Download Skeletal muscle

NAME: ATUNRASE ABIODUN
MATRIC NUMBER: 13/MHS01/031
DEPARTMENT: MEDICINE AND SURGERY
COURSE: HISTOLOGY
COURSE CODE: ANA 203
LECTURER: MR ADESHINA
Histology of muscle tissue
Despite its complexity, the human body is composed of only four basic types of tissue:
epithelial, connective, muscular, and nervous. These tissues, which are formed by cells and
molecules of the extracellular matrix, exist not as isolated units but rather in association with
one another and in variable proportions, forming different organs and systems of the body
Muscle tissue is categorized on the basis of a functional property: the ability of its cells to
contract. In muscle tissue, the bulk of the cytoplasmic volume consists of the contractile protein
myosin and the cytoskeletal protein actin, where they are the primary structural components
that bring about contraction. Muscle is responsible for movement of the body and changes in
the size and shape of internal organs.
Muscle cells are generally referred to as muscle fibres. (Note that the term "fibre" is used both
for muscle cells, and for the extracellular elements, e.g. collagen, produced by connective tissue
cells.) Muscle fibres are typically arranged in parallel arrays, allowing them to work together
effectively. In addition to movement, muscle contraction also fulfills some other important
functions in the body, such as posture, joint stability, and heat production.
Muscle cells, like neurons, can be excited chemically, electrically, and mechanically to produce
an action potential that is transmitted along their cell membranes. Unlike neurons, they
respond to stimuli by activating a contractile mechanism.
*Most of the muscle cells are mainly of mesodermal origin
REGENERATION OF MUSCLE TISSUE
The three types of adult muscle have different potentials for regeneration after injury. In
skeletal muscle, although the nuclei are incapable of undergoing mitosis, the tissue can
undergo limited regeneration. The source of regenerating cells is the sparse population of
mesenchymal satellite cells that lies within the external lamina of each mature muscle fiber.
Satellite cells are inactive, reserve myoblasts that persist after muscle differentiation. After
injury or certain other stimuli, the normally quiescent satellite cells become activated,
proliferating and fusing to form new skeletal muscle fibers. A similar activity of satellite cells has
been implicated in muscle growth after extensive exercise, a process in which they fuse with
their parent fibers to increase muscle mass beyond that occurring by cell hypertrophy. The
regenerative capacity of skeletal muscle is limited, however, after major muscle trauma or
degeneration.
Cardiac muscle lacks satellite cells and has virtually no regenerative capacity beyond early
childhood. Defects or damage (eg, infarcts) in heart muscle are generally replaced by fibroblast
proliferation and growth of connective tissue, forming myocardial scars.
Smooth muscle, composed of simpler, mononucleated cells, is capable of a more active
regenerative response. After injury, viable smooth muscle cells undergo mitosis and replace the
damaged tissue. Contractile pericytes from the walls of small blood vessels participate in the
repair of vascular smooth muscle.
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
1. Striated muscle: Muscle which has a large number of cross-striations (transverse lines).
Skeletal muscle and cardiac muscle belong to this category.
2. Non striated or smooth muscle or plain muscle: Muscle which does not have cross-striations.
It is found in the wall of the visceral organs.
- Functional classification: There are two types of muscle based on a functional classification
system
1. Voluntary-Voluntary muscle is the muscle that is controlled by the will.
2.
Involuntary- Involuntary muscle is the muscle that cannot be controlled by the will
Depending upon situation, there are generally considered to be three types of muscle in the
human body- skeletal muscle, cardiac muscle and smooth muscle About 40 per cent of the
body is skeletal muscle, and perhaps another 10 per cent is smooth and cardiac muscle
Skeletal muscle
Skeletal muscle constitutes the muscle that is attached to the skeleton and controls motor
movements and posture. There are a few instances where this type of muscle is restricted to
soft tissues: the tongue, pharynx, diaphragm and upper part of the esophagus.
Structure
Skeletal muscle cells contain similar components and structures as other cells but different
terms are used to describe those components and structure in skeletal muscle cells. The plasma
membrane of skeletal muscle is called the sarcolemma; its cytoplasm is known as sarcoplasm;
the endoplasmic reticulum is called the sarcoplasmic reticulum.
