Muscular Tissue
Bryan Lee
Overview of Muscular Tissue
Types of Muscular Tissue
Skeletal Muscle Tissue: is named so because the function of most skeletal muscle tissue
is to move bones of the skeleton. Skeletal muscle is linked to bone by bundles of
collagen fibers known as tendons. Skeletal muscle is made up of individual components
known as muscle fibers. These fibers are formed from the fusion of developmental
myoblasts. The myofibers are long, cylindrical, multinucleated cells composed of actin
and myosin myofibrils repeated as a sarcomere, the basic functional unit of the cell and
responsible for skeletal muscle's striated (alternating bands of light and dark)
appearance and forming the basic machinery necessary for muscle contraction. Skeletal
muscles tissue works in a primarily voluntary (somatic) manner – that is, in a conscious
manner.
Cardiac Muscle Tissue: is a type of involuntary (autonomic) striated muscle found in
the walls of the heart, specifically the myocardium. Cardiac muscle cells are known as
cardiac myocytes (or cardiomyocytes). The cells that comprise cardiac muscle are
sometimes seen as intermediate between these two other types in terms of appearance,
structure, metabolism, excitation-coupling and mechanism of contraction. Cardiac muscle
shares similarities with skeletal muscle with regard to its striated appearance and
contraction, with both differing significantly from smooth muscle cells. Cardiac muscle
is adapted to be highly resistant to fatigue: it has a large number of mitochondria,
enabling continuous aerobic respiration via numerous myoglobins (oxygen-storing
pigment) and a good blood supply, which provides nutrients and oxygen. A simple way
to distinguish cardiac muscle tissue from skeletal or smooth muscle tissue is to: 1.) Look
for striations. 2.) Look for intercalated discs. Intercalated discs are complex adhering
structures which connect single cardiac myocytes to an electrochemical syncytium (in
contrast to the skeletal muscle, which becomes a multicellular syncytium during
mammalian embryonic development) and are mainly responsible for force transmission
during muscle contraction. Intercalated discs also support the rapid spread of action
potentials and the synchronized contraction of the myocardium. Within the intercalated
discs are gaps which are known as gap junctions. The heart beats because it has a
pacemaker that initiates each contraction; this built-in rhythym is autorhythmicity.
Smooth Muscle Tissue: is an involuntary (autonomic) non-striated muscle, found
within the tunica media layer of large and small arteries and veins, the bladder, uterus,
male and female reproductive tracts, gastrointestinal tract, respiratory tract, arrector pili
of skin, the ciliary muscle, and iris of the eye. The glomeruli of the kidneys contain a
smooth muscle-like cell called the mesangial cell. Smooth muscle is fundamentally
different from skeletal muscle and cardiac muscle in terms of structure, function,
excitation-contraction coupling, and mechanism of contraction. Smooth muscle fibers
have fusiform shape, and, like striated muscle, can tense and relax. Smooth muscle does
not contain the protein troponin; instead calmodulin (which takes on the regulatory role
in smooth muscle), caldesmon and calponin are significant proteins expressed within
smooth muscle. Some smooth muscle tissue has autorhythmicity.
Functions of Muscular Tissue

Producing Body Movements:
Total body movements such as walking and
running, and localized movements such as grasping a pencil or nodding the head
as a result of muscular contraction, rely on the integrated functioning of skeletal
muscles, bones, and joints.

Stabilizing Body Positions: Skeletal muscle contractions stabilize joints and
help maintain body positions, such as standing or sitting.
Postural muscles
contract continuously when a person is awake; for example, sustained
contractions in neck muscles hold the head upright.

Storing and Moving Substances within the Body: Sustained contractions of
ringlike bands of smooth muscles called sphincters may prevent outflow of the
contents of a hollow organ. Temporary storage of food in the stomach or urine in
the urinary bladder is possible because smooth muscle sphincters close off the
outlets of these organs. Cardiac muscle contractions pump blood through the
body’s blood vessels. Contraction and relaxation of smooth muscle in the walls
of blood vessels help adjust their diameter and thus regulate the rate of blood
flow. Smooth muscle contractions also move food and substances such as bile
and enzymes through the gastrointestinal tract, push gametes (sperm and oocytes)
through the reproductive systems, and propel urine through the urinary system.
Skeletal muscle contractions promote the flow of lymph and aid the return of
blood to the heart.

