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An Introduction to Muscle Tissue
 Muscle Tissue
 A primary tissue type, divided into
 Skeletal muscle
 Cardiac muscle
 Smooth muscle
An Introduction to Muscle Tissue
 Skeletal Muscles
 Are attached to the skeletal system
 Allow us to move
 The muscular system
 Includes only skeletal muscles
Functions of Skeletal Muscles
 Produce skeletal movement
 Maintain body position
 Support soft tissues
 Guard openings
 Maintain body temperature
 Store nutrient reserves
Skeletal Muscle Structures
 Muscle tissue (muscle cells or fibers)
 Connective tissues
 Nerves
 Blood vessels
Skeletal Muscle Structures
 Organization of Connective Tissues
 Muscles have three layers of connective tissues
 Epimysium:
– exterior collagen layer
– connected to deep fascia
– Separates muscle from surrounding tissues
 Perimysium:
– surrounds muscle fiber bundles (fascicles)
– contains blood vessel and nerve supply to fascicles
 Endomysium:
– surrounds individual muscle cells (muscle fibers)
– contains capillaries and nerve fibers contacting muscle cells
– contains myosatellite cells (stem cells) that repair damage
Skeletal Muscle Structures
Figure 10–1 The Organization of Skeletal Muscles.
Skeletal Muscle Structures
 Organization of Connective Tissues
 Muscle attachments
 Endomysium, perimysium, and epimysium come
together:
– at ends of muscles
– to form connective tissue attachment to bone matrix
– i.e., tendon (bundle) or aponeurosis (sheet)
Skeletal Muscle Structures
 Nerves
 Skeletal muscles are voluntary muscles, controlled by
nerves of the central nervous system (brain and spinal cord)
 Blood Vessels
 Muscles have extensive vascular systems that
 Supply large amounts of oxygen
 Supply nutrients
 Carry away wastes
Skeletal Muscle Fibers
 Are very long
 Develop through fusion of mesodermal cells
(myoblasts)
 Become very large
 Contain hundreds of nuclei
Skeletal Muscle Fibers
Figure 10–2 The Formation of a Multinucleate Skeletal Muscle Fiber.
Skeletal Muscle Fibers
Figure 10–2a The Formation of a Multinucleate Skeletal Muscle Fiber.
Skeletal Muscle Fibers
Figure 10–2b The Formation of a Multinucleate Skeletal Muscle Fiber.
Skeletal Muscle Fibers
 Internal Organization of Muscle Fibers
 The sarcolemma
 The cell membrane of a muscle fiber (cell)
 Surrounds the sarcoplasm (cytoplasm of muscle fiber)
 A change in transmembrane potential begins
contractions
Skeletal Muscle Fibers
 Internal Organization of Muscle Fibers
 Transverse tubules (T tubules)
 Transmit action potential through cell
 Allow entire muscle fiber to contract simultaneously
 Have same properties as sarcolemma
Skeletal Muscle Fibers
 Internal Organization of Muscle Fibers
 Myofibrils
 Lengthwise subdivisions within muscle fiber
 Made up of bundles of protein filaments
(myofilaments)
 Myofilaments are responsible for muscle contraction
 Types of myofilaments:
– thin filaments:
» made of the protein actin
– thick filaments:
» made of the protein myosin
Skeletal Muscle Fibers
 Internal Organization of Muscle Fibers
 Sarcoplasmic reticulum (SR)
 A membranous structure surrounding each myofibril
 Helps transmit action potential to myofibril
 Similar in structure to smooth endoplasmic reticulum
 Forms chambers (terminal cisternae) attached to T
tubules
Skeletal Muscle Fibers
 Internal Organization of Muscle Fibers
 Triad
 Is formed by one T tubule and two terminal cisternae
 Cisternae:
– concentrate Ca2+ (via ion pumps)
– release Ca2+ into sarcomeres to begin muscle
contraction
Skeletal Muscle Fibers
Figure 10–3 The Structure of a Skeletal Muscle Fiber.
