Chapter 10:
Skeletal (striated) muscle:
o Consists of muscle fibers:
Long, cylindrical, multinucleated cells
o Mesenchymal myoblasts (In embryonic mesenchyme) fuse mytotubes with many
nuclei which differentiate striated muscle fibers
o Nuclei are found on periphery under sarcolemma
o Satellite cells: progenitor cells that remain adjacent to most fibers of differentiated
skeletal muscle.
o Organization of skeletal muscle:
Connective tissue organizes contractile fibers in 3 types of muscle:
Epimysium: external sheath of dense connective tissue that surrounds
the entire muscle. Its septa extend inward, carrying large nerves, blood
vessels and lymphatic of vessels.
Perimysium: thin connective tissue layer that immediately surrounds
each bundle of muscle fibers termed as fascicle (each fascicle is one
functional unit). Nerves, blood vessels and lymphatics penetrate the
permysium to supply each fascicle.
Endomysium: reticular fibers and scattered fibroblasts that surround the
external lamina of individual muscle fibers. This brings oxygen to muscle
fibers.
Myotendinous junctions: where the epimysium and tendon of a muscle fuse.
o Organization within muscle fibers:
Light (I bands) and dark bands (A bands)
Z disc bisects the I band.
These bands together are called a sarcomere which extends from 1 Z disc to
another. They are about 2.5 micro meters long in resting muscle.
Sarcoplasm has little RER and contains long cylindrical filament bundles called
myofibrils.
Mitochondria and sarcoplasmic reticulum (known as SER in other cells) are
found between the myofibrils.
The band pattern is due to arrangement of thick and thin myofilaments
composed of myosin (A band) and F-actin (I band)
Each G-actin monomer contains a binding site for myosin.
Actin filaments are anchored perpendicularly on the Z disc by a protein alpha-
actinin and have different polarity on each side of this disc.
The thin filaments (actin) are associated with 2 proteins:
Tropomyosin: 2 polypeptide chain in the groove between the 2 twisted
actin strands
Troponin: 3 subunits
o TnT which attaches to tropomyosin
o TnC which binds Ca2+
o TnI which regulates actin-myosin interaction
, Titin: an important protein in the I band and is the largest protein in the body
and has elastic properties.
Nebulin: binds each thin myofilament laterally and helps anchor them to alpha-
actinin and specifies the length of the actin polymers during myogenesis.
A bands contain both thick filaments and the overlapping portions of the thin
filaments
Close observation of the A bands shows a lighter zone in the center (H zone)
where only the rodlike portions of myosin molecules and no thin filaments are.
It has myomesin : protein that holds the thick filaments in place
It also has creatine kinase which catalyzes transfer of phosphate groups
from phosophocreatine to ADP, helping to supply ATP for muscle
contraction.
o Sarcoplasmic reticulum (SER of muscle tissue) and T tubules
Specialized for Ca2+ sequestration.
Sarcolemma is folded into a system of T tubules
Long fingerlike invaginations of the cell membrane to penetrate deeply
into the sarcoplasm and encircle every myofibril near the aligned A and I
band boundaries of sarcomeres.
Terminal cisterns: part of the sarcoplasmic reticulum on each side of every T
tubule
Triad: complex of a T tubule with 2 closely associated small cisterns of
sarcoplasmic reticulum.
Ca2+ is concentrated in these cisternae after sarcoplasmic reticulum is
depolarized and are released through calcium channels in the membrane into
cytoplasm surrounding the thick and thin filaments.
Calcium binds troponin and allows bridging between actin and myosin.
When depolarization ends, sarcoplasmic reticulum pumps calcium back into the
cisternae ending contraction
o Mechanisms of contraction:
Thick and thin filaments do not change their length
Contraction results from overlapping thin and thick filaments as they slide past
one another.
In resting muscle, myosin heads cannot bind G-actin because the binding sites
are blocked by troponin-tropomyosin complex on the F-actin filaments.
Ca2+ binds troponin when depolarization occurs, changing shape and moving
tropomyosin on the F=actin to expose the myosin-binding active sites.
Energy for pulling of actin is provided by ATP on myosin heads after which
myosin binds another ATP and detaches from actin.