Producing Heat: As muscular tissue contracts, it also produces heat. Much of
the heat released by muscle is used to maintain normal body temperature.
Involuntary contractions of skeletal muscles, known as shivering, can
dramatically increase the rate of heat production.
Properties of Muscular Tissue

Electrical Excitability: a property of both muscle cells and neurons, is the
ability to respond to certain stimuli by producing electrical signals – for example,
action potentials. The action potentials travel along the plasma membrane due to
the presence of specific ion channels. The stimuli that trigger action potentials
may be electrical signals arising in the muscle tissue itself, as in the heart’s
pacemaker, or chemical stimuli, such as neurotransmitters released by neurons,
hormones distributed by the blood, or even local changes in pH.

Contractility:
is the ablilty of muscular tissue to contract forcefully when
stimulated by an action potential. When muscle contracts, it generates tension
(force of contraction) while pulling on its attachment points. In some muscle
contractions, the muscle develops tension but does not shorten. Example: Holding
a book in an outstretched hand.
In other muscle contractions, the tension
generated is great enough to overcome the load (resistance) of the object being
moved so the muscle shortens and movements occurs. Example: Lifting a book
off a table.

Extensibility: is the ability of muscular tissue to stretch without being damaged.
Extensibility allows a muscle to contract forcefully even if it is already stretched.
Normally, smooth muscle is subject to the greatest amount of stretching.
Example:
Each time the stomach fills with food, the muscle in the wall is
stretched.
Example: Cardiac muscle is stretched each time the heart fills with blood.

Elasticity: is the ability of muscular tissue to return to its original length and
shape after contraction or extension.
Skeletal Muscle Tissue
Structure of a Skeletal Muscle
Although skeletal muscles come in different shapes and sizes the main structure
of a skeletal muscle remains the same. If you were to take one whole muscle and cut
through it, you would find the muscle is covered in a layer of connective tissue known as
the epimysium. The Epimysium protects the muscle from friction against other muscles
and bones.
It also continues at the end of the muscle to form (along with other
connective tissues) the muscles tendon. Looking at the cross section of the muscle you
can see bundles of fibers, known as fasciculi, which are surrounded by another
connective tissue, called the perimysium. Each muscle fiber is surrounded by a layer of
connective tissue known as the endomysium. Each Fasciculi contains anywhere between
10 and 100 muscle fibers, depending on the muscle in question. A large strong muscle,
such as those forming your Quadriceps would have a large number of fibers within each
bundle. A smaller muscle used for precision movement, such as those in the hand would
contain far fewer fibers per Fasciculi. The reddish or meatlike appearance that we
associate with muscular tissue arises from the large population of well-vascularized
muscle cells in the muscle belly. The belly of a muscle can be an elongate, thick,
rounded mass, or a thin, flat sheet of muscular tissue. Tendons, tough, glistening white
dense regular connective tissue structures that attach the muscle belly to the bones, are
minimally vascular, lack muscle cells, and consist primarily of parallel arrangements of
collagen fibers. Like the muscle belly, tendons display a great variety of shapes: Some
are ropelike structures, while others are flat sheets called aponeurosis. Example: Galea
Aponeurotica, located between the occipitalis and the frontalis. The various skeletal
muscles of the body are further grouped together and protected by large connective tissue
sheets, called faschia, which wrap around groups of muscles much like a sock encircles
your foot.
Microscopic Anatomy of a Skeletal Muscle Fiber
Muscle is mainly composed of muscle cells. Within the cells are myofibrils;
myofibrils contain sarcomeres, which are composed of actin, myosin, troponin,
tropomyosin and titin. Individual muscle fibres are surrounded by endomysium. Muscle
fibers are bound together by perimysium into bundles called fascicles; the bundles are
then grouped together to form muscle, which is enclosed in a sheath of epimysium.
Muscle spindles are distributed throughout the muscles and provide sensory feedback
information to the central nervous system.
Skeletal muscle is arranged in discrete muscles, an example of which is the biceps
brachii. It is connected by tendons to processes of the skeleton. Cardiac muscle is similar
to skeletal muscle in both composition and action, being comprised of myofibrils of
sarcomeres, but anatomically different in that the muscle fibers are typically branched
like a tree and connect to other cardiac muscle fibers through intercalcated discs.
