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York University
Kinesiology & Health Science
KINE 1020
Jennifer Kuk

Chapter 17: Muscular Structure and Function Lecture 25, 26, 27 and 28 Categories of muscle  Skeletal muscle o Striated  Lines aligned o Not electrically coupled o Voluntary  Contract if want o 600 skeletal muscles  40-50% of total body weight o Functions  Locomotion and breathing  Postural support  Heat production during cold stress/shivering  Cardiac muscle o Striated o Controlled by autonomic nervous system o interconnected o Electrically coupled cells  1 cell stimulated it can stimulated one beside it  Smooth muscle o Non striated o Responsible for blood vessel tone (vasoconstriction/vasodilation) Structure of a skeletal Muscle  Muscle is composed number of fasciculi that are bundled together  Within fasciculi there are muscle fibres (cells) o Contains striation areas – alignment of proteins held together  Muscle fibres are made up myofibrils (contraction units) o Contain contractile proteins that generate force and are numerous  Med gastrocnemius 1,033,000 fibres…lumbar 10, 250 fibres  Anatomy o Muscle (600)  Composed of many cell types – muscle, vascular, satellite o fasciculi  Bundles of muscle cells – number in 10s o Muscle cell (fiber)  Elongated structure – 80% myofibrils, multi nucleated o Myofibrils  2,500 - 5000 running along length of cell Anatomy of Sarcomere: Bands, Lines and filaments  Myofibrils can be broken down into sarcomeres (basic contractile unit) – action happening/tension produced o 5000 – 10,000 o One Z line left and one Z line on the right  Z-line is centre of I-band o Different zones/regions in sarcomere because amount of proteins  Darker region  A-Band (anisotropic) o More protein, not as much light gets through  Light region  Center of A-Band is the H-Band and M-line at the centre  External to A-Band is the I-Band (isotropic unit)  Span border of two sarcomeres Muscle Proteins  Myosin and Actin are leaders in force production, Z line comes closer together, reacts in movement  Filaments o Thin Filaments  Actin, tropomysoin and troponin o Thick filament  Myosin (responsible for light and dark regions) o Intermediate filaments are the structure or scaffolding that the actin and myosin attach to allow for generation  Titan  Larger protein and is located from Z-line to M-line  Allows for the force transmission at the z-line and contributes to passive stiffness of muscle  Desmin  Acts as scaffolding anchoring the z line to z line of next lateral sarcomere  Provides support lengthwise and is like the cord that binds all sarcomere chains together with z-lines  C-protein  In the middle of each of A-band  Elastic bands binding myosin tail region in bundle of 200 to 400 to form thick filaments  Actin o Globular protein (G-actin) binds together in chain to form F-actin; microfilament o Two F-actins (filamentous), winded together in double helix form, with troponin and tropomyosin to form thin filament  Tropomyosin o Long double stranded protein weaves around F-actin molecules o Blocks active sites on actin so myosin can’t bind  Troponin o Globular protein made up of 3 sub units (Troponin C, I and T)  T bind the troponin to tropomyosin  Calcium bind to troponin-C (during muscle contraction) and cause it to change shape and remove tropomyosin exposing active sites  I - inhibits this process during resting state  Myosin o Two globular head regions and rod like tail region and is major protein in thick filaments o Myosin’s form thick filament with tail regions coming together in a thick rod like structure with the heads extending outwards o Each head has a binding site for actin and other for ATP Muscle Contraction (actin and myosin interaction)  Actin and myosin must interact for force production to occur  CNS control voluntary skeletal muscle contractions o Brain sent messages down spinal cord to the nerves and motor neurons o Action potential travels down motor neuron and causes release of acetylcholine, travels down sarcolemma o Action potential enters t-tubules, causing the voltage calcium channels on sarcoplasmic reticulum to open and release calcium ions into sarcoplasm o Calcium binds to tropinin C, causing tropinin T to change shape allows tropomyosin to move and unblock binding site on actin  Regulation over actin and myosin o ATP  Primarily focussed on myosin molecule, needs energy to bind with Actin  Head of myosin o Calcium  Prepare the thin filament, prepare actin for myosin  Binds to specific group of proteins  Calcium bind to troponin-C (during muscle contraction) and cause it to change shape and remove tropomyosin exposing active sites o Alignment of proteins – protein-protein interactions (mechanotransduction)  Force being generated at one sarcomere must be aligned at all the others ones  Process Force production o Myosin molecule, close to the Actin  Myosin must take ATP and transform to mechanical energy from chemical to bind  Myosin ATPase – hydrolyze ATP  Hydrolyzed to ATP to ADP and extra phosphate  Energizes myosin o No calcium myosin and actin don’t bind o Calcium is added actin and myosin bind  After process calcium goes back to sarcoplasmic reticulum o Bending occurs – adp and phosphate leave and become available for next myosin o Rigor complex is available  Net equation  (ATP + H2O  ADP + PI + H + Eh) o Mechanical energy and heat energy (by product) Calcium’s role  Opens up binding site, where myosin can bind to actin  Calcium is stored in sarcoplasmic reticulum o Internal membrane system o On the other side of T tubule  T-tubule o Interacts with outside membrane of the cell  “I wanna move my arm” – nerve impulse goes on the membrane into t-tubule, which releases calcium – action potential goes down and releases calcium  Calcium is taken out of membrane system and goes into sarcoplasm  Calcium associates with tropinin, opens up the site so myosin can interact  Calcium then dissociates and translocates, calcium reuptake going back into sarcoplasmic reticulum Sarcomere Protein-Protein Interactions: Force Transmission  Titan – very large - intermediate o Located from z-line to M-line – with 244 protein regions/domains  Another molecules can regulate its shape o Function  Force transmission at Z-line  Resting tension at I-band  Contributes to passive stiffness  Adhesion template – scaffold  Myosin binding Protein C (MYBPC) – located in middle of each half of A-band o Function  Binds to myosin tail region  Maintains thick filaments in bundle of 200 to 400 molecules o Types  Slow – MYBPC1  Fast – MBPC2  Cardiac – MYBPRC3 Sarcomere – Dynamic Structure  Resting muscle  Z line disruption – not aligned  Focal disruption  Moderate disruption Muscle Contractile Activity: Physiological Profile Twitch Response for a Single Muscle Cell or Fibre  In order for muscle to contract, there needs to be a stimulus (action potential_ that is followed by a response (force production)  Latent period - When a nerve stimulates a muscle there is an action potential o 1-2 msec  Contraction time – contraction begins and gradual increase in force produced in the muscle
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