Muscular System Flashcards
the regular repeating region between successive Z disks and constitutes the
functional unit of contraction in skeletal muscle.
Sarcomere
The enzyme that cleaves the heavy meromyosin, releasing the short tail
(S2 fragment) and the two globular heads (S1 fragments). These S1 fragments have
adenosine triphosphatase (ATPase) activity but require interaction with actin to
release the noncovalently bound adenosine diphosphate (ADP) and Pi
Papain
It is a long, inelastic protein. Two molecules of it wrap around each thin filament and assist in anchoring it to the Z disk.
(1) Each nebulin molecule is embedded in the Z disk by its carboxy terminal but does not span the entire Z disk.
(2) The amino terminal of each molecule ends in the A band, at or near the free end of its thin filament.
(3) In skeletal muscle, it is thought to determine the length of its associated thin
filament, although in cardiac muscle it extends only one-quarter of the length of the
thin filament.
Nebulin
A narrow space between the presynaptic membrane of the axon terminal and the postsynaptic membrane (also known as the motor end plate) of the
muscle cell. The synaptic cleft contains an amorphous external lamina, a basal lamina-like material, derived from the muscle cell.
Synaptic cleft
It binds to actin, inhibiting interaction of myosin and actin.
Troponin I (TnI)
It possesses four binding sites for calcium. It may be related to calmodulin.
Troponin C (TnC)
They are nonstriated, fusiform cells that range in length from 20 μm in small blood vessels to 500 μm in the uterus of pregnant women. They contain a single nucleus and actively divide and regenerate. They
are surrounded by an external lamina and a reticular fiber network and may be arranged in layers,
small bundles, or helices (in arteries).
Smooth muscle cells
It is composed of two identical heavy chains and two pairs of light chains.
Myosin
It is relatively small proteins, approximately 60 kDa, that form bonds not only with dystrophins but also with desmin and dystrobrevin.
Syntrophins
It is marked by degeneration of motor neurons of the spinal cord, resulting in muscle atrophy. Death is usually due to respiratory muscle failure.
Amyotrophic lateral sclerosis (ALS, or Lou Gehrig disease)
A synapse between a branch of a motor nerve
axon and a skeletal muscle cell
Myoneural junction
It is an irreversible necrosis of cardiac muscle cells due to prolonged ischemia. It may result in death if the cardiac muscle damage is extensive.
Myocardial infarct
It is a pencil-shaped molecule that is the lynchpin between the extracellular
matrix and the cytoskeleton attached to the peripheral-most myofibril of the striated muscle fiber. It is dystrophin that plays the greatest role in strengthening and
maintaining the integrity of the sarcolemma during the process of contraction.
Dystrophin
They include thick filaments (15 nm in diameter and 1.5 μm long) and thin filaments
(7 nm in diameter and 1.0 μm long). They lie parallel to the long axis of the myofibril in a precise arrangement that is responsible for the sarcomere banding pattern.
Myofilaments
- Arise from mesenchymal cells and possess vimentin
as their characteristic intermediate filaments as well as caldesmon and cytokeratins. - Although they resemble fibroblasts, they possess higher amounts of actin and myosin and are
capable of contraction. - They may contract during wound healing to decrease the size of the defect (wound contraction).
Myofibroblasts
They are isotropic with polarized light and appear lightly stained in routine histologic preparations. They contain only thin filaments.
I bands
It is caused by a sex-linked, recessive genetic defect that results in the inability to synthesize dystrophin, an actin-binding protein normally present in small amounts in the sarcolemma. Dystrophin also stabilizes the sarcolemma and acts as a link between the cytoskeleton and the extracellular matrix.
- This common, serious degenerative disorder occurs in young men and results in death
usually before 20 years of age. - It is characterized by the replacement of degenerating skeletal muscle cells by fatty
and fibrous connective tissue, but it may also affect cardiac muscle.
Duchenne muscular dystrophy (DMD)
It is the membrane on the synaptic surface of the axon terminal
Presynaptic membrane
They are long, cylindrical, multinucleated and are enveloped by an external lamina and reticular fibers. Their cytoplasm is called sarcoplasm, and their
plasmalemma is called the sarcolemma and forms deep tubular invaginations, or T (transverse) tubules
Skeletal muscle cells
It surrounds individual muscle cells and is composed of reticular fibers and an
external lamina.
