Download Muscles and Muscles Tissue

9: Muscles and Muscles Tissue
Outline of Chapter
I. Overview of Muscle Tissues (pp. 276–277; Table 9.3)
A. Types of Muscle Tissue (p. 276; Table 9.3)
1. Skeletal muscle is associated with the bony skeleton, and consists of large cells that bear striations
and are controlled voluntarily.
2. Cardiac muscle occurs only in the heart, and consists of small cells that are striated and under
involuntary control.
3. Smooth muscle is found in the walls of hollow organs, and consists of small elongated cells that are
not striated and are under involuntary control.
B. Special Characteristics of Muscle Tissue (p. 276)
1. Excitability, or irritability, is the ability to receive and respond to a stimulus.
2. Contractility is the ability to contract forcibly when stimulated.
3. Extensibility is the ability to be stretched.
4. Elasticity is the ability to resume the cells’ original length once stretched.
C. Muscle Functions (pp. 276–277; Table 9.3)
1. Muscles produce movement by acting on the bones of the skeleton, pumping blood, or propelling
substances throughout hollow organ systems.
2. Muscles aid in maintaining posture by adjusting the position of the body with respect to gravity.
3. Muscles stabilize joints by exerting tension around the joint.
4. Muscles generate heat as a function of their cellular metabolic processes.
II. Skeletal Muscle (pp. 277–305; Figs. 9.1–9.25; Tables 9.1–9.3)
A. Gross Anatomy of Skeletal Muscle (pp. 277–278; Fig. 9.2; Tables 9.1, 9.3)
1. Each muscle has a nerve and blood supply that allows neural control and ensures adequate nutrient
delivery and waste removal.
2. Connective tissue sheaths are found at various structural levels of each muscle: endomysium
surrounds each muscle fiber, perimysium surrounds groups of muscle fibers, and epimysium
surrounds whole muscles.
3. Attachments span joints and cause movement to occur from the movable bone (the muscle’s
insertion) toward the less movable bone (the muscle’s origin).
4. Muscle attachments may be direct or indirect.
B. Microscopic Anatomy of a Skeletal Muscle Fiber (pp. 278–284; Figs. 9.2–9.6; Tables 9.1, 9.3)
1. Skeletal muscle fibers are long cylindrical cells with multiple nuclei beneath the sarcolemma.
2. Myofibrils account for roughly 80% of cellular volume, and contain the contractile elements of the
muscle cell.
3. Striations are due to a repeating series of dark A bands and light I bands.
4. Myofilaments make up the myofibrils, and consist of thick and thin filaments.
5. Ultrastructure and Molecular Composition of the Myofilaments
a. There are two types of myofilaments in muscle cells: thick filaments composed of bundles of
myosin, and thin filaments composed of strands of actin.
b. Tropomyosin and troponin are regulatory proteins present in thin filaments.
6. The sarcoplasmic reticulum is a smooth endoplasmic reticulum surrounding each myofibril.
7. T tubules are infoldings of the sarcolemma that conduct electrical impulses from the surface of the
cell to the terminal cisternae.
C. The sliding filament model of muscle contraction states that during contraction, the thin filaments slide
past the thick filaments. Overlap between the myofilaments increases and the sarcomere shortens (p.
284; Fig. 9.6).
D. Physiology of a Skeletal Muscle Fiber (pp. 284–289; Figs. 9.6–9.12; Table 9.3)
1. The neuromuscular junction is a connection between an axon terminal and a muscle fiber that is the
route of electrical stimulation of the muscle cell.
2. A nerve impulse causes the release of acetylcholine to the synaptic cleft, which binds to receptors
on the motor end plate, triggering a series of electrical events on the sarcolemma.
3. Generation of an action potential across the sarcolemma occurs in response to acetylcholine binding
with receptors on the motor end plate. It involves the influx of sodium ions, which makes the
membrane potential slightly less negative.
4. Excitation-contraction coupling is the sequence of events by which an action potential on the
sarcolemma results in the sliding of the myofilaments.
5. Ionic calcium in muscle contraction is kept at almost undetectable levels within the cell through the
regulatory action of intracellular proteins.
6. Muscle fiber contraction follows exposure of the myosin binding sites, and follows a series of
events.
E. Contraction of a Skeletal Muscle (pp. 289–296; Figs. 9.13–9.18)
1. A motor unit consists of a motor neuron and all the muscle fibers it innervates. It is smaller in
muscles that exhibit fine control.
2. The muscle twitch is the response of a muscle to a single action potential on its motor neuron.
3. There are three kinds of graded muscle responses: wave summation, multiple motor unit
summation (recruitment), and treppe.
4. Muscle tone is the phenomenon of muscles exhibiting slight contraction, even when at rest, which
keeps muscles firm, healthy, and ready to respond.
5. Isotonic contractions result in movement occurring at the joint and shortening of muscles.
6. Isometric contractions result in increases in muscle tension, but no lengthening or shortening of the
muscle occurs.
