BMI04 - Histology of Muscle

What is the basic classification of muscle tissue?

Muscle tissue is classified into three types: skeletal muscle, cardiac muscle, and smooth muscle. Skeletal and cardiac muscle are striated, while smooth muscle is non-striated.

What percentage of the body is composed of skeletal muscle?

About 40% of the body is skeletal muscle.

What percentage of the body is composed of cardiac and smooth muscle combined?

About 5–10% of the body is composed of cardiac and smooth muscle combined.

What is another name for a skeletal muscle cell?

A skeletal muscle cell is called a muscle fiber.

What are the key characteristics of skeletal muscle fibers?

Skeletal muscle fibers are multinucleated, striated, contain myofibrils, and are under voluntary control.

What is a myofibril?

A myofibril is the major organelle of striated muscle and is composed of assemblies of contractile myofilaments.

What are thin filaments composed of?

Thin filaments are composed of actin.

What are thick filaments composed of?

Thick filaments are composed of myosin.

What is the sarcomere?

The sarcomere is the basic structural and functional unit of striated muscle and consists of repeating units between two Z lines.

Can skeletal muscle contract without nerve stimulation?

No. Skeletal muscle cannot contract without nerve stimulation.

How are skeletal muscles innervated and controlled?

Skeletal muscles are innervated by motor nerves and are under conscious, voluntary control.

What intermediate filaments are found in muscle cells?

Muscle cells contain desmin and vimentin intermediate filaments.

What is the function of desmin and vimentin in muscle cells?

They help organize myofibrils and keep them in register in striated muscle.

What is the epimysium?

The epimysium is dense irregular connective tissue that surrounds the entire gross skeletal muscle.

What structures pass through the epimysium?

Major blood vessels and nerves penetrate the epimysium.

What is the perimysium?

The perimysium is dense irregular connective tissue that surrounds muscle fascicles.

What is a fascicle?

A fascicle is a group of 10–100 muscle fibers.

What structures travel in the perimysium?

Larger blood vessels and nerves travel in the perimysium.

What is the endomysium?

The endomysium is loose connective tissue with reticular fibers that surrounds each individual muscle fiber.

What structures are found in the endomysium?

Capillaries and small nerve branches are found in the endomysium.

How does skeletal muscle stain with H&E stain?

Skeletal muscle is eosinophilic and stains pink-red with H&E stain.

What are the two main orientations in which skeletal muscle can be sectioned histologically?

Skeletal muscle can be sectioned longitudinally or transversely.

How are skeletal muscle fibers formed during embryonic development?

About 100 myoblasts fuse together to form one multinucleated skeletal muscle fiber.

Where are the nuclei located in skeletal muscle fibers?

The nuclei are pushed to the periphery of the cell just beneath the sarcolemma.

What causes the striated appearance of skeletal muscle fibers?

Alternating light and dark bands arranged perpendicular to the long axis of the fiber cause the striated appearance.

What is the A band?

The A band is the darker region of the myofibril containing thick filaments and overlapping thin filaments.

How does the A band appear histologically?

The A band is electron dense in TEM and darker staining in light microscopy with H&E.

What proteins are found in the A band?

The A band contains myosin thick filaments and overlapping actin thin filaments.

What changes in the A band during contraction?

The relationship between thick and thin filaments changes during muscle contraction.

What is the H band?

The H band is the pale region in the center of the A band that contains only thick filaments.

How does the H band appear histologically?

The H band is electron lucent in TEM and cannot be seen with light microscopy.

What is located in the H band?

Only thick filaments (myosin) are located in the H band.

What is the M band or M line?

The M band is the electron dense line in the center of the H zone that organizes thick filaments.

What protein is associated with the M line?

Myomesin is a major protein of the M line.

What is the significance of the M line?

The M line marks the center of the sarcomere and organizes thick filaments.

How does the M line appear histologically?

It is electron dense in TEM and usually cannot be seen in light microscopy.

What is the I band?

The I band is the lighter region containing only thin filaments.