Sarcolemma, Transverse Tubules and Sarcoplasm
The sarcolemma is the cell membrane of a muscle cell (skeletal, cardiac and
smooth muscle). It consists of a true cell membrane, called the plasma membrane, and an
outer coat made up of a thin layer of polysaccharide material that contains numerous thin
collagen fibrils. At each end of the muscle fiber, this surface layer of the sarcolemma
fuses with a tendon fiber, and the tendon fibers in turn collect into bundles to form the
muscle tendons that then insert into bones. The membrane is designed to receive and
conduct stimuli.
T-tubule (or transverse tubule) is a deep invagination of the sarcolemma, which
is the plasma membrane, only found in skeletal and cardiac muscle cells. These
invaginations allow depolarization of the membrane to quickly penetrate to the interior of
the cell.
The sarcoplasm of a muscle fiber is comparable to the cytoplasm of other cells
fluid of the muscle, but it houses unusually large amounts of glycosomes (granules of
stored glycogen) and significant amounts of myoglobin, an oxygen binding protein. The
calcium concentration in sarcoplasma is also a special element of the muscular fiber by
means of which the contractions takes place and regulates. Other than the fact that it
contains mostly myofibrils its contents are otherwise comparable to those of the
cytoplasm of other cells. It has a Golgi apparatus, near the nucleus, mitochondria just on
the inside of the cytoplasmic membrane or sarcolemma, as well as a smooth endoplasmic
reticulum organized in an extensive network.
Myofibrils and Sarcoplasmic Reticulum
Myofibrils are the small structures that are the contractile elements of skeletal
muscle. In striated muscle, such as skeletal and cardiac muscle, the actin and myosin
filaments each have a specific and constant length on the order of a few micrometers, far
less than the length of the elongated muscle cell (a few millimeters in the case of human
skeletal muscle cells). The muscle cell is nearly filled with myofibrils running parallel to
each other on the long axis of the cell.
The sarcoplasmic reticulum (SR), is a special type of smooth ER found in
smooth and striated muscle. The only structural difference between this organelle and the
SER is the medley of proteins they have, both bound to their membranes and drifting
within the confines of their lumens. This fundamental difference is indicative of their
functions: the SER synthesizes molecules while the SR stores and pumps calcium ions.
The SR contains large stores of calcium, which it sequesters and then releases when the
muscle cell is stimulated. The SR's release of calcium upon electrical stimulation of the
cell plays a major role in muscle contraction.
Muscular Atrophy, Hypertrophy and Dystrophy
Muscle atrophy is defined as a decrease in the mass of the muscle; it can be a partial or
complete wasting away of muscle. Individual muscle fibers decrease in size as a result of
progressive loss of myofibrils. When a muscle atrophies, it becomes weaker, since the
ability to exert force is related to mass. When atrophy occurs because muscles are not
used is disuse atrophy. Bedridden individuals and people with casts experience disuse
atrophy because the flow of nerve impulses to inactive skeletal muscle is greatly reduced.
The condition is reversible. However, if the nerve supply to a muscle is disrupted or cut,
the muscle undergoes denervation atrophy. Over a period of 6 months ~ 2 years, the
muscle shrinks to about one-fourth its original size, and the muscle fibers are irreversibly
replaced by fibrous connective tissue.
Muscular hypertrophy is the increase of the size of muscle cells due to
increased production of myofibrils, mitochondria, sarcoplasmic reticulum, and other
organelles. Muscular hypertrophy can be increased through strength training and other
short duration, high intensity anaerobic exercises, although those kind of exercises have
little effect strengthening the muscles involved in respiration. Lower intensity, longer
duration aerobic exercise generally does not result in very effective tissue hypertrophy,
instead endurance athletes enhance storage of fats and carbohydrates within the muscles,
as well as neovascularization. An adequate supply of amino acids is essential to produce
muscle hypertrophy. Because hypertrophied muscles contain more myofibrils, they are
capable of more forceful contractions.
Muscular dystrophy (abbreviated MD) refers to a group of genetic, hereditary
muscle diseases that weaken the muscles that move the human body.
Muscular
dystrophies are characterized by progressive skeletal muscle weakness, defects in muscle
proteins, and the death of muscle cells and tissue
Filaments and the Sarcomere
Within myofibrils are smaller structures called filaments. Thin filaments consists
primarily of the protein actin and thick filaments consists primarily of the protein
myosin, held in place by titin filaments. Both thin and thick filaments are directly
involved in the contractile process. There are two thin filaments for every thick filament
in the regions of filament overlap. The filaments inside a myofibril do not extend the
entire length of a muscle fiber.
