Download unitary smooth muscle

NAME: KOLADE-ERNEST EXCELLENCE
MATRIC NO: DIRECT ENTRY
LEVEL: 200
COURSE: HISTOLOGY OF BASIC TISSUES.
THE HISTOLOGY OF THE MUSCLE TISSUES
INTRODUCTION:
Tissues are known to be made up of cells with function together in a
specific way to form tissues. Muscles tissues are mostly of mesodermal
origin and are also made up of cells that are differentiated and
altogether function to enable CONTRACTILITY which they do by
differentiating through a gradual process of cell lengthening with
simultaneous synthesis of myofibrillar proteins.
Muscle cells have organelles such as the sarcoplasm which is relatively
the same as the cytoplasm in other tissue types, also they posses a
sarcoplasmic reticulum which is the same as the smooth endoplasmic
reticulum and also the plasma membrane called the sarcolemma.
Muscle cells can either be under voluntary or involuntary control. For
example, skeletal muscles are usually under involuntary control while
cardiac muscles are under voluntary control.
TYPES OF MUSCLE TISSUES.
Muscle tissues are classified based on morphology , functional
characteristics and how their structure is adapted to their physiologic
role. There are however three muscle tissue types which are as follows:
1. Skeletal muscle
2. Cardiac muscle
3. Smooth muscle
SKELETAL MUSCLES
Skeletal muscle fibres are usually under voluntary control. They are
made up of bundles that are very long, cylindrical and mutinucleated
cell which shoe cross striations. Also their contraction is relatively
strong, quick and forceful. This is actually caused by the interaction of
the thin Actin filaments nad the thick Myosin filaments whose
molecular configuration allows them to slide upon one another. The
forces necessary for sliding are generated by weak interaction in the
bridges between actin and myosin.
Their muscle fibres are long, cylindrical and multinucleated cells with
diameters of about 10-100 micro meters. This mutinucleation is as a
result of fusion of embryonic mesenchymal cells called myoblasts.
Their long nuclei are found just below the cell membrane . this is
helpful in separating skeletal muscle from cardiac and smooth muscles
which both have a centrally located nuclei.
The medical application of this is that the difference in the skeletal
muscle fibres depends on factors like specific muscle and the age and
sex , state of nutrition, and physical training of the individual. It is a
common ocservation that exercise enlarges the musculature and
decreases fat depots. The increase in muscles is caused by the
formation of myofibrils and a pronounced growth in the diameterof the
individual muscle fibres. This is called Hypertrophy (characterised by an
increase in cell volume). Also, tissue growth by an increase in the
number of cells is termed Hyperplasia.
Skeletal muscle begins to differentiate when mesenchymal cells called
myoblasts align and fuse together to make longer, multinucleated
tubes called myotubes.
Myotubes synthesize the proteins to make up myofilaments and
gradually begin to show cross striations by light microscopy. Myotubes
continue differentiating to form
functional myofilaments and the nuclei are displaced against the
sarcolemma. Part of the myoblast population does not fuse and
differentiate, but remains as a group of
mesenchymal cells called muscle satellite cells located on the external
surface of muscle fibers inside the developing external lamina. Satellite
cells proliferate and produce new muscle fibers following muscle injury
.
Mechanism of Contraction
Resting sarcomeres consist of partially overlapping thick and thin
filaments. During contraction, neither the thick nor thin filaments
changes their length.
Contraction is the result of an increase in the amount of overlap
between the filaments caused by the sliding of thin and thick filaments
past one another.
Contraction is induced by an action potential produced at a synapse,
the neuromuscular junction, between the muscle fiber and a terminus
of a motor axon.
Although a large number of myosin heads extend from the thick
filament, at any one time during the contraction only a small number of
heads align with available
actin-binding sites. As the bound myosin heads move the actin,
however, they provide for alignment of new actin-myosin crossbridges.
The old actin-myosin
bridges detach only after the myosin binds a new ATP molecule; this
action also resets the myosin head and prepares it for another
contraction cycle. If no ATP is
available, the actin-myosin complex becomes stable, which accounts
for the extreme muscular rigidity (rigor mortis) that occurs after death.
A single muscle
contraction is the result of hundreds of bridge-forming and bridgebreaking cycles. The contraction activity that leads to a complete
overlap between thin and thick
filaments continues until Ca2+ ions are removed and the troponin–
tropomyosin complex again covers the myosin-binding site.
During contraction, the I band decreases in size as thin filaments
penetrate the A band. The H band—the part of the A band with only
thick filaments—diminishes in
width as the thin filaments completely overlap the thick filaments. A
net result is that each sarcomere, and consequently the whole cell
(fiber), is greatly shortened.
Skeletal muscle fibers of humans are classified into three types based
on their physiological, biochemical, and histochemical characteristics
All three fiber types are normally found throughout most muscles.
Type I or slow, red oxidative fibers contain many mitochondria and
abundant myoglobin, a protein with iron groups that bind O2 and
produce a
dark red color. Red fibers derive energy primarily from aerobic
oxidative phosphorylation of fatty acids and are adapted for slow,
continuous
contractions over prolonged periods, as required for example in the
postural muscles of the back.