Skeletal Muscle Fibers
 Internal Organization of Muscle Fibers
 Sarcomeres
 The contractile units of muscle
 Structural units of myofibrils
 Form visible patterns within myofibrils
 Muscle striations
 A striped or striated pattern within myofibrils:
– alternating dark, thick filaments (A bands) and light, thin filaments
(I bands)
Skeletal Muscle Fibers
 Internal Organization of Muscle Fibers
 Sarcomeres
 M Lines and Z Lines:
– M line:
» the center of the A band
» at midline of sarcomere
– Z lines:
» the centers of the I bands
» at two ends of sarcomere
Skeletal Muscle Fibers
 Internal Organization of Muscle Fibers
 Sarcomeres
 Zone of overlap:
– the densest, darkest area on a light micrograph
– where thick and thin filaments overlap
 The H Band:
– the area around the M line
– has thick filaments but no thin filaments
Skeletal Muscle Fibers
 Internal Organization of Muscle Fibers
 Sarcomeres
 Titin:
– are strands of protein
– reach from tips of thick filaments to the Z line
– stabilize the filaments
Skeletal Muscle Fibers
Figure 10–4a Sarcomere Structure.
Skeletal Muscle Fibers
Figure 10–4b Sarcomere Structure.
Skeletal Muscle Fibers
Figure 10–5 Sarcomere Structure.
Skeletal Muscle Fibers
Figure 10–6 Levels of Functional Organization in a Skeletal Muscle.
Skeletal Muscle Fibers
Figure 10–6 Levels of Functional Organization in a Skeletal Muscle.
Skeletal Muscle Fibers
 Sarcomere Function
 Transverse tubules encircle the sarcomere near
zones of overlap
 Ca2+ released by SR causes thin and thick
filaments to interact
Skeletal Muscle Fibers
 Muscle Contraction
 Is caused by interactions of thick and thin
filaments
 Structures of protein molecules determine
interactions
Skeletal Muscle Fibers
 Four Thin Filament Proteins
 F-actin (Filamentous actin)
 Is two twisted rows of globular G-actin
 The active sites on G-actin strands bind to myosin
 Nebulin
 Holds F-actin strands together
 Tropomyosin
 Is a double strand
 Prevents actin–myosin interaction
 Troponin
 A globular protein
 Binds tropomyosin to G-actin
 Controlled by Ca2+
Skeletal Muscle Fibers
Figure 10–7a, b Thick and Thin Filaments.
Skeletal Muscle Fibers
 Initiating Contraction
 Ca2+ binds to receptor on troponin molecule
 Troponin–tropomyosin complex changes
 Exposes active site of F-actin
Skeletal Muscle Fibers
 Thick Filaments
 Contain twisted myosin subunits
 Contain titin strands that recoil after stretching
 The mysosin molecule
 Tail:
– binds to other myosin molecules
 Head:
– made of two globular protein subunits
– reaches the nearest thin filament
Skeletal Muscle Fibers
Figure 10–7c, d Thick and Thin Filaments.
Skeletal Muscle Fibers
 Myosin Action
 During contraction, myosin heads
 Interact with actin filaments, forming cross-bridges
 Pivot, producing motion
Skeletal Muscle Fibers
 Skeletal Muscle Contraction
 Sliding filament theory
 Thin filaments of sarcomere slide toward M line,
alongside thick filaments
 The width of A zone stays the same
 Z lines move closer together
Skeletal Muscle Fibers
Figure 10–8a Changes in the Appearance of a Sarcomere during the
Contraction of a Skeletal Muscle Fiber.
Skeletal Muscle Fibers
Figure 10–8b Changes in the Appearance of a Sarcomere during the
Contraction of a Skeletal Muscle Fiber.
Skeletal Muscle Fibers
 Skeletal Muscle Contraction
 The process of contraction
 Neural stimulation of sarcolemma:
– causes excitation–contraction coupling
 Cisternae of SR release Ca2+:
– which triggers interaction of thick and thin filaments
– consuming ATP and producing tension
Skeletal Muscle Fibers
Figure 10–9 An Overview of Skeletal Muscle Contraction.
The Neuromuscular Junction
 Is the location of neural stimulation
 Action potential (electrical signal)
 Travels along nerve axon
 Ends at synaptic terminal
 Synaptic terminal:
– releases neurotransmitter (acetylcholine or ACh)
– into the synaptic cleft (gap between synaptic
terminal and motor end plate)
The Neuromuscular Junction
Figure 10–10a, b Skeletal Muscle Innervation.