Skeletal muscle fibres (cells) are actually a multinucleated syncytium formed by the fusion of
individual small cylindrical muscle cells or myoblasts, during development. They are filled with
longitudinally arrayed subunits called myofibrils. The myofibrils are made up of the
myofilaments myosin (thick filaments) and actin (thin filaments). The striations reflect the
arrangement of actin and myosin filaments and support structures. The individual contractile
units are called sarcomeres. A myofibril consists of many sarcomeres arranged end to end. The
entire muscle exhibits cross-striations because sarcomeres in adjacent myofibrils and muscle
fibers are in register.
Transmission Electron Microscopy of Skeletal Muscle
The most obvious feature in longitudinal sections of skeletal muscle is the alternating pattern of
dark and light bands, called respectively the A (anisotropic) and I (isotropic) band. The I band is
bisected by a dense zone called the Z line (Zwischenschieben), to which the thin filaments of
the I band are attached. The thin filaments are made up of actin, tropomyosin, and troponin.
These thin filaments are polymers made up of two chains of actin that form a long double helix
The thick filaments, which are about twice the diameter of the thin filaments, are made up of
myosin. The form of myosin found in muscle is myosin-II, with two globular heads and a long
tail. The heads of the myosin contain an actin-binding site and a catalytic site that hydrolyzes
ATP. The thick filaments are lined up to form the A bands. The lighter H bands/zone (Heller) sits
within each A-band .The M-line (Mittelschiebe) bisects each A-band (and, in doing so, bisects
each H-band). The I- and H-bands are areas where thick and thin filaments do not overlap (this
is why these bands appear paler under the microscope). The I-band exclusively contains thin
filaments whereas the H-band contains exclusively thick filaments. When muscle contracts, I
and H bands shorten, while A bands stay same.
T tubules or transverse tubules are narrow tubules formed by the invagination of the
sarcolemma. These tubules penetrate all the way from one side of the muscle fibre to another
side. The nuclei are located peripherally, immediately under the plasma membrane
(sarcolemma). The thickness of individual muscle fibres varies from 10 μ to 100 μ (depending
for example on location in the body and exercise) but each fibre is of uniform thickness
throughout its length. Skeletal muscle fibres do not branch.
Origin
Development of skeletal muscle from mesoderm occurs by mononucleated myoblasts
(individual progenitor cells) fusing together to form multinucleated myotubes (multinucleated,
but undifferentiated contractile apparatus -sarcomere) that express contractile proteins
forming sarcomeres within myofibers (multinucleated and differentiated sarcomeres).
Myofibers are of 2 types: Primary myofibres - first-formed, act as a structural framework upon
which myoblasts proliferate, fuse in linear sequence. Secondary myofibers- second later
population of myofibers that form surrounding the primary fibres.
Skeletal muscle cells (fibers), like other body cells, are soft and fragile.
Tissues
Connective tissue elements surround muscle fibres. The connective tissue covering furnish
support and protection for the delicate cells and allow them to withstand the forces of
contraction. The coverings also provide pathways for the passage of blood vessels and nerves.
Each individual muscle fibres are surrounded by a delicate layer of reticular fibres called the
endomysium. The endomysium contains only capillaries and the finest neuronal branches..
Groups of fibres are bundled into fascicles by a thicker CT layer called the perimysium. The
collection of fascicles that constitutes one muscle is surrounded by a sheath of dense CT called
the epimysium, which continues into the tendon.
Commonly, the epimysium, perimysium, and endomysium extend beyond the fleshy part of the
muscle, the belly or gaster, to form a thick ropelike tendon or a broad, flat sheet-like
aponeurosis. The tendon and aponeurosis form indirect attachments from muscles to the
periosteum of bones or to the connective tissue of other muscles. Typically a muscle spans a
joint and is attached to bones by tendons at both ends. One of the bones remains relatively
fixed or stable while the other end moves as a result of muscle contraction.
Innervations
Before a skeletal muscle fiber can contract, it has to receive an impulse from a somatic nerve
cell. Generally, an artery and at least one vein accompany each nerve that penetrates the
epimysium of a skeletal muscle. Branches of the nerve and blood vessels follow the connective
tissue components of the muscle of a nerve cell and with one or more minute blood vessels
called capillaries. Because the axons of the spinal motor neurons supplying skeletal muscle each
branch to innervate several muscle fibres, the smallest possible amount of muscle that can
contract in response to the excitation of a single motor neuron is not one muscle fibre but all
the fibres supplied by the neuron. Each single motor neuron and the muscle fibres it innervates
constitute a motor unit.
Contraction
Motor axons terminate in a neuromuscular junction on the surface of skeletal muscle fibers.
The neuromuscular junction is composed of a pre-synaptic nerve terminal and a post-synaptic
muscle fiber. Upon depolarization, the pre-synaptic vesicles containing the neurotransmitter
acetylcholine fuse with the membrane, releasing acetylcholine into the cleft. Acetylcholine
binds to receptors on the post-synaptic membrane and causes depolarization of the muscle
fiber, which leads to its contraction. Typically, one action potential in the neuron releases
enough neurotransmitter to cause one contraction in the muscle fiber.
Cardiac muscle Cardiac muscle is the type of muscle found in the heart, and at the base
of the vena cava as they enter into the heart. Cardiac muscle is intrinsically contractile but is
regulated by autonomic and hormonal stimuli.
Structure
Cardiac muscle exhibits striations because it also has actin and myosin filaments arranged into
sarcomeres. Generally these striations do not appear as well-defined as in skeletal muscle. The
actin filaments are thin, causing the lighter appearance of the I bands in striated muscle,
whereas the myosin filament is thicker, lending a darker appearance to the alternating A bands
as observed with electron microscopy. However, in contrast to skeletal muscle, cardiac muscle
cells are typically branch-like instead of linear.
At the ultrastructural level, some differences in the arrangement of the sarcoplasmic
retiuculum and T tubules can be seen. Cardiac muscle also has a much greater number of
mitochondria in its cytoplasm.
At the light microscope level, a number of features distinguish cardiac from skeletal muscle.
Cardiac muscle cells have only one or two nuclei, which are centrally located. The myofibrils
separate to pass around the nucleus, leaving a perinuclear clear area (not always evident in
standard preparations). This clear area is occupied by organelles, especially mitochondria
(which are of course not visible in LM). As in skeletal muscle, individual muscle fibres are
surrounded by delicate connective tissue. Numerous capillaries are found in the connective
tissue around cardiac muscle fibres.
Cardiac muscle cells are joined to one another in a linear array. The boundary between two
cells abutting one another is called an intercalated disc. The intercalated discs contain three
types of membrane-to-membrane contact: fascia adherens, which are connected to actin
filaments to transmit contraction, desmosomes, which connect to intermediate filaments of
the cytoskeleton and gap junctions, which are sites of low electrical resistance that allow the
spread of excitation They permit cardiac muscle to function as if it were a syncytium, even
though no protoplasmic bridges are present between cells. Intercalated discs help to facilitate
the passage of an electrical impulse from cell to cell and to keep the cells bound together
during constant contractile activity. They provide a strong union between fibres, maintaining
cell-to-cell cohesion, so that the pull of one contractile cell can be transmitted along its axis to
the next.
The T system in cardiac muscle is located at the Z lines rather than at the A–I junction, where it
is located in mammalian skeletal muscle. Although made up of individual fibres, heart muscle
acts as a functional syncytium during contraction for the efficient pumping of blood.
Cardiac muscle is generally slow and has relatively low ATPase activity. Its fibres are dependent
on oxidative metabolism and hence on a continuous supply of O2. The human heart contains
both α and β isoforms of the myosin heavy chain (α MHC and β MHC). β MHC has lower myosin
ATPase activity than α MHC. Both are present in the atria, with the α isoform predominating,
whereas the β isoform predominates in the ventricle. The spatial differences in expression
contribute to the well-coordinated contraction of the heart.
Until recently, it was commonly believed that cardiac muscle cells could not be regenerated.
However, a study reported in the April 3, 2009 issue of Science contradicts that belief. One way
that cardiomyocyte regeneration occurs is through the division of pre-existing cardiomyocytes
during the normal aging process. In addition, certain growth factors promote the self-renewal
of endogenous cardiomyocytes and cardiac stem cells.
Smooth muscle
Like cardiac muscle, smooth muscle fibres are intrinsically contractile but responsive to
autonomic and hormonal stimuli. Smooth muscle forms the contractile portion of the wall of
the digestive tract from the middle portion of the esophagus to the internal sphincter of the
anus. It is also found in the iris and ciliary body of the eye and associated with hair follicles
(arrector pili). It is found in the walls of the ducts in the glands associated with the alimentary
tract, in the walls of the respiratory passages from the trachea to the alveolar ducts, and in the
urinary and genital ducts. The walls of the arteries, veins, and large lymph vessels contain
smooth muscle as well.