Endomysium
- Contract spontaneously and display a rhythmic beat, which is modified by hormonal and neural (sympathetic and parasympathetic) stimuli.
- May branch at their ends to form connections with adjacent cells.
- Contain one centrally located nucleus, or occasionally two nuclei.
- Contain glycogen granules, especially at either pole of the nucleus, and the sarcoplasm is rich
in myoglobin. - Possess thick and thin filaments arranged in poorly defined myofibrils.
- Exhibit a cross-banding pattern identical to that in skeletal muscle.
- Do not regenerate; injuries are repaired by the formation of fibrous
connective (scar) tissue by fibroblasts
Cardiac muscle cells
It is a form of food poisoning caused by ingestion of Clostridium botulinum toxin,
which inhibits acetylcholine release at myoneural junctions. It is marked by muscle paralysis, vomiting, nausea, and visual disorders and is fatal if untreated.
Botulism
It is a narrow, dark region at the center of H band formed by several cross-connections
(M-bridges) at the centers of adjacent thick filaments.
M lines
- In certain glands, these cells share basal laminae of secretory and duct cells.
- They arise from ectoderm and can contract to express secretory material from glandular epithelium into the ducts and out of the gland.
- Although generally similar in morphology to smooth muscle cells, they have a basketlike shape and several radiating processes.
- They are attached to the underlying basal lamina via hemidesmosomes.
- They contain actin, myosin, and intermediate filaments, as well as cytoplasmic and peripheral
densities to which these filaments attach. - Contraction is similar to that of smooth muscle and occurs via a calmodulin-mediated process.
In lactating mammary glands, they contract in response to oxytocin. In lacrimal glands, they
contract in response to acetylcholine.
Myoepithelial cells
It is located in tendons, counteracts the effects of muscle spindles
- It is composed of encapsulated collagen fibers that are surrounded by terminal branches of type Ib sensory nerves.
- It is stimulated when the muscle contracts too strenuously, increasing tension on the tendon.
Impulses from type Ib neurons inhibit α-efferent (motor) neurons to the muscle, preventing
further contraction.
Golgi tendon organ
It is about 40 nm in length. They bind head to tail, forming filaments that are located in the grooves of the F-actin helix.
Tropomyosin
They are light regions transecting A bands; they consist of thick filaments only
H bands
A modified smooth endoplasmic reticulum (SER) that surrounds myofilaments and forms a meshwork around each myofibril.
a. The SR forms a pair of dilated terminal cisternae, which encircle the myofibrils at the junction of each A and I band.
b. It regulates muscle contraction by sequestering calcium ions (leading to relaxation) or
releasing calcium ions (leading to contraction).
sarcoplasmic reticulum (SR)
It caps the minus end of each thin filament and prevents the addition of more G-actin molecules to the growing end; thus, it limits the length of the F-actin filament.
Tropomodulin
They are modified cardiac muscle cells located in the bundle of His. They are specialized for conduction and contain a few peripheral myofibrils.
a. These large, pale cells are rich in glycogen and mitochondria.
b. They form gap junctions, fasciae adherentes, and desmosomes with cardiac muscle cells
(but not through typical intercalated disks).
Purkinje fibers
It is a polymer of G-actin monomers arranged in a double helix.
(1) Each monomer possesses an active site that can interact with myosin.
(2) It is present as filaments (with a diameter of 5–7 nm) that exhibit polarity, having
a plus (+) and a minus (–) end, where the plus end is tethered to Cap Z of the Z disk and the minus end, capped by tropomodulin
(3) It loses and gains back G-actin molecules at both its plus and minus ends, but this turnover rate is very slow, occurring over a period of several days, whereas in other cells
this turnover occurs every few minutes.
F-actin
It is a complex of two transmembrane proteins whose extracellular domains bind to laminin of the external lamina of the muscle cells. Intracellularly dystroglycan binds to dystrophin as well as to syntrophins.
Dystroglycan complex
It is an enzyme that is responsible for removing a phosphate group from creatine phosphate and phosphorylating ADP converting it to the energy-rich compound adenosine
triphosphate (ATP).
Creatinine kinase
It has binding sites with syntrophins and desmin
Dystrobevin
They are anisotropic with polarized light; they usually stain dark. They contain both thin and thick filaments, which overlap and interdigitate. Six thin filaments surround each thick filament
A bands
It surrounds fascicles (small bundles) of muscle cells.
