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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
10-5 Tension Production and
Contraction Types
• 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
Tension (percent of maximum)
Figure 10-14 The Effect of Sarcomere Length on Active Tension
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.
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
•
10-5 Tension Production and
Contraction Types
Tension Production by Muscles Fibers
–
Twitches
1.
2.
3.
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
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.
Figure 10-15b The Development of Tension in a Twitch
Tension
Maximum tension
development
Stimulus
Resting
phase
Latent Contraction
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.
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
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
Figure 10-16ab Effects of Repeated Stimulations
 Stimulus
Tension
Maximum tension (in tetanus)
Maximum tension (in treppe)
Time
Time
Treppe. Treppe is an increase in
Wave summation. Wave
peak tension with each
successive stimulus delivered
shortly after the completion of
the relaxation phase of the
preceding twitch.
summation occurs when
successive stimuli arrive
before the relaxation phase
has been completed.
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
– Complete tetanus
• If stimulation frequency is high enough, muscle never
begins to relax, and is in continuous contraction
Figure 10-16cd Effects of Repeated Stimulations
Tension
Maximum tension (in tetanus)
Time
Time
Incomplete tetanus.
Complete tetanus. During
Incomplete tetanus occurs if the
stimulus frequency increases
further. Tension production rises
to a peak, and the periods of
relaxation are very brief.
complete tetanus, the stimulus
frequency is so high that the
relaxation phase is eliminated;
tension plateaus at maximal
levels.
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
10-5 Tension Production and
Contraction Types
• 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
10-5 Tension Production and
Contraction Types
• Motor Units and Tension Production
– 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
Figure 10-17a The Arrangement and Activity of Motor Units in a Skeletal Muscle
Axons of
motor neurons
Motor
nerve
KEY
SPINAL CORD
Muscle fibers
Motor unit 1
Motor unit 2
Motor unit 3
Muscle fibers of different motor units are
intermingled, so the forces applied to the
tendon remain roughly balanced regardless of
which motor units are stimulated.
Figure 10-17b The Arrangement and Activity of Motor Units in a Skeletal Muscle
Tension in tendon
Tension
Motor Motor Motor
unit 1 unit 2 unit 3
Time
The tension applied to the
tendon remains relatively
constant, even though
individual motor units cycle
between contraction and
relaxation.
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
10-5 Tension Production and
Contraction Types
• Motor Units and Tension Production
– Contraction are classified based on pattern of
tension production
• Isotonic contraction
• Isometric contraction
10-5 Tension Production and
Contraction Types
• 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)
Figure 10-18a Concentric, Eccentric, and Isometric Contractions
Tendon
Muscle
contracts
(concentric
contraction)
2 kg
2 kg
Muscle
tension
(kg)
Amount of
load
Muscle
relaxes
Peak tension
production
Contraction
begins
Resting length
Muscle
length
(percent
of resting
length)
Time
Figure 10-18b Concentric, Eccentric, and Isometric Contractions
Support removed
when contraction
begins
(eccentric contraction)
Muscle
tension
(kg)
Peak tension
production
Support removed,
contraction begins
6 kg
Resting length
6 kg
Time
Muscle
length
(percent
of resting
length)
10-5 Tension Production and
Contraction Types
• Isometric Contraction
– Skeletal muscle develops tension, but is prevented
from changing length
– iso- = same, metric = measure
Figure 10-18c Concentric, Eccentric, and Isometric Contractions
Amount of load
Muscle
tension
(kg)
Muscle
contracts
(isometric
contraction)
Muscle
relaxes
Peak tension
production
Contraction
begins
6 kg
Length unchanged
Muscle
length
(percent
of resting
length)
6 kg
Time
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
Figure 10-19 Load and Speed of Contraction
Distance shortened
Small load
Intermediate load
Large load
Time (msec)
Stimulus
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
10-5 Tension Production and
Contraction Types
• Muscle Relaxation and the Return to Resting
Length
– Gravity
• Can take the place of opposing muscle contraction to
return a muscle to its resting state
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
10-6 Energy to Power Contractions
• 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
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
10-6 Energy to Power Contractions
• 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
Table 10-2 Sources of Energy in a Typical Muscle Fiber
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
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.
