OF TYPES MUSCLE TISSUE
Skeletal muscle - associated with the skeleton
striated (exhibits alternating dark and light bands)
voluntary (innervated by the somatic nervous system and
subject to voluntary control).
Cardiac muscle - occurs in the wall of the heart
striated and involuntary (innervated by the autonomic
nervous system - contractions not under conscious control)
Smooth muscle - found in the walls of hollow
organs and tubes (blood vessels, stomach,
and I
ntestines)
nonstriated and involuntary
Types of Vertebrate Muscle Tissue
STRUCTURE OF SKELETAL MUSCLE:
A single skeletal muscle cell (muscle fiber) is formed by
fusion of undifferentiated, mononucleated cells (myoblasts),
into a single, cylindrical, multinucleated fiber
Muscle fibers do not divide
Each fiber is covered by a plasma membrane (sarcolemma)
Muscle fibers are arranged in bundles called fasciculi, to form
the muscle
Skeletal muscle fibers contain cylindrical myofibrils, each
consisting of alternating light and dark bands (I-bands and Abands), respectively
The bands of all the myofibrils lined up parallel to each other,
accounts for the striated appearance of a skeletal muscle fiber
ORGANIZATION OF
SKELETAL MUSCLE
ORGANIZATION OF THE MYOFIBRIL
Consists of - thick filaments, composed of the protein myosin
thin filaments, made up primarily of the protein actin and smaller
amounts of the proteins tropomyosin and troponin
Thick and thin filaments are specifically arranged in contractile
units called sarcomeres, which are the basic functional units of
striated muscle fibers.
Z discs - narrow zones of dense material which separate
sarcomeres.
A band - dark area consisting of a stacked set of thick filaments
along with the portions of the thin filaments that overlap on both
ends of the thick filaments.
I band - light area consisting of the portion of thin filaments that
do not project into the A band.
THICK FILAMENTS ARE
COMPOSED OF MYOSIN
Each myosin molecule consists of 2 identical golf clubshaped molecules with their tails intertwined and their
globular heads projecting at one end:
ARRANGEMENT OF MYOSIN
MOLECULES INTO THICK FILAMENTS
The myosin heads project outward on the surface of the shaft of the
thick filament forming cross bridges
During muscle contraction these cross bridges make contact with the
thin filaments and exert force on them
The cross bridges are the force-generating sites in muscle cells
THIN FILAMENTS
Two chains of spherical-shaped actin molecules
are twisted together forming the thin filament
Troponin molecules, which consist of 3 small
spherical subunits, and tropomyosin, a rodshaped molecule whose length is about equal to
that of 7 actin molecules, lie along the two strands
of the actin double helix
Each actin molecule has a binding site for
attachment with a myosin cross bridge, but
binding of tropomyosin prevents this
MOLECULAR BASIS OF
MUSCLE CONTRACTION
Binding of actin and myosin molecules at
the cross bridges results in energy (ATP)dependent contraction of the muscle fiber
Although actin and myosin are termed
contractile proteins neither molecule
"contracts" The basis of contraction is
sarcomere shortening due to sliding of the
thin filaments inward between the thick
filaments (sliding-filament mechanism)
SCHEMATIC SHOWING ARRANGEMENT OF THIN
AND THICK FILAMENTS DURING CONTRACTION
THE SLIDING FILAMENT
MECHANISM
The steps occurring between binding of a cross bridge to a thin
filament and the time it again binds to a thin filament to repeat the
process is called a cross-bridge cycle Each cycle consists of 4
steps:
attachment of the cross bridge to a thin filament
movement of the cross bridge, producing movement of the thin filament
detachment of the cross bridge from the thin filament
movement of the cross bridge to a position where it can reattach to a
thin filament and repeat the cycle.
The cycle is dependent on hydrolysis of ATP
A molecule of myosin can bind a molecule of ATP and hydrolysis
of ATP by the myosin
ATPase generates an energized form of myosin
Formation of the energized state involves rotation of the globular
head about its flexible attachment site to the tail
The Cross-bridge Cycle
CALCIUM AND MUSCLE
CONTRACTION
Calcium ions trigger muscle
contraction by binding to troponin
Binding of calcium changes the shape
of troponin causing tropomyosin to
slide away from its blocking position
on actin molecules
This allows actin and myosin to bind
(cross-bridge formation), resulting in
muscle contraction
WHERE DOES CALCIUM
COME FROM ?
