MUSCLE & NERVOUS TISSUE

ANATOMY

What is the primary role of muscle tissue?

Contraction.

What is the origin of muscle tissue?

Mesodermal.

Define sarcolemma.

Cell membrane of the muscle tissue.

How does sarcoplasm differ from cytoplasm?

Sarcoplasm is equivalent to the cytoplasm of the cell, has little rough endoplasmic reticulum and many myofibrils.

What is the function of the sarcoplasmic reticulum?

A variety of smooth endoplasmic reticulum.

What are sarcosomes?

Large specialized mitochondria found in a striated muscle.

Describe the organization of skeletal muscle from macro to micro level.

Skeletal muscles are composed of muscle fascicles, which are composed of muscle fibers. Each muscle fiber is composed of myofibrils. Each myofibril is composed of myofilaments.

What are muscle fascicles composed of?

Muscle fibers.

What are muscle fibers composed of?

Myofibrils.

What are myofibrils composed of?

Myofilaments.

Differentiate between thick and thin filaments.

Thick filaments are composed of myosin II, and thin filaments are composed of actin.

How are muscles classified according to morphology?

Striated and Non-Striated/Smooth.

What are the characteristics of striated muscles?

Characterized with alternating light and dark bands (cross-striations). Darker bands are A-bands, and lighter bands are I-bands.

What are A-bands and I-bands?

A-bands stand for anisotropic and are birefringent in polarized light. I-bands stand for isotropic and do not alter polarized light.

What are the characteristics of non-striated muscles?

No striations.

How are muscles classified according to function/activity?

Voluntary and Involuntary.

What are voluntary muscles? Give an example.

Conscious control (i.e., skeletal and striated muscles).

What are involuntary muscles? Give examples.

No conscious control (i.e., cardiac and smooth muscles).

Which nervous systems control involuntary muscles?

ANS and PNS.

Compare the functions of skeletal, cardiac, and smooth muscles.

Skeletal muscle serves as a forceful, quick discontinuous voluntary contraction. Cardiac muscle serves as a strong, quick continuous involuntary contraction. Smooth muscle serves as a weak, slow involuntary contraction.

Describe the general features of skeletal muscle fibers.

Long cylindrical muscle fibers (up to 30 cm), diameter: 10-100 µm, multinucleated cells, oval nuclei found in the periphery of the cell just under the cell membrane (sarcolemma), presence of striations, and characterized as pink to red in color due to the rich vascular supply and presence of myoglobin pigments.

Where are the nuclei located in skeletal muscle cells?

In the periphery of the cell just under the cell membrane (sarcolemma).

Why are skeletal muscles pink to red in color?

Due to the rich vascular supply and presence of myoglobin pigments.

Where are blood vessels located in skeletal muscles?

Endomysium.

Describe the epimysium.

Connective tissue covering of the muscle, carries the larger nerves, blood vessels, and lymphatics of the muscles inwards, dense irregular connective tissue, ensheaths the entire muscle, and is continuous with the endomysium and perimysium.

Describe the perimysium.

Covers each muscle fascicle. Muscle fascicle is composed of bundles of muscle fibers.

Describe the endomysium.

Innermost covering, covers each muscle fiber (also called a muscle cell), delicate connective tissue, and a layer of connective tissue (composed mostly of reticular fibers and scattered fibroblasts) that ensheaths a muscle fiber which contains lymphatic capillary nerves.

What is the H-band and where is it located?

Located at the center of the A band, contains myosin (thick) filaments.

What bisects the H-band?

M line.

What is the Z-line and where is it located?

Dark band that bisects each I band, denotes borders of adjacent sarcomeres.

Define a sarcomere.

Smallest subunit of contractile apparatus.

What are the components of thick filaments?

Myosin.

What are the components of thin filaments?

F-actin.

What proteins tie adjacent sarcomeres together?

Alpha actinin and Desmin.

Describe the molecular structure of thick filaments.

Central portion (A band), bound together by the M line (myosin binding proteins).

Describe the molecular structure of thin filaments.

I band, composed of F-actin.

What are the 4 major muscle proteins?

Actin, Troponin, Tropomyosin, and Myosin II.

Describe the structure and function of actin.

For thin filaments, has two strands of globular (G-actin) monomers in double helical formation, and contains the binding site for myosin.

Describe the 3 subunits of troponin and their functions.

Troponin C - Calcium, Troponin T - Tropomyosin, and Troponin I - Actin.

Describe the structure and function of tropomyosin.

Double-helix of 2 polypeptide chains, runs in the groove between F-actin molecules.

Describe the structure and function of myosin II.

For thick filaments, composed of 2 heavy and 4 light chains. Heads of heavy chains have ATP binding sites and ATPase activity.

Explain the sliding filament mechanism of muscle contraction.

Two z discs upon contraction, are brought closer together, as thin filaments slide past the thick filaments and the light band disappears.

What happens to the I-band, H-band, Z-discs, and A-band during contraction?

I band becomes narrower, H band disappears, Z discs move closer together, and A band remains unaltered.

