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Chapter 10: Muscle Tissue
An Introduction to Muscle Tissue
Muscle Tissue
A primary tissue type, divided into:
Skeletal muscle tissue
Cardiac muscle tissue
Smooth muscle tissue
10-1 Functions of Skeletal Muscle Tissue
Skeletal Muscles
Are attached to the skeletal system
Allow us to move
The muscular system Includes only skeletal muscles
Six Functions of Skeletal Muscle Tissue
Produce skeletal movement
Maintain posture and body position
Support soft tissues
Guard entrances and exits
Maintain body temperature
Store nutrient reserves
10-2 Organization of Muscle
Skeletal Muscle
Muscle tissue (muscle cells or fibers)
Connective tissues
Nerves
Blood vessels
Organization of Connective Tissues
Muscles have three layers of connective tissues
Epimysium; Exterior collagen layer Connected to deep fascia, and 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, and Contains myosatellite cells (stem
cells) that repair damage
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)
Blood Vessels and Nerves
Muscles have extensive vascular systems that:
Supply large amounts of oxygen
Supply nutrients
Carry away wastes
Skeletal muscles are voluntary muscles, controlled by nerves of the central
nervous system (brain and spinal cord)
10-3 Characteristics of Skeletal Muscle Fibers
Skeletal Muscle Cells
© 2012 Pearson Education, Inc.
Are very long
Develop through fusion of mesodermal cells (myoblasts)
Become very large
Contain hundreds of nuclei
The Sarcolemma and Transverse Tubules
The sarcolemma
The cell membrane of a muscle fiber (cell)
Surrounds the sarcoplasm (cytoplasm of muscle fiber)
A change in transmembrane potential begins contractions
The Sarcolemma and Transverse Tubules
Transverse tubules (T tubules)
Transmit action potential through cell
Allow entire muscle fiber to contract simultaneously
Have same properties as sarcolemma
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
The 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
The Sarcoplasmic Reticulum (SR)
Triad
Is formed by one T tubule and two terminal cisternae
Cisternae
Concentrate Ca2+ (via ion pumps)
10-3 Structural Components of a Sarcomere
Sarcomeres
The contractile units of muscle
Structural units of myofibrils
Form visible patterns within myofibrils
A striped or striated pattern within myofibrils
Alternating dark, thick filaments (A bands) and light, thin filaments (I bands)
The A Band
M line
The center of the A band
At midline of sarcomere
The H Band
The area around the M line
Has thick filaments but no thin filaments
Zone of overlap
The densest, darkest area on a light micrograph
Where thick and thin filaments overlap
© 2012 Pearson Education, Inc.
The I Band
Z lines
The centers of the I bands
At two ends of sarcomere
Titin
Are strands of protein
Reach from tips of thick filaments to the Z line
Stabilize the filaments
Thin Filaments
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+
Initiating Contraction
Ca2+ binds to receptor on troponin molecule
Troponin–tropomyosin complex changes
Exposes active site of F-actin
Thick Filaments
Contain about 300 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
Myosin Action
During contraction, myosin heads:
Interact with actin filaments, forming cross-bridges
Pivot, producing motion
Sliding Filaments and 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 Contraction
The process of contraction
Neural stimulation of sarcolemma
Causes excitation–contraction coupling
Muscle fiber contraction
Interaction of thick and thin filaments
Tension production
© 2012 Pearson Education, Inc.