Instead, they are arranged in compartments called
sarcomeres, the basic functional units of a myofibril. The names of the various subregions of the sarcomere are based on their relatively lighter or darker appearance when
viewed through the light microscope. Each sarcomere is delimited by two very dark
colored bands called Z-discs or Z-lines (from the German zwischen meaning between).
These Z-discs are dense protein discs that do not easily allow the passage of light. The Ttubule is present in this area. The area between the Z-discs is further divided into two
lighter colored bands at either end called the I-bands, and a darker, grayish band in the
middle called the A band.
The I bands appear lighter because these regions of the sarcomere mainly contain
the thin actin filaments, whose smaller diameter allows the passage of light between
them. The A band, on the other hand, contains mostly myosin filaments whose larger
diameter restricts the passage of light. A stands for anisotropic and I for isotropic,
referring to the optical properties of living muscle as demonstrated with polarized light
microscopy.
The parts of the A band that overlaps the I bands are occupied by the both actin and
myosin filaments. Also within the A band is a relatively brighter central region called the
H-zone (from the German helle, meaning bright) in which there is no actin/myosin
overlap when the muscle is in a relaxed state. Finally, the A band is bisected by a dark
central line called the M-line (from the German mittel meaning middle).
Muscle Proteins
There are two contractile proteins in muscle: myosin and actin, which are the
main components of thick and thin filaments. Myosin functions as a motor protein in all
three types of muscle tissue. Motor proteins push or pull various cellular structures to
achieve movement by converting the chemical energy in ATP into mechanical energy of
motion or force production. About 300 molecules of myosin form a single thick filament
in skeletal muscular tissue. Each myosin molecule is shaped like two golf clubs twisted
together. The myosin tail (twisted golf club handles) points towards the M line in the
center of the sarcomere. Tails of neighboring myosin molecule are called myosin heads.
The heads project outward from the shaft in a spiraling fashion, each extending toward
one of the six thin filaments that surround each thick filament.
Thin filaments extend from anchoring points within the Z discs. Thin filaments
are primarily composed of the protein actin. Individual actin molecules join to form an
actin filament that is twisted into a helix. On each actin moelecule is a myosin-binding
site, where a myosin head can attach. Smaller amounts of two regulatory proteins:
tropomyosin and troponin are also part of the thin filament. In a relaxed muscle,
myosin is blocked from binding to actin because strands of tropomyosin cover the
myosin-binding site on the actin. The tropomyosin, in turn, is held in place by the
troponin molecules.
Besides contractile and regulatory proteins, muscle contains about a dozen
structural proteins, which contribute to the alignment, stablilty, elasticity, and
extensibility of myofibrils. Several key structural proteins are titin, myomesin, and
dystrophin.
Titin is a large abundant protein of striated muscle. A N-terminal Z-disc region
and a C-terminal M-line region bind to the Z-line and M-line of the sarcomere
respectively so that a single titin molecule spans half the length of a sarcomere. Titin also
contains binding sites for muscle associated proteins so it serves as an adhesion template
for the assembly of contractile machinery in muscle cells. It has also been identified as a
structural protein for chromosomes. This molecule’s name reflects its enormous size.
With a molecular weight of about 3 million daltons, titin is 50 times larger than an
average-sized proteins. Titin is very elastic, as it can stretch to at least four times its
resting length then spring back unharmed. Titin accounts for much of the elasticity and
extensibility of myofibrils. Titin probably helps the sarcomeres return to their resting
length after a muscle has contracted or been stretched, may help prevent overextension of
sarcomeres, and maintains the central location of the A bands.
Molecules of the protein myomesin form the M line. The M line proteins bind to
titin and connect adjacent thick filaments to one another. Myomesin holds the thick
filaments in register at the M line.
Dystrophin is a rod-shaped cytoplasmic protein, and a vital part of a protein
complex that connects the cytoskeleton of a muscle fiber to the surrounding extracellular
matrix through the cell membrane. Dystrophin and its associated proteins are thought to
reinforce the sarcolemma and help transmit the tension generated by the sarcomeres to
the tendons.
