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Muscles &Muscle Tissue
Chapter 10
Functions of Skeletal Muscles
1.
2.
3.
4.
5.
Produce skeletal movement
Maintain body position
Support soft tissues
Guard body openings
Maintain body temperature
Functional Characteristics of
Muscle
• Excitability (irritability)
– Can receive and respond to stimuli.
• Stimuli can include nerve impulses, stretch,
hormones or changes in the chemical
environment.
• Contractility – the ability to shorten with
increasing tension.
• Extensibility – the ability to stretch.
• Elasticity – the ability to snap back (recoil)
to their resting length after being
stretched.
Three types of muscle
Skeletal
Smooth
Cardiac
Characteristics of Skeletal Muscle
•
•
•
•
Striated
Multinucleate (it is a syncytium)
Voluntary
Parallel fibers
Organization
of
Connective
Tissues
Figure 10–1
• Skeletal muscle cells are called fibers
Formation of Skeletal Muscle Fibers
Organization of Skeletal Muscle Fibers
Anatomy of a myofibril
A Triad
• Is formed by 1 T tubule and 2 terminal
cisternae
Sarcomeres
Figure 10–4
Muscle Striations
• A striped or striated pattern within
myofibrils:
– alternating dark, thick filaments (A bands) and
light, thin filaments (I bands)
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 2 ends of sarcomere
Zone of Overlap
• The densest, darkest area on a light
micrograph
• Where thick and thin filaments overlap
The H Zone
• The area around the M line
• Has thick filaments but no thin filaments
Titin
• Are strands of protein
• Reach from tips of thick filaments to the Z
line
• Stabilize the filaments
Sarcomere Structure
Summary of skeletal muscle anatomy:
muscles are made of fascicles
Fascicles are made of fibers,
fibers are made of myofibrils
Level 1: Skeletal Muscle
Figure 10–6 (1 of 5)
Level 2: Muscle Fascicle
Figure 10–6 (2 of 5)
Level 3: Muscle Fiber
Figure 10–6 (3 of 5)
Level 4: Myofibril
Figure 10–6 (4 of 5)
Level 5: Sarcomere
Figure 10–6 (5 of 5)
Fibrils are divided into sarcomeres,
sarcomeres are made of myofilaments
Myofilaments
are made of
protein molecules
A Thin Filament
4 Thin Filament Proteins
1. F actin:
– is 2 twisted rows of globular G actin
– the active sites on G actin strands bind to
myosin
4 Thin Filament Proteins
2. Nebulin:
– holds F actin strands together
4 Thin Filament Proteins
3. Tropomyosin:
– is a double strand
– prevents actin–myosin interaction
4 Thin Filament Proteins
4. Troponin:
– a globular protein
– binds tropomyosin to G actin
– controlled by Ca2+
Troponin and Tropomyosin
Figure 10–7b
A Thick Filament
Figure 10–7c
Thick Filaments
• Contain twisted myosin subunits
• Contain titin strands that recoil after
stretching
The Mysosin Molecule
Figure 10–7d
Muscle Contraction:
the Sliding Filament Theory
• Muscle contraction requires:
– Stimulus – the generation of an action
potential.
– Crossbridge formation – interaction between
the thick and thin myofilaments. This is
triggered by Ca++ ions released from the
sarcoplasmic reticulum.
– Energy – ATP to energize the myosin
molecules.
Sliding
Filaments
Skeletal
Muscle
Contraction
T- tubules supply the stimulus,
Sarcoplasmic Reticulum supplies the Ca++,
Mitochondria supply the ATP.
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 Innervation
Skeletal Muscle Innervation
Figure 10–10c
The Neuromuscular Junction
• Is the location of neural stimulation
• Action potential (electrical signal):
– travels along nerve axon
– ends at synaptic terminal
A neuromuscular junction (NMJ).
acetylcholine
The actual synapse
Synaptic Terminal
• Releases neurotransmitter (acetylcholine
or ACh)
• Into the synaptic cleft (gap between
synaptic terminal and motor end plate)
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)
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
Exposing the Active Site
The Contraction Cycle
The Contraction Cycle
The Contraction Cycle
The Contraction Cycle
5 Steps of the Contraction Cycle
1.
2.
3.
4.
5.
Exposure of active sites
Formation of cross-bridges
Pivoting of myosin heads
Detachment of cross-bridges
Reactivation of myosin
Show the animation
A Review of Muscle Contraction
Excitation-Contraction coupling
• Stimulus or excitation is required for
muscles to contract.
• In skeletal muscle, the stimulus is from a
motor neuron.
• The stimulus is in the form of an action
potential.
• This action potential starts at the
neuromuscular junction (NMJ).