How does the I band appear histologically?

The I band is electron lucent in TEM and lighter staining in light microscopy.

How are thin filaments arranged in the I band?

Thin filaments are attached to the Z line and extend into the A band up to the edge of the H band.

What is the Z line or Z disc?

The Z line is the dark line in the center of the I band where thin filaments are anchored.

How does the Z line appear histologically?

The Z line is electron dense in TEM and appears as a dark staining line in light microscopy.

What proteins attach to the Z line?

Proteins anchor actin thin filaments to the Z line.

How is a sarcomere defined?

A sarcomere includes all proteins and bands between two adjacent Z lines of one myofibril.

Why is the sarcomere important?

The sarcomere is the structural unit of the myofibril and the functional unit of striated muscle contraction.

What are T-tubules in skeletal muscle?

T-tubules are invaginations of the sarcolemma into the cytoplasm of skeletal muscle fibers.

What fills the T-tubules?

T-tubules are filled with extracellular fluid.

What is the function of T-tubules?

T-tubules transmit action potentials from motor nerves deep into the muscle fiber to the sarcoplasmic reticulum.

What is the sarcoplasmic reticulum?

The sarcoplasmic reticulum is the equivalent of smooth ER in striated muscle and stores calcium ions.

What triggers skeletal muscle contraction?

An increase in cytosolic calcium triggers skeletal muscle contraction.

Describe the sequence of excitation-contraction coupling in skeletal muscle.

Neurotransmitter release from the nerve terminal causes an action potential in the muscle plasma membrane, the excitation spreads through T-tubules, signals the sarcoplasmic reticulum, and calcium is released to initiate contraction.

What are cisternae of the sarcoplasmic reticulum?

Cisternae are dilated sacs of the sarcoplasmic reticulum adjacent to T-tubules that store, release, and accumulate calcium ions.

What is a skeletal muscle triad?

A skeletal muscle triad consists of one T-tubule flanked by two terminal cisternae of the sarcoplasmic reticulum.

What is one major role of connective tissue in skeletal muscle?

Connective tissue mechanically transmits the forces generated by contracting muscle fibers.

Why is connective tissue necessary in skeletal muscle?

Most individual muscle fibers do not extend the full length of the muscle, so connective tissue helps transmit force throughout the muscle.

What additional function does connective tissue serve in skeletal muscle?

It provides pathways for innervation and blood supply to reach muscle fibers.

What happens to connective tissue at the ends of muscles?

The connective tissue layers coalesce to form tendons.

Where is cardiac muscle located?

Cardiac muscle is found in the heart wall and proximal portions of the pulmonary veins.

What are other names for a cardiac muscle cell?

A cardiac muscle cell may be called a muscle fiber, myocyte, or cardiomyocyte.

What are the characteristics of cardiac muscle fibers?

Cardiac muscle fibers are mononucleated, striated, involuntary, and connected by intercalated discs.

How is cardiac muscle contraction regulated?

Cardiac muscle contraction is involuntary and regulated by the autonomic nervous system.

Where is the nucleus located in cardiac muscle fibers?

The nucleus is centrally located in cardiac muscle fibers.

Where are organelles concentrated in cardiac muscle cells?

Organelles are concentrated around the sides of the nucleus.

How do cardiac muscle striations appear?

Cardiac muscle shows light and dark bands perpendicular to the long axis of the cell.

How are cardiac and skeletal muscle sarcomeres similar?

Both contain myofibrils with A bands, I bands, Z lines, and similar sarcomere morphology.

How are cardiac and skeletal muscle proteins different?

Cardiac and skeletal muscle contain different isoforms of myofilament proteins, such as different actin isoforms.

How are T-tubules and sarcoplasmic reticulum in cardiac muscle similar to skeletal muscle?

They perform the same functions of conducting excitation and regulating calcium for contraction.

What is a dyad in cardiac muscle?

A dyad consists of one T-tubule associated with one cisterna of sarcoplasmic reticulum at the Z line.

