed as a baby are still functional in the elderly Amitotic: during fetal development, mitotic activity is lost in most neurons except the olfactory epithelium of the nose and parts of the brain.

Answer 79

cell body: enclosed by a plasma membrane and contains cytoplasm surrounding a nucleus dendrites: relatively short, small, tapering, unmyelinated processes that branch off the cell body. axon: typically a longer process that stems from the cell body to make contact with other neurons, muscle cells, or gland cells.

Answer 80

Multipolar neuron: Multiple processes extend directly from the cell body; typically many dendrites and one axon; most common type of neuron All motor neurons; most interneurons Bipolar neuron: Two processes extend directly from the cell body; one dendrite and one axon; relatively limited in where they are located Some special sense neurons (e.g., retina of eye, olfactory epithelium in nose) Unipolar neuron: Single short process extends directly from the cell and looks like a T as a result of the fusion of two processes into one long axon Most sensory neurons Anaxonic neuron: Processes are only dendrites; no axon present Interneurons

Answer 81

sensory neurons (afferent): neurons of sensory nervous system. responsible for conducting sensory input from both somatic and visceral receptors to the CNS. motor neurons (efferent): neurons of motor nervous system. conducts motor output from the CNS to both somatic effectors and visceral effectors (all multipolar). interneurons: entirely within the CNS. receive stimulation from many other neurons and carry out the integrative function of the nervous system. (receive, process, store info, and decide). facilitate communication between sensory and motor neurons.

Answer 82

A synapse is the specific location where a neuron is functionally connected to either another neuron or an effector (muscle or gland). There are two types of synapses in the human body: chemical synapses and electrical synapses. Most synapses within the nervous system are chemical synapses.

Answer 83

Glial cells are sometimes referred to as neuroglia. They are found within both the CNS and the PNS. Glial cells are both smaller than neurons and capable of producing new glial cells through cell division. Glial cells do not transmit electrical signals, but they do assist neurons with their functions. The glial cells cooperate to physically protect and help nourish neurons as well as provide an organized, supporting scaffolding for all the nervous tissue. During development, glial cells form the framework that guides young, migrating neurons to their final destinations.

Answer 84

Four types of glial cells are located within the CNS: astrocytes, ependymal cells, microglia, and oligodendrocytes. Oligodendrocyte Functions 1. Myelinates and insulates CNS axons 2. Allows faster action potential propagation along axons in the CNS Astrocyte Functions 1. Helps form the blood-brain barrier 2. Regulates interstitial fluid composition 3. Provides structural support and organization to the central nervous system (CNS) 4. Assists with neuronal development 5. Replicates to occupy space of dying neurons Ependymal Cell Functions 1. Lines ventricles of brain and central canal of spinal cord 2. Assists in production and circulation of cerebrospinal fluid (CSF) Microglial Cell Functions 1. Phagocytic cell that moves through the CNS 2. Protects the CNS by engulfing infectious agents and other potentially harmful substances Two primary types of glial cells are located within the PNS, including satellite cells and neurolemmocytes. Satellite Cell Functions 1. Electrically insulates PNS cell bodies 2. Regulates nutrient and waste exchange for cell bodies in ganglia Neurolemmocyte Functions 1. Myelinates and insulates PNS axons 2. Allows for faster action potential propagation along an axon

Answer 85

the process by which part of an axon is wrapped with myelin. myelin is the insulating covering around the axon that consist of repeating concentric layers of plasma membrane of glial cells. The main purpose of a myelin sheath is to increase the speed at which impulses propagate along the myelinated fiber.

Answer 86

A neurolemmocyte in the PNS can myelinate only a 1-millimeter portion of a single axon. Thus, if an axon is longer than 1 millimeter (and most PNS axons are), it takes many neurolemmocytes to myelinate the entire axon. The axons in many of the nerves in the body have hundreds or thousands of neurolemmocytes along their entire length. The gaps between the neurolemmocytes are called neurofibril nodes, or nodes of Ranvier. An oligodendrocyte in the CNS, in comparison, can myelinate a 1-millimeter portion of multiple axons and not just one. The cytoplasmic extensions of the oligodendrocyte wrap repeatedly around a portion of each axon where plasma membrane layers of the oligodendrocyte form the myelin sheath.

Answer 87

1. the amount of damage 2. the distance between the site of the damaged axon and the structure it innervates. 1. oligodendrocytes do not release a nerved growth factor 2. the large number of axons crowded within the CNS tends to complicate regrowth activites 3. Both astrocytes and connective tissue coverings may form some scar tissue that obstructs axon regrowth.