Nerve and Blood Supply
Skeletal muscles are well supplied with nerves and blood vessels. Most of the
skeletal muscles in the body, especially those of the limbs and head, receive one main
nerve that supplies them with both motor and sensory functions. Other muscles, such as
the sheet like muscles of the body wall, receive multiple nerves. Nerves typically enter
the muscle with the main blood vessels of the muscle as a unit called a neuromuscular
bundle. These neuromuscular bundles enter the muscle body near the stable attachment
or tendon of origin and then spread through the muscle via the perimysial and endomysial
connective tissues. The motor fibers initiate the contractile function of muscle cells,
while the sensory fibers provide feedback to the nervous system to regulate motor
function. Somatic motor neurons directly innervate skeletal muscles, involved in
locomotion (such as muscles of the limbs, abdominal, and intercostal muscles). Somatic
motor neurons have a threadlike exntesion, called an axon, which travels from the neuron
cell body in the brain or spinal cord to a group of skeletal muscle fibers in a muscle of the
body. A muscle fiber contracts in respons to one or more action potentials propagating
along its sarcolemma and through its transverse tubule system. A neuromuscular
junction (NMJ) is the synapse or junction of the axon terminal of a motor neuron with
the motor end plate, the highly-excitable region of muscle fiber plasma membrane
responsible for initiation of action potentials across the muscle's surface, ultimately
causing the muscle to contract. At most synapses a small gap, called the synaptic cleft,
seperates the two cells. In humans, the signal passes through the neuromuscular junction
via the neurotransmitter acetylcholine.
At the NMJ, the somatic motor neuron axon terminal divides into a cluster of
synaptic end bulbs. Suspended in the cytosol within each synaptic end bulb are
hundreds of membrane-enclosed sacs called synaptic vesicles. Inside each synaptic
vesicle are thousands of molecules of Ach. The region of the sarcolemma opposite of the
synaptic end bulbs is called the motor end plate. It conatins 30-40 million Ach
receptors, which are integral transmembrane proteins that bind specifically to Ach. These
receptors are abundant in junctional folds, deep grooves in the motor end plate that
provide a large surface area for Ach.
A motor unit is a single α-motor neuron and all of the corresponding muscle
fibers it innervates; all of these fibers will be of the same type (either fast twitch or slow
twitch). When a motor unit is activated, all of its fibers contract. Groups of motor units
often work together to coordinate the contractions of a single muscle; all of the motor
units that subserve a single muscle are considered a motor unit pool. Larger motor units
have stronger twitch tensions. The number of muscle fibers within each unit can vary:
thigh muscles can have a thousand fibers in each unit, eye muscles might have ten. In
general, the number of muscle fibers innervated by a motor unit is a function of a
muscle's need for refined motion. The smaller the motor unit, the more precise the action
of the muscle. Muscles requiring more refined motion are innervated by motor units that
synapse with fewer muscle fibers. Nerve cell axons are very thin, about 1 micrometer.
However, they are extraordinarily long. For many motor neurons the axon is over a meter
long, extending from the spinal column to a muscle cell. They stretch the spinal column
to increase height.
Generally, an artery and one or two veins accompany eaceh nerve that penetrates
a skeletal muscle. Microscopic blood vessels called capillaries are plentiful in muscle
tissue; each muscle fiber is in close contact with one or more capillaries. Capillary blood
brings oxygen and nutrients to the muscle fibers and removes heat and the waste products
of muscle metabolism. Expecially during contraction, a muscle fiber synthesizes and
uses considerable ATP; these reacions require considerable oxygen, glucose, fatty acids,
and other substances that are supplied in the blood.
Contraction and Relaxation of Skeletal Muscle
Fibers
Myosin is a molecular motor that acts like an active ratchet. Chains of actin proteins
form high tensile passive 'thin' filaments that transmit the force generated by myosin to
the ends of the muscle. Myosin also forms 'thick' filaments. Each myosin 'paddles' along
an actin filament repeatedly binding, ratcheting and letting go, sliding the thick filament
over thin filament. This is known as the sliding filament mechanism.
1. Myosin heads bind to the passive actin filaments at the myosin binding sites.
2. Upon strong binding, myosin and actin undergo an isomerization (myosin rotates
at the myosin-actin interface) extending an extensible region in the neck of the
myosin head.
3. Shortening occurs when the extensible region pulls the filaments across each
other (like the shortening of a spring). Myosin remains attached to the actin.
