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03.05 Muscle Physiology

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  1. Muscle physiology
    1. “Motor unit”
      1. 1 nerve cell and all muscles cells that it innervates
        1. 2-2,000 muscle cells
    2. Isolated muscle contraction
      1. Types
        1. Isotonic contraction
          1. When stimulated, muscle moves and changes shape
          2. Can do work since free end can move
            1. W = F x D
            2. 5 lbs x 4 ft. = 20 ft. lbs of work
          3. Movement
        2. Isometric contraction
          1. When stimulated, cannot move, shorten, or change
          2. Can’t do work
          3. For stability, rigidity, and maintenance of posture
    3. Phases of contraction
      1. Latent/Resting phase
        1. No (or weak) stimulus, no contraction
      2. Depolarization/Contraction phase
        1. Strong enough stimulus initiates a contraction
      3. Relaxation/Repolarization phase
        1. Calcium leaving cell, cells begin relaxing
      4. Refractory phase
        1. A new stimulus will NOT produce a new contraction
      5. Return to Latent phase

Nursing Points


  1. Stimulus leads to twitch
    1. Threshold stimulus (liminal)
      1. Lowest stimulus that causes a reaction
    2. Simple summation
      1. 2 consecutive stimuli
        1. Not during refractory period or after relaxation
      2. Second twitch superimposed and stronger
        1. 1st twitch overcomes 100% of elastic forces
        2. 2nd twitch has less forces to overcome
  2. Tetanus
    1. Conditions – Multiple rapid stimuli
    2. Incomplete tetanus
      1. Constant contraction interrupted by partial relaxations
    3. Complete tetanus
      1. Frequency of stimuli is so great that muscle is in sustained contraction without relaxation
      2. Muscles in body
      3. Tetanic
  3. Fatigue
    1. Complete fatigue
      1. Overstimulation of muscle
      2. Muscle loses:
        1. Irritability (doesn’t react to stimulus)
        2. Conductivity (no action potentials)
        3. Contractility
      3. In real life – complete fatigue doesn’t usually occur
    2. Chemicals in cells converted to fatigue substances
      1. Glycogen → Pyruvic acid
      2. Glucose → Lactic acid
      3. ATP → ADP + Pi
      4. Creatine Phosphate → Creatine
      5. Oxygen → Carbon Dioxide
  4. Muscle tonus
    1. Spinal cord sends continual action potentials to muscles
      1. Slight state of tension
    2. Increase tonus
      1. Anxiety, fear, stress
    3. Decrease tonus
      1. Sleep

Betts, J.G., et al. (2017). Anatomy and physiology. Houston, TX: OpenStax, Rice University. Retrieved from

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Video Transcript

All right guys in this lesson, we’re going to talk about muscle physiology and how it works at a microscopic level

So, before we get started we have to understand this concept of motor units. What a motor unit is, is that it’s that you can have one particular nerve cell that comes in and innervates a specific number of muscle cells. So you can have like maybe two muscle cells that it’s innervating or you can even have up to like 2,000 muscle cells that it innervates, so you have all these muscle cells here. So if you look at this image here here’s a good example so you can this nerve coming in and it’s actually innervating it all these specific different types or different areas of those muscle fibers. Now just because that one thing happened what what you are getting is one specific motor unit.

Looking at different types of muscle contractions, we have to that we’re going to compare. You have an isotonic and then you have something called an isometric. Now, the difference between isotonic and isometric is that first off we’re looking at shape. There’s another lesson that talks about this specifically but when you have muscle fibers that are going to shorten what happens is they move towards the center of the cell which is right here. So that means that the length is actually going to get shorter in the diameters going to get wider. So here if you look at this bicep muscle every time the muscle contracts, it gets actually get shorter at the same time it’ll execute wider. The other thing about isotonic contractions is that the work requires movement. So here we’re looking at the articulation of the elbow as a arm moves and the forearm moves up and down your actually getting this movement. You have this free end, so imagine that this person here has a dumbbell and every time they lift it the dumbbell is moving in so it’s got a free end. When you’re looking at isometric contractions are quite the opposite. The muscles aren’t going to shorten it where they’re going to change shape so you’ll stay the contraction that happens but there’s no movement that’s actually occurring. This is really important for things like stability, rigidity, maintenance of posture. If you look at this person they’re doing a plank, you’re actually going to get shortening in the contraction of like the abdominal muscles to the erector muscles in the back, You’re can get some contraction to the glutes and the quads. But all of these are for stability and rigidity and it’s helping to to create that that core support here.

Another important thing to think about are the phases of contraction. When we look at the phases of contraction we also look at the action potential. We have four different ones we’ve got. Latent which is resting, depolarization and that’s where contraction actually happens. You’ve got relaxation which is a repolarization and then you got the refractory period and we’re going to go through each one of these. When we’re talking about the latent phase, we are talking about that there’s no or there’s a weak stimulus and so no contractions going to happen. The important thing here too, is when we’re talking about the action potential there’s a certain threshold that has to be met by the by the action potential and if it doesn’t pass the threshold no stimulus is going to occur. So this is where we talked about a weak stimulus. When we move along to depolarization where the contraction phase is which is right here basically we hit this threshold and the depolarization happens and there’s a strong enough stimulus to initiate the contraction.

Once we get that contraction what’s going to happen is it you’re going to have the relaxation and repolarization. So you going to have the calcium that leaves the cell. Calcium is in the in the cell doing its job working with the myosin and actin to create those the shortening of sarcomeres. There’s an excellent lesson on that and I encourage you to check that out and see how calcium plays in all of that but when calcium leaves the cell, the muscles going to begin to relax and then you’re going to get this repolarization here. In the refractory period is really important because what’s going to happen is you’re actually going to hit this this bottoming out and what happens if there’s there’s no new stimulus it’s going to override this. So even if you have all the all cylinders firing in terms of the nerve stimulus is just not going to happen and it’s not going to happen until you get back to this resting state and the latent phase.

