Download NAME: IWORH CHINYERE MIRACLE MATRIC NO: 14/MHS02/031

NAME: IWORH CHINYERE MIRACLE
MATRIC NO: 14/MHS02/031
DEPARTMENT: NURSING SCIENCE.
COURSE: ANA 203
ASSIGNMENT: HISTOLOGY OF THE MUSCLE
Muscles are multicellular contractile units.
Muscle Function:
1. Contraction for locomotion and skeletal movement.
2. Contraction for propulsion.
3. Contraction for 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
1. Striated.
2. Non striated or smooth.
Functional classification
There are two types of muscle based on a functional classification system.
1. Voluntary.
2. Involuntary.
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.
II Cardiac muscle: Which is striated and Involuntary.
III Smooth muscle: Which is non striated and Involuntary.
SKELETAL MUSCLE.
Skeletal muscle is mainly responsible for the movement of the skeleton, but is also found in organs such
as the globe of the eye and the tongue. It is a voluntary muscle, and therefore under conscious control.
Skeletal muscle is specialized for rapid and forceful contraction of short duration.
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.
Each muscle cell is defined by a sarcolemma and contains many nuclei along its length. The nuclei are
displaced peripherally within a cross section of the sarcoplasm while a large number of longitudinal
myofibrils, groups of arranged contractile proteins, occupy most of the center space. The myofibril
contains several important histological landmarks:
The myofibril is composed of alternating bands. The I-bands(isotropic in polarized light) appear light in
color and the A-bands(anisotropic in polarized light) appear dark in color. The alternative pattern of
these bands results in the striated appearance of skeletal muscles.
The Z-lines(Zwischenschieben) bisect the I-bands.
A light band called the H-band(Heller) sits within each A-band.
The M-line(Mittelschiebe) bisects each A-band(and, in doing so bisects each H-band).
Each myofibril can be understood as a series of contractile units called sarcomeres that contains two
types of filaments: thick filaments, composed of myosin, and thin filaments, composed of actins. The
individual filaments do not change in length during muscle contraction ; rather the thin filaments slide
over the thick filaments to shorten the sarcomere. The nature of these filaments can be understood in
the context of the histological landmarks of the myofibril.
The thick filaments are a bipolar array of polymerized myosin motors. The motors on one side of the
filament are oriented in the same direction whereas the motors on the other side of the filaments are
oriented in the opposite direction. The center of the filament lacks motors; it contains only the coiledcoil region of the myosin. A set of proteins crosslink's each myosin filament to its neighbor to the center
of the filament. These proteins make up the M-line.
The thin filaments are attached to a disc-like zone that appears histological as the z-line. The Z-lines
contain proteins that bind and stabilize the plus ends of actins filaments. Z-lines also define borders of
each sarcomere.
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.
Skeletal muscles are divided into two muscle fiber types:
Slow-twitch(type l) muscle fibers contract more slowly and rely aerobic metabolism. they contain large
amounts of mitochondria and myoglobin, an oxygen-storage molecule. the reddish color on myoglobin
is why these fibers may be referred to as red fibers. These muscles can maintain continuous contraction
and are use in activities such as the maintenance of posture.
Fast-twitch(type II)muscle fibers contract more rapidly due to presence of a faster myosin. Type II fibers
can be subdivided into those that have a large amounts of mitochondria and myoglobin and those that
have few mitochondria and little myoglobin. The form primarily utilize aerobic respiration to generate
energy, whereas the latter rely on glycolysis. The lack of myoglobin results in a pale color than the slowtwitch muscles, and fast-twitch fibers may therefore be referred to as white fibers. These muscles are
important for intense but sporadic contractions.
Most muscles contain a mixture of these extreme fiber types. In humans, the fiber types cannot be
distinguished based on gross examination, but requires specific stains or treatments to differentiate the
fibers.
SMOOTH MUSCLE
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 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 is involuntary muscle.
Smooth muscle is specialized for slow and sustained contractions of low force. 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 involuntary muscle.
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 (actins) filaments are scattered throughout the sarcoplasm and are
attached to adhesion densities on the cell membrane and focal densities within the cytoplasm. since the
contractile proteins of these cells are not arranged into myofibrils like those of skeletal and cardiac
muscle, they appear smooth rather than striated.
Smooth muscle fibers are bound together in irregular branching fasciculi that vary in arrangement from
organ to organ. these fasciculi are the functional contractile units. there is also network of supporting
collagenous tissues between the fibers and the fasciculi.
CARDIAC MUSCLE
Cardiac muscle shares important characteristics with both skeletal and smooth muscle. Functionally,
cardiac muscle produces strong contractions like skeletal muscles. however, it has inherent mechanism
to initiate continuous contraction is not subject to voluntary control, but is influenced by the autonomic
nervous system and hormones.
Histologically, cardiac muscle appears striated like the skeletal muscle due to arrangement of contractile
proteins. it also has several unique structural characteristics:
The fibers of cardiac muscle are not arranged in a simple parallel fashion. instead, they branch at the
ends to form connections with multiple adjacent cells, resulting in a complex, three-dimensional
network.
Cardiac muscle fibers are long cylindrical cells with one or two nuclei. the nuclei are centrally situated
like that of smooth muscle.
Cardiac muscle sarcoplasm has a great amount of mitochondria that meet the energy demands.
Similar to the skeletal muscle, cardiac muscle cells have an invigilating network of T-tubules and
sarcoplasmic reticulum.
In atrial cardiac muscle cells, secretory granules can be seen. the granules contain atrial natriuretic
factor(ANF), which is released upon excess filling of the atria and opposes the action of angiotensin II in
production of aldosterone.
Collagenous tissues are found surrounding individual cardiac muscle fibers. there is abundance
vascularisation within this supporting tissue, which is required to meet the high metabolic demands of
cardiac muscle.
The cardiac muscle fibers are joined end to end by specialized functional regions called the intercalated
discs. the intercalated discs provide anchorage for myofibrils and allow rapid spread of contractile
stimuli between cells. such rapid spread of contraction allows the cardiac muscles to act as a functional
syncytium. the intercalated discs contain three types of membrane-to-membrane contact:
Fascia adherens, which are connected to actins filaments to transmit contraction.
Desmosomes, which connect to intermediate filaments of the cytoskeleton.
Gap junctions, which are sites of low electrical resistance that allow the spread of excitation.
In addition to the contractile cells, there is a specialized system made up of modified muscle cells whose
function is to generate the stimulus for heartbeat and conduct the impulse to various parts of the
myocardium. this system consists of sinoatrial node, atrioventricular node, bundle of His, and Purkinje
fibers.
SOURCE: INTERNET