Chapter 11
*
Muscular Tissue
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Introduction to Muscle
• Movement is a fundamental characteristic of all living things
• Cells capable of shortening and converting the chemical energy of
ATP into mechanical energy
11-2
The Functions of Muscles
• Movement of body parts and organ contents
• Body movement, breathing, blood circulation, digestion
• Maintain posture and prevent movement
• Communication - speech, expression and writing
• Control of openings and passageways
• Mouth, eyelid and pupil
• Internal muscular rings – food, blood
• Heat production - 85% of body heat
• Glycemic control
11-3
Characteristics of Muscle
• Responsiveness (excitability)
• Respond to chemical signals, stretch and electrical changes across the plasma
membrane
• Conductivity
• Local electrical change triggers a wave of excitation that travels along the
muscle fiber
• Contractility - shortens when stimulated
• Extensibility - capable of being stretched
• Elasticity - returns to its original resting length after being stretched
11-4
Types of muscle
• Skeletal muscle - voluntary striated muscle
• Cardiac muscle - involuntary striated muscle
• Smooth muscle - involuntary nonstriated muscle
11-5
Connective tissue of the muscle muscle
• Each muscle is composed of muscle tissue,
blood vessels, nerve fibers, and connective
tissue
• The three connective tissue sheaths are:
Tendon
Deep fascia
Epimysium
• Epimysium – an overcoat of dense regular
connective tissue that surrounds the entire muscle
• Epimysium surrounds entire muscle
• Perimysium – fibrous connective tissue that
surrounds groups of muscle fibers called fascicles
• Perimysium surrounds fascicles of muscle fibers
• Endomysium – fine sheath of connective tissue
composed of reticular fibers surrounding each
muscle fiber
• Endomysium surrounds muscle fiber
Perimysium
Endomysium
11-6
Classification of muscles according to fascicle orientation
• Muscle belly (contractile part) and Tendon (no contractile part)
11-7
Muscle Attachments
• Direct (fleshy) attachment to bone
• epimysium is continuous with periosteum
• intercostal muscles
• Indirect attachment to bone
• epimysium continues as tendon or aponeurosis that merges into periosteum
as perforating fibers
• biceps brachii or abdominal muscle
• Attachment to dermis
• Stress will tear the tendon before pulling the
tendon loose from either muscle or bone
11-8
Skeletal Muscle
• Skeletal muscle - voluntary, striated muscle attached to one or more bones
• Made up of individual muscle cells = myocytes = muscle fiber
( myofibers)
• Cylindrical, As long as 30 cm
• Multinucleated
• Fusion of myoblasts (type of progenitor cells)
• Composed of myofibrils
• composed of actin and myosin filaments
• Striations - alternating light and dark
transverse bands
• Results from an overlapping of internal contractile proteins
• Voluntary - usually subject to conscious control
11-9
11-10
Muscle Fibers
• Sarcolemma - cell membrane of a muscle fiber
 Sarcolemma has tunnel-like infoldings or transverse (T) tubules that penetrate
the cell - carry electric current to cell interior
 Sarcoplasmic reticulum (SR) - smooth ER
• dilated end-sacs (terminal cisternea) store calcium
• Triad = T tubule and 2 terminal cisternae
11-11
Muscle Fibers
 Sarcoplasm - cytoplasm of a muscle fiber
• Myofibrils - long protein bundles that occupy the main portion of the
sarcoplasm
 Multiple flattened nuclei inside cell membrane
 fusion of multiple myoblasts during development
 unfused satellite cells nearby can multiply to produce a small number of new
myofibers

Myofibrils are bundles of myofilaments
• glycogen for stored energy
• myoglobin for binding oxygen
11-12
Thick Filaments - Myosin
• Made of 200 to 500 myosin molecules
• 2 entwined polypeptides (golf clubs)
• Arranged in a bundle with heads directed outward in a spiral array
around the bundled tails
• central area is a bare zone with no heads
11-13
Thin Filaments - Actin
• Two intertwined strands fibrous (F) actin
• String of globular (G) actin with an active site that can bind to head of
myosin molecule
• Tropomyosin molecules
• each blocking 6 or 7 active sites of G actins
• One small, calcium-binding troponin molecule on each
tropomyosin molecule
11-14
Overlap of Thick and Thin Filaments
11-15
Elastic filaments
• Springy proteins called Titin (connectin)