The Neuromuscular Junction
Figure 10–10c Skeletal Muscle Innervation.
The Neuromuscular Junction
Figure 10–10c Skeletal Muscle Innervation.
The Neuromuscular Junction
 The Neurotransmitter
 Acetylcholine or ACh
 Travels across the synaptic cleft
 Binds to membrane receptors on sarcolemma (motor
end plate)
 Causes sodium–ion rush into sarcoplasm
 Is quickly broken down by enzyme
(acetylcholinesterase or AChE)
The Neuromuscular Junction
Figure 10–10c Skeletal Muscle Innervation.
The Neuromuscular Junction
 Action Potential
 Generated by increase in sodium ions in
sarcolemma
 Travels along the T tubules
 Leads to excitation–contraction coupling
 Excitation–contraction coupling:
– action potential reaches a triad:
» releasing Ca2+
» triggering contraction
– requires myosin heads to be in “cocked” position:
» loaded by ATP energy
The Neuromuscular Junction
Figure 10–11 The Exposure of Active Sites.
The Contraction Cycle
 Five Steps of the Contraction Cycle
 Exposure of active sites
 Formation of cross-bridges
 Pivoting of myosin heads
 Detachment of cross-bridges
 Reactivation of myosin
The Contraction Cycle
Figure 10–12 The Contraction Cycle.
The Contraction Cycle
[INSERT FIG. 10.12, step 1]
Figure 10–12 The Contraction Cycle.
The Contraction Cycle
Figure 10–12 The Contraction Cycle.
The Contraction Cycle
Figure 10–12 The Contraction Cycle.
The Contraction Cycle
Figure 10–12 The Contraction Cycle.
The Contraction Cycle
Figure 10–12 The Contraction Cycle.
The Contraction Cycle
 Fiber Shortening
 As sarcomeres shorten, muscle pulls together, producing
tension
 Contraction Duration
 Depends on
 Duration of neural stimulus
 Number of free calcium ions in sarcoplasm
 Availability of ATP
The Contraction Cycle
Figure 10–13 Shortening during a Contraction.
The Contraction Cycle
 Relaxation
 Ca2+ concentrations fall
 Ca2+ detaches from troponin
 Active sites are re-covered by tropomyosin
 Sarcomeres remain contracted
 Rigor Mortis
 A fixed muscular contraction after death
 Caused when
 Ion pumps cease to function; ran out of ATP
 Calcium builds up in the sarcoplasm
The Contraction Cycle
 Skeletal muscle fibers shorten as thin filaments slide
between thick filaments
 Free Ca2+ in the sarcoplasm triggers contraction
 SR releases Ca2+ when a motor neuron stimulates the
muscle fiber
 Contraction is an active process
 Relaxation and return to resting length are passive
The Contraction Cycle
ATP and Muscle Contraction
 Sustained muscle contraction uses a lot of ATP
energy
 Muscles store enough energy to start
contraction
 Muscle fibers must manufacture more ATP as
needed
ATP and Muscle Contraction
 ATP and CP Reserves
 Adenosine triphosphate (ATP)
 The active energy molecule
 Creatine phosphate (CP)
 The storage molecule for excess ATP energy in resting muscle
 Energy recharges ADP to ATP
 Using the enzyme creatine phosphokinase (CPK or CK)
 When CP is used up, other mechanisms generate ATP
ATP and Muscle Contraction
 ATP Generation
 Cells produce ATP in two ways
 Aerobic metabolism of fatty acids in the mitochondria
 Anaerobic glycolysis in the cytoplasm
ATP and Muscle Contraction
 ATP Generation
 Aerobic metabolism
 Is the primary energy source of resting muscles
 Breaks down fatty acids
 Produces 34 ATP molecules per glucose molecule
 Anaerobic glycolysis
 Is the primary energy source for peak muscular activity
 Produces two ATP molecules per molecule of glucose
 Breaks down glucose from glycogen stored in skeletal muscles
ATP and Muscle Contraction
ATP and Muscle Contraction
 Energy Use and Muscle Activity
 At peak exertion
 Muscles lack oxygen to support mitochondria
 Muscles rely on glycolysis for ATP
 Pyruvic acid builds up, is converted to lactic acid
ATP and Muscle Contraction
Figure 10–20 Muscle Metabolism.