Structure
Well-defined myofibrils and sarcomere are absent in smooth muscles, so the alternate dark and
light bands are absent. Since the contractile proteins-actin and myosin filaments of these cells
are not arranged into myofibrils like those of skeletal and cardiac muscle, they appear smooth
rather than striated.
Instead of Z lines, there are dense bodies in the cytoplasm and attached to the cell membrane
and these are bound by α-actinin to actin filaments. Dense bodies are the special structures of
smooth muscle fibres to which the actin and tropomyosin molecules of thin filaments are
attached. The dense bodies are scattered all over the sarcoplasm in the network of
intermediate filaments, which is formed by the protein desmin and vimentin. Smooth muscle
contains tropomyosin, but troponin appears to be absent. Instead calmodulin (which takes on
the regulatory role in smooth muscle), caldesmon and calponin are significant proteins
expressed within smooth muscle. The isoforms of actin and myosin differ from those in skeletal
muscle. A sarcoplasmic reticulum is present, but it is less extensive than those observed in
skeletal or cardiac muscle. In general, smooth muscles contain few mitochondria and depend,
to a large extent, on glycolysis for their metabolic needs.
Smooth muscle fibers are elongated spindle-shaped cells with a single nucleus. In general, they
are much shorter than skeletal muscle cells. The nucleus is located centrally and the sarcoplasm
is filled with fibrils. The thick (myosin) and thin (actin) filaments are scattered throughout the
sarcoplasm and are attached to adhesion densities on the cell membrane and focal densities
within the cytoplasm.
Smooth muscle fibres do not branch. They range enormously in size, from 20 (in wall of small
blood vessels) to 500 (in wall of uterus during pregnancy) micrometers. Smooth muscle fibres
lie over one another in a staggered fashion (tapered part of one fibre over thicker part of
another).
In longitudinal sections, it is often not possible to distinguish the fibre boundaries, and smooth
muscle may closely resemble connective tissue (bundles of collagen). Where smooth muscle
bundles are interlaced with bundles of connective tissue (e.g. in the uterus), one can distinguish
the smooth muscle by the orientation of the nuclei (all oriented in the same direction), and the
greater abundance of nuclei per unit area (every smooth muscle cell has a nucleus, fibroblast
nuclei are more scattered in bundles of CT). Also, smooth muscle nuclei often have a corkscrew
shape in longitudinal section due to contraction of the muscle fibre during fixation. In cross
section, smooth muscle appears as profiles of various sizes, depending on whether the cut went
through the thick central part or tapered end of any individual fibre. Nuclei are seen only in the
thicker profiles.
*One distinguishing physiological feature of smooth muscle is its ability to secrete connective
tissue matrix. In the walls of blood vessels and the uterus in particular, smooth muscle fibres
secrete large amounts of collagen and elastin and proteoglycans, extracellular matrix (ECM)
components normally synthesized by fibroblasts
Contraction of smooth muscle is not under voluntary control, but is regulated by autonomic
nerves, certain hormones, and local physiological conditions such as the degree of stretch. 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.
Innervations
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.
Contraction
Smooth muscle is specialized for slow and sustained contractions of low force because of poor
development of ‘L’ tubules (sarcoplasmic reticulum). So, the calcium ions, which are responsible
for excitation-contraction coupling, must be obtained from the extracellular fluid which makes
the process of excitation-contraction coupling slow. Instead of having motor units, all cells
within a whole smooth muscle mass contract together. Smooth muscle has inherent
contractility, and the autonomic nervous system, hormones and local metabolites can influence
its contraction. Since it is not under conscious control, smooth muscle is an involuntary muscle.
Diseases
"Smooth muscle condition" is a condition in which the body of a developing embryo does not
create enough smooth muscle for the gastrointestinal system. This condition is fatal. Antismooth muscle antibodies (ASMA) can be a symptom of an auto-immune disorder, such as
hepatitis, cirrhosis, or lupus. Vascular smooth muscle tumors are very rare. They can be
malignant or benign, and morbidity can be significant with either type.
References
http://medcell.med.yale.edu/histology/muscle_lab.php
Wikipedia
Junqueira’s Basic Histology Text & Atlas 13TH EDITION
Essential Medical Physiology BY SEBULINGAM 7TH EDITION