4. The binding of ATP allows myosin to detach from actin. While detached, ATP
hydrolysis occurs "recharging" the myosin head. If the actin binding sites are still
available, myosin can bind actin again.
5. The collective bending of numerous myosin heads (all in the same direction),
combine to move the actin filament relative to the myosin filament. This results in
muscle contraction.
All muscle cells are composed of a number of actin and myosin filaments in series.
The basic unit of organisation of these contractile proteins in striated muscle cells (i.e.,
the cells that compose cardiac and skeletal muscle, but not in smooth muscle tissue) is
called the sarcomere. It consists of a central bidirectional thick filament flanked by two
actin filaments, orientated in opposite directions. When each end of the myosin thick
filament ratchets along the actin filament with which it overlaps, the two actin filaments
are drawn closer together. Thus, the ends of the sarcomere are drawn in and the
sarcomere shortens. Sarcomeres are connected together by so-called 'Z lines', which
anchor the ends of actin filaments in such a way that the filaments on each side of the Z
line point in opposite directions. By this means, sarcomeres are arranged in series. When
a muscle fiber contracts, all sarcomeres contract simultaneously so that force is
transmitted to the fiber ends.
If the process of movement were to continue constantly, all muscles would constantly
be contracted. Therefore, the body needs a way to control the ability of myosin to bind to
the actin. This is accomplished by the introduction of calcium into the cytoplasm of the
muscle cell.
1. When the muscle does not need to contract (is in a resting state), thin strands of a
protein called tropomyosin are wrapped around the actin filaments, blocking the
myosin binding sites. This inhibits the myosin from binding to actin, and therefore
causing a chain of events leading to muscle contraction.
2. Molecules called troponin are attached to the tropomyosin.
3. When calcium is introduced into the muscle cell (fiber), calcium ions bind to
troponin molecules.
4. Calcium binding changes the shape of troponin, causing tropomyosin to be moved
deeper into the groove of the actin dimer, therefore causing the myosin binding
sites on the actin to be exposed.
5. Myosin binds to the now-exposed binding sites, and muscles contract via the
sliding-filament mechanism.
Nerve impulses affect the way in which calcium bonds to the troponin.
Muscle Tone
Muscle tone is the continuous and passive partial contraction of the muscles. It
helps maintain posture, and it declines during REM sleep. Unconscious nerve impulses
maintain the muscles in a partially contracted state. If a sudden pull or stretch occurs, the
body responds by automatically increasing the muscle's tension, a reflex which helps
guard against danger as well as helping to maintain balance. The presence of nearcontinuous innervation makes it clear that tonus describes a "default" or "steady state"
condition. There is, for the most part, no actual "rest state" insofar as activation is
concerned. In terms of skeletal muscle, both the extensor and flexor muscles, under
normal innervation maintain a constant tone while "at rest" that maintains a normal
posture. Cardiac muscle and smooth muscle, although not directly connected to the
skeleton, also have tonus in the sense that although their contractions are not matched
with those of antagonist muscles; their non-contractive state is characterized by
(sometimes random) innervation.
Isotonic and Isometric Contractions
In an isotonic contraction, tension remains unchanged and the muscle's length
changes. Lifting an object off a desk, walking, and running involve isotonic contractions.
A near isotonic contraction is known as Auxotonic contraction. There are two types of
isotonic contractions: (1) concentric and (2) eccentric. In a concentric contraction, the
muscle tension rises to meet the resistance, then remains the same as the muscle shortens.
In eccentric, the muscle lengthens due to the resistance being greater than the force the
muscle is producing.
In an isometric contraction, the tension generated is not enough to exceed the
resistance of the object to be moved and the muscle does not change its length. An
example would be holding a book steady using an outstretched arm. These contractions
are important for maintaing posture and for supporting objects in a fiixed position.
Although isometric contractions do not result in body movement, energy is still
expended. Most activities include both isotonic and isometric contractions.
Types of Skeletal Muscle Fibers
Among other differences, skeletal muscle fibers vary in their content of
myoglobin, the red protein that binds oxygen in muscle fibers. Those with a high
myoglobin content are called red muscle fibers, while those that have a low myoglobin
content are called white muscle fibers.
Red muscle fibers also contain more
mitochondria and are supplied by more blood capillaries than white muscle fibers.
Skeletal muscle fibers are classified into three main types: slow oxidative fibers, fast
oxidative-glycolytic fibers, and fast glycolytic fibers.
