Muscular System 1. Define: endomysium, perimysium, epimysium, fascicles, tendons, sarcolemma, sarcoplasm, sarcoplasmic reticulum, myoglobin, sarcomere, Z line, H zone, A band, I band, Triad, motor unitEach muscle fiber (skeletal muscle cell) is wrapped in a delicate CT membrane called endomysium. A fascicle is a group of endomysium – covered muscle fibers wrapped in a coarse CT membrane called perimysium. Skeletal muscle consists of a group of fascicles wrapped in a tough T membrane called epimysium. When the skeletal muscle is attached indirectly, the epimysium blends into a tendon, which attaches the skeletal muscle to the surface of the bone. Most skeletal muscles in the human bodyare attached indirectly.Skeletal muscles are made of myofibrils, which contain 2 types of myofilaments (thick and thin).Each thin filament (made of actin) is bisected by a line called the Z line or z disc. The differencebetween 2 successive z lines is called the sacromere, which is the smallest contractile unite in skeletal muscle. Sacromeres are the structural unites of skeletal muscle. The Z lines anchor the thick filaments. Locations of the thick filaments (made of myosin): inside the sacromeres where they occupy the center of the sacromere and are referred to as A-bands. The A band overlaps with the thin filaments. - The region of the thin filaments not overlapping with the A-band is called the I-band. The I-band is the part of the thin filament attached by the Z-lines. - The region of the A-band not overlapping with the thin filaments is called the H-zone. The linebisecting the H-zone is called the M-line, which anchors the A-bands in a sacromere. To remember the order, I-band, Z-line, A-band, H-zone, and M-line: Ian Ziggey A Handsome Man. Sarcolemma – plasma membrane of the muscle fiberSarcoplasm – cytoplasm of the muscle fiberSarcoplasmic reticulum (SR) – specialized smooth ER that stores/releases calcium into the sarcoplasm; the expanded ends of the SR are called terminal cisternaeMyoglobin – a red pigment that binds and stores oxygenTriad – composed of a transverse tubule in between 2 terminal cisternae:- Transverse tubule: involutions of the sarcolemma into the sarcoplasm- Terminal cisternae - t-tubule – terminal cisternae - Function: for the release of calcium ions into the sarcoplasm when the sarcolemma depolarizes - Excitation Coupling- Each skeletal muscle is innervated (“served”) by a motor neuron, which gives off several axon terminals. Axon terminals make contact with several skeletal muscle fibers in that skeletal muscle. The motor neuron and all skeletal muscle fibers that innervate in a skeletal muscle are referred to as a motor unit. Motor unit- There are different sizes of motor units - Small motor units occur when a motor neuron makes contact with a few skeletal muscle fibers via its axonal terminals (around 3 contacts)- Medium sized motor units have more contact with skeletal muscle fibers than the small motor units (around 50 contacts)- Large units have contact with many skeletal muscle fibers in the skeletal muscles (100 or more)2. Describe the neuromuscular junction. What is the motor end plate? - the point of close contact between the axonal terminal (axon terminal) and the skeletal muscle fiber is the neuromuscular junction (myoneural junction) - the narrow space between the axonal terminal and the sarcolemma of the skeletal muscle fiber is called the neuromuscular cleft- the region of the sarcolemma involved in forming the neuromuscular junction is highly folded and is called the motor endplate - the motor end plate will have increased surface area to accommodate more acetylcholine receptors, which are located on the surface of the sarcolemma (motor endplate)3. Name the 3 proteins in a thin filament. What is the function of tropomyosin in relaxed skeletal muscle?Each fiber is composed of rod-like structures called myofibrils, which run the entire length of theskeletal muscle fiber. Approximately 80% of the volume of skeletal muscle fiber is made up of myofibrils. Each myofibril contains 2 types of myofilaments – thin filaments and thick filaments. The thin and thick filaments are arranged in an alternating pattern, resulting in the striations in skeletal muscle.Each thin filament is composed of 3 types of proteins.- 1. Actin: forms the framework of the thin filament, which is a double stranded protein.- 2. Troponin: a regulatory protein composed of three sub-units- 3. Tropomyosin: a rod-like regulatory protein that spirals around the actin blocking myosin binding sites on actin (in the relaxed skeletal muscle, the myosin-binding sites areblocked by tropomyosin) Name the 3 subunits of troponin. Which troponin subunit is bound to actin? - Troponin: a regulatory protein composed of three sub-units- - Troponin T (TnC) – binds to tropomyosin- - Troponin C (TnC) – binds to Ca2+- - Inhibitory Troponin (Tnl) – binds to actin 4. How are the myosin heads activated?- 1. The motor neuron is activated and the axon of the motor neuron generates and transmits action potentials (“electrical signals”)- 2. The action potentials cause the release of neurotransmitters, acetylcholine, from the axonal terminals into the neuromuscular clefts.- 3. Acetylcholine binds to acetylcholine receptors located on the surface of the motor endplate. Binding of acetylcholine to its receptors stimulates the generation and transmission of actionpotentials, which spread across the entire sarcolemma, including the transverse tubules (T tubules) in the triads- 4. The action potentials in the T-tubules stimulate the release of calcium ions from the SR (sarcoplasmic reticulum) by depolarization of the terminal cisternai (the expanded ends of the SRin the triads). Ca2+ are added to the sarcoplasm.- 5. The Ca2+ binds to the TnC which causes a conformational change in the troponintropomyosin complex via TnT. This ends the blockade of tropomyosin because the tropomyosin is removed from blocking the myosin-binding sites on the actin (these sites are nowexposed) - 6. Activated myosin heads can now bind to their sites on actin -** An activated myosin head: ATPase splits ATP ADP + Pi- A myosin head with ADP and Pi attached is an activated myosin head - 7. Activated myosin head binds to its site on actin in an upright position-8. The attached, activated myosin heads are called cross bridges. The ADP + Pi dissociates from the cross bridges. Once the dissociation
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