F. Muscle Metabolism (pp. 296–300; Figs. 9.19–9.20)
1. Muscles contain very little stored ATP, and consumed ATP is replenished rapidly through
phosphorylation by creatine phosphate, glycolysis and anaerobic respiration, and aerobic
respiration.
2. Muscles will function aerobically as long as there is adequate oxygen, but when exercise demands
exceed the ability of muscle metabolism to keep up with ATP demand, metabolism converts to
anaerobic glycolysis.
3. Muscle fatigue is a problem in excitation-contraction coupling or within the muscle cells
themselves.
4. Oxygen deficit is the extra oxygen needed to replenish oxygen reserves, glycogen stores, ATP and
creatine phosphate reserves, as well as conversion of lactic acid to pyruvic acid glucose after
vigorous muscle activity.
5. Heat production during muscle activity is considerable. It requires release of excess heat through
homeostatic mechanisms such as sweating and radiation from the skin.
G. Force of Muscle Contraction (pp. 300–302; Figs. 9.21–9.22)
1. As the number of muscle fibers stimulated increases, force of contraction increases.
2. Large muscle fibers generate more force than smaller muscle fibers.
3. As the rate of stimulation increases, contractions sum up, ultimately producing tetanus and
generating more force.
4. There is an optimal length-tension relationship when the muscle is slightly stretched and there is
slight overlap between the myofibrils.
H. Velocity and Duration of Muscle Contraction (pp. 302–303; Figs. 9.23–9.25; Tables 9.2–9.3)
1. There are three muscle fiber types: slow oxidative fibers, fast oxidative fibers, and fast glycolytic
fibers.
2. Muscle fiber type is a genetically determined trait, with varying percentages of each fiber type in
every muscle, determined by specific function of a given muscle.
3. As load increases, the slower the velocity and shorter the duration of contraction.
4. Recruitment of additional motor units increases velocity and duration of contraction.
I.
Effect of Exercise on Muscles (pp. 304–305)
1. Aerobic, or endurance, exercise promotes an increase in capillary penetration, the number of
mitochondria, and increased synthesis of myoglobin, leading to more efficient metabolism, but no
hypertrophy.
2. Resistance exercise, such as weight lifting or isometric exercise, promotes an increase in the number
of mitochondria, myofilaments and myofibrils, and glycogen storage, leading to hypertrophied
cells.
III. Smooth Muscle (pp. 305–311; Figs. 9.26–9.29; Table 9.3)
A. Microscopic Structure of Smooth Muscle Fibers (pp. 305–307; Figs. 9.26–9.28; Table 9.3)
1. Smooth muscle cells are small, spindle-shaped cells with one central nucleus, and lack the coarse
connective tissue coverings of skeletal muscle.
2. Smooth muscle cells are usually arranged into sheets of opposing fibers, forming a longitudinal
layer and a circular layer.
3. Contraction of the opposing layers of muscle leads to a rhythmic form of contraction, called
peristalsis, which propels substances through the organs.
4. Smooth muscle lacks neuromuscular junctions, but has varicosities instead, numerous bulbous
swellings that release neurotransmitters to a wide synaptic cleft.
5. Smooth muscle cells have a less developed sarcoplasmic reticulum, sequestering large amounts of
calcium in extracellular fluid within caveolae in the cell membrane.
6. Smooth muscle has no striations, no sarcomeres, a lower ratio of thick to thin filaments compared
with skeletal muscle, and has tropomyosin but no troponin.
7. Smooth muscle fibers contain longitudinal bundles of noncontractile intermediate filaments
anchored to the sarcolemma and suurounding tissues via dense bodies.
B. Contraction of Smooth Muscle (pp. 307–311; Fig. 9.29; Table 9.3)
1. Mechanism and Characteristics of Contraction
a. Smooth muscle fibers exhibit slow, synchronized contractions due to electrical coupling by gap
junctions.
b. Like skeletal muscle, actin and myosin interact by the sliding filament mechanism. The final
trigger for contraction is a rise in intracellular calcium level, and the process is energized by ATP.
c. During excitation-contraction coupling, calcium ions enter the cell from the extracellular space,
bind to calmodulin, and activate myosin light chain kinase, powering the cross bridging cycle.
d. Smooth muscle contracts more slowly and consumes less ATP than skeletal muscle.
2. Regulation of Contraction
a. Autonomic nerve endings release either acetylcholine or norepinephrine, which may result in
excitation of certain groups of smooth muscle cells, and inhibition of others.
b. Hormones and local factors, such as lack of oxygen, histamine, excess carbon dioxide, or low pH,
act as signals for contraction.
3. Special Features of Smooth Muscle Contraction
a. Smooth muscle initially contracts when stretched, but contraction is brief, and then the cells relax
to accommodate the stretch.
b. Smooth muscle stretches more and generates more tension when stretched than skeletal muscle.
c. Hyperplasia, an increase in cell number through division, is possible in addition to hypertrophy,
an increase in individual cell size.