ATP and Muscle Contraction
Figure 10–20a Muscle Metabolism.
ATP and Muscle Contraction
Figure 10–20b Muscle Metabolism.
ATP and Muscle Contraction
Figure 10–20c Muscle Metabolism.
ATP and Muscle Contraction
 Muscle Fatigue
 When muscles can no longer perform a required activity,
they are fatigued
 Results of Muscle Fatigue
 Depletion of metabolic reserves
 Damage to sarcolemma and sarcoplasmic reticulum
 Low pH (lactic acid)
 Muscle exhaustion and pain
ATP and Muscle Contraction
 The Recovery Period
 The time required after exertion for muscles to
return to normal
 Oxygen becomes available
 Mitochondrial activity resumes
ATP and Muscle Contraction
 The Cori Cycle
 The removal and recycling of lactic acid by the liver
 Liver converts lactic acid to pyruvic acid
 Glucose is released to recharge muscle glycogen reserves
 Oxygen Debt
 After exercise or other exertion
 The body needs more oxygen than usual to normalize metabolic
activities
 Resulting in heavy breathing
ATP and Muscle Contraction
 Skeletal muscles at rest metabolize fatty acids and
store glycogen
 During light activity, muscles generate ATP through
anaerobic breakdown of carbohydrates, lipids, or
amino acids
 At peak activity, energy is provided by anaerobic
reactions that generate lactic acid as a byproduct
ATP and Muscle Contraction
 Heat Production and Loss
 Active muscles produce heat
 Up to 70% of muscle energy can be lost as heat, raising
body temperature
 Hormones and Muscle Metabolism
 Growth hormone
 Testosterone
 Thyroid hormones
 Epinephrine
ATP and Muscle Contraction
 Muscle Performance
 Power
 The maximum amount of tension produced
 Endurance
 The amount of time an activity can be sustained
 Power and endurance depend on
 The types of muscle fibers
 Physical conditioning
Muscle Fiber Types
 Three Types of Skeletal Muscle Fibers
 Fast fibers
 Slow fibers
 Intermediate fibers
Muscle Fiber Types
 Three Types of Skeletal Muscle Fibers
 Fast fibers
 Contract very quickly
 Have large diameter, large glycogen reserves, few
mitochondria
 Have strong contractions, fatigue quickly
Muscle Fiber Types
 Three Types of Skeletal Muscle Fibers
 Slow fibers
 Are slow to contract, slow to fatigue
 Have small diameter, more mitochondria
 Have high oxygen supply
 Contain myoglobin (red pigment, binds oxygen)
Muscle Fiber Types
 Three Types of Skeletal Muscle Fibers
 Intermediate fibers
 Are mid-sized
 Have low myoglobin
 Have more capillaries than fast fibers, slower to fatigue
Muscle Fiber Types
Figure 10–21 Fast versus Slow Fibers.
