Download Name: Ibezim Vivian Somtoochuwu Medicine and Surgery 14

Name: Ibezim Vivian Somtoochuwu
Medicine and Surgery
14/MHS01/062
Histology Assignment
HISTOLOGY OF MUSCLE AS A TISSUE AND ITS TYPES.
Histology of The muscle as a tissue
The muscle is a type of tissue composed of cells that optimize the universal cell property
of contractility. As in all cells, actin microfilaments and associated proteins generate the
forces necessary for the muscle contraction which drives movement within the
body.Essentialy, all muscle cells are of mesodermal origin and differentiate by a gradual
process of cell lengthening with abundant synthesis of the myofibrillar proteins actin
and, myosin.
Generally, muscles enable contraction for locomotion and skeletal movement,
propulsion and pressure regulation.
Muscle classification: muscle tissue may be classified according to a morphological
classification or a functional classification.
Morphological Classification (Based on Structure) :There are two types of muscle
based on the morphological classification system, they are :
1. Striated
2. Non striated or smooth.
Striated muscles have cross-striations while smooth muscles do not.
Functional classification: There are two types of muscle based on a functional
classification system, they are
1. Voluntary
2. Involuntary.
Voluntary muscles can be controlled by our human will, they are innervated by Somatic
nerves and Involuntary muscle cannot be controlled by human will they are innervated
by the autonomic nerves.
Types of muscle:
There are generally considered to be three types of muscle in the human body.
1) Skeletal muscle: which is striated and voluntary
2) Cardiac muscle: which is striated and involuntary
3) Smooth muscle: which is non striated and involuntary
SKELETAL MUSCLES
Skeletal muscles are striated muscles. They contain bundles of very long cylindrical
multinucleated cells with diameters of 10 to 100 micrometers. Their contraction is very
quick, forceful and usually under voluntary control. Their nucleus is located at the
periphery of the cell.
ORGANIZATION OF SKELETAL MUSCLE
Skeletal muscles consist of long muscle fibers which can also be called myocytes or
myofibers and are surrounded by thin layers of connective tissues that organize the
fibers.
a) The Epimysium: this is an external sheath of dense connective tissue, it surround
the entire muscle. Septa of this tissue extend inward, carrying the larger nerves,
blood vessels and lymphatics of the muscle.
b) The Perimysium: This is the thin connective tissue layer that immediately
sorrounds each bundle of muscle fibers termed a FASCICLE; this is functional unit
in which the fibers work together. Nerves, blood vessels and lymphatics enter the
perimysium to supply each fascicle.
c) Endomysium: Within the fascicle, this is a very thin delicate layer of reticular
fibers and scattered fibroblasts which surrounds the individual muscle fibers.
ORGANIZATION WITHIN MUSCLE FIBRES
A myofibril is a cylindrical bundle of contractile proteins found within the muscle cell.
Note that there are several myofibrils within each muscle cell. It is the arrangement of
the contractile proteins within the myofibril that cause the striated appearance of
skeletal and cardiac muscle. Myofibrils are composed of individual contractile proteins
called myofilaments. These myofilaments are generally divided into thick and thin
myofilaments.
The thin myofilaments are composed mainly of a protein known as F-actin. F-Actin
filaments are anchored into the z-line of a sarcomere Actin filaments are tightly
associated with two regulatory proteins : Tropomyosin a 40n long coil of two
polypeptide chains located in the groove between the two twisted actin strands. The
second protein is Troponin which is a complex of three subunits such as Troponin C, I
and T. The thick myofilaments are composed mainly of the protein myosin. Myosin is a
large complex with two identical heavy chains and two light chains twisted together as
myosin tails. It is the orderly overlapping of the actin and myosin filaments that give
cardiac and skeletal muscle their striated appearance (light and dark bands).
The A band is the dark band and corresponds to the length of a bundle of myosin
filaments. Because muscle contraction is a sliding of the myofilaments past each other
we do not see any of the myofilaments actually shorten. However the width of the
banding patterns change as the degree of overlap changes. Because the A band
corresponds to the length of the myosin filaments, and these filaments do not shorten,
the width of the A band also does not shorten.