Answer 88

pump: maintains specific concentration gradients by moving substances up a concentration gradient, a process that requires cellular energy. channel: provide the means for a substance to move down its concentration gradient. (Na+, K+, Cl-)

Answer 89

chemically-gated or ligand-gated channels, voltage-gated channels, and mechanically-gated channels Three States of Voltage-Gated Na+ Channels 1. Resting state. Although the inactivation gate is open, the activation gate is closed, and entry of Na+ is prevented. 2. Activation state. Both the inactivation gate (which remains open) and the activation gate are open (activation gate opens in response to a voltage change); Na+ moves into the cell through the open channel. 3. Inactivation state. Although the activation gate is open, the inactivation gate is temporarily closed (for several milliseconds) following activation of the Na+ channel— during this time, it cannot be stimulated to reopen, and entry of Na+ is prevented.

Answer 90

The receptive segment includes both dendrites and the cell body, which are the regions of the neuron that receive stimuli to excite the neuron. Chemically gated channels (cation channels, K+ channels, and Cl– channels) are located in this segment; no significant numbers of voltage-gated channels are present. (Note that cation channels allow the passage of both Na+ into the neuron and K+ out of the neuron. However, more Na+ moves into the neuron than K+ moves out.) The initial segment is commonly considered to be the region of the axon hillock. This segment contains both voltage-gated Na+ channels and voltage-gated K+ channels. The conductive segment is equivalent to the length of the axon. Like the initial segment, it contains both voltage-gated Na+ channels and voltage-gated K+ channels. The transmissive segment includes the synaptic knobs and contains both voltage- gated Ca2+ channels and Ca2+ pumps.

Answer 91

Ion concentration gradients exist for K+, Na+, and Cl– across the plasma membrane along the entire neuron. At the plasma membrane, there is relatively more K+ in the cytosol than in the interstitial fluid (IF) surrounding the neuron, whereas there is more Na+ and Cl– in the IF than in the cytosol. These gradients are established by Na+/K+ pumps that move three Na+ out of the neuron for every two K+ moved in. (Chloride ion follows the movement of Na+.) ∙ A Ca2+ concentration gradient exists at the synaptic knob. Calcium pumps within this segment continuously pump Ca2+ from within the synaptic knob to the surrounding IF. Thus, there is more Ca2+ in the IF outside the synaptic knob than within the cytosol in the synaptic knob. ∙ Gated channels are closed. These channels include the chemically gated channels in the receptive segment, the voltage-gated Na+ channels and voltage-gated K+ channels in both the initial segment and the conductive segment, and voltage-gated Ca2+ channels in the transmissive segment. ∙ There is an electrical charge difference (an electrical gradient) across the plasma membrane; the cytosol adjacent to the plasma membrane is relatively negative in comparison to the IF outside of the cell. This electrical charge difference is called a membrane potential. When the neuron is at rest, the membrane potential is more specifically called the resting membrane potential (RMP). The RMP of a neuron is typically –70 millivolts (mV), but can range between –40 mV and –90 mV.

Answer 92

RMP - Degree of the difference of eletrical charge between points. Typical value for neuron is -70mV RMP Established via - permeability to ions. Mainly Na+ and K+ leaky channels and maintained by the Na+/K+Pumps. -- K+: most important factor, if ONLY K+ leaky channels RMP= -90mV. -- Na+: movement of sodium changes RMP to -70mV. moves into the cell due to chemical and electrical gradient. -- Negatively charged proteins (A-): adds to the electrical gradient. Maintaining RMP - Na+/K+Pumps: help maintain 3 Na+out, 2 K+ in. [2/3 total energy expenditure of the Neuron]

Answer 93

Receptive segment Binding of neurotransmitter released from presynaptic neurons; production of graded potentials Initial segment Summation of graded potentials; initiation of action potential Conductive segment Propagation of action potential Transmissive segment Action potential causes release of neurotransmitter

Answer 94

Graded potentials occur in the receptive segment of a neuron (dendrites and cell bodies) and are due to the opening of chemically gated channels. The chemically gated channels open temporarily to allow passage of a relatively small amount of a specific type of ion across the plasma membrane. This results in the membrane potential becoming either more positive (depolarization) or more negative (hyperpolarization) than the resting membrane potential.

Answer 95

An excitatory postsynaptic potential creates a local depolarization in the membrane of the postsynaptic neuron that brings it closer to threshold. An inhibitor postsynaptic potential does the opposite; it hyper-polarizes the membrane and brings it farther away from threshold.

Answer 96

1. The unstimulated axon has a resting membrane potential of –70 mV. 2. Graded potentials reach the initial segment and are added together (–70 mV –55 mV). 3. Depolarization occurs when the threshold (–55 mV) is reached; voltage-gated Na+ channels open and Na+ enters rapidly, reversing the polarity from negative to positive (–55 mV +30 mV). 4. Repolarization occurs due to closure of voltage-gated Na+ channels (inactivation state) and opening of voltage-gated K+ channels. K+ moves out of the cell and polarity is reversed from positive to negative (+30 mV –70 mV). 5. Hyperpolarization occurs when voltage-gated K+ channels stay open longer than the time needed to reach the resting membrane potential; during this time the membrane potential is less than the resting membrane potential (–70 mV –80 mV). 6. Voltage-gated K+ channels are closed, and the plasma membrane has returned to resting conditions by activity of Na+/K+ pumps (–80 mV –70 mV).