Type IIa or fast, intermediate oxidative-glycolytic fibers have many
mitochondria and much myoglobin, but also have considerable
glycogen.
They utilize both oxidative metabolism and anaerobic glycolysis and are
intermediate between the other fiber types both in color and in energy
metabolism. They are adapted for rapid contractions and short bursts
of activity, such as those required for athletics.
Type IIb or fast, white glycolytic fibers have fewer mitochondria and
less myoglobin, but abundant glycogen, making them very pale in color.
They
depend largely on glycolysis for energy and are adapted for rapid
contractions, but fatigue quickly. They are typically small muscles with a
relatively
large number of neuromuscular junctions, such as the muscles that
move the eyes and digits.
CARDIAC MUSCLES
The cardiac muscles exhibit many structural and functional
characteristics intermediate between those of skeletal muscles. Its
contractions are also strong and utilize a great deal of energy but they
do not contract continuously they relax at some point to prevent
tetany. Cardiac muscles are referred to as synctium because of their
inter connection.
The fibres are essentially long, cylindrical with one or at most two
nuclei which are centrally located in the cell. Between these fibres are
delicate collagenous tissue analogous to the endomysium of skeletal
muscle supports the extremely rich capillary network necessary to meet
the high metabolic demands of continous activity. Betwwnthe ends of
adjacent cardiac muscle cells are specialized intercellular junctions
called INTERCALATED DISCS which not only provide points of anchorage
for the myofibrils but permit extremely rapid spread of contractile
stimuli from one cell to another. Thus, adjacent fibres caused to
contract simultaneously , thereby acting as a functional syncytium. In
addition, a system of highly modified cardiac muscle cells constitute the
pacemaker regions of the heart and ramify through out the organ as
the purkinje system thus co-ordinating contraction of the myocardium
as whole in each cardiac cycle.
A few differences in structure exist between atrial and ventricular
muscle. The arrangement of myofilaments is the same in both, but
atrial muscle has markedly
fewer T tubules, and the cells are somewhat smaller. Membranelimited granules, each about 0.2–0.3 m in diameter, are found at the
poles of atrial muscle nuclei
and are associated with Golgi complexes in this region (Figure 10–18).
These granules release the peptide hormone atrial natriuretic factor
(ANF) which acts on
target cells in the kidney to affect Na+ excretion and water balance. The
contractile cells of the heart's atria thus also serve an endocrine
function.
SMOOTH MUSCLES
Smooth muscle cells have an elaborate array of 10-nm intermediate
filaments. Desmin is the major intermediate filament protein in all
smooth muscles and vimentin is an additional component in vascular
smooth muscle. Both intermediate filaments and F-actin filaments
insert into dense bodies which can be membrane-associated or
cytoplasmic. Dense bodies contain -actinin and are thus functionally
similar to the Z discs of striated and cardiac muscles. The
attachments of thin and intermediate filaments to the dense bodies
helps transmit contractile force to adjacent smooth muscle cells and
their surrounding network in reticular fibres.
Smooth muscle fibers are elongated, tapering, and nonstriated cells,
each of which is enclosed by a thin basal lamina and a fine network of
reticular fibers.
The connective tissues serve to combine the forces generated by each
smooth muscle fiber into a concerted action, e.g, peristalsis in the
intestine
Smooth muscle cells may range in length from 20 m in small blood
vessels to 500 m in the pregnant uterus. Each cell has a single nucleus
located in the center of the cell's broadest part. To achieve the tightest
packing, the narrow part of one cell lies adjacent to the broad parts of
neighboring cells. Such an arrangement viewed in cross section shows a
range of
diameters, with only the largest profiles containing a nucleus. The
borders of the cell become scalloped when smooth muscle contracts
not under voluntary control, but is regulated by autonomic nerves,
certain hormones, and local physiological conditions such as the degree
of stretch.and the nucleus becomes distorted.
The cells occur either as multiunit smooth muscle, in which each cell is
innervated and can contract independently, or more commonly as
unitary smooth muscle, in which only a few cells are innervated but all
cells are interconnected by gap junctions. Gap junctions allow the
stimulus for contraction to spread as a synchronized wave among
adjacent cells. Smooth muscle lacks neuromuscular junctions like those
in skeletal muscle. Instead axonal swellings with synaptic vesicles
simply lie in close contact with the sarcolemma, with little or no
specialized structure to the junctions.
Because smooth muscle is usually spontaneously active without
nervous stimuli, its nerve supply serves primarily to modify activity
rather than initiate it. Smooth muscle receives both adrenergic and
cholinergic nerve endings that act antagonistically, stimulating or
depressing its activity. In some organs, the cholinergic endings activate
and the adrenergic nerves depress; in others, the reverse occurs.
In addition to contractile activity, smooth muscle cells also synthesize
collagen, elastin, and proteoglycans, extracellular matrix (ECM)
components normally synthesized by fibroblasts.