Slow Oxidative Fibers
These fibers, also called type I fibers, contain large amounts of myoglobin, many
mitochondria and many blood capillaries. Type I fibers are red, split ATP at a slow rate,
have a slow contraction velocity, very resistant to fatigue and have a high capacity to
generate ATP by oxidative metabolic processes. Such fibers are found in large numbers
in the postural muscles of the neck.
Fast Oxidative-Glycolytic Fibers
These fibres, also called type II A fibers, contain very large amounts of
myoglobin, very many mitochondria and very many blood capillaries. Fast oxidativeGlycolytic fibers are red, have a very high capacity for generating ATP by oxidative
metabolic processes, split ATP at a very rapid rate, have a fast contraction velocity and
are resistant to fatigue. FOG fibers contribute to activities such as walking and sprinting.
Fast Glycolytic Fibers
These fibers, also called type II B fibers, contain a low content of myoglobin,
relatively few mitochondria, relatively few blood capillaries and large amounts glycogen.
Type II B fibers are white, geared to generate ATP by anaerobic metabolic processes, not
able to supply skeletal muscle fibers continuously with sufficient ATP, fatigue easily,
split ATP at a fast rate and have a fast contraction velocity. Such fibers are found in large
numbers in the muscles of the arms.
Excersice and Skeletal Muscle Tissue
The relative ratio of FG and SO fibers in each muscle is genetically determined
and helps account for individual differences in physical performance. For example,
people with a higher proportion of FG fibers often excel in activities that require periods
of intense activity, such as weight lifting or sprinting. SO fibers are better at activities
that require endurance, such as long distance running.
Although the total number of skeletal muscle fibers usually does not increase, the
characteristics of those present can change to some extent. Various types of exercises
can induce changes in the fibers in a skeletal muscle.
Cardiac Muscle Tissue
The principal tissue in the heart wall is cardiac muscle tissue. Although it is
striated like skeletal muscle, its activity cannot be controlled voluntarily, thus it is
autonomic. Also, certain cardiac muscle fibers display autorhythmicity, the ability to
repeatedly generate spontaneous action potentials. In the heart, these action potentials
cause alternating contraction and relaxation of the heart muscle fibers. Compared to
skeletal muscle fibers, cardiac muscle fibers are shorter in length and less circular in
transverse section.
Usually one centrally located nucleus is present, although an
occasional cell may have two nuclei. The ends of cardiac muscle fibers connect to
neighboring fibers by irregular transverse thickenings of the sarcolemma referred to as
intercalated discs. The discs contain desmosomes, which hold the fibers together, and
gap junctions, which allow muscle action potentials to spread from one muscle fiber to
its neighbors. Cardiac muscle tissue has an endomysium, but lacks a perimysium and
epimysium. Mitochondria are larger and more numerous in cardiac muscle fibers than in
skeletal muscle fibers. Cardiac muscle fibers have the same arrangement of actin and
myosin, and the same bands, zones, and Z discs, as skeletal muscle fibers.
One
histological difference between cardiac muscle and skeletal muscle is that the T-tubules
in cardiac muscle are larger, broader and run along the Z-Discs. There are fewer Ttubules in comparison with skeletal muscle. Additionally, cardiac muscle forms dyads
instead of the triads formed between the T-tubules and the sarcoplasmic reticulum in
skeletal muscle.
The sarcoplasmic reticulum of cardiac muscle fibers is somewhat
smaller than the SR of skeletal muscle fibers.
Skeletal muscle tissue contracts only when stimulated by Ach released by an
action potential in a motor neuron. In contrast, cardiac muscle tissue can contract without
extrinsic nervous or hormonal stimulaton. Its source of stimulation is a conducting
network of specialized cardiac muscle fibers within the heart. Stimulation from the
body’s nervous or endocrine system merely causes the conducting fibers to increase or
decrease their rate of discharge. Cardiac muscle tissue remains contracted 10~15 times
longer than skeletal muscle tissue, allowing time for the heart chambers to relax and fill
with blood in between beats. This permits the heart rate to increase significantly but
prevents the heart from undergoing tetanus (sustained contraction), which would stop
blood flow. Cardiac muscle tissue can undergo hypertrophy in response to increased
workload.
Smooth Muscle Tissue
Like cardiac muscle tissue, smooth muscle tissue is usually activated
involuntarily. Of the two types of smooth muscle tissue, the more common type is
visceral smooth muscle tissue. It is found in wraparound sheets that form part of the
walls of small arteries and veins and of hollow viscera such as the stomach, intestines,
uterus, and urinary bladder.