The light bands are known as I bands. The I bands are composed mainly of actin
filaments. Each I band is bisected by a protein disc known as the Z-line. Actin filaments
are anchored into the Z-line. During muscle contraction the actin filaments slide over
the myosin filaments which results in a shortening of the I band.
In the middle of the A band is a somewhat lighter area known as the H zone. This zone
corresponds to the area where we have myosin not overlapped by actin (the area
between the thin filaments). During muscle contraction the actin sliding over the myosin
encroaches into this area so that the H zone shortens. In the middle of the H zone we
see a dark band known as the M line. The M line is comprised of protein fibers that
function to anchor the myosin filaments.
The area between two Z lines is known as a SARCOMERE. The sarcomere is the
functional or contractile unit of muscle.
ENDOPLASMIC RECTICULUM AND TRANSVERSE TUBULAR SYSTEM
In skeletal muscle fibers, the smooth ER is known as the Sarcoplasmic Reticulum. It is
specialized for Calcium ions sequestration. It releases Calcium ions when triggered and
thus causing a uniform contraction in all myofibrils.
The sarcolemma is folded into a system of Transverse or T- tubules. These long
invaginations penetrate deeply into the Sarcoplasmic and encircle the myofibrils.
Adjacent to each tubule are expanded Terminal Cisterns of the SR on each side forming
a TRIAD.
MECHANISM OF CONTRACTION
During contraction, neither the actin nor myosin filaments change their length.
Contraction occurs as the overlapping thick and thin filaments of each sarcomeres slide
past each other. Contraction is induced when an action potential arrives at a synapse,
the neuromuscular junction (NMI) and is transmitted along the T tubules to the
Sarcoplasmic reticulum to trigger the Ca ions release. In a resting muscle, the myosin
heads cannot bind with G actin because the binding sites are blocked by troponintropomyosin complex on the F-actin filaments. Calcium ions released upon neural
stimulation binds troponin changing its shape and moving tropomyosin on the F-actin to
expose the myosin binding sites and enable cross bridges to form thus pulling actin
filaments in the A band towards Z disk.
Energy for the pivot and pulling is provided by the hydrolysis of ATP bound to the
myosin heads. When neural impulse stops and level of free calcium diminishes,
tropomyosin again, covers the myosin binding sites on actin, filaments slide back and
sarcomeres return to their original length. In the absence of ATP the myosin cross
bridges become stable, which accounts for the rigidity of skeletal muscles (rigormortis)
occurring as mitochondrial activity stops after death.
CARDIAC MUSCLE
During embryonic development, mesodermal cells of the primitive heart tube align in
chain-like arrays. Cardiac muscle cells form complex junctions between interdigitating
processes. Cells within s fiber often branch and bind to cells in adjacent fibers.
Consequently, the heart consists of tightly knit bundles of cells, interwoven in a fashion
that provides for characteristic wave of contraction that resembles wringing out of the
ventricles.
Characteristics
1) Mature cardiac muscle cells are approximately 15micrometer in diameter and
from 85 to 100 micrometer in length.
2) They exhibit a cross-striated banding pattern comparable to that of skeletal
muscle.
3) Unlike the multi-nucleated skeletal cell, each cardiac cell consists of one or two
pale staining nuclei.
4) Surrounding the muscle cells is a sheath of Endomysium rich in capillaries
5) They have dark staining transverse lines that cross the chains of cardiac cells at
irregular intervals where the cells join, known as Intercalated Discs. They
represent the interface between adjacent muscle cells and contain many
junctional complexes.
Transverse regions of these step-like discs have many desmosomes and fascia
adherents together, these serve to bind the cardiac muscle cells firmly together
and prevent their pulling apart under constant contractile conductivity.
Longitudinal oriented portions have many gap junctions providing ionic
continuity between cells. These serve as ionic synapses and allow cells of cardiac
muscle to act like a multinucleated syncytium as in skeletal muscle with
contraction signals passing in a wave from cell to cell.
The structure and function of contractile proteins are the same as in skeletal
muscles. The T-tubule and Sarcoplasmic reticulum however are not as regularly
arranged in the cardiac fiber. T-tubules are more in number than the
Sarcoplasmic reticulum. Cardiac muscle cells contain a lot of mitochondria which
occupy 40% of the cytoplasm. Fatty acids transported by lipoproteins are the
major fuels of the heart and are stored as Triglycerides in numerous lipid
droplets.
Muscles of the ventricles are thicker than the muscles of the atria of the heart
reflecting to it’s use of pumping systemic blood. Atria cells are somewhat smaller
with fewer t-tubules than the ventricular cells. Membrane limited cytoplasmic
granules each about 0.2 to 0.3 micrometer in diameter are found near atria
muscle nuclei and are associated with small Golgi complexes. These granules
release the peptide hormone Atrial Natriuretic Factor (ANF) that acts on target
cells in the kidney to affect the Na ions excretion and water balance. The
contractile cells of the heart’s atria thus also serve in endocrine functions.
Cardiac Muscle Fiber Contraction is intrinsic and spontaneous. Impulses for the rhythmic
contraction or heartbeat are initiated, regulated, and coordinated locally by nodes of
unique myocardial fibers specialized for impulse generation and conduction. As with
skeletal muscle fibers, contraction of individual cardiac muscle fibers is all-or-none. The
rate of contraction is modified by autonomic innervations of the nodes of conducting
cells, with the sympathetic nerve supply accelerating and then parasympathetic supply
decreasing the frequency of the impulses.
SMOOTH MUSCLES
Smooth muscles are specialized for slow and steady contractions and is controlled by
involuntary mechanisms. Fibers of smooth muscle also called visceral muscle are
elongsted, tapering and nonstriated cells,each of which is enclosed by a thin basal
lamina and a fine network of reticular fibers, the Endomysium. The connective tissue
serve to combine the forces generated by each smooth muscle fiber into a concered
action example, peristalsis in the small intestine.
Characteristics
1) Smooth muscle cells may range in length from 20 micrometer in small blood
vessels to 500 micrometer in the pregnant uterus.
2) Each cell has a single long nucleus located in the center of the cell.
3) The cells stain uniformly along their lengths .
4) All cells are linked by numerous gap junctions.
5) The borders of the cell becomes scalloped when the smooth muscle contracts
and the nucleus becomes distorted.
6) In addition to contractility, smooth muscle cells supplement fibroblast activity ,
synthesizing collagen, elastin and proteoglycans with a major influence on the
extra cellular matrix where the contractile cells are abundant.
Concentrated near the nucleus are the mitochondria, polyribosome, RER, Golgi
apparatus. The short membrane invaginations, called Caveoleae are usually found on
the smooth muscle surface. Fibers have rudimentary Sarcoplasmic reticulum that lack ttubules their function Is unnecessary in smaller tapering cells with many gap junctions.
Caveoleae contain several pumps and ion channels and may serve to organize protein
signaling calcium release in myofibrils.
Contraction Of Smooth Muscles.
The contractile activity is generalized by myofibrillar array of actin and myosin fiiaments.
Bundles of thick and thin myofilaments crisscross obliquely through the cell. Myosin
filaments have a less regular arrangement among the thin filaments and fewer cross
bridges than in striated muscles. The actin filaments lack troponin, using instead,
Calmodulin and Calcium sensitive myosin light-chain kinase (MLCK) in the contraction
mechanism. Sliding filament mechanism of contraction is similar to that in striated
muscle. Contraction is regulated differently in the smooth muscle of the viscera, the
respiratory airways or large and small blood vessels. it could involve autonomic nerves,
a variety of hormones and local physiological conditions such as the degree of stretch .
Whether they contract as small groups or throughout an entire muscle is determined by
the degree of autonomic innervations and the density of the gap junctions, both
conditions vary in different organs.
Smooth muscle cells have intermediate filaments usually composed of Desmin. These
filaments and F-actin filaments insert into the cell associated dense bodies. Dense
bodies contain alpha-actinin and are functionally similar to Z – discs of striated and
cardiac muscles. The attachment of thin and intermediate filaments to the dense bodies
helps to transmit contractile force to adjacent smooth muscle cells and surrounding
network of reticular fibers.
INNERVATION
Smooth muscles are not under voluntary control and its fibers lack MEPs. They are
often active without nervous stimuli; its nerve supply serves primarily to modify and not
to initiate it. They receive both adrenergic and cholinergic nerve endings that act
antagonistically, stimulating or depressing its activity. In some organs, cholinergic
activate while the adrenergic stimulate, in others, the reverse is the case.