Download Properties of Muscle Tissue

MUSCULAR TISSUE
Muscle Tissue:
Function:
 Movement
-integration between bones, joints, and muscles
 Body position
-sitting, standing, posture
 Regulation of organ volume
-sphincters regulate flow
-ring-like bands of muscle
-smooth muscle in stomach, bladder
 Movement of substances in the body
-heart pumps blood –cardiac
-vasoconstriction and vasodilation
-release of bile and enzymes
 Producing heat
-contraction of muscle requires energy
-inefficiency of ATP production releases HEAT
-heat produced helps maintain body temperature
Properties of Muscle Tissue
 Electrical Excitability
-respond to stimuli by producing action potentials
-action potentials result from
*auto-rhythmic signals from the tissue itself
*chemical stimuli –neurotransmitters,
hormones, pH
 Contractility
-forcefully contracts when stimulated by an action
potential
*isometric contraction –tension no shortening
*isotonic contraction –muscle shortens and
movement occurs
*tension on the muscle remains constant
during shortening
 Extensibility
-stretches without damage
-smooth muscle stretches the most -full stomach!!!!
-cardiac muscle stretches when heart full of blood
-stretch on skeletal muscle relatively constant
 Elasticity
-returns to original length and shape after
contraction
3 Types:
 Skeletal
-involved in movement and attached to skeleton
-striated
-voluntary -enervated by somatic nerves
-some involuntary control (i.e. posture)
 Cardiac
-heart
-striated
-involuntary action
-auto-rhythmic
-pacemaker controls rhythm
*influenced by hormones and
neurotransmitters
-regulated by the ANS and hormones
 Smooth
-walls of viscera (hollow organs)
-blood vessels and airways
-non-striated
-usually involuntary
-some are auto-rhythmic
-regulated by ANS and hormones
Organization of Skeletal Muscles
Skeletal Muscle:
 Separate organ
 100s-1000s of elongated cells called fibers
 CT surrounds fibers and whole muscles
 Nerves and blood vessels supply it
 CT components
-fascia
*general term for sheet or wrapping of CT
*sheets support and surround organs
*3 types –2 which relate to muscles
1.-superficial fascia
*separates skin from the layers underneath
*lets skin and underlying structures move
independently
*adipose stores triglycerides
-insulates
-reduction from heat loss
-protection from trauma
2.-deep fascia
*made of many layers that run in different
directions – like plywood
*very tough and resistant to force
*lines the body wall and limbs
*holds muscles with similar functions together
*components are “interwoven”
-deep fascia around the muscle blends
into the tendon which mingles with the
periosteum
MICROSCOPIC ANATOMY OF SKELETAL MUSCLE FIBERS
Basics:
 muscle fibers arise from cells called myoblasts
 myoblasts fuse creating individual muscle
fibers
 once fusion occurs muscle cells do not undergo
cell division
 each muscle fiber has 100 + nuclei
 muscle growth results from enlargement of
existing fibers
 satellite cells are myoblasts that persist
-can fuse with damaged muscle fibers
-regenerate functional muscle fibers
 Mature muscle cells lie parallel
 Range from 10 to 100 micrometers in
diameter
 Most are 100 mm long
 Some are as much as 30 cm long
Terminology:

Sarcomere
-functional unit of myofibril
-separated by Z discs
-compartments
 Sarcolemma
-muscle fibers plasma membrane
 Transverse (T) tubules
-thousands of invaginations of the sarcolemma
-extend from the sarcolemma to the interior of the
muscle fiber
-open to the outside of the fiber
-filled with extra-cellular fluid
-muscle action potentials propagate along
sarcolemma through the T tubules
-all parts of the muscle fiber can be affected by the
action potential at the same time
 Sarcoplasm
-cytoplasm of a muscle fiber
 Sarcoplasmic reticulum
-similar to SER
-stores calcium in a relaxed fiber
-release of Ca+ from terminal cisterns triggers
muscle contraction
 Myoglobin
-specialized oxygen binding protein
-found only in muscle cells