C. Types of Smooth Muscle (p. 311)
1. Single-unit smooth muscle, called visceral muscle, is the most common type of smooth muscle. It
contracts rhythmically as a unit, is electrically coupled by gap junctions, and exhibits spontaneous
action potentials.
2. Multiunit smooth muscle is located in large airways to the lungs, large arteries, arrector pili muscles
in hair follicles, and the iris of the eye. It consists of cells that are structurally independent of each
other, has motor units, and is capable of graded contractions.
IV. Developmental Aspects of Muscles (pp. 311–312; Fig. 9.30)
A. Nearly all muscle tissue develops from specialized mesodermal cells called myoblasts (p. 311).
B. Skeletal muscle fibers form through the fusion of several myoblasts, and are actively contracting by
week 7 of fetal development (p. 311).
C. Myoblasts of cardiac and smooth muscle do not fuse but form gap junctions at a very early stage (p.
312).
D. Muscular development in infants is mostly reflexive at birth, and progresses in a head-to-toe and
proximal-to-distal direction (p. 312).
E. Women have relatively less muscle mass than men due to the effects of the male sex hormone
testosterone, which accounts for the difference in strength between the sexes (p. 312).
F. Muscular dystrophy is one of the few disorders that muscles experience, and is characterized by
atrophy and degeneration of muscle tissue. Enlargement of muscles is due to fat and connective tissue
deposit (p. 312).
Answers to End-of-Chapter Questions
Multiple Choice and Matching Question answers appear in Appendix G of the main text.
Short Answer Essay Questions
15. The functions are: excitability—the ability to receive and respond to a stimulus; contractility—the ability to
shorten; extensibility—the ability to be stretched; and elasticity—the ability to resume normal length after
contraction or having been stretched. (p. 276)
16. a. -In direct attachment, the epimysium of the muscle is fused to the periosteum of a bone, and in indirect
attachment, the muscle connective tissue sheaths extend beyond the muscle as a tendon; the tendon
anchors to the periosteum of a bone. (pp. 277–278)
b. A tendon is a ropelike mass of fibrous tissue; an aponeurosis is a flat, broad sheet. (p. 278)
17. a. -A sarcomere is the region of a myofibril between two successive Z-lines and is the smallest contractile
unit of a muscle cell. The myofilaments are within the sarcomere. (p. 281; Fig. 9.2)
b. The theory proposes that the thin filaments slide toward the center of the sarcomere through the
ratchetlike action of the myosin heads. The process is energized by ATP. (p. 284; Fig. 9.6)
18. AChE destroys the ACh after it is released. This prevents continued muscle fiber contraction in the absence
of additional stimulation. (p. 285)
19. A slight, but smooth contraction involves rapid stimulation of a few motor units and affects only a few
muscle fibers of the muscle, whereas a strong contraction would involve many (or all) motor units
stimulated technically. (p. 295)
20. Excitation-contraction coupling is the sequence of events by which an action potential traveling along the
sarcolemma leads to the contraction of a muscle fiber. (p. 288; Fig. 9.11)
21. A motor unit is the motor neuron and all the muscle fibers it controls. (p. 289; Fig. 9.13)
22. Table 9.2, p. 302, illustrates the structural and functional characteristics of the three types of skeletal muscle
fibers.
23. False. Most body muscles contain a mixture of fiber types that allows them to exhibit a range of contractile
speeds and fatigue resistance. However, certain muscle fiber types may predominate in specific muscles,
e.g., white fibers predominate in the occular muscles. (p. 303)
24. Muscle fatigue is the state of physiological inability to contract. It occurs due to ATP deficit, lactic acid
buildup, and ionic imbalance. (p. 300)
25. Oxygen deficit is defined as the additional amount of oxygen that must be taken in by the body to provide
for restorative processes, and it represents the difference between the amount of oxygen needed for totally
aerobic respiration during muscle activity and the amount that is actually used. (p. 300)
26. Smooth muscle is located within the walls of hollow organs and around blood vessels. The tissue is under
involuntary control. These characteristics are essential because the vessels and hollow organs must
respond slowly, fill and expand slowly, and avoid expulsive contractions. (pp. 308–311)
Critical Thinking and Clinical Application Questions
1. Regular resistance exercise leads to increased muscle strength by causing muscle cells to hypertrophy, or
increase in size. The number of myofilaments increases in these muscles. (p. 304)
2. The reason for the tightness is rigor mortis. The myosin cross bridges are “locked on” to the actin because
of the lack of ATP necessary for release. No, peak rigidity occurs at 12 hours and then gradually dissipates
over the next 48 to 60 hours as biological molecules begin to degrade. (p. 289)
3. Chemical A. By blocking binding of ACh to the motor end plate, neural stimulation of the cell is blocked,
and the muscle cell cannot depolarize. Chemical B would actually increase contraction of the muscle cell by
increasing the availability of calcium ions that bind to troponin, contributing to actin-myosin cross
bridging. (pp. 285, 288–289)
4. The calcium actually binds to troponin, which changes shape and moves the tropomyosin to expose the
myosin head binding sites. (p. 281)