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 unit Each 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 body are 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 difference between 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 line bisecting 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 fiber Sarcoplasm cytoplasm of the muscle fiber Sarcoplasmic reticulum SR specialized smooth ER that stores releases calcium into the sarcoplasm the expanded ends of the SR are called terminal cisternae Myoglobin a red pigment that binds and stores oxygen Triad 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 the skeletal 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 are blocked 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 action potentials 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 SR in 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 now exposed 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 occurs there is a change in orientation from the upright position to an oblique position The upright position is referred to as the powerstroke which results in the sliding of the thin filaments into the H zone toward the M line this is the reason for the sliding
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