-“likes” oxygen better than hemoglobin
 Mitochondria
-arranged in rows
-close to proteins that need ATP for contraction
 Myofibrils
-contractile elements of skeletal muscle
-2 microns in diameter
-extend the entire length of the fiber
-prominent striations
Filaments and the Sarcomere:
 Smaller structures within the myofibrils
 Filaments -2 types
-thin filaments –8 nm diameter (mostly actin)
-thick filaments- 16 nm (myosin)
 Arranged in sarcomere compartments
 The functional unit of the myofibril
 Thick and thin filaments overlap one another
 Characteristic zones are characterized in electron
microscopy:
Z disc
M line
Z disc
I band
H Zone
A band
I band
Sarcomere
Muscle Proteins:
3 types:
1. Contractile proteins- generate force during contraction
 Myosin
-motor protein in all 3 types of muscle
-pushes or pulls –chemical energy (ATP)
becomes mechanical energy
-molecule shaped like 2 golf clubs twisted
together
-myosin tails (handles) point to M line
-tails lie parallel to each other- form filament of
the shaft
-heads project outward in a spiral
-form crossbridges
 Actin
-thin filaments
-anchoring points within the Z discs
-actin filament is a twisted helix
-each actin molecule has a myosin binding site
-myosin head attaches to binding site
-tropomyosin covers the myosin-binding site in
relaxed muscle
-troponin holds tropmyosin in place
2. Regulatory proteins
 Tropomyosin
 Toponin
CONTRACTION AND RELAXATION OF
SKELETAL MUSCLE FIBERS
Sliding filament mechanism
 sarcoplasmic reticulum releases Ca2+
 Ca2+ binds to troponin
 Troponin-tropomyosin complexes move away
from myosin binding sites on actin
 When sites are free, contraction cycle begins
1. Myosin head energized by hydrolysis of ATP
-ADP and phosphate remain attached
2. Energized myosin attaches to myosin-binding site
on actin, releasing the phosphate group.
3. Power stroke
 results from the release of the phosphate
group
 Pocket containing ADP opens
 ADP released from the pocket on the myosin
head
 Myosin head rotates toward the center of the
sarcomere
 Thin filament slides toward the M line
4. Myosin head remains attached to actin at the end
of the power stroke until it binds another ATP
 ATP binds the pocket
 Myosin head detaches from actin
5. Contraction cycle repeats as long as ATP and
calcium ions are available
6. Each power stroke pulls the filaments toward the
M line
Excitation/Contraction Coupling
1. An increase in calcium concentration starts
contractions.
2. A decrease in calcium concentration stops it.
3. Calcium concentration of the cytosol of a relaxed
muscle is low
4. Calcium is stored in the SR
5. Action Potential travels through the T tubules to
the SR causing the release of calcium through the
Ca2+ channels.
6. Calcium floods the region around the thick and
thin filaments, increases 10X or more.
7. Calcium binds with the troponin causing the
formation of the troponin-tropomyosin complex
which allows the cycle to begin.
8. SR has Ca2+ active transport pumps to transport
calcium back into the SR.
Rigor Mortis: Calcium leaks out of the SR through
“leaky” membranes. Initiates the excitation,
contraction coupling. No ATP to detach heads from the
actin. Muscles contract and stay that way until
proteolytic enzymes break crossbridges.
MUSCLE METABOLISM
Requirements for ATP:
 Contraction cycle
 Pumping Ca2+ into SR (= relaxation)
 Basic cell metabolism
Sources of ATP
 Available ATP lasts only seconds
 Three basic sources of ATP production
1. creatine phosphate
-unique to muscle fibers
-first source of ATP
-synthesized from ATP when muscle is resting
-creatine kinase (CK) transfers a phosphate
from ATP to creatine resulting in creatine
phosphate and ADP.
-creatine phosphate is 3-6 times as plentiful as
ATP in sarcoplasm
-when ATP requirements go up, CK transfers
the phosphate from creatine phosphate back
to the ATP.
-provides enough ATP for 15 sec. strenuous
exercise
-supplementation of creatine to increase
atheletic performance is debatable.