Figure 10-20a Muscle Metabolism
Fatty acids
Blood vessels
Glucose
Mitochondria
Glycogen
Creatine
Resting muscle: Fatty acids are catabolized; the
ATP produced is used to build energy reserves of ATP,
CP, and glycogen.
Figure 10-20b Muscle Metabolism
Fatty acids
Glucose
Glycogen
Pyruvate
To myofibrils to support
muscle contraction
Moderate activity: Glucose and fatty acids are
catabolized; the ATP produced is used to power
contraction.
Figure 10-20c Muscle Metabolism
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.
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
10-6 Energy to Power Contractions
• The Recovery Period
– The time required after exertion for muscles to
return to normal
– Oxygen becomes available
– Mitochondrial activity resumes
10-6 Energy to Power Contractions
• 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
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)
10-6 Energy to Power Contractions
• Heat Production and Loss
– Active muscles produce heat
– Up to 70% of muscle energy can be lost as heat,
raising body temperature
10-6 Energy to Power Contractions
• 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
10-7 Types of Muscles Fibers and
Endurance
• Three Major Types of Skeletal Muscle Fibers
1. Fast fibers
2. Slow fibers
3. Intermediate fibers
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
10-7 Types of Muscles Fibers and
Endurance
• Slow Fibers
– Are slow to contract, slow to fatigue
– Have small diameter, more mitochondria
– Have high oxygen supply
– Contain myoglobin (red pigment, binds oxygen)
10-7 Types of Muscles Fibers and
Endurance
• Intermediate Fibers
– Are mid-sized
– Have low myoglobin
– Have more capillaries than fast fibers, slower to
fatigue
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
LM  783
Table 10-3 Properties of Skeletal Muscle Fiber Types
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
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
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
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
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
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
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
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
10-8 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
Figure 10-22a Cardiac Muscle Tissue
Cardiac
muscle cell
Intercalated
discs
Nucleus
Cardiac muscle tissue
LM  575
A light micrograph of cardiac muscle tissue.
Figure 10-22b Cardiac Muscle Tissue
Cardiac muscle
cell (intact)
Intercalated disc
(sectioned)
A diagrammatic view of
cardiac muscle. Note
the striations and
intercalated
discs.
Mitochondria
Nucleus
Myofibrils
Intercalated
disc
Cardiac muscle cell
(sectioned)
Figure 10-22c Cardiac Muscle Tissue
Entrance to T tubule
Sarcolemma
Mitochondrion
Myofibrils
Contact of sarcoplasmic
reticulum with
T tubule
Sarcoplasmic
reticulum
Cardiac muscle tissue showing short, broad
T-tubules and SR that lacks terminal cisternae.
10-8 Cardiac Muscle Tissue
• 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
10-9 Smooth Muscle Tissue
• Structural Characteristics of Smooth Muscle
Tissue
– Nonstriated tissue
– Different internal organization of actin and myosin
– Different functional characteristics
Figure 10-23a Smooth Muscle Tissue
Circular
muscle layer
Longitudinal
muscle layer
Smooth muscle tissue
LM  100
Many visceral organs contain several layers of
smooth muscle tissue oriented in different
directions. Here, a single sectional view shows
smooth muscle cells in both longitudinal (L) and
transverse (T) sections.
Figure 10-23b Smooth Muscle Tissue
Relaxed (sectional view)
Dense body
Myosin
Actin
Relaxed (superficial view)
Intermediate
filaments (desmin)
Adjacent smooth muscle cells are
bound together at dense bodies,
transmitting the contractile forces
from cell to cell throughout the tissue.
Contracted
(superficial
view)
A single relaxed smooth muscle cell is spindle
shaped and has no striations. Note the changes in
cell shape as contraction occurs.
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
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
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
10-9 Smooth Muscle Tissue
• Length–Tension Relationships
– Thick and thin filaments are scattered
– Resting length not related to tension development
– Functions over a wide range of lengths (plasticity)
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
10-9 Smooth Muscle Tissue
• Smooth Muscle Tone
– Maintains normal levels of activity
– Modified by neural, hormonal, or chemical factors
Table 10-4 A Comparison of Skeletal, Cardiac, and Smooth Muscle Tissues