moving on to the microscopic structure of scal muscle and the most important part of a muscle is the muscle fibers or the muscle cells themselves so a muscle fiber or a muscle cell is a long cylindrical multinucleated cell made up of many myof fibral surrounding each muscle fiber is the sarola which is the plasma membrane of a muscle cell surrounding the Sara is then the endom mesum within the sarala So within that muscle cell plasma membrane are thousands of tiny infoldings or indents called transverse or t tubules these indents tunnel in from the surface of the membrane towards the center of each muscle fiber because the T tubules are open to the outside of the muscle fiber they're actually filled with interstitial fluid when a muscle fiber is stimulated so it receives an action potential from a motor neuron that action potential in the muscle travels along the cyma and through the T tubules quickly spreading through the muscle fiber this arrangement of the T tubules ensures that an action potential will excite or stimulate all parts of the muscle fiber at essentially the same time within the cyma or within the muscle cell is also the psychop plasm and the psychop plasm is just the cytoplasm of the muscle fiber the psychop includes a large amount of glycogen which is a stored form of glucose so the glycogen and the glucose will help the muscle produce ATP also within the psychop we find myoglobin which is a special type of protein only found in muscles which help bind to oxygen which diffuses into the muscle through those capillaries so to point out some of those structures to you on an image and so our muscle fiber or muscle cell is this part here made up of our many myof fibral and the muscle fiber is surrounded by ouro Lemma which is just the plasma membrane of a muscle cell surrounding or encapsulating that muscle cell and the psychop is the endom mesum so that's this one here so we've got the SOP pyemma is the plasma membrane of that muscle fiber or cell and then surrounding that is that connective tissue layer that we call the endom mesum within the sarala are thousands of infoldings or indents which we call T tubules so you can see the T tubu here if this is the S LMA we have those indents going to wrap around each of our myof fibral these are those interstitial filled channels that run deep into the muscle fiber to help spread an incoming muscle action potential now looking at the components which make up a muscle fiber so we're going smaller and deeper within the muscle now and within the muscle fiber or the muscle cell are many myof fibral which are the contractile organel of sceletal muscle myof fibral are only very very narrow in diameter but they extend the entire length of the muscle their prominent striations which are those stripes which we'll speak more about shortly is what gives the sceletal muscle that stripe appearance now wrapped around each myop fibral is a fluid filled system of membran a sax which we call the sarcoplasmic reticulum the psychop plasmic reticulum is similar to the smooth endoplasmic reticulum in cells that are not muscle cells the dilated ends of the sarcoplasmic reticulum are called terminal Cy turns and they butt up against a t tubule on either side in a relaxed muscle the psychop reticulum stores calcium ions when that muscle is stimulated the release of calcium ions from the terminal systems of the psychop plasmic reticulum is what will trigger muscle contraction so again looking at those structures now on an image and this is our myop fibral here this this is our zoomed in version down the bottom and remember lots of myop fibral will make up a muscle fiber or muscle cell wrapped around each myof fibral is a pyop plasmic reticulum so that's this yellow structure here and then also in this image here here and here and the dilated ends of the psychop plasmic reticulum are called the terminal sister or terminal Cy a as a single so that's this part here and here and here and here and you can see that they lie either side of our T tub together the two terminal Cy and the T tub form what we call a Triad now in a relaxed muscle this psychop reticulum will store calcium ions when that muscle is stimulated which remember will run through the CMA and down those T tubules calcium is released from these terminal cists now again continuing to look deeper and deeper into the muscle and a myof fibral is made up of smaller protein structures called filaments or myofilaments thin filaments are unsurprisingly the thinner of the two filaments and are mostly made up of the protein actin the thick filaments are thicker in diameter and are mostly composed of the protein mein both our thin and our thick filaments are directly involved in muscle contraction now the filaments of a myop fibral do not extend the entire length of a muscle so our myof fibral do extend the length of a muscle but not the filaments within that myof fibral instead they are arranged into sections called sarom and a saramia is considered the basic functional unit of a myop fibral now narrow z-shaped proteins called Z diss which are these sections here and here consider the end of each SAR and that's what separates one Saria from the next and the next so looking at the structure of a sar and we've already established the sarir runs from one zisk to the other the region on either side of the Z disk is what we call the eye band and the ey band is a lighter less dense area of the saramia because it contains our thin filaments only so our thick filament is this dark purple line running through the middle of the sarir our thin filaments are these green lines that are extending out from either side of that Z disk so the I band is the region either side of our Z disk that only contains those thin filaments the a band is then the darker middle zone of a saram it extends the entire length of our thick filament so this dark purple line and towards each end of the a band is a zone of overlap where our thick and our thick filaments will actually lie side by side so you can see in this section here and in this section here we have overlap of that thick filament and that thin filament the H zone is this narrow region in the center of a sarir this contains our thick filaments only but no thin filaments and in the very very center of the Saria which lies within the H Zone it's not labeled on this image here it's labeled um down here that very middle line is what we call the mline and so the mline anchors the thick filaments to the center of the saramia so it keeps it in this center of the Saria which when we start talking about how the muscle actually contracts and the sliding filament theory you'll understand how important this is now the very last structures that I want to mention which are also the smallest components in a muscle are some specific muscle proteins and we have two contractile proteins which we've already mentioned we have mein which forms most of the thick filament and actin which forms most most of the thin filament so mein is this protein here making up most of that thick filament it runs through the center of each sarir and it's anchored to the middle of that Saria by the mline now each mein molecule is shaped a little bit like two golf clubs with their handles Twisted together the ends of the mein protein which is the part that looks like the head of a golf club so these end bits here they project out from the center of the Saria and again when we start talking about muscle contraction in the next lot of lecture recordings it's these meas and heads which will bind to Acton for contraction to occur our actin is in this part here it's the protein that makes up most of our thin filaments and the thin filaments are anchored to the end of each Saria by these Z diss on each Acton molecule is a mein binding side where the mein head can then attach now looking at this bottom image which is a zoomed in version of our thin filament and we have two regulatory proteins called tropomyosin and troponin and in a Rel reled muscle myosin is blocked from binding to actin because strands of tropomyosin which are these Brown strands along here cover The myin Binding SES on actin so these little dark dots on the Acton molecules are the mein binding sides when a muscle is relaxed this tropomyosin protein will hide those binding spots so the M heads and the actin can bind the tropomyosin strands are then in turn held in place when that muscle is relaxed by our troponin molecules which are these blue Parts here now again in the next module when we speak more about muscle contraction we will learn that when calcium ions bind to chonin it will change its shape this moves tropomyosin out of the way and so the actin and the myin can then bind and muscle contraction can occur so recapping some of that for you in text and giving you another a zoomed in image and in scal muscle about 300 molecules of myosin form our single thick filament here each myin molecule as I described looks a little bit like a golf club with the the handles all wrapped around each other and then the heads extending out toward each Z disk each mein head contains a binding s for actin and a binding site for ATP the thin filament is composed mostly of actin as well as smaller amounts of our troponin and tropomyosin and actin which is our yellow part of this thin filament here contains binding SES for mein and when the mein heads and the actin bind is when our muscle contraction actually occurs when a muscle is relaxed mein is blocked from binding to actin because of these strands of the tropomyosin the tropomyosin strands are held in place by the troponin molecules