• Anchor each thick filament to Z disc
• Help stabilize the thick filament
• Center it between the thin filaments
• Prevents overstretching of sarcomere
11-16
Striations Organization of Filaments
Sarcomere
• Sarcomere – the smallest unit of a myofibril
• from one Z disc (Z line) to the next
• Z disc protein called connectin, anchoring elastic and thin
filaments
• Dark A bands (regions) alternating with lighter I bands (regions)
• A band is thick filament region
• H band - lighter, central area contains no thin filaments
• I band is thin filament region
• bisected by Z disc
11-17
Striations and Sarcomeres
11-18
Contractile and Regulatory Proteins
• Myosin and actin are contractile proteins
• Tropomyosin and troponin = regulatory proteins
• Switch that starts and stops shortening of muscle cell
• Contraction activated by release of calcium into sarcoplasm and its binding
to troponin,
• troponin moves tropomyosin off the actin active sites
11-19
Relaxed and Contracted Sarcomeres
• Muscle cells shorten because
their individual sarcomeres
shorten
• pulling Z discs closer together
• pulls on sarcolemma
• Notice neither thick nor thin
filaments change length during
shortening
• Their overlap changes as
sarcomeres shorten
Animation
11-20
Neuromuscular Junctions (Synapse)
• Functional connection between nerve fiber and muscle cell
• Neurotransmitter (acetylcholine/ACh) released from nerve fiber
stimulates muscle cell
• Components of synapse (NMJ)
• synaptic knob is swollen end of
nerve fiber (contains ACh)
• junctional folds region of
sarcolemma with ACh receptors
• synaptic cleft = tiny gap between
nerve and muscle cells
• basal lamina = thin layer of collagen
and glycoprotein over all of muscle
fiber
11-21
Electrically Excitable Cells
• Plasma membrane is polarized or charged
• Resting membrane potential due to Na+ outside of cell and K+ and other
anions inside of cell
• Difference in charge across the membrane - resting membrane potential (-90
mV)
• Stimulation opens ion gates in membrane (both in muscle and nerve
cells)
• Ion gates open (Na+ rushes into cell and K+ rushes out of cell)
• Quick up-and-down voltage shift - action potential
• Spreads over cell surface
11-22
Muscle Contraction and Relaxation
• Four actions involved in this process
• Excitation - nerve action potentials lead to action potentials in muscle fiber
• Excitation-contraction coupling - action potentials on the sarcolemma
activate myofilaments
• Contraction - shortening of muscle fiber
• Relaxation - return to resting length
• Images will be used to demonstrate the steps of each of these
actions
11-23
Excitation (steps 1 and 2)
1.Nerve signal opens voltage-gated calcium channels – Ca2+ ions enter the
synaptic knob
2. Calcium stimulates exocytosis of synaptic vesicles containing Acetylcholine - ACh release into synaptic cleft.
11-24
Excitation (steps 3 and 4)
3. ACh binds to the receptor proteins on the sarcolemma (ligandgated ion chenals)
4. When opens - Na+ and K+ channels resulting in jump in resting
membrane potential (RMP) from -90mV to +75mV forming an
end-plate potential (EPP).
11-25
Excitation (step 5)
5. Voltage change in end-plate region (EPP) opens nearby voltagegated channels producing an action potential
11-26
Excitation-Contraction Coupling
(steps 6 and 7)
6. Action potential spreading over sarcolemma enters T tubules
7. voltage-gated channels open in T tubules causing calcium
gates to open in SR
11-27
Excitation-Contraction Coupling
(steps 8 and 9)
8. Calcium released by SR binds to troponin
9. Troponin-tropomyosin complex changes shape and exposes
active sites on actin
11-28
Contraction (steps 10 and 11)
10. Myosin ATPase in myosin head hydrolyzes an ATP molecule,
activating the head and “cocking” it in an extended position
11. It binds to actin active site forming a cross-bridge
11-29
Contraction (steps 12 and 13)
12. Power stroke - myosin head
releases ADP and phosphate as it
flexes pulling the thin filament past the
thick
13. With the binding of more ATP, the
myosin head extends to attach to a
new active site
 half of the heads are bound to a
thin filament at one time
preventing slippage
 thin and thick filaments do not
become shorter, just slide past
each other (sliding filament
theory)
11-30
Relaxation (steps 14 and 15)
14. Nerve stimulation ceases and acetylcholinesterase removes ACh
from receptors.
