4. muscle

Muscle Fibers: Long cylindrical, multinucleated cells made up of myofibrils.
Sarcolemma: Plasma membrane surrounding each muscle fiber; contains T tubules for action potential transmission.
Endomysium: Connective tissue surrounding the sarcolemma.
Sarcoplasm: Cytoplasm of muscle fiber; contains glycogen and myoglobin (oxygen-binding protein).

T Tubules: Infoldings of the sarcolemma; filled with interstitial fluid; crucial for action potential propagation.
Sarcoplasmic Reticulum (SR): Fluid-filled membranous sac surrounding myofibrils; stores calcium ions; releases calcium upon muscle stimulation for contraction.
Terminal Cisterns: Dilated ends of the SR that lie next to T tubules; together form a triad with T tubules.

Myofibrils: Contractile organelles extending the entire muscle length; consist of myofilaments (thin and thick).
Thin Filaments: Primarily composed of actin.
Thick Filaments: Primarily composed of myosin.
Sarcomeres: Basic functional units of myofibrils; arranged between Z discs.

Z Discs: Define the boundaries of sarcomeres.
I Band: Light region containing only thin filaments on either side of Z discs.
A Band: Dark region containing thick filaments; includes zones of overlap with thin filaments.
H Zone: Central region of A band containing only thick filaments; no thin filaments.
M Line: Center of the sarcomere; anchors thick filaments.

Myosin: Thick filament protein; head binds to actin for contraction.
Actin: Thin filament protein; contains binding sites for myosin.
Regulatory Proteins: Tropomyosin (blocks myosin binding sites) and troponin (holds tropomyosin in place).

In relaxed muscles, tropomyosin blocks actin's myosin binding sites.
Calcium binding to troponin causes tropomyosin to shift, allowing myosin and actin to bind and trigger contraction.
Each myosin head binds actin and ATP for muscle contraction to occur.

Muscle Fibers: Long tube-like cells that are very long and have many nuclei (the parts that hold information). They help muscles move.
Sarcolemma: A thin layer around each muscle fiber; like a plastic wrap that helps send messages when you want to move.
Endomysium: A soft wrapper around the outside of the muscle fiber, like a cozy blanket.
Sarcoplasm: The jelly inside the muscle cell that stores energy and helps transport oxygen (a special gas needed by muscles).

T Tubules: Tiny tubes that go deep into muscle fibers, helping send messages quickly, like a phone signal.
Sarcoplasmic Reticulum (SR): A balloon-like storage place next to myofibrils; it keeps calcium (a mineral that helps muscles work) and sends it out when needed.
Terminal Cisterns: The ends of the balloon that are next to T tubules; together they form a triangle to help the muscle work.

Myofibrils: Tiny string-like parts inside muscles that help them squeeze; they are very long and thick.
Thin Filaments: Made mostly of a protein called actin; these are like the small threads that work with the thick ones.
Thick Filaments: Made mostly of a protein called myosin; these threads are bigger and help pull on the thin ones.
Sarcomeres: The small sections of myofibrils that act as building blocks; they help muscles work together.

Z Discs: The walls or ends that keep the sarcomeres together.
I Band: The light area where only thin threads are found, like the empty space between two buildings.
A Band: The darker area with thick threads, where the thick and thin threads mix.
H Zone: The middle part of the A band where only thick threads are; it’s like a quiet place without anyone else.
M Line: The center line of the sarcomere that holds the thick threads in place.

Myosin: The thick thread that can grab onto the thin one to help muscles squeeze.
Actin: The thin thread that can be held by myosin to help in movement.
Regulatory Proteins: Tropomyosin helps block actin from being grabbed too early, while troponin helps hold tropomyosin in place.

When muscles are relaxed, tropomyosin blocks the actin threads.
When calcium attaches to troponin, it moves tropomyosin away, letting myosin and actin connect, which makes the muscle squeeze.
Each myosin head needs to grab onto actin and use ATP (an energy source) so the muscle can work.