How does the sarcoplasmic reticulum in cardiac muscle differ from skeletal muscle?

Cardiac muscle has a less extensive SR, stores less calcium, and has larger T-tubules.

What are intercalated discs?

Intercalated discs are specialized junctional complexes connecting adjacent cardiac muscle cells into long chains.

What types of junctions contribute to intercalated discs?

Intercalated discs contain proteins from adherens junctions, desmosomes, and gap junctions.

What is the function of intercalated discs?

They allow mechanical and electrical coupling between cardiac muscle cells.

Why is cardiac muscle considered a functional syncytium?

Gap junctions allow ions and action potentials to move freely between cells so the myocardium contracts as a coordinated unit.

What is the function of fascia adherens in intercalated discs?

Fascia adherens anchor actin thin filaments from adjacent sarcomeres between neighboring cells.

What is the function of macula adherens (desmosomes) in intercalated discs?

Desmosomes link adjacent cells through desmin and vimentin intermediate filaments for strong adhesion.

What is the function of gap junctions in intercalated discs?

Gap junctions allow ions to move between adjacent cardiac cells to synchronize contraction.

What are the three layers of the heart wall?

The heart wall consists of the epicardium, myocardium, and endocardium.

What is the epicardium?

The epicardium is the outer layer of the heart and is equivalent to the visceral layer of the serous pericardium.

What tissue lines the epicardium?

The epicardium is lined by simple squamous epithelium called mesothelium.

What connective tissue features are found in the epicardium?

Loose connective tissue and significant adipose tissue may be present.

Where is the epicardium thickest?

The epicardium tends to be thicker over the atria.

What is the myocardium?

The myocardium is the muscular layer of the heart composed of cardiac muscle cells.

Why is atrial myocardium thinner than ventricular myocardium?

Atrial myocardium only pumps blood into the ventricles, requiring less force.

Why is the left ventricular myocardium thicker than the right?

The left ventricle must generate higher pressure to pump blood through the systemic circulation.

What is the endocardium?

The endocardium is the inner layer of the heart wall closest to the blood.

What epithelium lines the endocardium?

The endocardium is lined by simple squamous epithelium called endothelium.

What structures are found in the connective tissue of the ventricular endocardium?

Elements of the cardiac conduction system are found there.

How does atrial endocardium compare to ventricular endocardium?

The atrial endocardium is thicker than the ventricular endocardium.

What structures make up the cardiac conduction system?

The cardiac conduction system includes the SA node, AV node, Bundle of His, left and right bundle branches, and Purkinje fibers.

Where are Purkinje fibers located?

Purkinje fibers are located in the ventricular endocardium.

How do Purkinje fibers compare in size to normal cardiac muscle cells?

Purkinje fibers are 2–3 times larger than normal cardiac muscle cells.

What are the histologic features of Purkinje fibers?

Purkinje fibers are pale staining, contain few myofibrils, have few or no intercalated discs, and contain many gap junctions.

Where is smooth muscle found?

Smooth muscle is found in blood vessel walls, tubular organs such as the stomach and uterus, the iris, and associated with hair follicles.

What are the characteristics of smooth muscle cells?

Smooth muscle cells are mononucleated, non-striated, and contain actin and myosin.

How is smooth muscle contraction controlled?

Smooth muscle contraction is controlled by intrinsic, neural, or hormonal stimuli.

Where is the nucleus located in smooth muscle cells?

The nucleus is centrally located.

What histologic feature commonly identifies smooth muscle?

Smooth muscle nuclei often appear twisted or corkscrew-shaped.

How does smooth muscle cytoplasm stain?

The cytoplasm is uniformly eosinophilic with no visible striations.

What are dense bodies in smooth muscle?

Dense bodies are structures in the cytoplasm or attached to the plasma membrane that function similarly to Z lines.

How are filaments arranged in smooth muscle?

Thin and thick filaments connect to dense bodies.

What intermediate filaments are found in smooth muscle?

Smooth muscle contains vimentin and desmin intermediate filaments.