10-4 Components of the Neuromuscular Junction
The Control of Skeletal Muscle Activity
The neuromuscular junction (NMJ)
Special intercellular connection between the nervous system and skeletal
muscle fiber
Controls calcium ion release into the sarcoplasm
Excitation–Contraction Coupling
Action potential reaches a triad
Releasing Ca2+
Triggering contraction
Requires myosin heads to be in “cocked” position
Loaded by ATP energy
10-4 Skeletal Muscle Contraction
The Contraction Cycle
Contraction Cycle Begins
Active-Site Exposure
Cross-Bridge Formation
Myosin Head Pivoting
Cross-Bridge Detachment
Myosin Reactivation
Fiber Shortening
As sarcomeres shorten, muscle pulls together, producing tension
Muscle shortening can occur at both ends of the muscle, or at only one end of
the
muscle
This depends on the way the muscle is attached at the ends
10-4 Skeletal Muscle Relaxation
Relaxation
Contraction Duration
Depends on:
Duration of neural stimulus
Number of free calcium ions in sarcoplasm
Availability of ATP
Ca2+ concentrations fall
Ca2+ detaches from troponin
Active sites are re-covered by tropomyosin
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
Summary
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
© 2012 Pearson Education, Inc.
10-5 Tension Production and Contraction Types
Tension Production by Muscles Fibers
As a whole, a muscle fiber is either contracted or relaxed
Depends on:
The number of pivoting cross-bridges
The fiber’s resting length at the time of stimulation
The frequency of stimulation
Length–Tension Relationships
Number of pivoting cross-bridges depends on:
Amount of overlap between thick and thin fibers
Optimum overlap produces greatest amount of tension
Too much or too little reduces efficiency
Normal resting sarcomere length
Is 75% to 130% of optimal length
The Frequency of Stimulation
A single neural stimulation produces:
A single contraction or twitch
Which lasts about 7–100 msec.
Sustained muscular contractions
Require many repeated stimuli
Twitches
Latent period
The action potential moves through sarcolemma
Causing Ca2+ release
Contraction phase
Calcium ions bind
Tension builds to peak
Relaxation phase
Ca2+ levels fall
Active sites are covered and tension falls to resting levels
Treppe
A stair-step increase in twitch tension
Repeated stimulations immediately after relaxation phase
Stimulus frequency <50/second
Causes a series of contractions with increasing tension
Wave summation
Increasing tension or summation of twitches
Repeated stimulations before the end of relaxation phase
Stimulus frequency >50/second
Causes increasing tension or summation of twitches
Incomplete tetanus
Twitches reach maximum tension
If rapid stimulation continues and muscle is not allowed to relax, twitches
reach maximum level of tension
Complete tetanus
If stimulation frequency is high enough, muscle never begins to relax, and
is in continuous contraction
Depends on:
Internal tension produced by muscle fibers
External tension exerted by muscle fibers on elastic extracellular fibers
© 2012 Pearson Education, Inc.
Total number of muscle fibers stimulated
Motor Units and Tension Production
Motor units in a skeletal muscle:
Contain hundreds of muscle fibers
That contract at the same time
Controlled by a single motor neuron
Recruitment (multiple motor unit summation)
In a whole muscle or group of muscles, smooth motion and increasing
tension are produced by slowly increasing the size or number of motor
units stimulated
Maximum tension
Achieved when all motor units reach tetanus
Can be sustained only a very short time
Sustained tension
Less than maximum tension
Allows motor units rest in rotation
Muscle tone
The normal tension and firmness of a muscle at rest
Muscle units actively maintain body position, without motion
Increasing muscle tone increases metabolic energy used, even at rest
Contraction are classified based on pattern of tension production
Isotonic contraction
Isometric contraction
Isotonic Contraction
Skeletal muscle changes length
Resulting in motion
If muscle tension > load (resistance):
Muscle shortens (concentric contraction)
If muscle tension < load (resistance):
Muscle lengthens (eccentric contraction)
Isometric Contraction
Skeletal muscle develops tension, but is prevented from changing length
iso- = same, metric = measure
Load and Speed of Contraction
Are inversely related
The heavier the load (resistance) on a muscle
The longer it takes for shortening to begin
And the less the muscle will shorten
Muscle Relaxation and the Return to Resting Length
Elastic Forces
The pull of elastic elements (tendons and ligaments)
Expands the sarcomeres to resting length
Opposing Muscle Contractions
Reverse the direction of the original motion
Are the work of opposing skeletal muscle pairs
Gravity
Can take the place of opposing muscle contraction to return a muscle to
its resting state
© 2012 Pearson Education, Inc.