Like cardiac muscle, visceral smooth muscle is
autorhythmic. The fibers connect via gap junctions, therefore muscle action potentials
spread throughout the network.
The second type of smooth mucle tissue is multiunit smooth muscle tissue. It
consists of individual fibers, each with its own motor neuron terminals and with few gap
junctions in between neighboring fibers. Stimulation of one visceral muscle fiber causes
contraction of many adjacent fibers, but stimulation of one multiunit fiber causes
contraction of that fiber only. The walls of large arteries, the airways to the lungs, the
arrector pili muscles that attach to hair follicles, the muscles of the iris that adjust pupil
diameter, and the ciliary body that adjusts focus of the lens in the eye all contain
multiunit smooth muscle tissue.
Smooth muscle fibers are smaller than skeletal muscle fibers. Within each fiber is
a single, oval, centrally located nucleus.
The sarcoplasm of smooth muscle fibers
contains both thick and thin filaments, in rations between 1:10 and 1:15, but they are not
arranged in orderly sarcomeres as in striated muscle. Smooth muscle fibers also contain
intermediate filaments. Intermediate filaments (IFs) are a family of related proteins that
share common structural and sequence features. Intermediate filaments have an average
diameter of 10 nanometers, which is between that of actin (microfilaments) and
microtubules, although they were initially designated 'intermediate' because their average
diameter was between those of narrower microfilaments (actin) and wider myosin
filaments. These filaments contain the protein desmin and appear to have a structural
role. Because the various filaments have no regular pattern of overlap, smooth muscle
fibers do not have striations. Smooth muscle fibers also lack transverse tubules and have
little sarcoplasmic reticulum for storage of calcium ions. Smooth muscle tissue has an
endomysium, but lacks a perimysium and epimysium.
In smooth muscle fibers, intermediate filaments attach to structures called dense
bodies, which are functionally similar to Z discs in striated msucle fibers. Bundles of
intermediate filaments stretch from one dense body to another. During contraction, the
sliding filament mechanism involving thick and thin filaments generates tension that is
transmitted to intermediate filaments. These in tun pull on the dense bodies attached to
the sarcolemma, causing a shortening of the muscle fiber. When a smooth muscle fiber
contracts, it turns like a corkscrew; it rotates in the opposite direction as it relaxes.
Compared with contraction in a skeletal muscle fiber, contraction in a smooth
muscle fiber starts more slowly and lasts much longer. In addition, smooth muscle can
both shorten and stretch to a greater extent than other muscle types.
As in striated muscle, an increase in the concentration of calcium in the cytosol of
smooth muscle initiates contraction. Theres is far less sarcoplasmic reticulum in smooth
muscle than in skeletal muscle.
There are no T Tubules in smooth musclefibers,
therefore it takes longer for calcium ions to reach the filaments in the center of the fiber
and trigger the contractile process. This accounts for the slow onset and prolonged
contraction of smooth muscle.
Calcium ions also move out of the muscle fiber slowly, which delays relaxation.
The prolonged presence of calcium ions in the cytosol provides for smooth muscle tone,
a state of continuous partial contraction. Smooth muscle tissue than thus sustain longterm tone which is important in the gastrointestinal tract, where the walls maintain a
steady pressure on the contents of the tract, and in the walls of blood vessels called
arterioles, which maintain a steady pressure on blood.
Unlike striated muscle fibers, smooth muscle fibers can stretch considerably and
maintain their contractile function.
When smooth muscle fibers are stretched, they
initially contract, developing increased tension. Whithin a minute or so, the tension
decreases.
This phenomenon, called the stress-relaxation response allows smooth
muscle to undergo great changes in length while still retaining the ability to contract
effectively. Even though the smooth muscle in the walls of blood vessels and hollow
organs can stretch, the pressure on the contents within them changes very little. After the
organ empties, however, the smooth muscle rebounds, and the wall of the organ retains
its firmness.
Certain smooth muscle fibers retain capacity for division and can thus grow by
hyperplasia. New smooth muscle fibers can arise from cells falled pericytes, stem cells
found in association with blood capillaries and small veins. Smooth muscle fibers can
also proliferate in certain pathological condition, such as occur in the development of
atherosclerosis.