Download HISTOLOGY OF MUSCLES

HISTOLOGY OF MUSCLES
OBJECTIVES
At the end of this lecture, students should be able to:
1. Be able to distinguish the three types of muscle at the light and electron
microscope levels, including distinctive features of each muscle fiber.
2. Describe the structural basis of muscle striations.
3. Know the structural elements that harness muscle contraction to the movement of
a body part.
4. Know the function and organization of the connective tissue in muscle.
LECTURE OUTLINE:
Muscle Function
1. Contraction for locomotion and skeletal movement
2. Contraction for propulsion
3. Contraction for pressure regulation
Muscle classification
1. muscle tissue may be classified according to a morphological classification or a
functional classification.
Morphological classification (based on structure)
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There are two types of muscle based on the morphological classification system
Striated
Non striated or smooth.
Functional classification
There are two types of muscle based on a functional classification system
Voluntary
Involuntary.
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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
Characteristics of skeletal muscle
1. Skeletal muscle cells are elongated or tubular.
2. multiple nuclei located on the periphery of the cell.
3. Skeletal muscle is striated.
Characteristics Of Cardiac Muscle
1. Cardiac muscle cells are not as long as skeletal muscles cells and often are
branched cells.
2. Cardiac muscle cells may be
3. mononucleated or binucleated.
4. nuclei are located centrally in the cell.
5. Cardiac muscle is also striated.
6. In addition cardiac muscle contains intercalated discs.
Characteristics Of Smooth Muscle
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Smooth muscle cell are described as spindle shaped. Wide in the middle and
narrow to almost a point at both ends.
Single centrally located nucleus.
Smooth muscle cells do not have visible striations although they do
Contains the same contractile proteins as skeletal and cardiac muscle, but laid out
in a different pattern.
Shapes Of Skeletal Muscles
1. Parallel or fusiform: as their name implies their fibers run parallel to each other.
These muscles contract over a great distance and usually have good endurance but
are not very strong.
EXAMPLES: Sartorius muscle and rectus abdominus muscle.
2. Convergent: the muscle fibers converge on the insertion to maximize the force of
muscle contraction.
EXAMPLES: Deltoid muscle and Pectoralis Major muscle.
Muscle Terminology
Myofiber Or Myocyte: a muscle cell
1. Sarcolemma: the plasma membrane of a muscle cell
2. Sarcoplasm: the cytoplasm of the muscle cell
3. Sarcoplasmic reticulum: the endoplasmic reticulum of a muscle cell
4. Sarcosome: the mitochondria of a muscle cell
5. Sarcomere: the contractile or functional unit of muscle
 Skeletal Muscle have three major areas:
1. A belly or Gaster
2. An origin: a tendinous connection of the muscle to a bone, usually the bone that is
stabilized.
3. An insertion: a tendinous connection of the muscle to a bone, usually the bone to
be moved.
 Skeletal muscle is designed as a bundle within a bundle arrangement called
Fasciculi.
Microscopic Structure Of Skeletal Muscle
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The entire muscle is surrounded by a connective tissue called the epimysium.
The muscle is made up of smaller bundles known as fascicles. Fascicles are
actually bundles of individual muscle cells (myofibers or myocytes).
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These bundles are surrounded by a connective tissue sheath called the
perimysium.
Each fascicle is made up of several muscle cells known as myocytes. They may
also be called myofibers or delicate muscle fibers.
Each muscle Cell is surrounded by a connective tissue sheath known as the
endomysium. This sheath is very important in the physiology of muscle
contraction because it electrically insulates the individual muscle cells from each
other.
At the ends of the muscle all of the connective tissue sheaths (epimysium,
perimysium, and endomysium) converge to form a tendon which will connect the
muscle to its attachment site.
Each muscle fiber (muscle cell) contains all of the organelles that we find in other
cell types.
Although these organelles are the same as in other cells they are given special
names. Note that the prefixes sarco and myo both refer to muscle.
Therefore if you see a word with either of these prefixes you should immediately
think MUSCLE.
Organization Of Skeletal Muscle Fibers
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Sarcoplasm is filled with long cylindrical filamentous bundles called myofibrils
which are 1-2 μm in diameter and run parallel to the long axis of muscle fiber and
consist of an end to end chain like arrangement of sarcomere. The sarcomere in
adjascemt myofibrils cause the entire muscle fiber to exhibit a characteristic
pattern of transverse striations.
The muscle fibers show the cross-section of alternating light and dark bands. The
dark bands are called „A‟ bands and light bands are called „I‟bands.
In electron microscope, each „I‟ band is bisected by a dark transverse „Z‟ line.
The smallest unit of contractile apparatus, the sarcomere extends from „Z‟ line to
„Z‟ line and is about 2.5μm in resting muscles.
Sarcomere pattern is due to two types of filaments, thick and thin.
Thick filaments are 1.6μm long and 15nm wide, they occupy „A‟ band.
thin filaments are 0.1μm long and 8nm wide. The „I‟ band consists of thin
filaments. The thick „A‟ band overlap the thin „I‟band filaments, but „I‟ band do
not overlap thick filament.
The „A‟ band shows the presence of light zones in center, the „H‟ band
containing only Myosin molecules.
The thin filaments run parallel to thick filament and have one line attached to the
„Z‟ line. The „H‟ band is bisected by „M‟ line.
The M‟ line is creatine kinase. The creatine kinase transfers the phosphate group
from phosphocreatine to ADP, then converts ADP to ATP for muscle.
Contraction
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Striated muscle filaments contain 4 types of proteins; Actin, Tropo-myosin,
Troponin and Myosin.
Thin filaments are composed of First three proteins and thick filament of Myosin.
Thin filaments are formed by filamentous protein called as F-actin. F-actin is
composed of two strands of G-actin.
Thin filaments also contain other proteins Tropo-myosin and Troponin.
MYOSIN: myosin is a much larger complex, can be dissociated into two pairs of
heavy and light chains.
Heavy chains are thin rod like molecules that form the Heads which are ATP
binding sites able to bind Actin.
In each thick filament several hundred molecules are arranged.
Analysis of thin section of Striated muscles show presence of cross bridges b/w
thin and thick filaments. The bridges are at head of myosin molecules involved in
the conversion of chemical energy into mechanical energy.
 The nucleus contains the genetic material of the muscle cell.
 The sarcolemma is the name given to the plasma membrane of the muscle cell.
 There are specialized invaginations of the sarcolemma that run transversely across
the cell known as T tubules (transverse tubules).
 The T tubules are essential for carrying the depolarization brought to the cell by a
motor nerve impulse down into the muscle cell where it can have an affect on the
terminal cisternae
 The cytosol is the cytoplasm of the muscle cell.
 The sarcoplasmic reticulum is the endoplasmic reticulum of the muscle cell.
There are sac-like regions of the sarcoplasmic reticulum known as Terminal
Cisternae.
 The terminal cisternae act as calcium storage sites. The calcium ions stored in the
terminal cisternae are essential in muscle contraction. NOTE: this is not calcium
storage for use in general body physiology as we would see with bone tissue, but
rather is calcium storage
Mechanism of Muscle Contraction
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When they receive a motor impulse from a motor nerve.
These nerve impulses serve only a limited number of muscle fibers.
The muscle fibers served by a single motor neuron make up a structure known as
a motor unit.
Motor units allow for selective contraction of muscle fibers so that we may
control the strength and extent of muscle contraction.
Without motor units a nerve impulse to the muscle would result in the entire
muscle contracting to its full extent.
After depolarization of cytoplasmic reticulum membrane, calcium releases on
surface of muscle cells.
Calcium will release from internal sarcoplasmic membranous cisternae.
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Adjacent to opposite sides of T-tubule are expanded terminal cisternae of
sarcoplasmic reticulum.
This specialized complex consist of T-tubule with two lateral portion of
sarcoplasmic reticulum and is known as TRIAD.
At the triad, depolarization of sarcolema is transmitted to sarcoplasmic reticulum
membrane.
The muscle contraction depends on the availability of Ca++ ion and relaxation
depends on Non-availablity.
Mitochondria are sites of energy production (ATP synthesis) in the muscle cell
as in all other cells of the body, except for mature red blood cells.
Blood supply of skeletal muscles
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Rich capillary supply.
Nerve supply.
One motor nerve ending form a somatic nerve.
One nerve fiber innervate a single muscle fiber.
A motor nerve fiber and muscle fiber are collectively known as a motor unit.
Cardiac Muscles
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Involuntary .
Striated,
Nucleus is oval.
Found in Myocardium.
Present in the form of chains.
Cells within each chain bifurcate or branch giving an appearance of syncytial
network.
7. Unique character of cardiac muscle is presence of darkly stained transverse lines
that cross the chain of cardiac cells, these bands often follow an irregular step like
course and are known as Intercalated discs.
8. The structure and function of contractile protein is same as skeletal muscle.
9. The T-tubule are large in ventricular muscles..
10. Cardiac t-tubule are found at the level of „Z‟ band, rather than A-I junction.
11. The sarcoplasmic reticulum is not well developed.
12. Triads are not common in cardiac muscles but T-tubule is associated with one
lateral endoplasmic reticulum cisternae are called Diads.
13. In old age, lipofuscin pigment is deposited around nucleus.
Smooth Muscles
1. Smooth muscle is found in stomach, intestine, urinary and genital tract.
2. Fusiform cells.
3. may range from 20μm to 500μm in pregnant uterus. During pregnancy, uterine
muscle cells undergo a marked increase in size and number.
4. Striations are longitudinal only.
5. Longitudinal striations run throughout length.
6. Single central nucleus.
7. Sarcoplasmic reticulum is rudimentary.
8. T-tubules are not present.
9. Characteristic contractile activity depends upon the Actin and Myosin filaments.
10. There is little connecting tissue b/w adjacent myofibrils.