We have several different concepts of muscle stimulus and so the first one to talk about is simple summation. Basically you get the you got that threshold stimulus I talked about in the previous slide where basic lowest stimulus that can cause a reaction. If we don’t hit that threshold there’s going to be no muscle contraction. But with a simple summation what happens is you’ll get a contraction and then before you get complete relaxation you’re going to have another full contraction. So you get these two consecutive stimuli. The important thing to know here is that that second stimuli can’t happen during that refractory period. But you’ll get that one contraction, and just as you start to rest, you’ll get another one. The idea behind it is called a second twitch and they’re superimposed and it creates a stronger stronger contraction. First twitch is going to overcome 100% of that elastic force and the second twitch has actually less of a resistance to overcome and you’re going to create a much stronger contraction.

This is another type of contraction that you’re going to see and it’s basically this idea multiple rapid stimuli. What’s happening is you get this. There two types of we look at. With the incomplete tetanus and complete tetanus is as it goes along you’re eventually going to hit complete tetanus unless you get this for relaxation. But what happens in incomplete tetanus is you get this constant contraction of these partial relaxations so you’ll get this the contraction that are relaxed just a little and then go get another one and then another one and they started firing rapid sequence. And then finally you get to the point where the frequency of that stimuli is so great and there’s no way for the muscle to relax that it ends up basically locking out and you’re going to get this sustained contraction which is here, which is known as complete tetanus

Then there’s a concept of fatigue and so you have basically you have the normal twitch you’ll get contraction-relaxation contraction-relaxation contractions-relaxation. But at some point there’s not going to be any ATP and is going to be no energy for use and you’re going to get this complete fatigue and happens is just as overstimulation of this muscle. So as muscles lose their irritability, they lose their conductivity and they lose a contractility. If you look here you’ll get you’ll get kind of the normal contraction-relaxation contraction-relaxation and you finally get to the point where you have there at this heat buildup in and the residual calcium. Then finally in once you get to the point where hey I can’t I there’s absolutely no way for me to contract this muscle any longer and you’re going to have no ATP here. You’re going to have this is a phase of incomplete fatigue. So basically you’re really weak and then you got complete fatigue. There are a couple things to think about. Number one you’ve got “in real life” because there’s constant circulation in the removal of waste. You have all these waste products and build up these fatigue substances and what happens is you’re going to get the circulation comes in a removes the waste and then it also brings a new ATP and calcium. It brings in new wastes for these muscles to to happen to a real life it’s just it’s not a possibility but it’s part of the theory of fatigue. The other thing that’s really important and I encourage you to check this out in some of the resources we talked about this fatigue substances. So your energy products essentially get converted to fatigue substances and what happens if one of these fatigue substances buildup that’s going to contribute to fatigue.

We have this idea of muscle tonus and what muscle tonus is that you have the spinal cord so you have the brain here and you have the spinal cord. And you’re going to be sending out constantly signals to the whole body because what happens is if you didn’t have this idea muscle tonus what happened was you would have met you never have the idea of proprioception, so where you are in space you would never have the idea of you need to be upright, you need to be sitting down. These are all kind of innate processes that we don’t think about but something that the body immediately does. So the spinal cord is going to constantly send out these continual action potentials to muscles and it helps keep us upright, right? So we have this slight state of tension and all of our muscles all the time for them to be on the ready to move

We know we’re going to grab something our body can automatically reach over. And if we know we are about to fall or so start to lean forward we know that our back muscles can try to pull us back and keep us up in space. Now there’s a couple of different ideas behind this that kind of the going either spectrum if you will. Let’s say you have you have normal tonus here and then you have increased tonus and then you have decreased tonus. Well, an increased tonus basically it’s an increase in the the action potential so they’re being stimulated in and kicked out even more frequently. This is caused by increased anxiety, fear or stress. If you are constantly anxious or do you feel like you have to you know your your fight-or-flight response is a little bit more heightened all the time and that’s going to increase the overall tonus.

Then you have decreased one is which happens during sleep. When we’re sleeping our muscles need to relax as much as possible for repair and restoration and that’s what happens during decreased tonus.
Okay so let’s recap.

When we’re talking about contraction we’re talking about two different types of contraction. We have the isometric and isotonic. With your isometric, this is the one where you have movement and you have a shortening and a change the muscle where isotonic doesn’t and isotonic is also responsible for stability.

Then you have the phases of contraction. You get the resting, the depolarization, repolarization and refractory. Remember nothing can happen during the refractory; there’s no stimulus that is going to be able to overcome that

When you have summation, you have basically two consecutive stimuli that is going to make that overall contraction greater.

With tetanus, you get the multiple stimuli resulting in that sustained contraction. Basically this is that lock out that we talked about.

When you have fatigue you have is overstimulation in the muscle. You’re going to have that decreased irritability, decreased conductivity and the decrease contractility. What happens is the muscle just overwhelmed with the amount of stress and is not going to work. Remember that this doesn’t happen in real life because we have a circulation which helps in that. But it’s important to understand because muscles do get fatigued and at some point they do need to to rest and relax and restore themselves. But you’re never going to get complete fatigue.

That’s it for our lesson on muscle physiology. Make sure you check out all the resources attached to this lesson. Now, go out and be a best selves today and as always happy nursing.