MUSCULAR TISSUE
Essentials of Histology
Muscular tissue is responsible for locomotion and for the movements of the various parts of the body. This function is assumed by specialized cells called muscle fibers, which contract upon appropriate stimulation. This system has the ability to transform chemical into mechanical energy through the enzymatic splitting of ATP. In the vertebrate body, there are three types of muscle based on the appearance and location of their constituent cells: smooth, skeletal and cardiac. All three types are composed of asymmetric cells, or fibers, with the long axis arranged in the direction of movement.
SMOOTH MUSCLE
Mature smooth
fibers are spindle-shaped cells, with a single central ovoid nucleus. The
smooth muscle has mesenchymal origin and is also referred to as involuntary
muscle and is found in the walls of hollow viscera, walls of blood vessels,
large ducts of compound glands, respiratory passages, and small bundles
within the dermis. The sarcoplasm at the nuclear poles contain abundant
mitochondria, some RER, a large Golgi complex and inclusions such as glycogen.
Each fiber produces its own external lamina, consisting of proteoglycan-rich
material and type III collagen fibers. Additionally, an extensive array
of interweaving thin and thick filaments are present. The thin filaments
are composed of actin (with its associated tropomyosin but with the notable
absense of troponin) and are anchored by ?-actinin dense bodies associated
with the plasma membrane, whereas the thick filaments are composed of myosin.
The filaments run mostly parallel to the long axis of smooth muscle fibers,
but they overlap to various degrees and attach to one another by fusing
their endomysial sheaths. The sheaths are interrupted by many gap junctions,
which transmit the ionic currents that initiate contraction. The ratio
of thin to thick filaments in smooth muscle is about 12:1. Lying just beneath
the cell membrane are structures that may be associated with the
sparse sarcoplasmic reticulum, known as caveolae. These vesicles may function
in the release and sequestering of calcium ions.
The mechanism
of smooth muscle contraction is a modification of the sliding-filament
mechanism. At the beginning of the contraction, the myosin filaments appear
and the actin filaments are pulled toward and between them. The sliding
actin filaments pull the attached dense bodies closer together, shortening
the cell. Individual muscle fibers may undergo partial peristaltic contractions.
During relaxation, the myosin filaments decrease in number, desintegrating
into soluble cytoplasmatic components. The smooth muscle fibers are capable
of spontaneous contraction that may be modulated by autonomic innervation.
Both sympathetic and parasympathetic endings are present and exert antagonistic
effects. In some organs, contractile activity is enhanced by cholinergic
nerves and decreased by adrenergic nerves, whereas in others the opposite
occurs.
SKELETAL MUSCLE
The unit of
skeletal muscle is the fiber, a long cylindrical multinucleate cell, unbranched,
of mesenchymal origin. The flattened, peripheral nuclei lie just under
the sarcolemma; most of the organelles and sarcoplasm are near the poles
of the nuclei. The sarcoplasm contains many mitochondria, glycogen granules,
and an oxygen-binding protein called myoglobin. Mature fibers cannot divide.
Skeletal muscles present besides the fibers, supporting connective tissue
that is organized into epimysial, perimysial and endomysial sheats. The
connective tissue transmit the pull of contractions, convey nerve fibers,
blood vessels, and lymphatics, and nourish the muscle fibers through diffusion.
With the light
microscope, skeletal muscle exhibits alternating light and dark staining
bands running perpendicular to the long axis of the muscle fibers. The
dark bands are known as A bands (anisotropic with polarized light) and
the light bands as I bands (isotropic with polarized light). The center
of each A band is occupied by a pale area, the H band, which is bisected
by a thin M line. Each I band is bisected by a thin dark line, the Z line.
The region of the myofibril between two sucessive Z lines, known as a sarcomere,
is 2,5 ?m in length and is considered to be the contactile unit of skeletal
muscle fibers.
At the electron
microscope level the sarcolemma is continued within the skeletal muscle
fiber as numerous T tubules (transverse tubules), long, tubular invaginations
that intertwine among the myofibrils. T tubules pass transversely across
the fiber and lie specifically in the plane of the junction of the A and
I bands in mammalian skeletal muscle. These tubules branch and anastomose
but usually remain in a single plane. Hense each sarcomere possesses two
sets of T tubules. Associated with this system of T tubules is the sarcoplasmic
reticulum, which is maintained in close register with the A and I bands
as well as with the T tubules. This structure stores intracellular calcium
and forms a meshwork around each myofibril and displays dilated terminal
cisternae at each A-I junction. Thus two of these cisternae are always
in close opposition to a T tubule, forming a triad in which a T tubule
is flanked by two cisternae. The organization of skeletal muscle fiber
shows longitudinal contractile filaments (myofilaments) that are of two
distinct types. The thin filaments, contain actin, together with troponin
and tropomyosin, which are both proteins that mediate the regulation of
contraction by Ca2+ ions. The major component of thin filament is filamentous
F-actin, a polymer of globular G-actin. Each thin filament contains 2 of
F-actin strands wound in a double helix. Tropomyosin is a long, thin, double-helical
polypeptide that wraps around the actin double helix, lies in the grooves
on its surface, and spans 7 G-actin monomers. Troponin is a complex of
3 globular proteins: TnT (troponin T) attaches each complexes to a specific
site on each tropomyosin molecule; TnC binds calcium ions and TnI inhibits
the interaction between thin and thick filaments. The thick filaments,
contain myosin. A myosin molecule is a long, golf-club-shaped polypeptide.
