Download Resting muscle

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
Tension Production by Muscles Fibers
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
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Figure 10-14 The Effect of Sarcomere Length on Active Tension
Tension (percent of maximum)
A muscle fiber is either “on” (producing tension) or “off” (relaxed).
Skeletal muscle fibers contract
most forcefully when stimulated
over a narrow range of lengths.
Normal
range
Decreased length
Increased sarcomere length
Optimal resting length:
The normal range of
sarcomere lengths in the
body is 75 to 130 percent of
the optimal length.
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10-5 Tension Production and Contraction Types
Tension Production by Muscles Fibers
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
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10-5 Tension Production and Contraction Types
Tension Production by Muscles Fibers
Twitches
1.
Latent period
The action potential moves through sarcolemma
Causing Ca2+ release
2.
Contraction phase
Calcium ions bind
Tension builds to peak
3.
Relaxation phase
Ca2+ levels fall
Active sites are covered and tension falls to resting levels
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Figure 10-15b The Development of Tension in a Twitch
Tension
Maximum tension
development
Stimulus
Resting Latent Contraction
phase period
phase
Relaxation
phase
The details of tension over time for a single
twitch in the gastrocnemius muscle. Notice the
presence of a latent period, which corresponds
to the time needed for the conduction of an
action potential and the subsequent release of
calcium ions by the sarcoplasmic reticulum.
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Figure 10-15a The Development of Tension in a Twitch
Eye muscle
Gastrocnemius
Tension
Soleus
Stimulus
Time (msec)
A myogram showing differences in
tension over time for a twitch in
different skeletal muscles.
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10-5 Tension Production and Contraction Types
Tension Production by Muscles Fibers
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
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10-5 Tension Production and Contraction Types
Tension Production by Muscles Fibers
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
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10-5 Tension Production and Contraction Types
Tension Production by Muscles Fibers
Incomplete tetanus
Twitches reach maximum tension
If rapid stimulation continues and muscle is not allowed to relax,
twitches reach maximum level of tension
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Tension Production and Contraction Types
Complete tetanus
If stimulation frequency is high enough,
muscle never begins to relax, and is in
continuous contraction
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10-5 Tension Production and Contraction Types
Tension Production by Skeletal Muscles
Depends on:
Internal tension produced by muscle fibers
External tension exerted by muscle fibers on elastic extracellular fibers
Total number of muscle fibers stimulated
Motor units (all muscle fibers controlled by a single motor neuron) in
a skeletal muscle:
Contain hundreds of muscle fibers
That contract at the same time
Controlled by a single motor neuron
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10-5 Tension Production and Contraction Types
Motor Units and Tension Production
Recruitment (multiple motor unit summation)
• n 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
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10-5 Tension Production and Contraction Types
Motor Units and Tension Production
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
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10-5 Tension Production and Contraction Types
Motor Units and Tension Production
Contraction are classified based on pattern of tension production
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)
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Figure 10-18a Concentric, Eccentric, and Isometric Contractions
Tendon
In an isotonic
contraction, tension
rises and the skeletal
Muscle’s length
changes.
Muscle
contracts
(concentric
contraction)
2 kg
2 kg
Examples: Lifting an object off a desk, walking, running.
Muscle
tension
(kg)
Amount of
load
Muscle
relaxes
Peak tension
production
Contraction
begins
Resting length
Muscle
length
(percent
of resting
length)
Time
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Figure 10-18b Concentric, Eccentric, and Isometric Contractions
In an eccentric contraction, the peak tension developed is less than the load, and the
muscle elongates due to the contraction of another muscle or the pull of gravity.
Support removed
when contraction
begins
(eccentric contraction)
Muscle
tension
(kg)
Peak tension
production
Support removed,
contraction begins
6 kg
Resting length
6 kg
Examples:
Bicep curls
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Time
Muscle
length
(percent
of resting
length)
10-5 Tension Production and Contraction Types
In an isometric contraction, the muscle as a whole does not change length, and
The tension produced never exceeds the load.
•
Isometric Contraction
•
Skeletal muscle develops tension, but is prevented from changing length
•
iso- = same, metric = measure
Examples: Carrying a bag of groceries; holding head up.
