muscle

Three types of muscle: Skeletal, Cardiac, and Smooth

All muscle cells are long cylindrical cells

All muscle cells are specialized to shorten and contract

All muscle cells have an acidophilic cytoplasm with oval or cigar-shapedeuchromatic nuclei.

All muscle cells contain large amounts of contractile filaments (myofilaments) including actin
microfilaments (thin filaments) and myosin (thick filaments).

All muscle cells are anchored to the connective tissue by a basal lamina (secreted by muscle cell)

All muscle cells are capable of enlarging hypertrophy

All muscle cells require a large vascular supply

All muscle cells develops in mesenchyme derived from mesoderm (except for some in head and neck)

Skeletal muscle has large unbranched, cylindrical fibers with visible cross-striations in an acidophilic cytoplasm.

In skeletal muscle, each fiber contains many nuclei (multinucleated) located peripherally, that is
immediately underneath the sarcolemma (cell membrane).

Connective tissue wrappings

Actin (thin) and myosin (thick) filaments are organized into repeating units called sarcomeres.

Sarcomeres contain light (I band) and dark (A band) regions. The actin filaments are anchored in a region called the Z line.

Sarcomeres extend from Z line to Z line.

End to end sarcomeres form long “sticks” called myofibrils that insert into the sarcolemma. This
arrangement of filaments into dark and light bands is visible in the LM and EM.

Contraction of all of the sarcomeres in ALL of the myofibrils (by sliding filaments) in the cell causes the cell to shorten and contract.

Skeletal muscle contains abundant mitochondria and two
specialized membrane systems

The sarcolemma (cell membrane) and the sarcoplasmic recticulum (smooth endoplasmic recticulum)

The extensive sarcoplasmic reticulum (SER) contains swollen ends, called terminal
cisternae.

Calcium

Muscle contraction

Invaginations of the sarcolemma (cell membrane) form transverse-tubules or T-tubules that carry excitation or depolarization of the membrane inward.

In skeletal muscle, Surrounding each sarcomere at the A/I band junction is a Triad consisting of
two terminal cisternae and one T-tubule.

Excitation of the neuron controlling the muscle cell (lower motor neuron) causes excitation
(depolarization) of the muscle cell membrane at the motor plate. The excitation travels
inward via the T- tubule. This causes the terminal cisternae to release calcium starting the
sequence that causes contraction of all the sarcomeres in each myofibril in the cell.

Continuous with the sarcolemma
Carries excitation inside

Store calcium, releases it with excitation and begins contraction

Lower motor neurons (LMN) from the spinal cord or brain stem innervate each skeletal
muscle cell by a special junction called a synapse.

Skeletal muscle fibers contract ONLY when excited by the neuron. They are
completely dependent on the LMN for its function and survival.

Motor end plate = Neuromuscular junction or synapse = Myoneural junction

A lower motor neuron and all of the fibers it innervates

Acetylcholine, the neurotransmitter in the synaptic vesicles,
binds to the muscle membrane causing a depolarization or excitation.

The motor end plate consists of the axon terminal of the neuron, a small synaptic space, and the membrane of the muscle cell.

Denervation results in an inability of the LMN to “tell” the muscle fiber to contract. The
muscle is “paralyzed”, atrophies (shrinks) , and dies if not reconnected to a neuron

The muscle spindle, the stretch receptor, consists of nerve
endings and several small, modified muscle fibers surrounded by a CT sheath.

All muscle fibers in a motor unit are the same fiber type.

Muscles that have lost and then subsequently regained their innervation have altered
fiber type patterns.

As the lower motor neurons sprout and grow back into the
denervated muscle they form motor end plates on the first muscle fibers they encounter,
thereby forming new motor units

Increased use causes hypertrophy of each fiber while decreased use causes atrophy or
shrinkage of the muscle fiber.

If the muscle is used more (ex. exercise), then more
sarcomeres are made and each muscle fiber gets larger in diameter.

If the muscle use is
reduced (ex. bed ridden), then the sarcomeres are slowly broken down (by autophagy),
resulting in smaller diameter fibers.

1. Myoblasts develop from mesenchyme of somites and migrate to final location
2. Myotubes, large multi nucl. cells, are formed by fusion of myoblasts. MYOTUBES CANNOT DIVIDE.
3. Myofibers are young muscle fibers that develop when myotubes become innervated. Leads to production of sarcomere, membrane, and migration of nuclei peripherally

During the third week of development, a column of mesoderm (paraxial mesoderm) forms along each side of the neural tube.

The mesoderm condenses and forms into cuboidal blocks of mesoderm, called
somites.

Eventually 42 to 44 pairs
of somites develop.

1. Myotome - makes skeletal muscles of back, body wall, limbs
2. Sclerotome - gives rise to vertebrae and ribs
3. Dermatome - dermis of the skin

Lower motor neuron axons in the neural tube sprout and grow into the somites during Week 5

Muscle
cells that fail to become innervated die

If neurons fail to form motor
end plates on muscle then the neuron dies.

Although mature muscle fibers are
terminally differentiated and CANNOT replicate, there is a small pool of stem cells
hiding in the muscle that can be stimulated to replicate and form myoblasts.

Satellite cells are stem cells hidden between the basal lamina and the muscle cell. They replicate and differentiate into myoblasts in adults.

Duchenne muscular dystrophy is an X linked recessive disease caused by a mutation in
dystrophin, an actin binding protein in the cell membrane. Its presence in the cell
membrane links the actin filaments to the cell membrane and basal lamina. This mutation leads to death of the muscle fibers.

RHABDOMYOSARCOMA is a malignant soft tissue sarcoma in children and adolescents.
Tumor cells may arise directly from satellite cells of mesenchymal cells (embryonic CT).