-While it may increase muscle mass over time, it
may also turn off the body’s own synthesis of it.
When available ATP is gone, more will be
generated from glucose metabolism.
 Glucose passes easily from the blood to
the muscle
 Glycogen is stored in muscle and can be
broken down into glucose
 Glucose can be processed 2 ways
2. anaerobic cellular respiration
 glucose is broken down to pyruvate during
glycolysis (cytosol)
 if no oxygen is available pyruvate will go to
lactic acid
 a net 2 ATP are generated for one glucose
 30-40 seconds of maximum muscle activity
3. aerobic cellular respiration
 oxygen is required
 occurs in the mitochondria
 slower than glycolysis
 results in CO2 , H2O, and about 36 ATP
 provides enough ATP for prolonged muscle
activity as long as nutrients and oxygen are
available
-oxygen comes from the blood
-oxygen is stored in myoglobin (in the
muscle)
Muscle Fatigue
 inability of a muscle to contract forcefully after
prolonged contraction
 results from changes within muscle fibers
 central fatigue refers to tiredness that precedes
muscle fatigue
 skeletal muscle does not fatigue uniformly
 contributing factors to muscle fatigue
1. inadequate release of Ca2+ from the SR
2. decline in Ca2+ in the sarcoplasm
3. depletion of creatine phosphate
4. ATP levels are NOT necessarily lower
5. insufficient oxygen
6. depletion of glycogen and other nutrients
7. build up of ADP and lactic acid
8. failure of motor neuron action potentials to
release enough ACh
Oxygen Consumption after Exercise
1. oxygen delivery to the muscles increases during
strenuous exercise
 increased blood flow
 increased breathing effort
2. Increased breathing continues after muscle
contraction has ceased
3. Oxygen consumption will remain above resting
levels
4. Time frame varies
5. oxygen debt refers to the oxygen consumption
taken in above resting after exercise
-convert lactic acid back to glycogen stores in the
liver
-resynthesize creatine phosphate and ATP
-replace oxygen removed from myoglobin
6. Recovery oxygen uptake is a more appropriate
term.
- most of the lactic acid is not
resynthesized into glycogen (comes from
dietary CHO)
- most of the lactic acid gets converted
into pyruvate to fuel aerobic respiration
- increased body temperature that results
from exercise also increase reaction
rates and thus raises the oxygen
requirement.
- Heart and muscles still are working
above the resting rate
- Tissue repair rate increases
TWITCH CONTRACTION
 Brief contraction of all the muscle fibers in a
motor unit in response to a single action
potential
 Myogram =Record of a muscle contraction
 Parts of a contraction
-latent period –between stimulus and the
beginning of the
contraction
-contraction period – 10-100 msec
-relaxation period –Ca2+ actively transported
back into SR
FREQUENCY OF SUMMATION
When the second of 2 stimuli is applied after the
refractory period, the muscle will respond to both stimuli.
 wave summation
-stimuli arrive at different times causing larger
contractions
 fused tetanus
-stimulation at a rate higher than 80-100 stimuli
per second
-sustained contraction
-addition of Ca2+ to sarcoplasm that still has Ca2+
results in a build up of Ca2+ .