This lasts for 50 milliseconds
In absence of ATP, actin-myosin crossbridges become stable rigidity of
skeletal muscles (known as rigor mortis) and that occurs mainly after
mitochondria stop functioning (in death)
o Innervation:
Skeletal (striated) muscle:
o Consists of muscle fibers:
Long, cylindrical, multinucleated cells
o Mesenchymal myoblasts (In embryonic mesenchyme) fuse mytotubes with many
nuclei which differentiate striated muscle fibers
o Nuclei are found on periphery under sarcolemma
o Satellite cells: progenitor cells that remain adjacent to most fibers of differentiated
skeletal muscle.
o Organization of skeletal muscle:
Connective tissue organizes contractile fibers in 3 types of muscle:
Epimysium: external sheath of dense connective tissue that surrounds
the entire muscle. Its septa extend inward, carrying large nerves, blood
vessels and lymphatic of vessels.
Perimysium: thin connective tissue layer that immediately surrounds
each bundle of muscle fibers termed as fascicle (each fascicle is one
functional unit). Nerves, blood vessels and lymphatics penetrate the
permysium to supply each fascicle.
Endomysium: reticular fibers and scattered fibroblasts that surround the
external lamina of individual muscle fibers. This brings oxygen to muscle
fibers.
Myotendinous junctions: where the epimysium and tendon of a muscle fuse.
o Organization within muscle fibers:
Light (I bands) and dark bands (A bands)
Z disc bisects the I band.
These bands together are called a sarcomere which extends from 1 Z disc to
another. They are about 2.5 micro meters long in resting muscle.
Sarcoplasm has little RER and contains long cylindrical filament bundles called
myofibrils.
Mitochondria and sarcoplasmic reticulum (known as SER in other cells) are
found between the myofibrils.
The band pattern is due to arrangement of thick and thin myofilaments
composed of myosin (A band) and F-actin (I band)
Each G-actin monomer contains a binding site for myosin.
Actin filaments are anchored perpendicularly on the Z disc by a protein alpha-
actinin and have different polarity on each side of this disc.
The thin filaments (actin) are associated with 2 proteins:
Tropomyosin: 2 polypeptide chain in the groove between the 2 twisted
actin strands
Troponin: 3 subunits
o TnT which attaches to tropomyosin
o TnC which binds Ca2+
o TnI which regulates actin-myosin interaction
, Titin: an important protein in the I band and is the largest protein in the body
and has elastic properties.
Nebulin: binds each thin myofilament laterally and helps anchor them to alpha-
actinin and specifies the length of the actin polymers during myogenesis.
A bands contain both thick filaments and the overlapping portions of the thin
filaments
Close observation of the A bands shows a lighter zone in the center (H zone)
where only the rodlike portions of myosin molecules and no thin filaments are.
It has myomesin : protein that holds the thick filaments in place
It also has creatine kinase which catalyzes transfer of phosphate groups
from phosophocreatine to ADP, helping to supply ATP for muscle
contraction.
o Sarcoplasmic reticulum (SER of muscle tissue) and T tubules
Specialized for Ca2+ sequestration.
Sarcolemma is folded into a system of T tubules
Long fingerlike invaginations of the cell membrane to penetrate deeply
into the sarcoplasm and encircle every myofibril near the aligned A and I
band boundaries of sarcomeres.
Terminal cisterns: part of the sarcoplasmic reticulum on each side of every T
tubule
Triad: complex of a T tubule with 2 closely associated small cisterns of
sarcoplasmic reticulum.
Ca2+ is concentrated in these cisternae after sarcoplasmic reticulum is
depolarized and are released through calcium channels in the membrane into
cytoplasm surrounding the thick and thin filaments.
Calcium binds troponin and allows bridging between actin and myosin.
When depolarization ends, sarcoplasmic reticulum pumps calcium back into the
cisternae ending contraction
o Mechanisms of contraction:
Thick and thin filaments do not change their length
Contraction results from overlapping thin and thick filaments as they slide past
one another.
In resting muscle, myosin heads cannot bind G-actin because the binding sites
are blocked by troponin-tropomyosin complex on the F-actin filaments.
Ca2+ binds troponin when depolarization occurs, changing shape and moving
tropomyosin on the F=actin to expose the myosin-binding active sites.
Energy for pulling of actin is provided by ATP on myosin heads after which
myosin binds another ATP and detaches from actin.
This lasts for 50 milliseconds
In absence of ATP, actin-myosin crossbridges become stable rigidity of
skeletal muscles (known as rigor mortis) and that occurs mainly after
mitochondria stop functioning (in death)
o Innervation:
