BIO210: Chapter 10 – Muscle Tissue
10-1: Skeletal Muscle Performs Six Major Functions
• our bodies contain three types of muscle tissue:
o skeletal muscles – the tissue moves the body by pilling on the bones. Single, long, cylindrical, striated cells
o cardiac muscle – pushed blood through the cardiovascular system (branching, striated chains of cells)
o smooth muscle – pushes fluids ands solids along the digestive tract (single, fusiform, nonstriated cells)
• Skeletal muscles are organs composed mainly of skeletal muscle tissue, but they also contain connective tissues,
nerves, and blood vessels.
• Each cell in skeletal muscle tissue is a single muscle fiber.
• Skeletal muscles are directly or indirectly attached to the bones of the skeleton.
• skeletal muscles perform the following six functions:
1. Produce Skeletal Movement: Skeletal muscle contractions pull on tendons and move the bones of the skeleton.
Their effects range from simple motions such as extending the arm or breathing, to the highly coordinated
movements of swimming, skiing, or typing.
2. Maintain Posture and Body Position: Tension in our skeletal muscles maintains body posture—for example,
holding your head still when you read a book or balancing your body weight above your feet when you walk.
Without constant muscular activity, we could neither sit upright nor stand.
3. Support Soft Tissues: Layers of skeletal muscle make up the abdominal wall and the floor of the pelvic cavity.
These muscles support the weight of our visceral organs and shield our internal tissues from injury.
4. Guard Entrances and Exits: skeletal muscles encircle the openings of the digestive and urinary tracts. These
muscles give us voluntary control over swallowing, defecation, and urination.
5. Maintain Body Temperature. Muscle contractions use energy, and whenever energy is used in the body, some of it
is converted to heat. The heat released by working muscles keeps body temperature in the range needed for
6. Store Nutrient Reserves. When our diet contains too few proteins or calories, the contractile proteins in skeletal
muscles are broken down, and their amino acids released into the circulation. The liver can use some of these
amino acids to synthesize glucose, and others can be broken down to provide energy.
10-2: A Skeletal Muscle Contains Muscle Tissue, Connective Tissues,
Blood Vessels and Nerves
Organization of Connective Tissues
• each muscle layer has three layers of connective tissue:
o epimysium – is a dense layer of collagen fibers that surround, it separates the muscle from nearby tissues and
organs. It is connected to the deep fascia, a dense connective tissue layer
o perimysium divides the skeletal muscle into a series of compartments. Each compartment contains a bundle
of muscle fibers called a fascicle. In addition to collagen and elastic fibers, the perimysium contains blood
vessels and nerves that serve the muscle fibers within the fascicles. Each fascicle receives branches of these
blood vessels and nerves.
o endomysium – within the fascicle, it is a connective tissue that surrounds the individual skeletal muscle cells,
called muscle fibers, and loosely interconnects adjacent muscle fibers. This flexible, elastic connective tissue
layer contains (1) capillary networks that supply blood to the muscle fibers; (2) myosatellite cells, stem cells
that take part in the repair of damaged muscle tissue; and (3) nerve fibers that control the muscle. All these
structures are in direct contact with the individual muscle fibers.
• at each end of the muscle, the collagen fibers of the epimysium, perimysium and endomysium come together to form
either a bundle known as a tendon, or a broad sheet called an aponeurosis
• they attack skeletal muscles to bones Blood Vessels and Nerves
the connective tissues of the endomysium and perimysium contain the blood vessels and nerves that supply the muscle
muscles have extensive vascular systems that:
supply large amounts of oxygen
carry away wastes
they contract only when the central nervous system stimulates them
axons – nerve fibers extending from the cell, penetrate the epimysium, branch through the perimysium, and enter the
endomysium to innervate individual muscle fibers
skeletal muscles are voluntary muscles, controlled by the nerves of the central nervous system
10-3: Skeletal Muscle Fibers Have Distinctive Features
• skeletal muscle fibers are enormous, can be up to 30cm in length
• they also contain multinucleate – each skeletal muscle contains hundreds of nuclei just internal to the plasma
membrane, they control the production of enzymes and structural proteins required for normal muscle contraction
• myoblasts – group of embryonic cells that fuse during development forming individual multinucleate skeletal muscle
• each nucleus in a skeletal muscle fiber reflects the contribution of a single myoblast
The Sarcolemma and Transverse Tubules
• sarcolemma (plasma membrane of a muscle fiber) surrounds the sarcoplasm (cytoplasm of the muscle fiber)
• the sarcolemma has a characteristic transmembrane potential due to the unequal distribution of positive and negative
charges across the membrane
• all regions of the cell must contract at the same time, thus, the signal to contract must be distributes quickly
throughout the interior of the cell
• transverse tubules (T tubules) are narrow tubes that are continuous w. the sarcolemma and extend deep into the
sarcoplasm. They are filled w. extracellular fluid and form passageways through the muscle fiber
• electrical impulses (action potentials) conducted by the sarcolemma travel along the T tubules into the cell interior and
trigger muscle fiber contraction
• myofibrils are cylindrical structures which can shorten and are responsible for skeletal muscle fiber contraction
• a myofibril is 12µm in diameter and as long as the entire cell
• they consist of bundles of protein filaments called myofilaments, there are two types:
o thin – composed of actin
o thick – composed of myosin
• in addition, myofibrils contain titin, elastic myofilaments associated w. thick filaments
• At each end of the skeletal muscle fiber, the myofibrils are anchored to the inner surface of the sarcolemma.
• In turn, the outer surface of the sarcolemma is attached to collagen fibers of the tendon or aponeurosis of the skeletal
• As a result, when the myofibrils contract, the entire cell shortens and pulls on the tendon.
The Sarcoplasmic Reticulum
• it is a membrane complex that forms a tubular network around each individual myofibril
• wherever a T tubule encircles a myofibril, the tubule is tightly bound to the membrane of the SR
• on either side of a T tubule, the tubules of the SR enlarge, fuse, and form expanded chambers called terminal cisternae
• triad – combo of a pair of terminal cisternae + Ttubule • Although skeletal muscle fibers do pump Ca out of the cell, they also remove calcium ions from the sarcoplasm by
actively transporting them into the terminal cisternae of the SR.
• The sarcoplasm of a resting skeletal muscle fiber contains very low concentrations of Ca , around 10 mmol/L.
• The free Ca concentration levels inside the terminal cisternae may be as much as 1000 times higher.
• In addition, cisternae contain the protein calsequestrin, which reversibly binds Ca .
• Including both the free calcium and the bound calcium, the total concentration of Ca inside cisternae can be 40,000
times that of the surrounding sarcoplasm.
• A muscle contraction begins when stored calcium ions are released into the sarcoplasm. These ions then diffuse into
individual contractile units called sacromeres
• myofibrils are bundles of thin and thick myofilaments, which are organized into repeating functional units called
• sarcomeres are the smallest functional unit of the muscle fiber, interaction b/w the thick and thin filament of
sarcomeres are responsible for muscle contraction
• A myofibril consists of approximately 10,000 sarcomeres, end to end. Each sarcomere has a resting length of about 2
µm. A sarcomere contains (1) thick filaments, (2) thin filaments, (3) proteins that stabilize the positions of the thick
and thin filaments, and (4) proteins that regulate the interactions between thick and thin filaments.
• Each sarcomere has dark bands called A bands and light bands called I bands. The names of these bands are derived
from anisotropic and isotropic
• A Band: the thick filaments are at the center of each sarcomere, the A band is about as long as a typical thick filament
The A band also includes portions of thin filaments and contains three subdivisions:
o the M line: proteins of the M line connect the central portion of each thick filament to neighboring thick
filaments (m – middle). They help stabilize the position of the thick filaments
o the H band – in a resting sarcomere, the H band is a lighter region on either side of the M line, it contains
thick filaments and no thin filaments
o zone of overlap – is a dark region where thin filaments are located b/w the thick filaments, three thick
filaments surround each thin filament and six thin filaments surround each thick filament
• I Band: contains thin filaments but not thick filaments and extends from the A band of one sarcomere to the A band of
the next sarcomere
o Z lines mark the boundary b/w adjacent sarcomeres. it consists of proteins called actinins, which interconnect
thin filament of adjacent sarcomeres
o strands of titin extend from the tips of the thick filaments to attachment sites at the Z line, titin helps keep the
thick and thin filaments in proper alignment and aids in restoring resting sarcomere length after contraction. It
also helps the muscle fiber resist extreme stretching
o each z line is surrounded by a meshwork of intermediate filaments that interconnect adjacent myofibrils
o The myofibrils closest to the sarcolemma, in turn, are bound to attachment sites on the inside of the
o Because the Z lines of all the myofibrils are aligned in this way, the muscle fiber as a whole has a banded
o These bands, or striations, are visible with the light microscope, so skeletal muscle tissue is also known as
• Thin Filaments
o 56 nm in diameter and 1µm in length
o a single thin filament contains four proteins:
Filamentous actin (Factin) – is a twisted strand composed of two rows of 300400 individual
globular molecules of gactin. Each Gactin molecule contains an active site that can bind to myosin
(in the thick filaments). Under resting conditions this is stopped by the troponintropomysoin
Nebulin: long strand that extends along the Factin strand in the cleft b/w the rows of Gactin
molecules and holds the Factin strand together Tropomyosin: strands that cover the active sites on Gactin and prevent actinmyosin interaction, it is
a double stranded protein that covers seven active sites. It is bound to one molecule of troponin
midway along its length
Troponin: consists of three globular subunits. One subunit binds to tropomyosin, locking them
together as a troponintropomyosin complex and a second subunit binds two one Gactin holding the
troponintropomyosin complex in position. The third one has a receptor that binds two calcium ions
o a contraction can occur only when the troponintropomyosin complex changes position, exposing the active
sites on actin, the necessary change in position takes place when calcium ions bind to receptors on the
o The thin filaments are attached to the Z lines at either end of the sarcomere
o Although it is called a “line” because it looks like a dark line on the surface of the myofibril, the Z line in
sectional view is more like a disc with an open meshwork
• Thick Filaments:
o 1012 nm in diameter and 1.6 µm long
o contains about 300 myosin muscles, each made up of a pair of myosin subunits twisted around one another
o when the myosin head interacts with thin filaments during contraction, they are known as crossbridges
o the connection between the head and the tail functions as a hinge that lets the head pivot. When it pivots, the
head swings toward or away from the M line.
o All the myosin molecules are arranged with their tails pointing toward the M line
o The H band includes a central region where there are no myosin heads.
o Elsewhere on the thick filaments, the myosin heads are arranged in a spiral, each facing one of the
surrounding thin filaments.
o Each thick filament has a core of titin.
o On either side of the M line, a strand of titin extends the length of the thick filament and then continues across
the I band to the Z line on that side.
o The portion of the titin strand exposed within the I band is elastic, which means that it will recoil after
o In the normal resting sarcomere, the titin strands are completely relaxed. They become tense only when some
external force stretches the sarcomere.
Sliding Filaments and Muscle Contraction
• when a skeletal muscle fiber contracts:
o the H bands and I bands of the sarcomeres get smaller
o the zones of overlap get larger
o the Z lines move closer together
o the width of the A band remains constant
• the sliding filament theory: thin filaments slide toward the center of each sarcomere, alongside the thick filaments.
The concept that a sarcomere shortens as the thick and thin filaments slide past one another. The contraction weakens
w. the disappearance of the I bands, at which point the Z lines are in contact w. the ends of the thick filaments
• The pull, called tension, is an active force: Energy must be expended to produce it. Tension is applied to some object,
whether a rope, a rubber band, or a book on a tabletop. Tension applied to an object tends to pull the object toward the
source of the tension. However, before movement can occur, the applied tension must overcome the object’s load (or
resistance), a passive force that opposes movement. The amount of load can depend on the weight of the object, its
shape, friction, and other factors. When the applied tension exceeds the load, the object moves.
