so in the last video we defined what a muscle twitch was it was a single electrical event and the ensuing mechanical event that followed so we can further define a muscle twitch then as a single action potential from a motor neuron and the resulting contraction that occurs in the skeletal muscle now of course what a muscle twitch is the length of the muscle twitch can be different in different skeletal muscles as shown in this particular figure here so for example when you look at the muscles in the eye specifically the lateral rectus the period of contraction is going to be very very very very short right these are muscles that are involved in rapid eye movement but when you look at something like the gastrocnemius or the soleus the contraction of the muscle is going to be more protracted and so when we define or when we go into the specifics let's say focusing in on the electrical events the electrical events in a muscle fiber are not all that different from the electrical electrical events in the motor neuron the propagation is the same using the voltage-gated sodium channels and the voltage-gated potassium channels to respectively rapidly depolarize and rapidly repolarize the main difference is the muscle fiber is not insulated so it's not going to be propagating as swiftly right it's not using saltatory conduction that we talked about in neurophysio but despite that the propagation is still about two milliseconds as large as the muscle fiber is relative to the motor neuron that propagation velocity is pretty fast now once we transduce that electrical signal into the skeletal muscle we're going to break the next stages down into three separate phases as it relates to the muscle twitch so in the first phase we're going to talk about all the events of excitation and so that was the conversion of the epp into an action potential right and that was accomplished through the voltage-gated sodium channel reaching threshold because the epps were mostly super threshold the propagation of that action potential courtesy of that voltage-gated sodium channel and the voltage-gated potassium channel down the length of the t-tubules where you encounter the protein dhp now remember dhp was a voltage sensitive protein and the conformational change due to the depolarization of the membrane caused dhp to change shape and consequently the ryr protein was are unblocked and calcium was able to diffuse into the cytosol so all of this is considered the latent period there is no tension developed in the muscle there are no strong cross bridges that are formed we then move into the period of contraction so during this period of contraction the calcium has already bound to troponin we've moved aside tropomyosin and consequently you have that interaction between actin and myosin since the binding site on actin is now available now exposed since tropomyosin has been shifted away so as long as that calcium is in the cytosol that calcium is bound to troponin so you have your calcium troponin complexes to shift aside tropomyosin so that's the period of contraction the last phase of the muscle twitch is the period of relaxation and all of this is focusing on the removal of cytosolic calcium because once cytosolic calcium is removed you no longer have the calcium troponin complex and because there is no longer a calcium troponin complex the tropomyosin shifts back into place and you block actin and myosin interaction and so the blocking of actin and myosin interaction of course leads to the end of the power stroke so when actin and myosin are no longer interacting that strong cross bridge is gone you're not developing any kind of tension in the muscle to counteract any physical load that the muscle is trying to move now when we look at these three phases it is actually load dependent so if you are carrying something that is light the latent period is going to be very very short but when you start to cavi carry heavier and heavier objects the latent period can extend because you first before you move something you first have to generate a sufficient tension to counteract the load okay so we actually don't consider that part of the period of contraction even though you might have a few crossbridges form that is still a part of the latent period because you're not moving the load you're not doing the work sufficient work against the load so we can see that the majority of the time as we stated in the previous video the electrical events take milliseconds the contraction and relaxation is the more prolonged aspect of the muscle twitch and it can last for over a hundred milliseconds maybe even a little longer in different skeletal muscles so in the next video we're going to specifically focus on the contractile unit of the muscle as opposed to the muscle fiber now the contractile unit within the muscle fiber or muscle cell was the sarcomere and you could add more sarcomeres and in essence add more cross-bridge formation so that that muscle cell would be capable of generating larger tension in the next video we're going to focus on the motor unit so we're zooming out and looking at the contractile unit in the muscle as an organ as opposed to the muscle cell