-peak tension is 5-10X greater than a single
twitch
 unfused tetanus
-stimulation at a rate of 20-30 /sec
-muscle can only partially relax between stimuli
-sustained but wavering contraction
-smaller build up of Ca2+
Motor Unit Recruitment:
 motor units have thresholds
 lower thresholds are recruited (stimulated) first
 stronger (higher threshold) motor units added
later
 the more effort the task requires-the more motor
units are recruited
 thus some are active while some are relaxing
 precise tasks have motor units with few fibers
TYPES OF SKELETAL MUSCLE FIBERS
Muscle fibers are characterized by the following
characteristics:
1. myoglobin content (red vs white)
2. velocity of contraction and relaxation (slow vs fast)
3. metabolic reactions to generate ATP
Slow Oxidative (SO) Fibers
 smallest in diameter
 least powerful
 red –large amounts of myoglobin











many blood capillaries
large mitochondria
aerobic cell respiration used to generate ATP
called oxidative fibers
slow –ATPase in the myosin head hydrolyzes
ATP slowly compared to “fast” fibers
low contraction velocity
twitch contractions last 100-00 msec
longer to reach peak tension
resistant to fatigue
capable of prolonged, sustained contractions
for many hours
posture and endurance activities
Fast Oxidative –Glycolytic (FOG) Fibers
 intermediate in diameter
 large amounts of myoglobin –red
 many capillaries
 aerobic cellular respiration generates ATP
 moderately high resistance to fatigue
 intracellular glycogen level is high
 can use anaerobic glycolysis to generate ATP
 “fast” because ATPase in myosin heads
hydrolyzes ATP 3-5X faster than “slow” fibers
 velocity is faster
 reach peak tension more quickly
 briefer in duration (less than 100 msec)
 walking, sprinting
Fast Glycolytic (FG) Fibers
 largest in diameter
 highest number of fibrils
 can generate the most powerful contractions
 low myoglobin -white
 few capillaries
 large amounts of glycogen
 generate ATP mostly from glycolysis
 hydrolyze ATP rapidly
 contract strongly and rapidly
 used for intense anaerobic movement of short
duration –weight lifting or throwing a ball
 fatigue quickly
 Strength training programs increase the size,
strength, and glycogen content of FG fibers
 Muscle enlargement is due to hypertrophy of
FG fibers and synthesis of muscle protein
Skeletal muscle is a mixture of all 3 types
 SO fibers make up about ½
 Postural muscles have more SO fibers
 Shoulders and arms –lift things have FG fibers
 Legs SO and FOG fibers
 A motor unit has only one type of fiber
 Types of muscles are recruited in an order of need
-SO are weak and first
-FOB is next
-FG is last when maximal force is needed
CARDIAC MUSCLE:
Physical Characteristics:
 Similar arrangement of actin and myosin to
skeletal muscle
 Have the same identifiable bands (I, H, A, etc.)
 Intercalated discs:
-form connections between ends of cells
-thickening of the sarcolemma
-discs contain
1. desmosomes
-hold fibers together
2. gap junctions
-conduct muscle action potentials
 cardiac muscle contractions
-10-15 times longer than skeletal muscle
-results from prolonged Ca2+ delivery
-Ca2+ enters sarcoplasm from both:
1. SR
2. extracellular fluid (ECF)
 Contracts in response to auto-rhythmic fiber
stimulation
 Resting cardiac muscle
-contracts and relaxes 75 times per minute (about)
-continuous rhythmic activity
-requires constant supply of oxygen
-mitochondria are larger and more numerous
-depends heavily on aerobic respiration
-can use lactic acid made in skeletal muscle as an
energy source for aerobic respiration during
exercise
SMOOTH MUSCLE:
2 Types
1. Visceral (Single Unit) Smooth Muscle Tissue
 Most common type
 Walls of arteries and hollow viscera
 Wrap around sheets
 Auto-rhythmic
 Fibers connect by gap junctions
 Process of fiber stimulation
1. One fiber stimulated by:
-neurotransmitter
-hormone
-auto-rhythmic signal
-pH
2. Muscle action potential spreads to
neighboring fibers
3. Contract in unison as a single unit
4. Stimulation of one visceral muscle
fiber causes contraction of many adjacent
fibers.
