okay so let's go ahead and continue our discussion with okay let's go ahead and continue our discussion with the histology of the heart focusing on cardiac muscle histology this should be a little bit of a review as you discuss the three different types of muscle tissue in amp1 so as you recall some basic characteristics of cardiac muscle tissue is it's got this branched Network branched Arrangement uh with with typically a single centrally no located nucleus uh lots of mitochondria in here because remember the heart can't take a break when it gets exhausted since we have several interlocking branching muscle fibers we need to make sure these guys are locked together with one another they are locked together with one another at these interpolated discs uh with the use of these desmosomes which as you recall are the strongest of these cell Junctions and of course also Gap Junctions which allow for seamless communication uh so all of these individual cells can work as a unit now you also notice that like skeletal muscle cardiac muscle is striated these striations mean that skeleton cardiac muscle employ the same contracti me mechanism using the same basic contractile unit which as you recall is your sarcomere as you see here now if we compare the cardiac myof fibral to skeletal muscle we do see some similarities right we still see these T tubules we see the sarcoplasmic reticulum we see the sarom in the same Arrangement but we do notice some distinct differences first of all we notice that the cardiac muscle has uh less Saro plasmic reticulum the skeletal muscle right well hopefully you recall that this sarcoplasmic reticulum had a fun to store calcium and this stored calcium again um remember from1 uh was used to uh start the contraction uh contraction cycle well if cardiac muscle has less stored calcium inside of the cell well does that mean it doesn't need as much no it needs the same amount of calcium as cardia skeletal muscle but since it's not all stored inside the cell it's going to to get some from the extracellular fluid so again this is going to require cardiac muscle to bring calcium in from the extra cellular fluid for E this is made possible due to our larger T tubules remember our larger T tubules are filled with extracellular fluid this is made possible by our large larger T tubules remember our larger T tubules and our T tubules over here are both filled with extracellular fluid these T tubules not only carry the action potential into inner parts of the cell which you learned about N1 help for helped signal the entire cell to contract it also helps deliver that extracellular calcium to the inner parts of the cell to again Aid in that contractile process because remember calcium is the key for contractions it is what binds to the troponin moves that tropomyosin exposing the mein binding sites so this whole sarom Mir can contract well let's get into the fun stuff and the heart again is a pretty amazing organ we've talked about this uh at the beginning of our lecture which all discussing all of the work um that the heart has to do and how it never gets a break and again one of the cool things about the heart that that you know we often learn is that the heart is autorhythmic okay that the heart can beat on its own that it is self excitable um this is one of the things we learn about when we think about this idea that I can remove uh a a living heart from my body and it will continue to contract and this is 100% true but how can that be well that is because the heart is not just a muscle the heart itself is composed of not only muscle tissue but also a a a small percentage of modified muscle cells called autorhythmic cells and these autorhythmic cells as the name implies forms the conduction system for the heart these muscle cells have the ability to spontaneously depolarize to threshold uh creating this pacemaker potential and sets the basic Rhythm of the heart so let's take a look at what I mean by that and again let's break it down these autorhythmic cells which again account for about 1% of muscle tissue of cardiac muscle tissue give the ability for the heart to beat on its own now if we break that down even further and when we say the heart is beating on its own what we're basically saying is the heart can contract on its own well as you learned in amp1 in order for a muscle to contract it needs to experience an action potential so if it can create its own uh or if it can contract on its own then it must be able to create its own action potential the question is how does it do that well again rushing off the the electrical physiology you learned about an1 uh in order for an action potential to occur all you need to do is reach threshold well if that's the case then in order for these autor rithmic cells to function on their own all they need to do is reach threshold on their own so again just a quick review here right we typically start here down here at -70 and we're at rest right and then some kind of stimuli will cause this cell to slowly depolarize right well oh that stimulus only got it that far well that did nothing happen we come back down here we have another stimulus but this time that stimulus is strong enough to reach our threshold value here at 55 if we reach threshold remember Action potentials shall pay the All or Nothing phase All or Nothing principle which means we are going to depolarize followed by repolarize back down well how in the heart do we modify this to create this so-called autorhythmicity well well how do we do this well we do this thanks to the presence of special voltage gated C channels that exist only within this uh autor rithmic cells and again these are only found in the conduction system cells conduction system autor rithmic cells so as a review uh gated ch channels mean this is a protein Channel a transmembrane channel that has a door on it and with that door it's got an opened and a closed State okay if it's voltage gated that means changes in voltage are what open and close that door and finally if it's a cation Channel and finally if it's a cation panel we know that um it's positive ions that are going to go through it but typically it's going to be calcium and sodium which means opening of this channel is going to trigger depolarization so finally the last thing we have to ask ourselves is when are these gates open and when are these Gates closed well these pacemaker channels that as I call them are open when the cell membrane is below threshold they close when the cell membrane is above the threshold well how does this work to create the autor rithmic so again let's start down here at -70 at this point I am below threshold right because threshold right here is 55 all of this stuff should be reviewed okay uh previous for from here on out a capital t is going to be threshold well if I am below threshold remember below threshold means pacemaker channels are open and if they're open my calcium and sodium are entering the cell are going into cell well if they're going into the cell we're not going to stay at -70 we're going to slowly depolarize to threshold well once I reach threshold then my pacemakers close but do I care nope because I've reached threshold and according to the All or Nothing rule I will depolarize up down oh back below threshold my pace makers open and I begin to depolarize Boom threshold they close but I don't care I fire back below they open positives in you throw close but I don't care I've reached threshold and I fired what we've basically created is an automatic spontaneous repeating threshold the repeating action potential so again these pacemaker channels create this automatic spontaneous depolarization to threshold which creates the automatic action potential in the heart so again if we come back to this slide here these autor rithmic cells with their pacemaker channels create this so-called spontaneous depolarization to threshold this spontaneous depolarization to threshold occurs from here to here creating this change in voltage that we call the pacemaker potential now one of the things I like to point out here is don't kind of get confused with this term spontaneous because sometimes students will think spontaneous means more willy-nilly whenever it wants and that's not exactly an accurate use of the term here spontaneous is meant to signify that this is occurring without external stimuli okay the brain isn't telling this to do it there's no neurons neurons there's no neurotransmitters there's no hormones nothing is telling the heart to do it it's doing it all on its own okay as we see here and that is how the heart is able to create its own action potential we will come back in a minute to discuss how we can then take that base Rhythm and modify it to meet our needs