Muscle Biology Quiz - Anatomy and Functions

Muscle Tissue Structure

•  Muscle tissue is a type of excitable tissue.
•  Muscle tissue makes up a large portion of the body, approximately 50%.
•  Muscle tissue has three main types: skeletal, cardiac, and smooth.
•  Skeletal muscle is responsible for movement of the body and manipulation of objects.
•  Cardiac muscle powers the heart's pumping action.
•  Smooth muscle moves substances through hollow internal organs.

Four Tissue Types

•  Epithelial tissue lines and protects surfaces, including skin and mucous membranes.
•  Muscle tissue is responsible for movement. Types include skeletal, smooth, and cardiac muscles.
•  Nervous tissue is involved in communication and control, comprising the brain and spinal cord.
•  Connective tissue supports other tissues, including loose connective tissue (e.g., ligaments, tendons) and dense connective tissue (e.g., bone, cartilage, and blood).

Muscle Structure

•  Skeletal muscle cells are also called muscle fibers. They're long and cylindrical, and contain multiple nuclei.
•  Muscle fibers are made up of myofibrils.
•  Myofibrils are composed of myofilaments.  
•  Myofilaments, primarily actin and myosin, are responsible for muscle contraction.
•  Muscle cells are organized into fascicles, then bundles of fascicles form a muscle. 
•  Tendons connect muscles to bones while aponeuroses attach to bones, cartilages, or other connective tissues.
•  Structure includes sarcolemma (outer membrane), sarcoplasm (cytoplasm), and sarcoplasmic reticulum (specialized smooth ER important for calcium ion storage).

Skeletal Muscle Cells

•  A motor unit is comprised of a single motor neuron that innervates the entire muscle fiber.
•  Myoneural junction is the synapse between the motor neuron and muscle fiber.
•  Acetylcholine is the neurotransmitter released at the myoneural junction for muscle contraction.
•  Motor end plate is the specialized area of the sarcolemma under the axon terminal.
•  The innervation ratio varies depending on the muscle, from 1:100 to 1:2000.

Basic Units of Muscle Organisation

•  Sarcolemma is the plasma membrane, which has a well-developed basement membrane outside.
•  Sarcoplasm is the cytoplasm excluding the myofibrils.
•  Sarcoplasmic Reticulum is the smooth endoplasmic reticulum found within the muscle cell.
•  Epimysium is a thick connective tissue layer surrounding a bundle of fascicles. 
•  Perimysium is the connective tissue wrapping each fascicle.
•  Endomysium is a thin connective tissue layer encasing individual muscle fibers.
•  Fascicle is a bundle of muscle cells encased by perimysium.
•  Skeletal muscles are connected to bones via tendons.

Skeletal Muscle - Myofibrils

•  Myofibrils are made up of sarcomeres, repeating units arranged end-to-end.
•  Each sarcomere gives the skeletal muscle its striated appearance.
•  Sarcomeres contain myofilaments. 
•  Myofilaments are actin and myosin filaments, which interact for contraction.

Myofilament Components

•  Components include Z-discs, A band, I band, H zone, and M line.
•  Z-discs are dense membranes separating sarcomeres.
•  A band contains thick myosin filaments (sometimes overlapped by thin actin).
•  I band contains thin actin filaments.
•  H zone is the center part of A band, mainly containing thick filaments.
•  M line supports the thick filaments in the center of sarcomere.
•  Myofilaments include thick myosin and thin actin filaments.

Myofilaments (Actin and Myosin)

•  Actin is a thin filament protein that plays a key role in muscle contraction.
•  Thin filaments consist of 3 proteins: actin, tropomyosin, and troponin.
•  Myosin is a thick filament protein that binds to actin during contraction.
•  Muscle contraction involves binding myosin to actin with the use of ATP, leading to shortening of the sarcomeres.

Regulatory Proteins

•  Regulatory proteins include Tropomyosin and Troponin.
•  Troponin is a complex of 3 proteins: TnC (Ca2+ binding), TnI (inhibitory), and TnT (tropomyosin binding).
•  Under resting conditions, tropomyosin covers the myosin-binding sites on actin, preventing contraction. Calcium ion release causes tropomyosin to move, allowing myosin to bind to actin.
•  Tropomyosin binds to actin and prevents the binding of myosin to actin.

Thin Filaments

•  Thin filaments have a length of about 1 µm and a diameter of 5-8 nm.
•  Nebulin and Titin are elasticity-related proteins within thin filaments and are important during contraction. 

Thick Filaments

•  Thick filaments are made up of myosin protein molecules. Myosin has a head and a tail region.
•  The heads bind to actin molecules to initiate cross-bridge formation during muscle contraction.

Cardiac Muscle

•  Structure includes intercalated discs which are specific for cardiac muscle.
•  Cardiac muscle cells are also called cardiocytes. 
•  Cardiocytes are branched muscle cells and are connected via gap junctions.
•  Cardiac muscle is under involuntary control and responsible for the rhythmic contractions of the heart.

Smooth Muscle

•  Smooth muscle cells are spindle-shaped with a single nucleus.
•  Smooth muscle cells are typically located in walls of hollow organs, such as blood vessels, intestines, and the digestive system.
•  Smooth muscle is under involuntary control.
•  Smooth muscle contraction is often slow and sustained. - Smooth muscles exist in two main types, multi-unit and single-unit. They differ in their innervation(how they receive signaling) and their response to stimuli.