15. Stimulation of the muscle cell ceases.
11-31
Relaxation (step 16)
16. Active transport needed to pump
calcium back into SR to bind to
calsequestrin
• ATP is needed for muscle relaxation as
well as muscle contraction
11-32
Relaxation (steps 17 and 18)
17. Loss of calcium from
sarcoplasm moves troponintropomyosin complex over active
sites
• stops the production or
maintenance of tension
18. Muscle fiber returns to its
resting length due to recoil of
series-elastic components and
contraction of antagonistic
muscles
11-33
Rigor Mortis
• Stiffening of the body beginning 3 to 4 hours after death
• Deteriorating sarcoplasmic reticulum releases calcium
• Calcium activates myosin-actin cross-bridging and muscle contracts,
but can not relax.
• Muscle relaxation requires ATP and ATP production is no longer
produced after death
• Fibers remain contracted until myofilaments decay
11-34
Nerve-Muscle Relationships
• Skeletal muscle must be stimulated by a nerve or it will not
contract
• Axons of somatic motor neurons - somatic motor fibers
• terminal branches supply one muscle fiber
• Motor unit
• One motor neuron and all the muscle fibers it innervates
11-35
Motor Units
• A motor neuron and the muscle fibers it innervates
• dispersed throughout the muscle
• when contract together causes weak contraction over wide area
• provides ability to sustain long-term contraction as motor units take turns resting
(postural control)
• Fine control
• small motor units contain as few as
20 muscle fibers per nerve fiber
• eye muscles
• Strength control
• gastrocnemius muscle has 1000
fibers per nerve fiber
11-36
Length-Tension Relationship
• Amount of tension generated depends on length of muscle before it
was stimulated - Length-tension relationship
• Overly contracted (weak contraction results)
• thick filaments too close to Z discs and
can’t slide
• Too stretched (weak contraction
results)
• little overlap of thin and thick does not
allow for very many cross bridges too
form
• Optimum resting length produces greatest
force when muscle contracts
• Muscle tone - is the continuous and
passive partial contraction of the muscles,
during resting state
11-37
Muscle Twitch in Frog
• Twitch - a quick cycle of contraction and relaxation of muscle (lasting
less than 1/10 second)
• Threshold- a single brief stimulus at that voltage producing an action
potential
• Phases of a twitch contraction
• Latent period (2 msec delay)
• only internal tension is generated
• no visible contraction occurs –
elastic components are being
stretched
• Contraction phase
• external tension develops as muscle
shortens
• Relaxation phase
• loss of tension and return to resting
length as calcium returns to SR
11-38
Contraction Strength of Twitches
• Threshold stimuli produces twitches
• Twitches unchanged despite increased voltage
• “Muscle fiber obeys an all-or-none law” contracting to its
maximum or not at all
• not a true statement since twitches vary in strength
• Even for constant voltage twitches vary in strength
•
•
•
•
previous stretch of the muscle
Ca2+ concentration
Temperature
pH and hydration
• Closer stimuli produce stronger twitches
11-39
Recruitment and Stimulus Intensity
• Stimulating the whole nerve with higher and higher voltage
produces stronger contractions
• More motor units are being recruited
• called multiple motor unit summation
• lift a glass of milk versus a whole gallon of milk
11-40
Twitch and Treppe Contractions
• Muscle stimulation at variable frequencies
• low frequency (up to 10 stimuli/sec)
• each stimulus produces an identical twitch response
• moderate frequency (between 10-20 stimuli/sec)
• each twitch has time to recover but develops more tension than the
one before (treppe phenomenon)
• calcium was not completely put back into SR
• heat of tissue increases myosin ATPase efficiency
11-41
Incomplete and Complete Tetanus
Tetanus – twitches fuse into a single non-fluctuating contraction
• Higher frequency stimulation (20-40 stimuli/second) generates gradually more
strength of contraction
• each stimuli arrives before last one recovers
• temporal summation or wave summation
• incomplete tetanus = sustained fluttering contractions
• Maximum frequency stimulation (40-50 stimuli/second)
• muscle has no time to relax at all
