BCI201 Lecture Notes!.docx

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University of Maryland
Biological Sciences Program
BSCI 201
Justicia Opoku

BCI201 Lecture Notes 11/1/13 Skeletal Muscle Fiber In a RELAXED skeletal muscle, the overlap between 1. Myofibrils consist of 2 types of filaments [actin and thick/ thin filament is not complete myosin] - Incomplete overlap between the alternating thick/ 2. Myoglobin- red pigment that bind/ release oxygen thin filaments in the myofibril [oxygen required for aerobic catabolism of glucose - The thin filaments are anchored by a disc like 3. Mitochondria- sites for aerobic catabolism protein called the Z disc 4. Sarcoplasmic Reticulum- specialized smooth endoplasmic reticulum (sequester/ store/ release calcium - the distance between 2 successive z discs in a into sarcoplasm) myofibril= SARCOMERE o sarcomere repeats throughout the myofibril  - Calcium release is required for muscle contraction repeated units  structural units - Each SR has 2 expanded ends called Terminal o a sarcomere is the smallest contractile unit in a Cisternae 5. Sarcolemma has deep invagination called Transverse skeletal muscle  functional and structural units Tubules (T tubules) of skeletal muscle - Each T tubule passes through 2 terminal cisternae *Sarcomere description- part of a myofibril between 2 (of 2 SR) successive z discs 6. Triad- structure is one terminal cisternae, t- tubule, an- in center of sarcomere= thick filament A bands one terminal cisternae - thin filaments are anchored by the z discs and there’s - As action potential travels into the t- tubule of thincomplete overlap w/ the A bands triad, calcium is released from the terminal cisternae of the triad o the region of the thin filament NOT overlapping with 7. Myofilaments in the myofibrils the A bands  I band (attached to the Z disc) Thick (16nm)-composed of 300 molecules of a protein, o region of the A band (thick filament) NOT Myosin overlapping w/ thin filaments  H Zone (in middle) o the line that bisects the H zone  M line anchors - *Myosin Structure- rod like tail with 2 globular heads, and each myosin head has 2 binding sites the A bands in a sarcomere [actin binding site and ATP binding site- contains Skeletal muscle- skeletal muscle fibers run the entire length ATPase] of the skeletal muscle (organ) - Each myosin globular head contains the enzyme - myofibrils run entire length of skeletal muscle fibers ATPase ] ADP+Pi - sarcomeres arranged end to end through the entire length of the myofibril - ATPase- An enzyme that catalyzes the hydrolysis of ATP to ADP, releasing energy that is used in the cell How does a skeletal muscle contract= shorten? Thin filaments (8 nm)- composed of 3 types of proteins: - Sarcomeres shorten then Myofibrils shorten since the 1. Actin- forms the structural framework of the thin myofibrils run the entire length of the skeletal muscle fibers, filament, and has binding sites for the myosin the skeletal muscle fibers shorter globular heads o Since the skeletal muscle fibers run the entire length of the skeletal muscle, the skeletal muscle 2. Tropomyosin- rod- like and spirals around the actin, and blocks the sites for myosin globular shortens heads on actin in a RELAXED SKELETAL o More overlap between thin filaments and A band MUSCLE when muscle contract 3. Troponin- trip complex protein with 3 subunits: o TnC- binds CA^2+ [calcium] o TnT- binds tropomyosin o TnI- binds actin Arrangement of the myofilaments in a myofibril thick filaments alternate with the thin filaments - Thick/ Thin/ Thick/ Thin etc. - Gives the banded/ striated appearance of skeletal muscle tissue Lecture: November 4, 2013 Sliding Filament Mechanism- Muscle Contraction **Sequence of Events in the E- C Coupling - Skeletal muscle contracts/ shorten when sarcomeres 1. Activate/ excite a motor neuron action potentials are shorten due to sliding of thin filaments into the H zone generated and transmitted by the axon of the motor neuron toward M line - When action potentials get to the axon terminals, a - Sliding of thin filaments pulls the Z discs inward  neurotransmitter (Acetylcholine) is released into a dec the distance between 2 successive z discs neuromuscular cleft (tiny fluid filled space btwn [shorten the sarcomeres  shorten the myofibrils shorten skeletal muscle fibers  shorten skeletal axon terminal and motor end plate) 2. Acetylcholine diffuses to the MEP to bind to acetylcholine muscle] receptors to stimulate the generation/ transmission of action potential at the sarcolemma which spreads into the What happens to cause thin filaments to slide into the H T- tubules of the triads zone in a sarcomere - Calcium (Ca^2+) is released from the terminal  Excitation- Contraction (E-C) Coupling- a series of cisternae of the SR and into the sarcoplasm events which involves the activation/ excitation of a Motor Neuron which stimulates skeletal muscle to - Function of calcium in skeletal muscle contraction: contract so calcium ends tropomyosin blockade of myosin binding sites on actin - Each skeletal muscle (the organ) is innervated by - Calcium binds to TnC of troponin  conformational at least 1 motor neuron (motor neuron is part of change of troponin which includes TnT (attached to the somatic nervous system) tropomyosin) resulting in the removal of o The motor neuron contacts several tropomyosin from blocking the myosin binding site skeletal muscle fibers in the skeletal on actin muscle via its Axon Terminals o Relaxed skeletal muscle, tropomyosin