Perimysium
The primary regulator
of the sarcoplasmic Ca2+ pump
Phospholamban
It is an autoimmune disease in which antibodies block acetylcholine receptors of myoneural junctions, reducing the number of sites available for initiation of
sarcolemma depolarization.
- It is characterized by gradual weakening of skeletal muscles, especially
the most active ones (e.g., muscles of the eyes, tongue, face, and extremities). Death may result from respiratory compromise and pulmonary infections. - Clinical signs include thymic hyperplasia (thymoma) and the presence of circulating
antibodies to acetylcholine receptors.
Myasthenia gravis
They are myoblast-like cells and regenerative cells that are probably residual cells from embryonic development lie within the external lamina (basal lamina) of skeletal muscle cells and adhere to these cells via M-cadherin molecules.
Satellite cells
It is composed of several transmembrane proteins localized at the costameres of the sarcolemma. They form attachments to and reinforce the
dystroglycan complex
Sarcoglycan complex
It binds to thick filaments in the vicinity of M lines along much of their lengths
(between the M line and the end of the thin filament in the vicinity of the A–I junction). This region of the A ban dis referred to as the C zone.
C protein
It is a rare, inherited disease where a mutation is
responsible for the formation of desmin molecules that form desmin aggregates rather than
the normal desmin filaments. The absence of filamentous desmin results in disorganized
myofilaments and skeletal muscle fibers. The symptoms of the disease begin as progressive
weakness in the muscles of the legs, followed by weakness in the trunk and the rest of the
body. Because it affects cardiac and smooth muscles also, respiratory insufficiency,
heart failure, and gastrointestinal functions follow with possibly fatal consequences.
Desmin-related myopathy (DRM)
They are complex step-like junctions forming endto-end attachments between adjacent cardiac muscle cells.
a. Its transverse portion runs across muscle fibers at right angles and possesses three specializations: fasciae adherentes (analogous to zonula adherentes) to
which actin filaments attach, desmosomes (macula adherentes), and gap junctions.
b. Its lateral portion has desmosomes and numerous large gap junctions, which facilitate ionic coupling between cells and aid in coordinating contraction;
thus, cardiac muscle behaves as a functional syncytium
Intercalated disks
It is elongated, fusiform sensory organ within
skeletal muscle that functions primarily as a stretch receptor
Muscle spindle
They are specialized areas of the sarcolemma located at regions that correspond
to the Z-lines of myofibrils that are situated just beneath the striated muscle cell membrane. They are defined by the presence of peripheral and transmembrane
protein complexes whose principal moieties are localized on the sarcoplasmic aspect
of the striated muscle cell membrane.
Costameres
It is the dense region bisecting each I band.
a. contain alpha-actinin and Cap Z, two proteins that bind to thin filaments and anchor
them to Z disks with the assistance of nebulin.
b. Desmin, aided by plectin, anchors Z disks to each other. Peripherally located Z disks are anchored to regions of the sarcolemma, known as costameres, dystrophin-associated
glycoprotein complexes.
Z disks
Specialized complexes consisting of a narrow central T tubule flanked on each side by terminal cisternae of the SR. They are located at the A–I junction in mammalian skeletal
muscle cells and help provide uniform contraction throughout the muscle cell.
Triads
It surrounds an entire muscle and forms aponeuroses, which connect skeletal
muscle to muscle, and tendons, which connect skeletal muscle to bone.
Epimysium
A large linear protein that displays axial periodicity. It forms an elastic lattice that
parallels the thick and thin filaments, and two titin filaments anchor each thick filament to the Z disk, thus maintaining their architectural relationships to each other
Titin
It forms the tail of the molecule and functions in binding the troponin complex to tropomyosin.
Troponin T (TnT)
It is a postmortem rigidity appearing as hardening of skeletal muscles caused
by the inability of muscle cells to synthesize ATP. As a result, myosin remains bound to actin,
and the muscles remain contracted.
Rigor mortis
The enzyme that cleaves myosin into light meromyosin (part of the tail
portion) and heavy meromyosin (the two heads and the remainder of the tail)
Trypsin
It is located at the M line. It is a protein that crosslinks adjacent thick filaments to one another to maintain their spatial relations
Myomesin
