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Chapter 9 Muscles and Muscle Tissue9.1 There are three types of muscle 4ssue
Types of Muscle Tissue
Skeletal and smooth muscle cells (but not cardiac muscle cells) are elongated and are called muscle fibers.
Whenever you see the prefixes myo or mys or sarco, the reference is to muscle. For example, the plasma
membrane of muscle cells is called the sarcolemma, and muscle cell cytoplasm is called sarcoplasm.
Skeletal Muscle
Skeletal muscle 4ssue is packaged into the skeletal muscles, organs that aDach to and cover the skeleton.
Skeletal muscle fibers are the longest muscle cells and have obvious stripes called stria4ons. Although it is
oFen ac4vated by reflexes, skeletal muscle is called voluntary muscle because it is the only type subject to
conscious control.
Cardiac Muscle
Cardiac muscle 4ssue occurs only in the heart, where it cons4tutes the bulk of the heart walls. Key words to
remember for cardiac muscle are cardiac, striated, and involuntary.
Smooth Muscle
Smooth muscle 4ssue is found in the walls of hollow visceral organs, such as the stomach, urinary bladder,
and respiratory passages. Its role is to force fluids and other substances through internal body channels.
Smooth muscle also forms valves to regulate the passage of substances through internal body openings,
dilates and constricts the pupils of your eyes and forms the arrector pili muscles aDached to hair follicles.
Characteris6cs of Muscle Tissue
What enables muscle 4ssue to perform its du4es? Four special characteris4cs are key.
• Excitability, also termed responsiveness, is the ability of a cell to receive and respond to a s4mulus
by changing its membrane poten4al.
• Contrac4lity is the ability to shorten forcibly when adequately s4mulated.
• Extensibility is the ability to extend or stretch.
• Elas4city is the ability of a muscle cell to recoil and resume its res4ng length aFer stretching.
Muscle Func6ons
Muscles perform at least four important func4ons for the
body:
• Produce movement. Skeletal muscles are responsible for all locomo4on and manipula4on.
• Maintain posture and body posi4on. We are rarely aware of the skeletal muscles that maintain body
posture.
• Stabilize joints. Even as they pull on bones to cause movement, they strengthen and stabilize the
joints of the skeleton.
• Generate heat. Muscles generate heat as they contract, which plays a role in maintaining normal
body temperature.
9.2 A skeletal muscle is made up of muscle fibers, nerves, blood vessels, and connec4ve 4ssues
Each skeletal muscle is a discrete organ, made up of several kinds of 4ssues.
Nerve and Blood Supply
Skeletal muscle has a rich blood supply. This is understandable because contrac4ng muscle fibers use huge
amounts of energy and require almost con4nuous delivery of oxygen and nutrients via the arteries.
Connec6ve Tissue Sheaths
Let’s consider these connec4ve 4ssue sheaths from external to internal.
, • Epimysium. The epimysium is an
“overcoat” of dense irregular
connec4ve 4ssue that surrounds the
whole muscle. Some4mes it blends
with the deep fascia that lies
between neighboring muscles or the
superficial fascia deep to the skin.
• Perimysium and fascicles. Within
each skeletal muscle, the muscle
fibers are grouped into fascicles that
resemble bundles of s4cks.
Surrounding each fascicle is a layer of
dense irregular connec4ve 4ssue
called perimysium.
• Endomysium. The endomysium is a wispy sheath of connec4ve 4ssue that surrounds each individual
muscle fiber. It consists of fine areolar connec4ve 4ssue.
A=achments
When a muscle contracts, the movable bone, the muscle’s inser4on, moves toward the immovable or less
movable bone, the muscle’s origin.
Muscle aDachments, whether origin or inser4on, may be direct or indirect.
• In direct, or fleshy, aDachments, the epimysium of the muscle is fused to the periosteum of a bone
or perichondrium of a car4lage.
• In indirect aDachments, the muscle’s connec4ve 4ssue wrap- pings extend beyond the muscle
either as a ropelike tendon or as a sheetlike aponeurosis.
,9.3 Skeletal muscle fibers contain calcium-regulated molecular motors
Each skeletal muscle fiber is a long cylindrical cell with mul4ple oval nuclei just beneath its sarcolemma or
plasma membrane. Sarcoplasm, the cytoplasm of a muscle cell, is similar to the cytoplasm of other cells,
but it contains unusually large amounts of glycosomes and myoglobin, a red pigment that stores oxygen.
Myofibrils
A single muscle fiber contains hundreds to thousands of rodlike myofibrils that run parallel to its length.