o The axon terminal of a motor neuron blocking sites on actin so calcium ends forms a junction [NEUROMUSCULAR blockade by removing the tropomyosin, JUNCTION] with a highly folded region Actin- when tropomyosin blockade ends, myosin binding [MOTOR END PLATE] on the sites on actin are exposed  they are bound by ACTIVATED sarcolemma of the muscle fiber myosin globular heads - A skeletal muscle fiber can form only one neuromuscular junction - Each myosin globular head has a binding site for actin; binding site of ATP; containing enzyme The motor neuron and all the skeletal fibers it makes ATPase contact with via its axon terminal Motor Unit - Small motor unit- makes contact w/ few (10) What activated myosin globular head? skeletal muscle fibers via axon terminals, - When attached ATP is hydrolyzed by ATPase into generate least force ADP + iniorganic Phosphate (Pi)  attached to the - Median sized motor unit- makes contact with 50 globular heads muscle fibers via axon terminals - Activated globular heads have ADP + Pi attached, - Large motor unit- contact w/ at least 100 skeletal the activated myosin head binds to its accessible muscle fibers site on actin (made accessible by calcium binding Force generated by skeletal muscle from the small motor to TnC to end tropomyosin blockade and expose unitmedium  large motor unit which will generate the site on actin) most force Cross Bridge- Activated myosin globular head attaches to - Recruitment: activate the smaller motor units first, the actin filament (exposed myosin binding site on actin) then medium then large (doesn’t waste force by using large motor units first) LECTURE NOTES 11/6/13 Activated myosin globular head binds to the myosin How is ATP generated to support skeletal muscle binding site on actin to form a CROSS BRIDGE contraction? - The initial binding is at a right angle  when the. Stored ATP- support 5 seconds of skeletal muscle activity ADP and inorganic Phosphate (Pi) dissociate from 2. Direct Phosphorylation of ADP by creatine phosphate the cross bridge, the cross bridge undergoes a  ATP + Creatine change in orientation (referred to as the POWER STROKE- slides or pulls the actin into the H zone). Aerobic Catabolism/Breakdown of Glucose o Explains the sliding filament mechanism - yield 38 ATP per glucose of skeletal muscle contraction 4. Anaerobic Catabolism of Glucose (= glycolysis)- net 2 ATP per glucose After “sliding” of the thin filaments (resulting in shortening Aerobic Catabolism is more efficient than of sarcomeres) the skeletal muscle relaxes: anaerobic catabolism Factors that Affect STRENGTH/ FORCE of Skeletal Sequence of events in skeletal muscle relaxation (after Muscle Contraction shortening/ contraction) 1. halt motor neuron excitation (activation) 1. Size of motor units activated 2. sequestration of calcium (CA^2+) into SR via active 2. Number of motor units activated transport that requires ATP (energy input) 3. Frequency of skeletal muscle activation by the motor neuron  temporal summation 3. cross bridge detachment  NEW ATP binds to the ATP  increased frequency of activation binding site on the myosin globular head a. lack of ATP (complete absence) occurs when  inc calcium in the sarcoplasm a person dies  result in RIGORMORTIS  nc tropomyosin blockade (contraction of skeletal muscles w/ attached  inc shortening of skeletal muscle cross bridges- no means of detaching them)  inc strength/ force generated by the shortened 4. in a living person if there’s ATP deficit  demand for skeletal muscle ATP to support skeletal muscle contraction exceeds 4. Length of Sarcomeres prior to contraction rate of ATP production o Optimal sarcomere length= overlap between the A a. resulting in MUSCLE FATIGUE  physical band (=thick filament) and the thin filaments w/ inability of skeletal muscle to contract even adequate H zone to allow for sliding of the thin when stimulated b/c of ATP deficit filaments 1. When sarcomeres are shortened prior to contraction  complete overlap between thin filaments and A bands but NO H zone  no sliding of thin filaments o no shortening of sarcomeres or skeletal muscle so zero tension/ force generated 2. Stretched sarcomeres prior to contractiono overlap between thin filaments and A band but wide H zone o since the activated myosin globular heads cannot reach their binding sites on actin (in thin filaments)  no sliding of the thin filaments o so no shortening of sarcomeres or skeletal muscle and no force/ tension generated Lecture Notes 11/8/13 2 Main Categories/ Phases of Skeletal Muscle 3 Types of Skeletal Muscle Fibers CONTRACTION o Each skeletal muscle ( the organ) contains all 3 1. Isometric Contraction (same length) - recruitment types of skeletal muscle fibers  the number of where small motor units are activated first, if more force is required, medium sized motor units are recruited each type of skeletal fibers in each muscle is genetically determined o  inc force followed by large motor unieven Fast Oxidative Fibers more inc in force isometric contraction  inc - High speed at which Myosin ATPase hydrolyzes tension/ force generated by skeletal muscle ATP causes length of skeletal muscle to REMAIN THE - Oxidative fiber type: aerobic respiration SAME o Occurs when the weight exceeds the force Slow Oxidative Fibers - myoglobin content is HIGH (highest of the Hence, generated by the muscle SOF appear reddish in color (b/c myoglobin is red 2. Isotonic Contraction (same force) - enough force
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