Describe the role of acetylcholine in muscle contraction.

A nerve impulse triggers release of acetylcholine (Ach) from the synaptic knob into the synaptic cleft. Ach binds to Ach receptors in the motor end plate of the neuromuscular junction, initiating a muscle impulse in the sarcolemma of the muscle fiber.

What is the role of T-tubules and calcium ions in muscle contraction?

Muscle impulse spreads quickly from the sarcolemma to T-tubules, resulting in calcium (Ca2+) released from the terminal cisternae to the sarcoplasm.

Explain the interaction between troponin, tropomyosin, and actin during contraction.

Calcium (Ca2+) binds to troponin. Troponin changes shape, which moves tropomyosin on the actin to expose active sites on actin molecules of thin filaments. Myosin heads of thick filaments attach to exposed active sites to form cross-bridges.

How do myosin heads contribute to the sliding filament mechanism?

Myosin heads pivot, moving thin filaments toward the sarcomere center. ATP binds to myosin heads and is broken down to ADP and P. Myosin heads detach from thin filaments and return to their pre-pivot position. Attach-pivot-detach-return slides thick and thin filaments past one another as long as calcium ions remain bound to troponin to keep active sites exposed.

What is the role of ATP in muscle contraction?

ATP binds to myosin heads and is broken down to ADP and P.

What events lead to muscle relaxation?

When the impulse stops, Ca2+ ions are actively transported into the sarcoplasmic reticulum, tropomyosin re-covers active sites, and filaments passively slide back to a relaxed state.

Explain the phenomenon of rigor mortis.

Without ATP, the actin and myosin complex becomes stable, leading to muscle rigidity or stiffness that occurs during death.

What are the special characteristics of muscle tissue?

Excitability, contractility, extensibility, and elasticity.

What are the functions of muscle tissue?

1) Movement of bones or fluids (e.g., blood),

2) Maintaining posture and body position

3) Stabilizing joints

4) Heat generation (especially skeletal muscle).

Describe the microscopic anatomy of a skeletal muscle fiber.

Multiple peripheral nuclei, many mitochondria, glycosomes for glycogen storage, myoglobin for O2 storage, and contain myofibrils, sarcoplasmic reticulum, and T tubules.

What is the composition and function of myofibrils?

Majority of every muscle fiber is composed of long fibrous molecules known as myofibrils. Densely packed, rod-like elements, approximately 80% of cell volume, and exhibit striations: perfectly aligned repeating series of dark A bands and light I bands.

What percentage of cell volume do myofibrils occupy?

Approximately 80%.

Describe the striations in skeletal muscle.

Perfectly aligned repeating series of dark A bands and light I bands.

Explain the structure of a sarcomere.

Smallest contractile unit (functional unit) of a muscle fiber. The region of a myofibril between 2 successive Z discs. Composed of thick and thin myofilaments made of contractile proteins. Thick filaments run the entire length of an A band. Thin filaments run the length of the I band and partway into the A band.

What is the role of Z-discs, H-zone, and M-line?

Z disc: coin-shaped sheet of proteins that anchors the thin filaments and connects myofibrils to one another. H zone: lighter mid region where filaments do not overlap. M line: line of protein myomesin that holds adjacent thick filaments together.

Describe the ultrastructure of thick filaments.

Composed of the protein myosin. Myosin tails contain 2 interwoven, heavy polypeptide chains. Myosin head contain 2 smaller, light polypeptide chains that act as cross bridges during contraction. Binding sites for actin of thin filaments. Binding sites for ATP. ATPase enzymes.

Describe the ultrastructure of thin filaments.

Twisted double strand of actin. Actin bears active sites for myosin head attachment during contraction. Tropomyosin and Troponin: regulatory proteins bound to actin.

What are the functions of tropomyosin and troponin?

Regulatory proteins bound to actin.

What are encapsulated proprioceptors?

Send data from the musculoskeletal system to the CNS. Regulate the amount of effort required to perform movements.

What is the function of muscle spindles?

Respond to stretch.

What is the function of Golgi tendon organs?

Respond to tension.

Describe the key features of Duchenne Muscular Dystrophy (DMD).

Genetic disorder characterized by progressive muscle degeneration and weakness.

Absence of dystrophin, a protein that helps keep muscles intact.

Symptoms during early childhood, more common in males but rarely occurs in females.

What protein is absent in DMD?

Dystrophin.

Describe the key features of Myasthenia Gravis.

• Autoimmune and neuromuscular disease involves circulating antibodies against proteins of acetylcholine receptors.

• Causes skeletal muscle weakness (responsible for breathing and movement).

• Weakness of muscles involved in facial expression. Ptosis - drooping of eyelids and corners of the mouth.

What causes muscle weakness in Myasthenia Gravis?

Autoimmune reactions against proteins of acetylcholine receptors.

What are the general characteristics of cardiac muscle?

Length: _ to _ µm in length (mature)

Diameter: _ µm

Branching or Non-branching fibers

Banding pattern? ___

Nuclei located?

