Lab 9: Muscle Physiology and Introduction to the Muscular System

Overview of Muscle Tissue – Three types of cells in muscle tissue - skeletal, cardiac, and smooth – Generating force (muscle tension) is a basic function of all muscle tissue Structure of Muscle Cells Myocyte - muscle cell: – Sarcoplasm - myocyte’s cytoplasm – Sarcolemma - myocyte’s plasma membrane – Sarcoplasmic reticulum (SR) - modified endoplasmic reticulum • Myofibrils - Cylindrical organelles – Bundles of specialized proteins; allow for contraction – Other organelles (such as mitochondria) are packed between myofibrils Skeletal Muscle Fibers Skeletal muscle tissue consists of many fibers and surrounding endomysium (extracellular matrix) • Fibers are thin cylinders • Myofibrils - made up of mostly contractile proteins • Sarcoplasmic reticulum (SR) - surrounds myofibrils; stores and releases calcium ions • Transverse tubules (T-tubules) - extensions of sarcolemma; surround each myofibril – Form tunnel-like network within muscle fibers • Terminal cisternae - enlarged sections of SR; flank each T-tubule of the Myofibril Each myofibril contains hundreds to thousands of myofilaments: • Thick filaments - bundles of contractile protein myosin • Thin filaments - proteins actin, tropomyosin, and troponin • Elastic filaments - spring-like structural protein (titin); stabilizes myofibril structure; resists excessive stretching Thick filaments - contractile protein myosin: • Globular heads at each end linked by intertwining tails • Each head has active site that binds with actin Filaments Thin filaments - actin, tropomyosin, and troponin: • Two intertwined strings of actin in thin filament – Each bead-shaped actin has active site - binds with myosin heads • Tropomyosin - rope-like protein that twists around actin and covers active sites • Troponin - small protein that holds tropomyosin in place; assists with turning contractions on and off Sarcomere • I band - only thin filaments • Z disc - in middle of I band; anchor thin filaments in place and to one another and serve as attachment points for elastic filaments • A band - contains overlap of thick and thin filaments • H zone - middle of A band where only thick filaments exist • M line - dark line in middle of A band; proteins hold thick filaments in place • Multiple muscle fibers (each surrounded by endomysium) form fascicle • Each fascicle is surrounded by perimysium • Bundles of fascicles make up a skeletal muscle; surrounded by epimysium • Perimysium and epimysium come together at end of muscle to form a tendon • Skeletal muscles are surrounded by fascia – Anchors them to surrounding tissues – Holds groups of muscles together Contraction • Sarcomere extends from one Z-disc to next • Sliding filament mechanism explains tension generation during muscle contraction: – Myosin heads attach to actin; pull thin filaments toward M line; brings Z-discs closer together (shorten sarcomere) ▪ Both I band and H zone narrow; A band unchanged – Sarcomeres are arranged end to end within each myofibril; when simultaneously contracted, shorten whole muscle fiber Electrically Excitable Cells Membrane potentials are due to unequal distribution of ions near plasma membrane • Separation of charges creates an electrical gradient and represents a source of potential energy – Layer of negatively charged ions inside of cell; layer of positively charged ions outside of cell • When creating a flow of electrical charges, potential energy becomes kinetic energy • Electrical gradient can therefore be referred to as an electrical potential – Voltage - difference in charge (potential) between two points – Measured in millivolts (mV) • Resting membrane potential of a muscle fiber is -85mV Resting membrane potential is maintained by Na+/K+ pump: • Pump moves three Na+ ions out and two K+ ions into cell, per ATP • ATP is necessary because Na+/K+ pump moves ions against their concentration gradient which requires energy – Creates high concentration of Na+ outside of the cell while concentration inside the cell remains low – Creates high concentration of K+ inside the cell while concentration outside of the cell remains low • Action potentials- A brief change in the membrane potential – Resting membrane potential is -85mV – Action potential is generated by opening and closing of Na+ and K+ channels in the plasma membrane in response to stimuli ▪ The movement of Na+ into the cell and K+ out of the cell changes the membrane potential and produces an action potential An action potential occurs in two stages: 1. Depolarization - Na+ channels open and Na+ flows into the cell – Membrane potential to become less negative (depolarization) 2. Repolarization - Na+ channels close, K+ channels open and K+ flows out of the cell – Membrane potential becomes more negative and K+ channels close once cell reaches resting membrane potential Contraction and Relaxation • All skeletal muscles are innervated (connected to a neuron) – A single motor neuron, communicates with many muscle fibers ▪ Each connection is a synapse • Neuromuscular