ΜΥΙΚΟ ΣΥΣΤΗΜΑ
The formation and nuclei of a skeletal muscle
(a) The formation of muscle fibers by the fusion of myoblasts. Notice the multiple
nuclei. (b) A micrograph and diagrammatic view of one muscle fiber (LM X 612).
(F. Martini, A&P, 2004)
Levels of functional organization
in a skeletal muscle fiber
(F. Martini, A&P, 2004)
Photomicrograph of the capillary network surrounding skeletal muscle fibers
The arterial supply was injected with dark red gelatin to demonstrate the capillary bed.
The long muscle fibers are stained orange (30X).
(E. Marieb, HA&P, 2004)
Connective tissue sheaths of skeletal muscle. (a) Each muscle fiber is wrapped with a
delicate connective tissue sheath, the endomysium. Bundles, or fascicles, of muscle fibers
are bounded by a collagenic sheath called a perimysium. The entire muscle is strengthened
and wrapped by a coarse epimysium sheath. (b) Photomicrograph of a cross section of part
of a skeletal muscle (90X) (E. Marieb, HA&P, 2004).
Microscopic anatomy of
skeletal muscle fiber
(a) Photomicrograph of portions
of two isolated muscle fibers
(700X). Notice the obvious
striations (alternating light
and dark bands).
(b) Diagram of part of a muscle
fiber showing the myofibrils.
One myofibril extends from
the cut end of the fiber.
(c) A small portion of one
myofibril enlarged to show
the myofilaments responsible
for the banding pattern. Each
sarcomere extends from one
Z disc to the next.
(d) Enlargement of one
sarcomere (sectioned
lengthwise). Notice the
myosin heads on the thick
filaments.
(E. Marieb, HA&P, 2004)
Thick and thin filaments
(a) The structure of the thin
filament, showing the
attachment at the Z line.
(b) The organization of G
actin subunits in an F actin
strand, and the position of
the troponin-tropomyosin
complex.
(c) The thick filaments,
showing the orientation of
the myosin molecules.
(d) The myosin molecule.
(F. Martini, A&P, 2004)
The components of thin filaments
G-actin is assembled into singlestranded F-actin. F-actin strands are
twisted together to form the backbone
of a thin filament. Double helixes of
tropomyosin lie on the F-actin strands.
Each tropomyosin has a molecule of
troponin positioned at one end (D.
Moffett, HP, 1993).
The components of thick filaments
Single myosin units have a double
globular head at the end of a long rod.
The long rod is composed of two
heavy polypeptide chains; the ends of
the heavy chains, together with the
four light polypeptide chains of
myosin, form the heads. Myosin units
self-assembled into thick filaments.
The myosin heads project from the
thick filament at the proper angles to
interact with the hexagonal array of
thin filaments (D. Moffett, HP, 1993).
Composition of thick
and thin filaments
(a) An individual myosin
molecule has a rod-like tail,
from which two heads
“protrude”.
(b) Each thick filament consists
of many myosin molecules,
whose heads protrude at
opposite ends of the
filament.
(c) A thin filament contains a
strand of actin twisted into
a helix. Each strand is made
up of actin subunits.
Tropomyosin molecules coil
around the actin filaments,
helping to reinforce them. A
troponin complex is
attached to each
tropomyosin molecule.
(E. Marieb, HA&P, 2004)
Relationship of the sarcoplasmic reticulum (SR) and T tubules to myofibrils of skeletal
muscle. The tubules of the SR encircle each myofiber like a “holey” sleeve. The tubules fuse
to form communicating channels at the level of the H zone and abutting the A-I junctions,
where the saclike elements called terminal cisternae are formed. The T tubules, inward
invaginations of the sarcolemma, run deep into the cell between the terminal cisternae. Sites
of close contact of these three elements (terminal cisterna, T tubule, and terminal cisterna)
are called triads (E. Marieb, HA&P, 2004).
Levels of functional organization
in a skeletal muscle fiber
(F. Martini, A&P, 2004)
An overview of the process of
skeletal muscle contraction
(F. Martini, A&P, 2004)
Motor units
A motor unit is a group of skeletal
muscle fibers controlled by a
single motor neuron.
(A) A schematic diagram of three
different motor units in a single
muscle, consisting of 3 to 5
individual muscle fibers.
(B) The total tension in a muscle
is given by summating the
activity of all the active motor
units.
(D. Moffett, HP, 1993)
A motor unit consists of a motor neuron and all the muscle fibers it innervates. (a)
Schematic view of portions of two motor units. The cell bodies of the motor neurons reside in the
spinal cord, and their axons extend to the muscle. In the muscle, each axon divides into a number of
axonal terminals that are distributed to muscle fibers scattered throughout the muscle. (b)
Photomicrograph of a portion of a motor unit (80X). Notice the diverging axonal terminals and the
neuromuscular junctions formed by the terminals and muscle fibers (E. Marieb, HA&P, 2004).
The neuromuscular
junction
(E. Marieb, HA&P, 2004)
(b) ACh in
vesicles is
released
into the
synaptic
cleft when
the action
potential
reaches the
axonal
terminal.