__ staining nuclei?

How many nuclei?

How mitochondria?

SR is ___ organized and __ developed compared to skeletal muscles

____ CT has a rich capillary network.

80-100 µm length (mature)

15 µm diameter

Branching fibers

Cross-striated banding pattern.

1 or 2 Centrally-Located, Pale-staining nuclei

Numerous mitochondria

SR is less well-organized and developed compared to skeletal muscles

Endomysial CT has a rich capillary network

How many nuclei are present in cardiac muscle cells and where are they located?

1 or 2 centrally-located, pale-staining nuclei.

What is the significance of the numerous mitochondria in cardiac muscle?

Need for continuous aerobic metabolism.

How does the sarcoplasmic reticulum in cardiac muscle compare to skeletal muscle?

SR is less well-organized and developed compared to skeletal muscles.

What are intercalated discs?

Represent the interfaces between adjacent cells and consist of many junctional complexes.

Where are Purkinje fibers located?

At the subendocardial layer between myocardium and endocardium.

Myocardium > SUBENDOCARDIAL LAYER> Endocardium

What is the function of Purkinje fibers?

Specialized cells in the heart, impulse-conducting fibers.

How do Purkinje fibers differ from Purkinje (aka Cardiac) cells?

• Nucleus is round and larger than the cardiac cell

• cytoplasm is more pale-staining. Faint, not as tinted due to the abundant supply of glycogen.

What are the 3 main junctional specializations in cardiac muscle?

1) Fascia Adherens

2) Macula adherens/Desmosome

3) Gap junction.

Describe fascia adherentes and its function.

• Most prominent functional specialization

• Found in the transverse portion.

• Serves as anchoring sites for actin filaments of terminal sarcomeres.

Describe macula adherentes/desmosomes and its function.

• Found in the transverse portion.

• Bind individual cardiac cells to one another.

Describe gap junctions and its function.

Found in the lateral portions. Provide ionic continuity between adjacent cells.

Explain the T-tubules and sarcoplasmic reticulum relationship in cardiac muscle.

DIAD/DYAD - 1 SR + 1 T-tubule. Numerous and larger T-tubules to compensate for less developed sarcoplasmic reticulum.

What is a diad/dyad in cardiac muscle?

1 SR + 1 T-tubule.

What are the general characteristics of smooth muscle cells?

Elongated, non-striated, spindle-shaped cells (Fusiform). Centrally located cigar-shaped nucleus.

Where is smooth muscle tissue found in the body?

Muscularis externa of the intestinal wall, blood vessels, urinary passages of the GUT, prostate gland, and hollow and tubular organs (e.g., uterus).

What type of innervation controls smooth muscle?

Sympathetic and parasympathetic nerves of the autonomic nervous system.

How does smooth muscle innervation differ from skeletal muscle?

Lack neuromuscular junction

instead uses axonal swellings that lie in close contact with the muscle fibers.

Explain the structure of smooth muscles.

Thin filaments

Thick filaments

*****Intermediate filaments.

What are the components of thin filaments in smooth muscle?

Actin, tropomyosin, and calmodulin (instead of troponin).

What is the role of calmodulin and myosin light-chain kinase (MLCK) in smooth muscle contraction?

• Calmodulin is a calcium-binding protein involved in the contraction of non-muscle cells.

• Contraction of the sliding filament is also aided by the Ca2+ sensitive, myosin light-chain kinase (MLCK).

What are dense bodies and their function?

• Dense bodies are structures found in smooth muscle cells and platelets.

• Function: They anchor thin filaments (actin) in smooth muscle cells, similar to Z-discs in skeletal muscle.

  _______________________________

  ****Actin myofilaments insert into the cytoplasmic and plasmalemma-associated dense bodies.

What are the two types of dense bodies?

Membrane-associated and Cytoplasmic.

What is the composition of intermediate filaments in smooth muscle?

Desmin (skeleton) and vimentin (in vascular smooth muscles).

How is contractile force transmitted in smooth muscle?

Attachment to dense bodies help transmit contractile forces to adjacent smooth muscles and surrounding network of reticular fibers.

Give examples of visceral smooth muscles and their characteristics.

In the intestines, uterus, and ureters. Contains abundant gap junctions and poor nerve supply. Functions in syncytial fascia.

Give examples of multiunit smooth muscles and their characteristics.

In the iris of the eye, intrinsic muscles of the eye. Can produce precise and greater contractions.

Compare the regeneration capacity of skeletal, smooth, and cardiac muscle.

Skeletal muscle undergoes limited regeneration, smooth muscle has active regeneration, and cardiac muscle has virtually no regenerative capacity beyond early childhood.

What cells are responsible for skeletal muscle regeneration?

Satellite cells.

How does smooth muscle regenerate?

After injury, cells undergo mitosis and replace damaged tissue.

What happens to damaged cardiac tissue?

Defects/damage (e.g., infarcts) are replaced by proliferating fibroblasts and growth of connective tissue.

What are the two main structural components of nervous tissue?

Neurons/Nerve cells and Supporting/Glial cells.