• twitches fuse into smooth, prolonged contraction called complete tetanus
• rarely occurs in the body
11-42
Isometric and Isotonic Contractions
• Isometric muscle contraction
• develops tension without changing length
• important in postural muscle function and antagonistic muscle joint stabilization
• Isotonic muscle contraction
• tension while shortening = concentric
• tension while lengthening = eccentric
11-43
ATP Sources
• Short, intense exercise (100 m dash)
• Oxygen need is supplied by myoglobin (iron- and oxygen- binding protein found
in muscle)
• Phosphagen system
• myokinase transfers Pi groups from one ADP to another forming ATP
• creatine kinase transfers Pi groups from creatine phosphate to make ATP
• Result is power enough for 1 minute brisk walk or 6 seconds of sprinting
• Glycogen-lactic acid system takes over
• produces ATP for 30-40 seconds of maximum activity
• playing basketball or running around baseball diamonds
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Short-Term Energy Needs
• LONG-TERM ENERGY NEEDS
• Muscles obtain glucose from blood and stored glycogen
• Aerobic respiration needed for prolonged exercise
• After 40 seconds of exercise, respiratory and cardiovascular systems must deliver
enough oxygen for aerobic respiration
• oxygen consumption rate increases for first 3-4 minutes and then levels
off to a steady state
• Limits are set by depletion of glycogen and blood glucose, loss of
fluid and electrolytes
11-45
Fatigue
• Progressive weakness from use
• ATP synthesis declines as glycogen is consumed
• sodium-potassium pumps fail to maintain membrane potential
and excitability
• lactic acid inhibits enzyme function
• accumulation of extracellular K+ hyperpolarizes the cell
• motor nerve fibers use up their acetylcholine
11-46
Endurance
• Improves the fatigue resistance of the miscule
• Ability to maintain high-intensity exercise for >5 minutes
• determined by maximum oxygen uptake (VO2)
• VO2 max is proportional to body size, peaks at age 20, is larger in trained
athlete and males
• nutrient availability
• carbohydrate loading used by some athletes
• packs glycogen into muscle cells
• adds water at same time (2.7 g water with each gram/glycogen)
• side effects include “heaviness” feeling
11-47
Oxygen Debt
• Heavy breathing after strenuous exercise
• known as excess postexercise oxygen consumption (EPOC)
• typically about 11 liters extra is consumed
• Purposes for extra oxygen
• replace oxygen reserves (myoglobin, blood hemoglobin, in air in the lungs
and dissolved in plasma)
• replenishing the phosphagen system
• reconverting lactic acid to glucose in kidneys and liver
• serving the elevated metabolic rate that occurs as long as the body temperature
remains elevated by exercise
11-48
Slow and Fast Twitch Fibers
• Classification of fibers based on their twitch capability
•
•
•
•
Activity of enzyme ATPase
Ability of transition of action potential
Rapid level of Ca++ release
Proportions genetically determined
• Slow oxidative, slow-twitch fibers
• More mitochondria, myoglobin and capillaries
• Reach blood supply
• Adapted for aerobic respiration and resistant to fatigue
• Soleus and postural muscles of the back (100msec/twitch) are
predominantly this type
11-49
Slow and Fast-Twitch Fibers
• Fast oxidative (glycolytic), fast-twitch fibers
• Use ATP quickly
• Rich in enzymes for phosphagen and glycogen-lactic acid
systems
• Sarcoplasmic reticulum releases calcium quickly so
contractions are quicker (7.5 msec/twitch)
• Extraocular eye muscles, gastrocnemius and biceps brachii
11-50
Muscular Dystrophy
• Hereditary diseases - skeletal muscles degenerate and are replaced
with adipose
• Dystrophin links actin filaments to cell membrane
• leads to torn cell membranes and necrosis
• Fascioscapulohumeral MD
- facial and shoulder muscle only
• Disease of males
• appears as child begins to walk
• rarely live past 20 years of age
11-51
Myasthenia Gravis
• Autoimmune disease - antibodies
attack NMJ and bind ACh receptors in
clusters
• receptors removed
• less and less sensitive to ACh
• drooping eyelids and double vision,
difficulty swallowing, weakness of the
limbs, respiratory failure
• Disease of women between 20 and 40
• Treated with cholinesterase inhibitors,
thymus removal or
immunosuppressive agents
11-52