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10-5 Tension Production and Contraction Types
•
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
Small load
Distance
Intermediate load
Large load
Stimulus
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Time
10-5 Tension Production and Contraction Types
•
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
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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
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10-6 Energy to Power Contractions
ATP Generation
Cells produce ATP in two ways
1.
Aerobic metabolism of fatty acids in the mitochondria
2.
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
6 Where inside a skeletal muscle does glycolysis occur? Where does cellular respiration?
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Table 10-2 Sources of Energy in a Typical Muscle Fiber
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10-6 Energy to Power Contractions
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
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Figure 10-20 Muscle Metabolism
Fatty acids
Fatty acids
Blood vessels
Glucose
Glucose
Glycogen
Glycogen
Pyruvate
Mitochondria
Creatine
To myofibrils to support
muscle contraction
Resting muscle: Fatty acids are catabolized; the
Moderate activity: Glucose and fatty acids are
ATP produced is used to build energy reserves of ATP,
CP, and glycogen.
catabolized; the ATP produced is used to power
contraction.
Lactate
Glucose
Pyruvate
Glycogen
Creatine
Lactate
To myofibrils to support
muscle contraction
Peak activity: Most ATP is produced through glycolysis,
with lactate as a by-product. Mitochondrial activity
(not shown) now provides only about one-third of the
ATP consumed.
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10-6 Energy to Power Contractions
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
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10-6 Energy to Power Contractions
Lactic Acid Removal and Recycling
The Cori Cycle (The removal and recycling of lactic acid by the liver back
to muscle cells).
Liver converts lactate to pyruvate
Glucose is released to recharge muscle glycogen reserves
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During exertion, lactate diffuses out of the muscle fibers into the bloodstream. The liver
absorbs the lactate and converts it to pyruvate. About 30% of the pyruvate is broken
down in the Citric Acid Cycle, providing the ATP needed to convert the other pyruvate
molecules to glucose. Glucose is then released into circulation, where they are
absorbed by skeletal muscle fibers and used to rebuild their glycogen reserves. This is
the Cori Cycle.
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10-6 Energy to Power Contractions
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
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10-6 Energy to Power Contractions
Hormones and Muscle Metabolism
Growth hormone
Testosterone
Thyroid hormones
Epinephrine
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
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10-7 Types of Muscles Fibers and Endurance
Three Major Types of Skeletal Muscle Fibers
1.
Fast fibers
2.
Slow fibers
3.
Intermediate fibers
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10-7 Types of Muscles Fibers and Endurance
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
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Figure 10-21 Fast versus Slow Fibers
Slow fibers
Smaller diameter,
darker color due to
myoglobin; fatigue
resistant
LM  170
Fast fibers
Larger diameter,
paler color;
easily fatigued
LM  170
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LM  783
Table 10-3 Properties of Skeletal Muscle Fiber Types
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10-7 Types of Muscles Fibers and Endurance
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
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10-7 Types of Muscles Fibers and Endurance
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
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10-7 Types of Muscles Fibers and Endurance
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
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10-7 Types of Muscles Fibers and Endurance
Physical Conditioning
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
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10-7 Types of Muscles Fibers and Endurance
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
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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
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10-8 Cardiac Muscle Tissue
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
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10-8 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
• Coordination of cardiocytes
• Because intercalated discs link heart cells mechanically, chemically,
and electrically, the heart functions like a single, fused mass of cells
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10-8 Cardiac Muscle Tissue
Functional Characteristics of Cardiac Muscle Tissue
Automaticity (contracts 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
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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
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10-9 Smooth Muscle Tissue
Structural Characteristics of Smooth Muscle Tissue
Nonstriated tissue
Different internal organization of actin and myosin
Different functional characteristics
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10-9 Smooth Muscle Tissue
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
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10-9 Smooth Muscle Tissue
Functional Characteristics of Smooth Muscle Tissue
1.
Excitation–contraction coupling
2.
Length–tension relationships
3.
Control of contractions
4.
Smooth muscle tone
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10-9 Smooth Muscle Tissue
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)
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10-9 Smooth Muscle Tissue
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
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Table 10-4 A Comparison of Skeletal, Cardiac, and Smooth Muscle Tissues
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