Skeletal Muscle• Skeletal muscles are usually attached to bone by tendons composedof connective tissue. This connective tissue also ensheathe the entiremuscle & is called epimysium. Skeletal muscles consist of numeroussubunits or bundles called fasicles• Fascicles are also surrounded by connective tissue (called theperimysium) and each fascicle is composed of numerous musclefibers (or muscle cells).• Muscle cells, ensheathed by endomysium, consist of many fibrils (ormyofibrils), and these myofibrils are made up of long proteinmolecules called myofilaments.
Types of Myofilaments• Myofibrils; There are twotypes of myofilamentsActin and MyosinFilaments.• Each muscle fiber containsseveral hundred to severalthousand myofibrils,Each myofibril iscomposed of about 1500myosin filaments and3000 actin filaments,The thick filamentsare myosin, and thethin filaments areactin.
• myosin and actin filamentspartially interdigitate and thuscause the myofibrils to havealternate light and darkbands, The light bandscontain only actin filamentsand are called I bandsbecause they are isotropic topolarized light.• The dark bands containmyosin filaments, as well asthe ends of the actinfilaments where they overlapthe myosin, and are called Abands because they areanisotropic to polarized light.• The portion of the myofibril(or of the whole muscle fiber)that lies between twosuccessive Z discs is calleda sarcomere.
Electron micrograph of muscle myofibrils showing the detailed organization of actin andmyosin filaments. Note the mitochondria lying between the myofibrils.
Titin Filamentous Molecules.• The side-by-side relationship between themyosin and actin filaments is achieved bya large number of filamentous moleculesof a protein called titin. Also, because it isfilamentous, it is very springy. Thesespringy titin molecules act as a frameworkthat holds the myosin and actin filamentsin place
Sarcoplasm.• Myofibrils of each muscle fiber aresuspended side by side in the musclefiber. The spaces between the myofibrilsare filled with intracellular fluid calledsarcoplasm, containing large quantities ofpotassium, magnesium, and phosphate,plus multiple protein enzymes.• mitochondria that lie parallel to themyofibrils, supply large amounts of energyin the form of adenosine triphosphate(ATP.
• Sarcoplasmic Reticulum. in the sarcoplasmsurrounding the myofibrils of each muscle fiberis an extensive reticulum the sarcoplasmicreticulum. This reticulum has a specialorganization that is extremely important incontrolling muscle contraction.• The very rapidly contracting types of musclefibers have extensive sarcoplasmic reticula.
General Mechanism of Muscle Contraction• The initiation and execution of musclecontraction occur in the following sequentialsteps.• An action potential travels along a motor nerveto its endings on muscle fibers.• At each ending, the nerve secretes a smallamount of the neurotransmitter substanceacetylcholine.• The acetylcholine acts on a local area of themuscle fiber membrane to open multiple"acetylcholine-gated" channels through proteinmolecules floating in the membrane.• Opening of the acetylcholine-gated channelsallows large quantities of sodium ions to diffuseto the interior of the muscle fiber membrane.This initiates an action potential at themembrane.
• The action potential travels along the muscle fibermembrane in the same way that action potentials travelalong nerve fiber membranes.• The action potential depolarizes the muscle membrane,and much of the action potential electricity flows throughthe center of the muscle fiber. Here it causes thesarcoplasmic reticulum to release large quantities ofcalcium ions that have been stored within this reticulum.• The calcium ions initiate attractive forces between theactin and myosin filaments, causing them to slidealongside each other, which is the contractile process.• After a fraction of a second, the calcium ions arepumped back into the sarcoplasmic reticulum by a Ca++membrane pump, and they remain stored in thereticulum until a new muscle action potential comesalong; this removal of calcium ions from the myofibrilscauses the muscle contraction to cease.
Molecular Mechanism of Muscle Contraction• In the relaxed state, the endsof the actin filaments extendingfrom two successive Z discsbarely begin to overlap oneanother. Conversely, in thecontracted state, these actinfilaments have been pulledinward among the myosinfilaments, so that their endsoverlap one another to theirmaximum extent. Also, the Zdiscs have been pulled by theactin filaments up to the endsof the myosin filaments. Thus,muscle contraction occurs by asliding filament mechanism.
A muscle fiber is excitedvia a motor nerve thatgenerates an actionpotential that spreadsalong the surfacemembrane (sarcolemma)and the transverse tubularsystem into the deeperparts of the muscle fiber.A receptor protein (DHP)senses the membranedepolarization, alters itsconformation, andactivates the ryanodinereceptor (RyR) thatreleases Ca2+ from theSR. Ca2+ then bind totroponin and activates thecontraction process
Sarcoplasmic reticulum (SR) membranes in close proximity to a T-tubule. RyRare proteins the aid in the release of calcium from the SR, SERCA2 are proteinsthat aid in the transport of calcium into the SR