• Compression, or a push applied to an object, tends to force the object away from the source of the compression.
Again, no movement can occur until the applied compression exceeds the load of the object. Muscle cells can use
energy to shorten and gener ate tension through interactions between thick and thin filaments, but not to lengthen and
generate compression. In other words, muscle cells can pull, but they cannot push.
10-4: Nervous System Communicates W. Skeletal Muscles at the NMJ
The Control of Skeletal Muscle Activity
• skeletal muscle fibers begin contraction w. the release of their internal stories of calcium ions • the release in under the control of the nervous systen
• neuromuscular junction (NMJ) – communication b/w the nervous system and skeletal muscle fiber occurs here
• the neuron stimulates a muscle fiber through a series of steps (next page)
• The link between the generation of an action potential in the sarcolemma and the start of a muscle contraction is
called excitation–contraction coupling.
• This coupling occurs at the triads. On reaching a triad, an action potential triggers the release of Ca from the
cisternae of the sarcoplasmic reticulum.
• The change in the permeability of the SR to Ca is temporary but within within a millisecond, the concentration in
and around the sarcomere reaches 100 times resting levels.
• b/c the terminal cisternae are located at the zones of overlap, where the thick and thin filaments interact, the effect of
calcium release on the sarcomere is almost instantaneous.
• Troponin is the lock that keeps the active sites inaccessible. Calcium is the key to that lock.
• Each troponin molecule also has a binding site for calcium, and this site is empty when the muscle fiber is at rest.
Calcium binding changes the shape of the troponin molecule and weakens the bond between troponin and actin. The
troponin molecule then changes position, rolling the attached tropomyosin strand away from the active sites
• With this change, the contraction cycle begins.
The Contraction Cycle:
• To understand how the contraction cycle produces tension in a muscle fiber, imagine that you are on a tugofwar
team. You reach forward, grab the rope with both hands, and pull it in. This grabandpull corresponds to crossbridge
attachment and pivoting. You then let go of the rope, reach forward and grab it, and pull once again. Your actions are
not synchronized with the rest of your team. At any given time, some people are reaching and grabbing, some are
pulling, and others are letting go. (If everyone let go at the same time, your opponents would pull the rope away.) The
amount of tension produced depends on how many people are pulling at the same time.
• The situation is similar in a muscle fiber. The myosin heads along a thick filament work together in a similar way to
pull a thin filament toward the center of the sarcomere. Each myofibril consists of a string of sarcomeres, and in a
contraction all of the thin filaments are pulled toward the centers of the sarcomeres.
• If neither end of the myofibril is held in position, both ends move toward the middle. This kind of contraction seldom
occurs in an intact skeletal muscle, because one end of the muscle (the origin) is usually fixed in position during a
contraction, while the other end (the insertion) moves. In that case, the free end moves toward the fixed end.
• If neither end of the myofibril can move, thick and thin filament interactions consume energy and generate tension,
but sliding cannot occur.
10.6: ATP Provides Energy For Muscle Contraction
• each thick filament breaks down around 2500 ATP molecules per second.
• a resting muscle fiber contains only enough ATP and other highenergy compounds to sustain a contraction until
additional ATP can be generated.
ATP and CP Reserves
• the primary function of ATP is to transfer energy from one location to another
• at rest, a skeletal muscle fiber produces more ATP than it needs
• so, ATP transfers energy to creatine – a small molecule that muscle cells assemble from fragments of amino acids
• the energy transfer creates another highenergy compound, creatine phosphate (CP)
o ATP + creatine → ADP + creatine phosphate
• During a contraction, each myosin head breaks down ATP, producing ADP and phosphate. The energy stored in
creatine phosphate is then used to “recharge” ADP, converting it back to ATP through the reverse reaction:
o ADP + creatine phosphate → ATP + creatine • The enzyme that facilitates this reaction is creatine kinase (CK). When muscle cells are damaged, CK leaks across
the plasma membranes and into the bloodstream. For this reason, a high blood concentration of CK usually indicates
serious muscle damage.
• A resting skeletal muscl