Chapter 9 Joints
Organization of Muscle TissueAn aponeurosis is essentially a
thick fascia that connects two muscle bellies. This epicranial
aponeurosis connects the muscle bellies of the occipitalis and the
frontalis to form one muscle: The occipitofrontalis
Epicranial aponeurosisFrontal belly of the occipitofrontalis
m.1An aponeurosis is essentially a thick fascia that connects two
muscle bellies. This epicranial aponeurosis connects the muscle
bellies of the occipitalis and the frontalis to form one muscle:
The occipitofrontalisVeins, arteries, and nerves are located in the
deep fascia between muscles of the thigh.Organization of Muscle
Tissue
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Beneath the connective tissue endomysium is found the plasma
membrane (called the sarcolemma) of an individual skeletal muscle
fiberThe cytoplasm (sarcoplasm) of skeletal muscle fibers is
chocked full of contractile proteins arranged in myofibrilsThe
Skeletal Muscle Fiber3
You should learn the names of the internal structures of the
muscle fiberSarcolemmaSarcoplasmMyofibrilT-tubulesTriad (with
terminal cisternsSarcoplasmic reticulumSarcomereThe Skeletal Muscle
Fiber4The Skeletal Muscle FiberIncreasing the level of
magnification, the myofibrils are seen to be composed of
filamentsThick filamentsThing filaments
Sarcomeres are multi-protein complexes composed of three
different filament systems. The thick filament system is composed
of myosin protein which is connected from the M-line to the Z-disc
by titin. It also contains myosin-binding protein C which binds at
one end to the thick filament and the other to actin.The thin
filaments are assembled by actin monomers bound to nebulin, which
also involves tropomyosin (a dimer which coils itself around the
F-actin core of the thin filament) and troponin.Nebulin and titin
give stability and structure to the sarcomere.5
A scanning electron micrograph of a sarcomereThe basic
functional unit of skeletal muscle fibers is the sarcomere: An
arrangement of thick and thin filaments sandwiched between two Z
discsThe Skeletal Muscle Fiber Z-line (from the German
"Zwischenscheibe). Each myofibril is made up of thin filament
proteins and thick filament proteins, arranged (configured) in
sarcomeres.6
The Z line is really a Z disc when considered in 3 dimensions. A
sarcomere extends from Z disc to Z disc.Muscle contraction occurs
in the sarcomeresThe Skeletal Muscle FiberEach myofibril is made up
of thin filament proteins, and thick filament proteins, arranged
(configured) in sarcomeres7Myofibrils are built from three groups
of proteinsContractile proteins generate force during
contractionRegulatory proteins help switch the contraction process
on and offStructural proteins keep the thick and thin filaments in
proper alignment and link the myofibrils to the sarcolemma and
extracellular matrixMuscle Proteins8The thin filaments are
comprised mostly of the structural protein actin, and the thick
filaments are comprised mostly of the structural protein
myosinHowever, in both types of filaments, there are also other
structural and regulatory proteins
Muscle Proteins9In the thin filaments actin proteins are strung
together like a bead of pearls
In the thick filaments myosin proteins look like golf clubs
bound together
Muscle Proteins10
In this first graphic, the myosin binding sites on the actin
proteins are readily visible.
The regulatory proteins troponin and tropomyosin have been added
to the bottom graphic: The myosin binding sites have been
covered
Muscle Proteins11In this graphic the troponin-tropomyosin
complex has slid down into the gutters of the actin molecule
unblocking the myosin binding site
The troponin-tropomyosin complex can slide back and forth
depending on the presence of Ca2+
Myosin binding site exposedMuscle Proteins12Ca2+ binds to
troponin which changes the shape of the troponin-tropomyosin
complex and uncovers the myosin binding sites on actin
Muscle Proteins13Besides contractile and regulatory proteins,
muscle contains about a dozen structural proteins which contribute
to the alignment, stability, elasticity, and extensibility of
myofibrilsTitan is the third most plentiful protein in muscle,
after actin and myosin - it extends from the Z disc and accounts
for much of the elasticity of myofibrilsDystrophin is discussed
later as it relates to the disease of muscular dystrophyMuscle
ProteinsTitin is the largest known protein, consisting of 34,350
amino acids. Titin, also known as connectin is a protein that is
important in the contraction of striated muscle tissues.
Dystrophin, not Titin, is the protein absent in muscular
dystrophy.14With exposure of the myosin binding sites on actin (the
thin filaments)in the presence of Ca2+ and ATPthe thick and thin
filaments slide on one another and the sarcomere is shortenedThe
Sliding-Filament Mechanism
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