Stria&ons
Stria4ons, a repea4ng series of dark and light bands, are evident along the length of each myofibril. In an
intact muscle fiber, the dark A bands and light I bands are nearly perfectly aligned, giving the cell its striated
appearance.
• Each dark A band has a lighter region in its midsec4on called the H zone.
• Each H zone is bisected ver4cally by a dark line called the M line (M for middle) formed by
molecules of the protein myomesin.
• Each light I band also has a midline interrup4on, a darker area called the Z disc (or Z line).
Sarcomeres
The region of a myofibril between two successive Z discs is a sarcomere.
Myofilaments
These smaller structures, the myofilaments or filaments, are the muscle equivalents of the ac4n-containing
microfilaments and myosin motor proteins. There are two types of contrac4le myofilaments in a
sarcomere:
• The central thick filaments containing myosin (red) extend the en4re length of the A band. They are
connected in the middle of the sarcomere at the M line.
• The more lateral thin filaments containing ac4n (blue) extend across the I band and partway into
the A band. The Z disc, a protein sheet, anchors the thin filaments.
A close look at myofibril arrangement and banding paDerns reveals that: A hexagonal arrangement of six
thin filaments surrounds each thick filament, and three thick filaments enclose each thin filament.
• The H zone of the A band appears less dense because the thin filaments do not extend into this
region.
• The M line in the center of the H zone is slightly darker because of the fine protein strands there
that hold adjacent thick filaments together.
• The myofilaments are held in alignment at the Z discs and the M lines and are anchored to the
sarcolemma at the Z discs.
, Molecular Composi&on of Myofilaments
Thick filaments are composed primarily of the protein myosin. The thin filaments are composed chiefly of
the protein ac4n. Thin filaments also contain several regulatory proteins.
• Polypep4de strands of tropomyosin spiral about the ac4n core and help s4ffen and stabilize it.
Successive tropomyosin molecules are arranged end to end along the ac4n filaments, and in a
relaxed muscle fiber, they block myosin-binding sites on ac4n so that myosin heads on the thick
filaments cannot bind to the thin filaments.
• Troponin is a globular protein with three polypep4de subunits. One subunit aDaches troponin to
ac4n. Another subunit binds tropomyosin and helps posi4on it on ac4n. The third subunit binds
calcium ions.
The elas4c filament we referred to earlier is composed of the giant protein 44n. It holds the thick filaments
in place, maintaining the organiza4on of the A band, and helps the muscle cell spring back into shape aFer
stretching. Another important structural protein is dystrophin, which links the thin filaments to the integral
proteins of the sarcolemma.
Sarcoplasmic Re6culum and T Tubules
Skeletal muscle fibers contain two sets of intracellular tubules that help regulate muscle contrac4on: 1) the
sarcoplasmic re4culum and 2) T tubules.
Sarcoplasmic Re&culum
The SR regulates intracellular levels of ionic calcium. It stores calcium and releases it
on demand when the muscle fiber is s4mulated to contract. Others called terminal
cisterns form larger, perpendicular cross channels at the A band–I band junc4ons,
and they always occur in pairs.
T Tubules
At each A band–I band junc4on, the sarcolemma of the muscle cell protrudes deep
into the cell interior, forming an elongated tube called the T tubule. Along its length,
each T tubule runs between the paired terminal cisterns of the SR, forming triads.
Triad Rela&onships
At the triads, membrane-spanning proteins from the T tubules and SR link together
across the gap between the two membranes.
• The protruding integral proteins of the T tubule act as voltage sensors.
• The integral proteins of the SR form gated channels through which the terminal cisterns release Ca2+
Sliding Filament Model of Contrac6on
The sliding filament model of contrac4on states that during contrac4on, the thin filaments slide past the
thick ones so that the ac4n and myosin filaments overlap to a greater degree. Neither the thick nor the thin
filaments change length during contrac4on. Here’s how it works:
• When the nervous system s4mulates muscle fibers, the myosin heads on the thick filaments latch
onto myosin-binding sites on ac4n in the thin filaments, and the sliding begins.
• These cross bridge aDachments form and break several 4mes during a contrac4on, ac4ng like 4ny
ratchets to generate tension and propel the thin filaments toward the center of the sarcomere.
• As this event occurs simultaneously in sarcomeres through- out the cell, the muscle cell shortens.
At the microscopic level, the following things occur as a muscle cell shortens:
• The I bands shorten.
• The distance between successive Z discs shortens. As the thin filaments slide centrally, the Z discs to
which they aDach are pulled toward the M line.
• The H zones disappear.
• The con4guous A bands move closer together, but their length does not change.
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