2. Multiunit Smooth Muscle Tissue
 Walls of large arteries, airways, arrector pili
muscles, muscles of iris and ciliary body (lens
adjustment) of the eye
 Individual fibers
 Each has own motor neuron terminals
 Few gap junctions between neighboring fibers
 Stimulation of one multiunit fiber causes
contraction of that fiber only
Microscopic Anatomy of Smooth Muscle:
Basic Characteristics:
 Small –30-200 microns long
 Thick at the middle with tapered ends
 Single, oval, centrally located nucleus
 Contain thick and thin filaments in 1:10 –15 ratios
 Not arranged in an orderly fashion
 Also contain intermediate filaments
 Filaments have no orderly overlap –no striations
 Lack transverse tubules
 Scanty SR for Ca2+ storage
Smooth Muscle Contraction differences:
 Intermediate fibers attach to dense bodies
(functionally similar to Z discs)
 Dense bodies attached to sarcolemma or dispersed
through sarcoplasm
 Bundles of intermediate fibers stretch from one
dense body to another
 Sliding filament mechanism generates tension that
is transmitted to intermediate filaments
 Intermediate filaments pull on dense bodies
attached to sarcolemma
 Muscle fiber shortens lengthwise
 Produces a bubble-like expansion of sarcolemma
 Fiber twists in a helix as it contracts, relaxes by
rotating in the opposite direction
Physiology of Smooth Muscle:
Important differences between smooth and skeletal:
 Contraction starts more slowly and lasts longer
 Smooth can both shorten and stretch to a greater
extent
Process of Contraction:
 Increase in Ca2+ in cytosol initiates contraction
 Ca2+ flows into cytosol from SR (smaller amount)
and extracellular fluid
 Since there are no transverse tubules –takes longer
 Ca2+ initiate contractile process at the filaments
Regulation of Contraction and Relaxation:
 Several mechanisms involved
 One involves calmodulin
-calmodulin binds Ca (like troponin in striated
muscle)
-calmodulin activates myosin light chain kinase
-uses ATP to phosphorylate the myosin head
-once phosphate is attached to the head it can bind
actin
-contraction occurs
- myosin light chain kinase acts slowly –accounts
for the slowness of contraction.
Smooth Muscle Tone:
 state of continued partial contraction
 results from slowness of the contraction
process
 important for maintaining steady pressure on
intestinal contents and blood
Stress-relaxation response:
 smooth muscle can stretch and still contract
 when stretched smooth muscle fibers initially
contract
 within a minute the tension will decrease
 allows smooth muscle to have great changes in
length and still contract
 thus even though the muscles are stretched the
pressure on the contents within changes very
little
 when the organ empties the muscle will
rebound with firmness
Regeneration of Muscle Tissue:
Skeletal muscle:
 satellite cells fuse with muscle fibers to assist
in growth and repair
 additional cells will migrate from red bone
marrow to muscle to assist in repair due to
muscle injury or disease
 limited power of regeneration
 fibrosis (replacement of muscle tissue with
fibrous scar tissue) will occur with significant
muscle damage
Cardiac Muscle
 cardiac fibers are not repaired or replaced
 does not have cells comparable to satellite cells
 healing occurs by fibrosis
 athletes have enlarged hearts due to
hypertrophy of the fibers due to an increased
workload
Smooth Muscle
 has greater powers of regeneration
 uterine muscle fibers retain their capacity for
cell division (some others do as well)
 pericytes –stem cells associated with
capillaries can form new smooth muscle fibers
 can also proliferate in atherosclerosis and
result in a pathological condition
Clinical correlations:
1. Myasthenia gravis:
 autoimmune disease
 neuromuscular junction
 antibodies block ACh receptors
 muscles become increasingly weak
 fatigue more easily
 may cease to function
 treat with anticholinesterase drugs
2. Muscular Dystrophy
 group of genetic disorders causing progressive
degeneration of muscular tissue
 Duchene DMD most common
 Sex linked
 Gene for dystrophin is mutated
 Sarcolemma tears easily during contraction
 Damaged muscle fibers rupture and die
3. Abnormal contractions of Skeletal Muscle
 Spasm
-sudden involuntary contraction of a single muscle
in a large group of muscles
 Cramp
-painful spasmodic contraction
 Tic
-spasmodic twitching made involuntarily by muscles
that are ordinarily under voluntary control
-twitching eyelid
 Tremor
-rhythmic, involuntary, purposeless contraction
-produces quivering or shaking movement
 Fasciculation
-involuntary brief twitch of an entire motor unit that
is visible under the skin
-MS or Lou Gehrig’s disease
 fibrillation
-spontaneous contraction of a single muscle fiber not
visible under the skin
-can be recorded by electron myography
-may signal destruction of motor neurons