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Muscle
Lecture #10 Ch 10 Muscle
Muse 10/31/12
An Introduction to Muscle Tissue
Muscle Tissue
A primary tissue type, divided into
Skeletal muscle
Cardiac muscle
Smooth muscle
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: (not to be confused with paramecium)– 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.
Figure 9.1
Bone
Perimysium
Endomysium(between individualmuscle fibers)
Muscle fiber
Fascicle(wrapped by perimysium)
Epimysium
Tendon
Epimysium
Muscle fiberin middle ofa fascicle
Blood vessel
Perimysium
Endomysium
Fascicle(a)
(b)
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: Attachments
Muscles attach: Directly—epimysium of muscle is fused to the
periosteum of bone or perichondrium of cartilage
Indirectly—connective tissue wrappings extend beyond the muscle as a ropelike tendon or sheetlike aponeurosis
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
NucleusLight I bandDark A band
Sarcolemma
Mitochondrion
(b) Diagram of part of a muscle fiber showing the myofibrils. Onemyofibril is extended afrom the cut end of the fiber.
Myofibril
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.
Show video excitation coupling
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)
Sarcomere
Smallest contractile unit (functional unit) of a muscle fiber
The region of a myofibril between two successive Z discs
Composed of thick and thin myofilaments made of contractile proteins
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
I band I bandA bandSarcomere
H zone
Thin (actin)filament
Thick (myosin)filament
Z disc Z disc
M line
(c) Small part of one myofibril enlarged to show the myofilamentsresponsible for the banding pattern. Each sarcomere extends fromone Z disc to the next.
Z disc Z discM line
Sarcomere
Thin (actin)filament
Thick(myosin)filament
Elastic (titin)filaments
(d) Enlargement of one sarcomere (sectioned lengthwise). Notice the myosin heads on the thick filaments.
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- quite elastic, like
springs
– 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
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
Show video neuromuscular junction
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
Tension Production
The all–or–none principle As a whole, a muscle fiber is either contracted or
relaxed
Tension of a Single Muscle Fiber Depends on
The number of pivoting cross-bridges
The fiber’s resting length at the time of stimulation
The frequency of stimulation
Need more force? Recruit more fibers
Tension Production
Tension of a Single Muscle Fiber
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
Normal resting sarcomere length:
– is 75% to 130% of optimal length
Tension Production
Figure 10–14 The Effect of Sarcomere Length on Active Tension.
Tension Production
Tension of a Single Muscle Fiber
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 Production
Three Phases of Twitch Latent period before contraction
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
Tension falls to resting levels
Control of Muscle Tension
Tension Production
Figure 10–15 The Development of Tension in a Twitch.
Tension Production
Figure 10–15 The Development of Tension in a Twitch.
Tension Production
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
Tension Production
Tension of a Single Muscle Fiber
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
Tension Production
Tension of a Single Muscle Fiber 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
Tension Production
Figure 10–16 Effects of Repeated Stimulations.
Tension Production
Figure 10–16 Effects of Repeated Stimulations.
Tension Production
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
Tension Production
Tension Produced by Whole Skeletal Muscles
Motor units in a skeletal muscle
Contain hundreds of muscle fibers
That contract at the same time
Controlled by a single motor neuron
Tension Production
Tension Produced by Whole Skeletal Muscles
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
Tension Production
Figure 10–17 The Arrangement and Activity of Motor Units in a Skeletal Muscle.
Tension Production
Figure 10–17 The Arrangement and Activity of Motor Units in a Skeletal Muscle.
Tension Production
Tension Produced by Whole Skeletal Muscles
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
Tension Production
Two Types of Skeletal Muscle Tension
Isotonic contraction
Isometric contraction
Tension Production
Two Types of Skeletal Muscle Tension
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)
Tension Production
Two Types of Skeletal Muscle Tension
Isometric contraction Skeletal muscle develops tension, but is prevented
from changing length
Note: iso- = same, metric = measure
Tension Production
Figure 10–18a, b Isotonic and Isometric Contractions.
Tension Production
Figure 10–18c, d Isotonic and Isometric Contractions.
Tension Production
Resistance 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 After contraction, a muscle fiber returns to resting
length by Elastic forces
Opposing muscle contractions
Gravity
Tension Production
Figure 10–19 Load and Speed of Contraction.
Tension Production
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
Tension Production
Gravity
Can take the place of opposing muscle
contraction to return a muscle to its resting
state
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
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 10–20c 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 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
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
Muscles and Fiber Types White muscle
Mostly fast fibers (aka Fast twitch) Pale (e.g., chicken breast)
Red muscle Mostly slow fibers (aka slow twitch) Dark (e.g., chicken legs)
Most human muscles Mixed fibers (General proportions determined by genetics)
Pink
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
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