When treated with papain (a proteolytic enzyme) the myosin molecule is
cleaved into 2 pieces at a point near the head. The piece containing most
of the thin shaft is termed light meromyosin; the head and associated section
of the shaft make up heavy meromyosin. The head portion of heavy meromyosin
has na ATP-binding site and na actin-binding site, both necessary for contraction.
The mechanism
of contraction, according to the sliding-filament hypothesis, is initiated
when the nerve impulse is carried along the axon of the motor neuron by
the arrival of the nerve impulse and consequent depolarization of the presynaptic
membrane, causing fusion of the synaptic vesicles with the presynaptic
membrane and exocitosis of acetylcholine into the synaptic cleft. Acetylcholine
binds to receptors in the postsynaptic membrane, causing depolarization
of the sarcolemma, of the T tubules, and of the sarcoplasmic reticulum.
These events causes releasing of Ca2+ from the sarcoplasmic reticulum into
the sarcoplasm surrounding the myofibrils. Ca2+ binds to the TnC subunit
of troponin altering its conformation. Conformational change in troponin
shifts the position of tropomyosin deeper into the groove, unmasking the
active site on the actin molecule. ATP present in the myosin head is hydrolyzed
in ADP and Pi. Pi is released, resulting not only in an increased bond
strength between the actin and myosin but also in a conformational alteration
of the myosin head. ADP is also released, and the thin filament is dragged
toward the center of the sarcomere (power stroke). A new ATP molecule binds
to the myosin head, which causes the release of the bond between actin
and myosin.
CARDIAC MUSCLE
Heart muscle
is found only in the walls of the embryonic heart tube and adult heart
and is derived from a strictly defined mass of splanchnic mesenchyme. The
fibers are long, branched cells with one or two ovoid central nuclei. The
sarcoplasm near the nuclear poles contains many mitochondria that lie in
chains between myofilaments and glycogen granules. The arrangement of myofilaments
yields striations like those of skeletal muscle. The sarcoplasmic reticulum
in cardiac muscle is less organized than that of skeletal muscle. Cardiac
T tubules occur at the Z line. In most cells, cardiac T tubules associate
with a single expanded cisterna of the sarcoplasmic reticulum forming dyads
instead triads. Cardiac muscle cells form highly specialized end-to-end
junctions, referred to as intercalated disks. With electron microscopy,
intercalated disks exhibit 3 major components arranged in a stepwise fashion:
1) The fascia adherens, that is a half Z line found in the vertical (transverse)
portion of the step. Its ?-actinin anchors the thin filaments of the terminal
sarcomeres; 2) The macula adherens (desmosome) prevents detachment of the
cardiac muscle fibers from one another during contraction; 3) The gap junctions
that form the horizontal (lateral) portion of the step. They provide electrotonic
coupling between adjacent cardiac muscle fibers and pass the stimulus for
contraction from cell to cell.
There are two
types of cardiac muscle fibers. The atrial cardiac muscle fibers are small
and have fewer T tubules than ventricular cells. They contain small granules
with a precursor of atrial natriuretic factor, a hormone secreted in response
to increased blood volume and acts on the kidneys to cause sodium and water
loss. The ventricular cardiac muscle fibers are larger cells with more
T tubules and no granules.
Cardiac muscle
fibers contract spontaneously with an intrinsic rhythm. The heart receives
autonomic innervation through axons that terminate near, but never form
synapses with cardiac muscle cells. The autonomic stimulus cannot initiate
contraction but can speed up or slow down the intrinsic beat. The initiating
stimulus for contraction is normally provided by a collection of specialized
cardiac muscle cells in the sinoatrial node; it is delivered by other specialized
cells called Purkinje fibers to the other cardiac muscle cells. The stimulus
is passed between adjacent cells through the gap junctions that establish
an ionic continuity among cardiac muscle fibers that allows them to work
together as a functional syncytium.
