OBJECTIVES:
U1P10
MUSCLES
Muscle classification
Striated – skeletal and cardiac
 Cross-striations
 Due to arrangement of actin and myosin myofilaments
Smooth
 Actin and myosin not as precisely arranged
Function = contraction
 Movement of body
 Changes in size and shape of organs
Muscle cell = muscle fibre
 Elongated
 Parallel to each other
Some definitions
Sarcolemma = plasma membrane
Sarcoplasm = cytoplasm
Sarcoplasmic reticulum = sER
Muscle fibre cells
Cross-section view
 Polygonal shape
 10-100µ diameter
 2mm up to 1metre long
 Nucleus directly below plasma membrane
Actin + myosin
= myofilaments

Bundles of myofilaments
= myofibrils
Surrounded by sER, mitochondria and glycogen

Bundles of myofibrils
= fill sarcoplasm of muscle fibres

Bundles of fibres
= muscle
To know and understand the definitive light microscope features of:

Smooth muscle fibres
Occurs as bundles or sheets of elongated fusiform cells with finely tapered ends. The cells are also called fibers and
range in length from 20 microns in the walls of smooth blood vessels to about 200 microns in the wall of the intestine;
they may be as large as 500 microns in the wall of the uterus during pregnancy. It is specialised for slow prolonged
contraction. there are no direct neural endings on smooth muscle cells; nerve terminals in smooth muscles are in
immediately adjacent connective tissue, and the neurotransmitter diffuses to the muscle cells. Gap junctions +
communication junctions between smooth muscle cells allow propagation of small cells and ions of the contraction
within the muscle.

Skeletal muscle fibers
A skeletal muscle cell (commonly called a muscle fiber) is actually a multinucleated syncytium. Muscle fibers are
formed during development by the fusion of small individual muscle cells called myoblasts. When viewed in cross
section, the mature multinucleated muscle fiber reveals a polygonal shape with a diameter of 10 to 100 microns. Their
length varies from almost a meter, as in the sartorius muscle of the lower limb, to as little as a few millimeters, as in the
stapedius muscle of the middle ear. They are held together by connective tissue.
Side note: muscle fibers should not be confused with a connective tissue fiber; muscle fibers are skeletal muscle cells,
whereas connective tissue firbers are extracellular products of connective tissue cells.
Skeletal muscle are long multinucleated protoplsmic units arranged in parallel with their neighbours. the multiple nuclei
are at the periphery of the cell, just under the plasma membrane (called the sarcolemma- it represents the plasma
membrane, its external lamina and the surrounding reticular lamina); contractile elements fill the rest of the cell. the
highly ordered arrangement of the contractile filaments of actin & myosin accounts for the cross- striations
characteristics of a longitudinal section of skeletal muscle fibers (thus they are also called striated muscle). Skeletal
muscle has a rich blood supply, and each fiber is usually close to several capillaries.
SUMMARY
Cross-striations
 Seen in longitudional sections
 Due to the arrangement of the myofilaments
Functional unit of Muscle
= the Sarcomere
The Sarcomere
 Extends from Z-line to Z-line
 2-3µ long when relaxed
 4µ long at maximum stretch
 1µ long when contracted

Cardiac muscle fibres
Has the same types of arrangement of contractile filaments as skeletal muscle, Therefore, cardiac muscle cells and fibers
they form exhibit cross-striations evident in sections. They also exhibit densely staining cross-bands, called intercalated
discs --> these cross the fibers in a linear fashion or frequently in a way that resembles the risers of a stairway. The
intercalated discs represent highly specialised attachment sites between adjacent cells. This linear cell-to-cell attachment
of the cardiac muscle cells results in ‘fibers’ of variable length. Thus unlike skeletal cells & visceral striated muscle
fibers that represent multinucleated single cells, cardiac muscle fibers consist of numerous cylindrical cells arranged end
to end. Furthermore, some cardiac muscle cells in a fiber may join with two or more cells through intercalated discs, thus
leading to a branched
fiber.
To know the locations of the three types of muscle.
Smooth muscle: Vessels, organs and viscera
Skeletal muscle: Muscles of skeleton visceral striated (e.g. tongue, esophagus, diaphragm)
Cardiac muscle: Heart, superior & inferior vena cava, pulmonary veins
To know the ultrastructure of skeletal muscle including the location and role of:

Sacroplasmic reticulum is arranged as a repeating series of networks around the myofibrils. each network of the
reticulum extends from one A-I junction to the next A-I junction with a sarcomere. Thus one network of sarcoplasmic
reticulum surrounds the A band, and the adjacent network surrounds the I band. Where the two networks meet, at the
junction between A and I bands, the sarcoplasmic reticulum forms a slightly more regular ringlike channel called the
terminal cinsternae. This serves as reservoirs for Ca2+. To release Ca2+ into the sarcoplasm, the plasma membrane of
the terminal cisternae contains an abundance of gated Ca2+ -release channels.
Also located around the myofibrils in association with the sarcoplasmic reticulum are large numbers of mitochondria and
glycogen granules, both of which are involved on providing the energy necessary for the reactions involved on
contraction.
SUMMARY
 Arranged as a network around the myofibrils
 Resevoirs of Ca2+ mitochondria and glycogen associated with the sER
 Located at the A-I junction

T-tubules
Transverse tubular system, or T system consists of numerous tubular invaginations of the plasma membrane; each one is
called a T tubule. T tubules penetrate to all levels of the muscle fiber and are located between adjacent terminal cisternae
at the A-I junctions. They contain voltage sensor proteins, depolarization-sensitive transmembrane channels that are
activated when the plasma membrane depolarizes. Conformational changes of these proteins directly affect the gated
Ca2+ -release channels located in the adjacent plasma membrane of the terminal cisternae. THe complex of T tubule
and the two adjacent cisternae is called a triad.
Regulation of contraction involves Ca2+, sarcoplasmic reticulum and the transverse tubular system.
SUMMARY
 Invaginations of the sarcolemma
 Located at the A-I junction
 Depolarization of T-Tubule

 Ca2+ release from sarcoplasmic reticulum

 Initiates muscle contraction
To understand the role of the myofilament in the sliding filament mechanism of contraction
Myofilaments are the individual filamentous polymers of myosin II (thick filaments) and actin and its associated proteins
(thin filaments).
The contraction cycle: shortening of a muscle involves the rapid contraction cycles that move the thin filaments along the
thick filament . Each contraction cycle consists of five stages,: attachment, release, bending, force generation and
reattachment.
Attachment is the initial stage of the contraction cycle, in which the myosin head is tightly bound to the actin molecule of the
thin filament. Release is the second stage of the cycle, in which the myosin head is uncoupled from the thin filament.
Bending is the third stage of the cycle, in which the myosin head , as a result of hydrolysis of ATP, advances a short
distance in relation to the thin filament. Force generation is the fourth stage of the cycle, in which the myosin head
releases inorganic phosphate and the power stroke occurs. Reattachment is the fifth & last stage if the cycle in which the
myosin head binds tightly to a new actin molecule.
The sliding filament model postulates that the ratchetlike movements of the myosin heads bound to actin produce the
movement of the thin filaments relative to the thick filaments (which in turn cause the sarcomere to shorten). Although
the model can explain contractions in a single sarcomere, it cannot adequately explain the shortening of a myofibril or
muscular fiber, If the activity were to occur simultaneously in adjacent sarcomeres, no contraction would occur. Equal &
opposite forces would be exerted on either side of the Z line, and the contraction of any given sarcomere would be
prevented by the contraction of its two immediate serial neighbours. Recent studies have demonstrated that an
extrememly small temporal delay occurs between the contraction of adjacent sarcomeres, so that a wavelike contraction
actually occurs in each muscle fibril, & consequently, in each muscle fiber.
SUMMARY
2 types associated with contraction
Thin myofilaments
 6-8nm diameter
 1µ long
 Actin, troponin (Ca2+), tropomyosin
Thick filaments
 15nm diameter
 1.5µ long
 Myosin, ATP binding sites
To know the ultrastructure of cardiac muscle and the differences and similarities between cardiac and skeletal muscle.
Cardiac muscle: The central location of the nucleus in cardiac muscle cells is one feature that helps distinguish them from
multinucleated skeletal muscle fibers, whose nuclei lie immediately under the plasma membrane. The transmission
electrion microscope (TEM) reveals that the microfibrils of cardiac muscle separate to pass around the nucleus, thus
outlining a biconical juxtanuclear region in which the cell organelles are concentrated. This region is rich in
mitochondria & contains the Golgi apparatus, lipofuscin pigment granules, and glycogen. In the atria of the heart,
polypeptide atrial granules measuring 0.3 - 0.4 microns in diameter are also concentrated in the juxtanuclear cytoplasm.
These granules contain two polypeptide hormones: atrial natriuretic factor (ANF) and brain natriuretic factor (BNF).
BOth hormones are diuretics, affecting urinary excretion of sodium. they inhibit renin secretion by the kidney and
aldosterone secretion by the adrenal gland. They also inhibit contractions of vascular smooth muscle. In congestive heart
failure, levels of circulating BNF increase.
Cardiac muscle
Same arrangement of myofilaments as skeletal muscle
 Centrally located nucleus
 Myofibrils separate to pass the nucleus
 Organelles located around the nucleus
 Cytoplasmic granules (hormones)
 Large mitochondria, glycogen granules located between the
myofibrils
 Sarcoplasmic reticulum
Glycogen = stores energy
Mitochondria = releases energy
Myofibrils = use energy
What is different ?
= Intercalated Discs
1. Attachment sites between muscle fibres (cells)
2. Fibres of variable length
3. Fibres can branch
To know the ultrastructure of smooth muscle:

Myofilaments
Smooth muscle cells possess a contractile apparatus of thin & thick filaments & a cytoskeleton of desmin and vimentin
intermediate filaments. The remaining sarcoplasm is filled with thin filaments that form a part of the contractile
apparatus. Thick myosin filaments are scattered throughout the sarcoplasm of a smooth muscle cell. They are extremely
labile & tend to be lost during tissue preparation. Special techniques can be used, however, to retain the structural
integrity of the thick filaments & thus demonstrate them with the TEM. The thin filaments of smooth muscle cells are
attached to cytoplasmic densities or dense bodies that are visible among the filaments. These structures are distributed
throughout the sarcoplasm in a network of intermediate filaments containing the protein desmin (vascular smooth muscle
contains vimentin filaments in addition to desmin filaments), which are part of the cytoskeleton of the cell. The
components of the contractile apparatus in smooth muscle are:
thin filaments containing actin, the smooth muscle isoform of tropomyosin, and two smooth muscle-specific proteins
caldesmon and the calponin.
Thick filaments containing myosin II differ lightly from those found in skeletal muscle. They too are composed of two
polypeptide heavy chains & four light chains. Instead of a bipolar arrangement, the myosin II are orientated in one
direction on one side of the filament and in an opposite direction on the other side of the filament --> staggered in
parallel between two immediate neighbours & are also bound to an anti-parallel partner via an overlap at the very tip of
their tails.
Myosin light chain kinase (MLCK), α-actinin and calmodulin are other smooth muscle proteins associated with the
contractile apparatus.

Cytoplasmic densities
Smooth muscle cytoplasm stains rather evenly with eosin in routine H&E preparations because of the concentrations of
actin & myosin that these cells contain. The nuclei is located in the centre of the cell and often has a corkscrew
appearance in longitudinal sections - a characteristic that is a result of contraction of the cell during fixation. This a
useful differentiating feature between smooth muscle cells and fibroblasts. in the noncontracted cell, the nucleus appears
as an elongated structure with tapering ends, lying in the centre axis of the cell. In a cross section, the nucleus appears as
a circular or round profile whether the cell is contracted or relaxed.

Locations of organelles
The TEM shows most of the cytoplasmic organelles are concentrated at each end of the nucleus. These include numerous
mitochondria, some cisternae of the rER, free ribosomes, glycogen granules, and a small Golgi apparatus.
SUMMARY
Thin myofilaments = actin
Thick myofilaments = myosin
Sarcolemma
Invaginates to form vesicles
Sarcoplasmic reticulum
Does not form a T-system
 Arranged in bundles and sheets
 Elongated cells = fibres
 Tapered cytoplasmic ends
 20-200µ long
 Centrally located nucleus
 Actin and myosin myofilaments in no specific arrangement
 organelles concentrated around the nucleus