Answer 97

Specifically how an action potential is propagated along the axon is dependent upon whether the axon is unmyelinated or myelinated. Continuous conduction occurs in unmyelinated axons and involves the sequential opening of voltage-gated Na+ channels and voltage-gated K+ channels located within the axon plasma membrane along the entire length of the axon. When previously discussing the process of action potential conduction, we described it as it would occur in an unmyelinated axon. Saltatory conduction occurs in myelinated axons. Here, action potentials do not occur in regions of the axon that are myelinated—rather, they are propagated only at neurofibril nodes. This is due to anatomic differences in the two types of regions of a myelinated axon. Myelinated regions of an axon contain limited numbers of voltage-gated Na+ channels and voltage-gated K+ channels, and myelin is a great insulator that prevents ion movement across the plasma membrane even if additional channels are present.

Answer 98

refractory period: brief time after an action potential has been initiated during which an axon is either incapable of generating another action potential OR a greater than normal amount of stimulation is required to generate another action potential. Absolute refractory period: time (about 1 millisecond) after an action potential onset when no amount of stimulus, no matter how strong, can initiate a second action potential. Relative refractory period: occurs immediately after the absolute refractory period

Answer 99

1. Nerve signal reaches synaptic knob. 2. Voltage-gated Ca2+ channels open and Ca2+ enters the synaptic knob and binds with proteins associated with synaptic vesicles. 3. Synaptic vesicles fuse with the synaptic knob plasma membrane and neurotransmitter is exocytosed. 4. Neurotransmitter diffuses across synaptic cleft and attaches to receptors on a muscle, as shown (or to receptors of a neuron or gland)

Answer 100

Ca2+ triggers synaptic vesicle exocytosis by binding, thereby releasing the neurotransmitters contained in the vesicles and initiating synaptic transmission

Answer 101

Diameter of the axon. Nerve signal velocity is generally faster in axons with a larger diameter. This is because there is less resistance to the movement of ions within the larger axon, allowing these axons to reach threshold more rapidly than smaller axons. ∙ Myelination of the axon. Myelination of the axon is the more important factor influencing nerve signal velocity. Nerve signal velocity occurs more rapidly in myelinated axons than in unmyelinated axons

Answer 102

Action potentials are always propagated along an axon (as nerve signals) at the same amplitude (change in voltage). However, action potential frequency can vary and is dependent upon the stimulus strength. As the stimulus strength increases, the frequency of action potentials increases (up to the point of maximum frequency)

Answer 103

∙ Amino acids. These include glutamate, aspartate, serine, glycine, and gamma aminobutyric acid (GABA, a modified amino acid). ∙ Neuropeptides (or peptides). These are chains of amino acids that range in length from 2 to 40 amino acids. Examples of neuropeptides include the natural opiates (e.g., enkephalins, beta-endorphins), and substance P. Acetylcholine (ACh) Biogenic amines (also called monoamines). They are derived from certain amino acids by the removal of a carboxyl group (—COOH) and the addition of another functional group (e.g., an hydroxyl group) by enzymatic pathways within the cytosol.

Answer 104

Neurotransmitter classification based upon function reflects the specific effect that a neurotransmitter has on the membrane potential of a target cell. Neurotransmitters are considered excitatory if they induce an EPSP, whereas they are inhibitory if they induce an IPSP. (Note that some neurotransmitters may be either excitatory or inhibitory depending upon the specific response they cause in their target organs.) Another neurotransmitter classification based upon function reflects whether the target cell response is either direct (i.e., the neu- rotransmitter directly binds to the receptor of the target cell to cause opening of an ion channel) or indirect (i.e., the neurotransmitter binds to a receptor that activates the second messenger pathway). The second messenger ultimately can trigger much more diverse effects, including the opening of ion channels, the activation of an existing enzymatic pathway, or transcription of genes for the synthesis of new proteins.

Answer 105

1. ACh is synthesized from acetate and choline stored in the synaptic vesicles in the synaptic knob. 2. ACh is released from the synaptic knob via exocytosis into the synaptic cleft 3. Some ACh is immediately broken down into acetate and choline by the enzyme acetylcholinesterase (AChE), which resides in the synaptic cleft, and the choline is taken up into the synaptic knob from which it came where it can resynthesized into ACh. 4. Some ACh binds to receptors on the target cell and then dissociates. Upon dissociation, AChE breaks down the ACh as described in step 3.

Answer 106

Degradation: when the neurotransmitter is chemically inactivated in the synaptic cleft. Reuptake: when the neurotransmitter is reabsorbed by a neurotransmitter transport protein in the membrane of the presynaptic neuron. (i.e. The neurotransmitter molecules are "recycled" and "repackaged" into vesicles.)