BIOL 2160 : Test 4 Book Notes
Test 4 Book Notes 4/30/2013 10:02:00 PM
12.1 Skeletal Muscle Structure
• With few exceptions, skeletal muscles are connected to at least two
bones
o Exceptions→certain skeletal muscles that are connected to
the skin, to cartilage, or other muscles
• Muscles are connected to bones by tendons→cords of elastic
connective tissue that transmit force form the muscle to the bone
Structure at the Cellular Level
o Body→part of the muscle that generates force; constitutes
the meaty part of the muscle
o The connective tissue of the tendons is continuous with the
connective tissues that surrounds the body of the
muscle→epimysium
o Perimysium→extends into the body dividing it into bundles,
called fascicles, of individual muscle cells
▪ Each fascicle contains hundreds to thousands of muscle
cells, called muscle fibers because of their elongated
shape
Each muscle fiber runs the full length of the
muscle and is encased in a thin sheath of
connective tissue→endomysium
Muscle fibers have many nuclei because each
muscle fiber is formed during embryonic life from
the fusion of several cells
• These nuclei lie immediately before the
muscle fiber’s plasma membrane→
sarcolemma
o A muscle fiber’s semifluid cytoplasm, called sarcoplasm, is
packed with mitochondria and hundreds of banded, rod-like
elements called myofibrils, which contain the fiber’s
contractile machinery
▪ Each myofibril comprises a bundle of overlapping thick
and thin filaments made of the proteins myosin and
actin
▪ Saclike membranous network called the sarcoplasmic
reticulum surrounds each of the myofibrils and is
closely associated with other structures called t
(transverse) tubules which are continuous with the
sarcolemma and penetrate into the cell’s interior
▪ Near the T-tubule, the sarcoplasmic reticulum has
enlargements called lateral sacs/terminal cisternae,
which store calcium
▪ Each T-tubule is associated with two lateral sacs,
forming a triad
o The sarcoplasmic reticulum (SR) and T-tubules play important
poles in the activation of muscle contractions:
▪ They help transmit signals form the sarcolemma to the
myofibrils, enabling a muscle cell to respond to neural
input
o Function of the SR → STORE CALCIUM IONS
▪ These calcium ions are released in response to electrical
signals that travel from the sarcolemma to the T-
tubules, and they serve as chemical messengers that
carry these signals to the myofibrils to initiate
contraction
Structure at the Molecular Level
o Skeletal muscle cells have a striped appearance→striated
muscle
▪ These striations are created by the orderly arrangement
of protein fibers in the myofibrils called thick filaments
and thin filaments, which run parallel to the muscle
cell’s long axis
Thick and thin filaments exist in a 2:1 ratio
▪ Myofibrils are composed of a fundamental unit called a
sarcomere that repeats over and over
Each sarcomere is boarded on either end by Z
lines, which run perpendicular to the long axis
and anchor the thin filaments at one end
• Connects thin filaments, located in the
center of the I band
The thick filaments in a sarcomere are connected
by M lines, which also run perpendicular to the
long axis
The dark striation observed under the microscope
is due to the presence of thick filaments→A band
• In the center of the A band is a region that
is lighter than the sides because only tick
filaments are present; there are no thin
filaments overlapping the thick filaments
here→H zone
The light striation comprises areas where there
are thin filaments with no overlap with thick
filaments→I band
o Thin and thick filaments of the sarcomere are made up of two
proteins called actin and myosin, respectively
▪ Contractile proteins→ they constitute the machinery
that generates contractile force
▪ The basic components of each thin filament are actin
monomers (G actin), each of which has a myosin-
binding site
The ability of actin and myosin to bind together
under certain conditions is critical to a muscle’s
ability to generate force
G actins are linked together end to end to form
strands called F actin
• Two F actins are arranged in a double helix
to form the actin strands found in thin
filaments
Also present are two regulatory proteins that
enable muscle fibers to start or stop contracting
• Tropomyosin: long fibrous molecule that
extends over numerous actin monomers in
such a way that if blocks the myosin-
binding sites in the muscle at rest
• Troponin: complex of three proteins—one
that attaches to the actin strand, another