Cardiac muscle fibers are small, branched cells with striations. In cross section
they appear smaller and more variable in size than skeletal muscle fibers.

In cardiac muscle adjacent fibers are connected at their ends by specialized junctions, visible as a
dark, zigzagged line. This junctional complex is called an intercalated disc.

In cardiac muscle, each fiber typically has an oval, euchromatic nucleus in the center of the cell.

Fibers of cardiac muscle are attached to the CT by a basal lamina. Large amounts of capillaries and
lymphatic vessels are in the surrounding connective tissue

Cardiac muscle differentiates in mesoderm surrounding the endothelial lined heart
tubes.

Cardiac muscle fibers also contain actin and myosin filaments organized into sarcomeres and myofibrils.

In cardiac muscle, terminal cisterna and transverse tubules (T) are present but less developed than skeletal muscle.

In cardiac msucle there are diads,
consisting of one terminal cisternae and one T-tubule are present at each Z line.

Cardiac muscle consists of individual cells linked together both structurally and electrically
through a specialized junctional complex, the intercalated disc. This allows the linked
cardiac muscle cells to contract together as a single unit.

In skeletal muscle, the cells are fused together to form a single large multinucleated cell or fiber.

1. Fascia adherens (zolnula adheren) - anchors actin filament of last sarcomere to the sarcolema
2. Demosomes (macula aderens) - ) strongly attaches adjacent cells together and
keeps them from pulling apart
3. Gap or communicating junction - allow the diffusion of ions and electrical
excitation between cells.

In intercalcated disks, the linking of the individual cells structurally and ionically, allow the cells to contract together as a mass or synctium.

Cardiac muscle cells are autogenic, that is they exhibit a spontaneous rhythmic
contraction

The autonomic nervous system, endocrine system, and specialized cardiac
muscle fibers modify the pattern and rate of the intrinsic contraction of the cells.

There are NO motor end plates in cardiac muscle.

PURKINJE FIBERS are specialized cardiac muscle cells located in the endocardium of
the heart.

PURKINJE FIBERS are much larger than the typical cardiac muscle fibers and contain
abundant glycogen with few myofibrils

In H and E preparation, the glycogen of the purkinje fibers is dissolved away

Purkinje fibers help set the pace of contraction by quickly conducting and spreading waves of excitation to other cardiac muscle fibers by gap junctions.

Cardiac muscle can hypertropy. For example, in hypertension, the increased functional demands on the heart meaintain high blood pressure and lead to a massive increase in cell and heart size.

If a portion of the heart dies (myocardial infarct), the muscle tissue is replaced with connective tissue. Cardiac cells are terminally differentiated.

Smooth muscle is found throughout the body, commonly in the walls of tubes (blood
vessels, air ways) and hollow organs (gut, uterus, etc.)

Smooth muscle has small acidophillic spinde shaped cells often arranged in layers

In smooth muscle, each fiber has a single central nucleus

In smooth muscle, there are no striations

In smooth muscle, individual cells are hard to distinguish

In smooth muscle, each fiber secretes a basal lamina and a network of recticular fibers

Smooth muscle cells secrete elastic, type 1 collagen, recticular fibers, and proteoglycans

In smooth muscle, myofilaments (actin, myosin, and intermediate filaments, mostly desmin) are present but NOT organized into myofibrils and sarcomeres.

In smooth muscle, filaments are anchored to dense
bodies that in turn are anchored to the sarcolemma

Smooth muscle cells do NOT have a specialized membrane system

In smooth muscle, there are small invaginations of the
sarcolemma (caveolae) contain Ca²+ and are considered analogous to the terminal
cisternae in skeletal and cardiac muscle.

Smooth muscle cells do not have specialized motor end plates.

Smooth muscle is innervated by autonomic NS, Contraction modified by autonomic NS and hormones.

1. Single unit smooth muscle
2. Multi-unit smooth muscle

Single unit smooth muscle commonly forms the muscular wall of hollow organs.

Cells in single unit smooth muscle Cells
are electrically coupled by gap junctions, allowing excitation to spread from one cell
to the next. This causes all the connected cells to contract together as a single unit
or syncytium.

The single unit smooth muscle is responsible peristalsis

The single unit smooth muscle is sensitive to stretch.

Multi-unit smooth muscle is made up of individual smooth muscle cells not linked by
gap junctions. Contraction is usually initiated by nerve impulses

What are examples in the body of multi-unit smooth muscle? The sphincter
pupillae muscle of the iris is an example of this type of muscle.

Smooth muscle is the only muscle that we covered that can undergo MITOSIS

With increased use smooth muscle fibers can hypertrophy (cell enlarges) and undergo
hyperplasia (increase cell number by mitosis).

In Marfan
syndrome, an inherited disorder associated with defective elastic fibers, the aorta wall
weakens. This leads to fatal aortic aneurysms or dissection.

Smooth muscle develops from mesenchyme, mostly from mesoderm

Leiomyomas – are benign tumors that develop from smooth muscle. When they form
in the wall of the uterus they are called fibroids. This is the most common benign
tumor in woman

Leiomyosarcomas are malignant tumors derived from smooth muscle. They are most
often found in the pelvic organs (especially the uterus) and blood vessels. These rare
tumors usually occur in adults.

Myoepithelial cells are often present in glands, hiding between the secretory cell and its
basal lamina. Cell processes, rich in actin and myosin, surround the secretory cell so
that with contraction, the secretions are squeezed out. Myoepithelial cells are
prominent in sweat and mammary glands. The dilator pupillae muscle of the iris is
formed by myoepithelial cells.

Myofibroblasts resemble fibroblasts but have increased actin and myosin filaments.
These cells are active in wound contraction