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Biological Sciences
Jason Brown

SLIDE 2Animals are characterized by having 2 broad types of muscle tissues striated muscles and smooth musclesStriated muscles are characterized by having banding patterns There are alternating light and dark bands on the cellSmooth muscles lack that banding patternSmooth muscles and striated muscles are found in both vertebrates and invertebrates but the way theyre distributed may be differentThe striations that occur in striated muscles reflect the fact that the myofillaments actin and myosin are organized in a very regular wayIn smooth muscles the same proteins are present but theyre not organized in a regular fashion so you dont get the bandsSLIDE 3Here we have a muscle The basic building block of a muscle is a muscle cell which you see hereMuscle cells go by different names but the most important ones are myofibre or muscle fibreMyofibre a single muscle cell is bound by a plasma membrane In the muscle the plasma membrane is given the name sarcolemmaEach individual muscle cell is ensheathed by a layer of connective tissue called endomysiumA whole bunch of individual muscle cells are bound up by a second layer of connective tissue called perimysiumThe structure the bundles of muscle fibres create is called a muscle fascicleAll of these fascicles are bound by another layer of connective tissue called epimysium and then we have a single muscleSo if you think it the other way you have a muscle which is comprised of fascicles which are comprised of myofibres and all of these things are bound by a connective tissueThe epimysium is continuous with the tendon The tendons are connective tissues that attach the muscle tissue to the bone The connective tissue that makes up the tendon is continuous with the layers of connective tissue that surround the entire muscleWithin the myofibre are cylindrical structures called myofibril Within each muscle cell there are many myofibrils The myofibrils themselves are made up of the myofilaments actin and myosinThe myofibrils are bounded by the sarcoplasmic reticulumEach myofibril is surrounded by the sarcoplasmic reticulum which is the site that calcium is stored inside the muscle cells Calcium plays a role in regulating muscle contraction and so the distance that calcium has to travel from the storage site to the region of the myofibril needs to be kept as short as possible so contraction can occur quickMyofibrils are very small around 1 micron in diameter so you minimize the distance calcium has to travel back and forth between the deepest parts of the myofibrilActin and myosin are the major proteins involved in muscle contractionSLIDE 4Actin is referred to as thin filaments because its thinner in diameter than myosinEach actin filament found in the muscle is made up of a protein called Gactin globular actin which is an individual monomer actinIf you want to be specific the type of actin found in muscles is known as alpha actin Actin plays a major role within cell cytoskeleton in the muscle its a different kind of actin but in muscles its just alpha actinA single monomer an individual actin protein is referred to as GactinIn the muscle cells the individual actin molecules are built into polymers and make up Factin filamentous actin In Factin individual Gactin molecules are linked together to form this long filament and 2 of these filaments are wound around each other shown in blue and purple to form the FactinActin is a polymer so you can grow it in length or shorten it in length by adding or stripping off Gactins This is good in circumstances like in cells cytoskeleton But in the context of a muscle we want this to be precisely regulated and unchangingTo make sure Gactins arent popping on or off the filament the ends of the actin filament in the muscle are capped by proteins called capping proteins CapZ and TropomodulinTheres a bit of polarity in the actin polymer Theres a barbed end to which its easy for new Gactins to grab on and add themselves to the chain The barbed end is called the
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