10-6 Energy to Power Contractions
ATP Provides Energy For 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 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 kinase (CK)
When CP is used up, other mechanisms generate ATP
ATP Generation
Cells produce ATP in two ways
Aerobic metabolism of fatty acids in the mitochondria
Anaerobic glycolysis in the cytoplasm
Aerobic Metabolism
Is the primary energy source of resting muscles
Breaks down fatty acids
Produces 34 ATP molecules per glucose molecule
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
Energy Use and the Level of Muscular Activity
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
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
The Recovery Period
The time required after exertion for muscles to return to normal
Oxygen becomes available
Mitochondrial activity resumes
Lactic Acid Removal and Recycling
The Cori Cycle
The removal and recycling of lactic acid by the liver
Liver converts lactate to pyruvate
Glucose is released to recharge muscle glycogen reserves
© 2012 Pearson Education, Inc.
The Oxygen Debt
After exercise or other exertion:
The body needs more oxygen than usual to normalize metabolic activities
Resulting in heavy breathing
Also called excess postexercise oxygen consumption (EPOC)
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
10-7 Types of Muscles Fibers and Endurance
Muscle Performance
Force
The maximum amount of tension produced
Endurance
The amount of time an activity can be sustained
Force and endurance depend on:
The types of muscle fibers
Physical conditioning
Three Major Types of Skeletal Muscle Fibers
Fast fibers
Slow fibers
Intermediate fibers
Fast Fibers
Contract very quickly
Have large diameter, large glycogen reserves, few mitochondria
Have strong contractions, fatigue quickly
Slow Fibers
Are slow to contract, slow to fatigue
Have small diameter, more mitochondria
Have high oxygen supply
Contain myoglobin (red pigment, binds oxygen)
Intermediate Fibers
Are mid-sized
Have low myoglobin
Have more capillaries than fast fibers, slower to fatigue
Muscle Performance and the Distribution of Muscle Fibers
White muscles
Mostly fast fibers
Pale (e.g., chicken breast)
Red muscles
Mostly slow fibers
Dark (e.g., chicken legs)
Most human muscles
Mixed fibers
Pink
© 2012 Pearson Education, Inc.
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
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
Causes hypertrophy
Improves both power and endurance
Aerobic activities (prolonged activity)
Supported by mitochondria
Require oxygen and nutrients
Improves:
Endurance by training fast fibers to be more like intermediate
fibers
Cardiovascular performance
Importance of Exercise
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
10-8 Cardiac Muscle Tissue
Cardiac Muscle Tissue
Cardiac muscle cells are striated and found only in the heart
Striations are similar to that of skeletal muscle because the internal
arrangement
of myofilaments is similar
Structural Characteristics of Cardiac Muscle Tissue
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
© 2012 Pearson Education, Inc.
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
Coordination of cardiocytes
Because intercalated discs link heart cells mechanically, chemically, and
electrically, the heart functions like a single, fused mass of cells
Functional Characteristics of Cardiac Muscle 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
10-9 Smooth Muscle Tissue
Smooth Muscle in Body Systems
Forms around other tissues
In integumentary system
Arrector pili muscles cause “goose bumps”
In blood vessels and airways
Regulates blood pressure and airflow
In reproductive and glandular systems
Produces movements
In digestive and urinary systems
Forms sphincters
Produces contractions
Structural Characteristics of Smooth Muscle Tissue
Nonstriated tissue
Different internal organization of actin and myosin
Different functional characteristics
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
Functional Characteristics of Smooth Muscle Tissue
Excitation–contraction coupling
Length–tension relationships
Control of contractions
Smooth muscle tone
© 2012 Pearson Education, Inc.
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
Length–Tension Relationships
Thick and thin filaments are scattered
Resting length not related to tension development
Functions over a wide range of lengths (plasticity)
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 Tone
Maintains normal levels of activity
Modified by neural, hormonal, or chemical factors
© 2012 Pearson Education, Inc.
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