Muscle Fiber Types
 Muscle Hypertrophy
 Muscle growth from heavy training
 Increases diameter of muscle fibers
 Increases number of myofibrils
 Increases mitochondria, glycogen reserves
 Muscle Atrophy
 Lack of muscle activity
 Reduces muscle size, tone, and power
Muscle Fiber Types
 Physical Conditioning
 Improves both power and endurance
 Anaerobic activities (e.g., 50-meter dash,
weightlifting):
– use fast fibers
– fatigue quickly with strenuous activity
 Improved by:
– frequent, brief, intensive workouts
– hypertrophy
Muscle Fiber Types
 Physical Conditioning
 Improves both power and endurance
 Aerobic activities (prolonged activity):
– supported by mitochondria
– require oxygen and nutrients
 Improved by:
– repetitive training (neural responses)
– cardiovascular training
Muscle Fiber Types
 What you don’t use, you lose
 Muscle tone indicates base activity in motor units of
skeletal muscles
 Muscles become flaccid when inactive for days or
weeks
 Muscle fibers break down proteins, become smaller
and weaker
 With prolonged inactivity, fibrous tissue may replace
muscle fibers
Cardiac Muscle Tissue
 Structure of Cardiac Tissue
 Cardiac muscle is striated, found only in the
heart
Cardiac Muscle Tissue
 Seven Characteristics of Cardiocytes
 Unlike skeletal muscle, cardiac muscle cells
(cardiocytes)
 Are small
 Have a single nucleus
 Have short, wide T tubules
 Have no triads
 Have SR with no terminal cisternae
 Are aerobic (high in myoglobin, mitochondria)
 Have intercalated discs
Cardiac Muscle Tissue
 Intercalated Discs
 Are specialized contact points between cardiocytes
 Join cell membranes of adjacent cardiocytes (gap
junctions, desmosomes)
 Functions of intercalated discs
 Maintain structure
 Enhance molecular and electrical connections
 Conduct action potentials
Cardiac Muscle Tissue
Figure 10–22 Cardiac Muscle Tissue.
Cardiac Muscle Tissue
Figure 10–22a Cardiac Muscle Tissue.
Cardiac Muscle Tissue
Figure 10–22c Cardiac Muscle Tissue.
Cardiac Muscle Tissue
 Intercalated Discs
 Coordination of cardiocytes
 Because intercalated discs link heart cells mechanically,
chemically, and electrically, the heart functions like a
single, fused mass of cells
Cardiac Muscle Tissue
 Four Functions of Cardiac Tissue
 Automaticity
 Contraction without neural stimulation
 Controlled by pacemaker cells
 Variable contraction tension
 Controlled by nervous system
 Extended contraction time
 Ten times as long as skeletal muscle
 Prevention of wave summation and tetanic contractions
by cell membranes
 Long refractory period
Smooth Muscle Tissue
 Smooth Muscle in Body Systems
 Forms around other tissues
 In blood vessels
 Regulates blood pressure and flow
 In reproductive and glandular systems
 Produces movements
 In digestive and urinary systems
 Forms sphincters
 Produces contractions
 In integumentary system
 Arrector pili muscles cause “goose bumps”
Smooth Muscle Tissue
 Structure of Smooth Muscle
 Nonstriated tissue
 Different internal organization of actin and myosin
 Different functional characteristics
Smooth Muscle Tissue
Figure 10–23a Smooth Muscle Tissue.
Smooth Muscle Tissue
Figure 10–23b Smooth Muscle Tissue.
Smooth Muscle Tissue
 Eight Characteristics of Smooth Muscle Cells
 Long, slender, and spindle shaped
 Have a single, central nucleus
 Have no T tubules, myofibrils, or sarcomeres
 Have no tendons or aponeuroses
 Have scattered myosin fibers
 Myosin fibers have more heads per thick filament
 Have thin filaments attached to dense bodies
 Dense bodies transmit contractions from cell to cell
Smooth Muscle Tissue
 Functional Characteristics of Smooth Muscle
 Excitation–contraction coupling
 Length–tension relationships
 Control of contractions
 Smooth muscle tone
Smooth Muscle Tissue
 Functional Characteristics of Smooth Muscle
 Excitation–contraction coupling
 Free Ca2+ in cytoplasm triggers contraction
 Ca2+ binds with calmodulin:
– in the sarcoplasm
– activates myosin light–chain kinase
 Enzyme breaks down ATP, initiates contraction
Smooth Muscle Tissue
 Functional Characteristics of Smooth Muscle
 Length–Tension Relationships
 Thick and thin filaments are scattered
 Resting length not related to tension development
 Functions over a wide range of lengths (plasticity)
Smooth Muscle Tissue
 Functional Characteristics of Smooth Muscle
 Control of contractions
 Multiunit smooth muscle cells:
– connected to motor neurons
 Visceral smooth muscle cells:
– not connected to motor neurons
– rhythmic cycles of activity controlled by pacesetter cells
Smooth Muscle Tissue
 Functional Characteristics of Smooth Muscle
 Smooth muscle tone
 Maintains normal levels of activity
 Modified by neural, hormonal, or chemical factors