Junction (NMJ) - Synapse where a single motor neuron communicates with many muscle fibers – Transmits the action potential from neuron to sarcolemma Three components of NMJ: • Axon terminal – synaptic vesicles filled with acetylcholine (ACh) • Synaptic cleft - space between axon terminal and muscle fiber • Motor end plate - specialized region of muscle fiber plasma membrane – Folded surface has ligand gated Na+ channels – Ach is ligand that opens gates, allowing Na+ to diffuse into muscle cell Step 1: Excitation • Action potential from brain or spinal cord arrives at synaptic terminal of motor neuron • Acetylcholine is released into synaptic cleft and binds to ligand-gated ion channels of motor end plate – Ligand gated channels open and Na+ ions flow into the muscle fiber, generating endplate potential (depolarization of motor end plate) Step 2: Excitation-contraction coupling • End-plate potential (depolarization of motor end plate) opens voltage gated Na+ channels in sarcolemma triggers an action potential – • Action potentials propagate depolarization of one area of membrane triggers next few channels to open (like a chain reaction) Action potential signals terminal cisternae to open voltage-gated Ca2+ channels, releasing Ca2+ into the cytosol of the muscle fiber In preparation for muscle contraction: • Ca2+ released from terminal cisternae binds to troponin – When Ca2+ binds to troponin it triggers tropomyosin to move off actin and expose the active sites Step 3: Contraction Begins when actin’s active site is exposed, initiating crossbridge cycle: 1. Myosin head cocked once ATP is bound 2. Myosin head is now able to bind to active site of actin (crossbridge) 3. ATP’s energy is harnessed causing myosin heads to pull actin toward M line (power stroke) then it pivots to a relaxed position 4. Myosin can bind to another ATP and break the link with the actin active site – Detachment of myosin head from actin does not allow thin filament to slide backward because some myosin heads are still attached to Contraction cycle may repeat as long as stimulus to contract continues and ATP is available: • Myosin head recocked, binds to first actin molecule, and power stroke repeats, Myosin binds to second actin, and so on, over and over Isotonic contractions - tension generated is constant, but muscle length changes: • Isotonic eccentric contractions constant tension but muscle lengthens • Isotonic concentric contractions constant tension while muscle shortens Isometric contractions - muscle length remains unchanged because external force applied equals that generated by muscle Skeletal Muscle Relaxation Motor neuron action potentials stop signaling for release of acetylcholine from axon terminals 1. Acetylcholinesterase degrades remaining Ach; ligand-gated Na+ channels close; end plate potential ends; final repolarization begins 2. Sarcolemma returns to resting membrane potential 3. Ca2+ ions pumped back into sarcoplasmic reticulum ; returns cytosol concentration to resting level 4. In absence of calcium, troponin and tropomyosin block active sites of actin, and muscle relaxes; myofilaments slide back into original positions Structure of a Skeletal Muscle Endomysium- thin connective tissue layer surrounding each individual muscle cell (fiber) • Perimysium- connective tissue layer surrounding each fascicle – Fascicle- several (between 10-100) muscle cells bundled together • Epimysium- fibrous connective tissue wrapping around all fascicles • Fascia – most superficial connective tissue sheath; continuous with epimysium – Parallel – strap-like muscle; evenly spaced fascicles; muscle and tendon are same width – Convergent – broad triangular-shaped muscle that tapers down into single tendon – Pennate – fascicles attach to tendon at angle; feather-like appearance – Unipennate – fascicles only attached to one side of associated tendon – Bipennate – fascicles attached to both sides of associated tendon – Multipennate – several regions of fascicles joined by connective tissue; each section contributes to form single tendon • Sphincters – circular fascicle arrangements; surround body openings; provide voluntary control over defecation and urination • Spiral – muscles that wrap around another structure (bone) • Fusiform – muscle midsection (belly) is thicker than each tapered end • Muscle origin and insertion – skeletal muscles begin and end at distinct anatomical locations – Origin – anchoring point on bone, where skeletal muscle “originates from”; typically not involved directly with movement of joint – Insertion – moving end of muscle whose tendon attaches to bone or other structures; usually on far side of joint • Unit 12 Activity 1 – Identifying the Structural Components of a Skeletal Muscle • Unit 12 Activity 2 – Examining the Microscopic Anatomy of Skeletal Muscle Tissue and the Neuromuscular Junction • Unit 12 Activity 3 – SKIP! • Unit 12 Activity 4 – SKIP! • Unit 12 Activity 5 – Using Play Doh to Illustrate Muscle Contraction Physiology • Unit 12 Activity 6 – Describing the Process of Muscle Contraction