Excitation-contraction coupling
Show NMJ animation
Micrograph of an NMJ
A Synapse
Synaptic vesicles
Tension and Sarcomere Length
Figure 10–14
Length–Tension Relationship
• 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
Length–Tension Relationship
• Normal resting sarcomere length:
– is 75% to 130% of optimal length
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
Tension in a Twitch
• Length of twitch depends on type of muscle
Myogram
• A graph of twitch tension development
Figure 10–15b (Navigator)
3 Phases of Twitch
1. Latent period before contraction:
– the action potential moves through
sarcolemma
– causing Ca2+ release
3 Phases of Twitch
2. Contraction phase:
– calcium ions bind
– tension builds to peak
3 Phases of Twitch
3. Relaxation phase:
– Ca2+ levels fall
– active sites are covered
– tension falls to resting levels
Treppe
• A stair-step increase in twitch tension
Figure 10–16a
Treppe
• 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
Wave Summation
• 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
Figure 10–16c
Incomplete Tetanus
• If rapid stimulation continues and muscle
is not allowed to relax, twitches reach
maximum level of tension
Complete
Tetanus
Figure 10–16d
Complete Tetanus
• If stimulation frequency is high enough,
muscle never begins to relax, and is in
continuous contraction
Comparative speed of different muscles
Tension Produced by
Whole 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 in a Skeletal Muscle
Figure 10–17
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 is
produced by slowly increasing 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 to rest in rotation
2 Types of Skeletal
Muscle Tension
1. Isotonic contraction
2. Isometric contraction
Isotonic Contraction
Figure 10–18a, b
Isotonic Contraction
• Skeletal muscle changes length:
– resulting in motion
• If muscle tension > resistance:
– muscle shortens (concentric contraction)
• If muscle tension < resistance:
– muscle lengthens (eccentric contraction)
Isometric Contraction
Figure 10–18c, d
Isometric Contraction
• Skeletal muscle develops tension, but is
prevented from changing length
Note: Iso = same, metric = measure
Resistance and Speed of Contraction
Resistance and Speed
of Contraction
• Are inversely related
• The heavier the resistance on a muscle:
– the longer it takes for shortening to begin
– and the less the muscle will shorten
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 CP Reserves
• Adenosine triphosphate (ATP):
– the active energy molecule
• Creatine phosphate (CP):
– the storage molecule for excess ATP energy
in resting muscle
Recharging ATP
• Energy recharges ADP to ATP:
– using the enzyme creatine phosphokinase
(CPK)
• When CP is used up, other mechanisms
generate ATP
Energy Storage in Muscle Fiber
Table 10–2
ATP Generation
• Cells produce ATP in 2 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
Anaerobic Glycolysis
• Is the primary energy source for peak
muscular activity
• Produces 2 ATP molecules per molecule
of glucose
• Breaks down glucose from glycogen
stored in skeletal muscles
Anaerobic
Metabolism:
a losing
proposition
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
Muscle
Metabolis
m
Muscle Metabolism
Figure 10–20b
Muscle Metabolism
Figure 10–20c
Results of Muscle Fatigue
1. Depletion of metabolic reserves
2. Damage to sarcolemma and
sarcoplasmic reticulum
3. Low pH (lactic acid)
4. Muscle exhaustion and pain
The Recovery Period
• The time required after exertion for
muscles to return to normal
• Oxygen becomes available
• Mitochondrial activity resumes
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:
– the body needs more oxygen than usual to
normalize metabolic activities
– resulting in heavy breathing
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
3 Types of Skeletal Muscle
Fibers
1. Fast fibers
2. Slow fibers
3. 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 fiber,
slower to fatigue
Fast versus Slow Fibers
Figure 10–21
Comparing Skeletal Muscle Fibers
Muscles and Fiber Types
• White muscle:
– mostly fast fibers
– pale (e.g., chicken breast)
• Red muscle:
– mostly slow fibers
– dark (e.g., chicken legs)
• Most human muscles:
– mixed fibers
– pink
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
Structure of
Cardiac Tissue
• Cardiac muscle is
striated, found
only in the heart
Figure 10–22
7 Characteristics of Cardiocytes
• Unlike skeletal muscle, cardiac muscle
cells (cardiocytes):
– are small
– have a single nucleus
– have short, wide T tubules
7 Characteristics of Cardiocytes
– have no triads
– have SR with no terminal cisternae
– are aerobic (high in myoglobin, mitochondria)
– have intercalated discs
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
4 Functions of Cardiac Tissue
1. Automaticity:
– contraction without neural stimulation
– controlled by pacemaker cells
2. Variable contraction tension:
– controlled by nervous system
4 Functions of Cardiac Tissue
3. Extended contraction time
4. Prevention of wave summation and
tetanic contractions by cell membranes
Structure of Smooth Muscle
• Nonstriated tissue
Figure 10–23
Comparing Smooth
and Striated Muscle
• Different internal organization of actin and
myosin
• Different functional characteristics
8 Characteristics of
Smooth Muscle Cells
1. Long, slender, and spindle shaped
2. Have a single, central nucleus
3. Have no T tubules, myofibrils, or
sarcomeres
4. Have no tendons or aponeuroses
8 Characteristics of
Smooth Muscle Cells
5. Have scattered myosin fibers
6. Myosin fibers have more heads per thick
filament
7. Have thin filaments attached to dense
bodies
8. Dense bodies transmit contractions from
cell to cell
Functional Characteristics
of Smooth Muscle
1.
2.
3.
4.
Excitation–contraction coupling
Length–tension relationships
Control of contractions
Smooth muscle tone
Excitation–Contraction Coupling
• Free Ca in cytoplasm triggers
contraction
• Ca2+ binds with calmodulin:
2+
– 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
• Subdivisions:
– 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
Smooth
Muscle
Varicosities
Skeletal
Smooth
Diameter
10 - 100 m
3 - 8 m
Connective tissue
Epi-, Peri- & Endomysium only
Endomysium
SR
Yes, complex Barely, simple
T - tubules
yes
no
Sarcomeres
yes
no
Gap Junctions
no
yes
voluntary
yes
no
Neurotransmitters
Acetylcholine Ach, epinephrine,
(Ach)
norepinephrine, et al
Regeneration
Very little
Lots, for muscle
Future governors of Califorina?