(c) Ach
attaches to
receptors,
opening ion
channels and
initiating
depolarization
of the
sarcolemma.
Summary of events in the generation and propagation of an action potential in
a skeletal muscle fiber (E. Marieb, HA&P, 2004).
Excitationcontraction
(E-C)
coupling
(E. Marieb,
HA&P, 2004)
Role of ionic calcium in the contraction mechanism. Views (a-d) are cross-sectional views of the thin
(actin) filament. (a) At low intracellular Ca2+ concentration, tropomyosin blocks the binding sites on
actin, preventing attachment of myosin cross bridges and enforcing the relaxed muscle state. (b) At
higher intracellular Ca2+ concentrations, additional calcium binds to (TnC) of troponin. (c) Calciumactivated troponin undergoes a conformational change that move the tropomyosin away from actin’s
binding sites. (d) The displacement allows the myosin heads to bind and cycle, and contraction
(sliding of the thin filaments by the myosin cross bridges) begins (E. Marieb, HA&P, 2004).
Sequence of events
involved in the sliding
of the thin filaments
during contraction
(E. Marieb, HA&P, 2004)
Steps involved in of skeletal muscle contraction
(F. Martini, A&P, 2004)
Steps involved in of skeletal muscle contraction
(F. Martini, A&P, 2004)
The time relationships of the muscle action potential, rise and fall of
cytoplasmic Ca2+, and force development by sarcomeres during a twitch
(D. Moffett, HP, 1993)
The muscle twitch
(a) Myogram of an isometric twitch
contraction, showing its three
phases: the latent period, the
period of contraction, and the
period of relaxation.
(b) Comparison of the twitch
responses of extraocular,
gastrocnemius, and soleus
muscles.
(E. Marieb, HA&P, 2004)
Wave summation and tetanus in a whole muscle
(1) A single stimulus is delivered, and the muscle contracts and relaxes (twitch
contraction).
(2) Stimuli are delivered more frequently, so that the muscle does not have adequate
time to relax completely, and contraction force increases (wave summation).
(3) More complete twitch fusion (unfused or incomplete tetanus) occurs as stimuli are
delivered more rapidly.
(4) Fused or complete tetanus, a smooth, continuous contraction without any evidence
of relaxation occurs (E. Marieb, HA&P, 2004).
The effect of sarcomere length on tension
(a) Maximum tension is produced when the zone of overlap is large but the thin filaments do not
extend across the sarcomere’s center.
(b) If the sarcomeres are stretched too far, the zone of overlap is reduced or disappears, and
cross-bridge interactions are reduced or cannot occur.
(c) At short resting lengths, thin filaments extending across the center of the sarcomere interfere
with the normal orientation of thick and thin filaments, reducing tension production.
(d) When the thick filament contact the Z lines, the sarcomere cannot shorten -the myosin heads
cannot pivot- and tension cannot be produced. The light purple are represents the normal
range of resting sarcomere length. (F. Martini, A&P, 2004)
Isotonic (concentric) contraction
On stimulation, the muscle develops enough tension (force) to lift the load
(weight). Once the resistance is overcome, the tension remains constant for
the rest of the contraction and the muscle shortens.
(E. Marieb, HA&P, 2004)
Isometric contraction
The muscle is attached to a weight that exceeds the muscle’s peak tensiondeveloping capabilities. When stimulated, the tension increases to the
muscle’s peak tension-developing capability, but the muscle does not
shorten.
(E. Marieb, HA&P, 2004)
Factors influencing force, velocity, and duration of skeletal muscle
contraction
(E. Marieb, HA&P, 2004)
Muscle metabolism
(F. Martini, A&P, 2004)
Methods of regenerating ATP during muscle activity
The fastest mechanism is direc phosphorylation (a); the slowest is the
aerobic mechanism (c).
(E. Marieb, HA&P, 2004)
Energy system used
during sports at peak
activity level
Skeletal muscles rely
largely on glycolysis
(anaerobic respiration)
for ATP synthesis. The
initial activity burst is
energized by ATP and
CP reserves. Muscles
operating at peak levels
fatigue rapidly as lactic
acid accumulates.
(E. Marieb, HA&P, 2004)
Fast versus slow fibers
(a) The slender, slow fiber (R for red) has more mitochondria (M) and a more
extensive capillary supply (cap) than does the fast fiber (W for white). (b) Notice
the difference in the size of slow fibers, above, and of fast fibers, below.
(F. Martini, A&P, 2004)
The effect of changes in innervation on the contractile properties of muscle
The nerve running to a muscle composed predominantly of slow-twitch fibers is surgically
switched with a nerve innervating a muscle composed predominantly of fast twitch fibers. The
original twitch properties of the muscle are shown in A. The effects of crossed-innervation on
twitch properties re shown in B. The influence is exerted both by chemical messengers that travel
between the muscle and the nervous system via axoplasmic flow and by the frequency and pattern
of recruitment of the muscles by the motor neurons (D. Moffett, H&P, 1993).