The sarcoplasmic reticulum (SR) is a modified
endoplasmic reticulum, consisting of a network of
interconnected tubules surrounding each myofibril
Calcium (Ca2+) is stored in the SR
Generation of an action potential at the
neuromuscular junction triggers release calcium
from of the SR into the cytoplasm where it can then bind
to troponin
The sequence of events that link an action potential to
mechanical contraction is called excitationcontraction coupling
STEPS IN EXCITATION CONTRACTION
COUPLING IN SKELETAL MUSCLE
An action potential in the muscle cell membrane initiates depolarization of
the transverse tubules (T tubules), invaginations of the plasma membrane
into the muscle fiber occurring at each junction of an A band and I band
Depolarization of the T tubules opens calcium channels in the SR, releasing
calcium
Calcium binds to part of the troponin complex causing a conformational
change in troponin such that tropomyosin moves, allowing cross-bridge
cycling to begin
Actin and myosin bind, the heads of the cross bridges pivot (using energy of
ATP hydrolysis), and the thin and thick filaments slide over each other
Subsequently. the cross-bridges break, a new molecule of ATP binds to the
myosin head so that a new cycle can begin
Cross-bridge cycling continues as long as calcium is bound to troponin
Sequestration of calcium back into the SR causes tropomyosin to resume its
blocking position and the thin filaments slide back to their resting position as
relaxation occurs
SEQUENCE OF EVENTS ASSOCIATED WITH
EXCITATION CONTRACTION COUPLING
OPTIMIZING FORCE
PRODUCTION FROM MUSCLE
Skeletal muscle must be activated by the nervous
system before it can begin its contraction
A single muscle action potential propagates along
the length of each muscle fiber innervated by that
axon terminal, leading to a single, brief contraction
of the muscle (twitch) Normally muscle fibers
within muscles function cooperatively to produce
gradations in whole-muscle tension by:
varying the frequency of stimulation delivered to a
muscle fiber
varying the number of muscle fibers contracting
EFFECT OF STIMULATION FREQUENCY
A tetanus (sustained contraction lacking partial
relaxation between stimuli) occurs at high
frequency due to elevated calcium concentration
in the region of the contractile proteins
MOTOR UNITS AND MUSCLE
CONTRACTION
A single motor neuron and all the fibers it innervates
constitutes a motor unit
The total force produced by a muscle is dependent on the
number of motor units active at any given time
ANABOLIC STEROIDS
AND MUSCLE
Anabolic steroids (derivatives of testosterone,
a male sex hormone) in large amounts
increase muscle mass through hypertrophy
(enlargement of tissue without cell division)
Adverse effects include:
liver dysfunction
decreased sperm production in males,
sterility in females
increased risk of heart disease
aggresive behavior
SKELETAL MUSCLE
METABOLISM
ATP is required at 3 steps in the cross-bridge cycle:
Splitting of ATP by myosin ATPase provides energy for the power stroke (pulling of
thin filament to which it is attached, inward)
Binding of a fresh ATP molecule to myosin allows detachment of the cross bridge
from the actin filament at the end of a power stroke, as part of the cycle (this ATP is
subsequently split)
Active transport of calcium back into the SR during relaxation
Only limited amounts of ATP are available in muscle
Creatine phosphate, which like ATP contains a high energy phosphate group, is an
energy store in muscle
The high energy phosphate group of creatine phosphate is donated to ADP forming
ATP
Depletion of creatine phosphate levels leads to breakdown of stored glycogen to
glucose and glycolysis
Under aerobic conditions, oxidative phosphorylation occurs in muscle mitochondria
CARDIAC MUSCLE
Cardiac muscle cells have a single, centrally located nucleus,
and are joined together in a branching network
Cells (fibers) are separated at the ends by intercalated discs
(thickened portions of the sarcolemma), which contain gap
junctions (facilitate conduction of muscle action potentials)
Cardiac muscle contracts and relaxes rhythmically (approx. 75
times/min), requiring oxygen
Contains numerous mitochondria
Unlike skeletal muscle, cardiac muscle contracts without
external nerve stimulation, as it contains an intrinsic
pacemaker; external nerve stimulation modulates the rate of
discharge by the pacemaker
SMOOTH MUSCLE
Fibers are smaller than those of skeletal muscle and are spindle
shaped
Contain intermediate filaments which stretch between
structures called dense bodies, formed of the same proteins
found in Z lines
Smooth muscle filaments are not arranged in the sarcomere
pattern; sliding of the thin filaments past the thick filaments
during contraction causes the filament lattice to reduce in
length and expand laterally
Duration of contraction and relaxation of smooth muscle is
longer than in skeletal muscle This feature is important for
maintaining long-term, steady pressures in the GI tract, walls
of arterioles, and the urinary bladder