Basic Anatomy of Muscle
We are all relatively familiar with how a muscle belly looks as is evidenced in every anatomy text book on the planet. For us to see the impact of the neural impulse, however, we need to pull out our microscope and look much deeper than the external layer.
The axon of an alpha motor neuron actually travels quite a ways to reach the point of the previously discussed neuromuscular junction (Read: Neuromuscular Junction & Impulse Transmission). The muscle we are familiar with is actually made up of several layers that will be simplified in this article to save concentration for other, more important ideas later on.
Basically, the muscle belly we are familiar with is made up of fasciculus. Fasciculi (plural form of fasciculus) are to the muscle belly what muscle cells are to the fasciculus. Essentially, several fasciculi make up a muscle belly; they are just sub pieces that work together to make the full muscle belly. Now, from there, we can see that several muscle cells are housed inside each fasciculus like several fasciculi are housed in the muscle belly. Here is where the axon drops off its message for muscle contraction to occur.
If you are here from the Overview of Muscle Contraction or Neuromuscular Junction & Impulse Transmission then you are familiar with where we are located in the body and likely want a detailed understanding of how the muscle actually contracts to produce force. If you have not read the articles mentioned above, I encourage you to do so to get a complete picture of what will be discussed here.
Our neural impulse has travelled across the synaptic cleft and depolarized the motor end plate to then carry the message to its final destination. That final destination is the beginning of the muscle contraction process, but first, let us familiarize ourselves with our surroundings before we dive in.
Muscle Fiber (Muscle Cell)
At the muscle fiber, we need to zoom in further to see the layout of the fiber – this will allow for a fuller understanding of what happens in the chain reaction we have been following. The impulse has depolarized the muscle fiber end plate and is now traveling down across the sarcolemma.
The sarcolemma is the plasma membrane of each muscle fiber. It encases the muscle fiber giving it its general shape. It is generally a solid skin-like layer, but we see that it is occasionally studded with holes – this is no coincidence. The sarcolemma houses several key components to the muscle fiber from the transverse tubules, the sarcoplasmic reticulum, and the myofibrils.
These holes on the outside of the sarcolemma lead to linear tubing, that runs perpendicular to myofibrils (discussed in a second)located on the inside of the sarcolemma, known as the transverse tubules (also referred to as, t-tubules). These tubules carry the neural impulse that has spread across the sarcolemma (the outside) inside to a following structure known as the sarcoplasmic reticulum that functions to turn a largely electrical impulse into a far more chemical signal. Again, the t-tubules spread the electrical signal carried in from outside across the myofibrils with the help of the sarcoplasmic reticulum.
As the impulse travels along the t-tubules, the sarcoplasmic reticulum releases a key ingredient to the recipe of muscle contraction; namely, calcium (Ca2+). The reticulum, with the help of a structure located intimately with the t-tubules called the terminal cisternae, houses and releases calcium ions for a process we will discuss once we have labeled and located all the main players of the muscle fiber. This combination of t-tubules and terminal cisternae is often referred to as the triad.
The terminal cisternae are grooved into the myofibril for the quick release of calcium as you will soon see.
Now, this is quite a bit of information, so I would recommend following along with the pictures to get an accurate perspective of what all these structures look like before we zoom in further with our mind’s eye.
Okay, so we’ve mentioned myofibrils several times now, but what are they?
Myofibrils are protein strands that make up the inside of each muscle fiber. Every structure mentioned so far plays a role in activating these strands into contraction – let’s take a look.
The neural impulse comes through the alpha motor neuron to then release that impulse onto the sarcolemma, which then carries the electrical impulse across itself until it runs into series of holes that lead inside the fiber membrane via the t-tubules, which then activates the terminal cisternae/sarcoplasmic reticulum (that line the myofibrils of each muscle fiber) where calcium is stored.
So, calcium is now released into the myofibril, but why?
Before we tackle that issue, go ahead and get some lemonade, stretch, maybe do a jig, loosen up, and then get your mind’s eye ready to realize some more complexities.
Okay, let’s take a look.
Myofibrils are extremely long, so we are breaking this down into its subsection called a sarcomere. A sarcomere is just a section of the entire myofibril (many sarcomeres make up the length of the myofibril).
Now, although there are several structures (Z-disks, A bands, I bands, Nebulin, Titin, among others) that make up a sarcomere, we only need to familiarize ourselves with a few of these sections to understand muscle contraction.
The furthest ends of the sarcomere are called Z-disks. These are especially important anchor points (columns) off which the rest of the structures work.
This piece is, if the Z-disks are anchor points, the rope or chain by which the myosin filaments (discussed next) are pulled.
a-actinin is located at the Z-disk and offers an anchor point for actin filaments (discussed next).
Now, these are the main structures aside from the two we are about to go over that we need to know to explain the following processes. These next two filaments are the mechanisms of muscle contraction itself, so listen up, because this is to what this has all led.
Myosin is one of the two filaments that make up the inside of the myofibril and its main function is to act on actin during a process explained shortly. Myosin, also known as the thick filament, is held in place by titin, which itself is anchored to the aforementioned Z-disks on either side of the myosin. Detailing the myosin further we can see that each myosin filament is covered in a type of “hook” called a myosin head (detailed shortly).
Actin is the other of the two filaments that make up the inside of a myofibril. Structurally, actin is a thin line that connects to the Z-disks via the a-actinin. The actin filament itself is made up of three proteins:
It is with these proteins that we will continue our impulse (now a discharge of calcium).
The Sliding Filament Theory
So, actin filament is anchored to a-actinin and runs parallel to myosin filament (better seen in the diagram provided), but it is staggered. Myosin lies in the middle of both Z-disks (the middle of the sarcomere), and recall that myosin is equipped with myosin heads. Well, in a short, concise way of explaining the mechanics, these myosin heads connect to actin and shorten the sarcomere to cause contraction. However, that isn’t a good enough explanation for a true physiologist, so let us look at how that exactly works.
Back to the actin filament made up of the three proteins (Tropomyosin, Troponin, Actin), each protein has a specific function.
1. Tropomyosin: This protein actually coils around actin and as far as we know, it serves as a blockage so that the aforementioned myosin heads cannot connect to the binding sites on actin to then force contraction.
2. Troponin: This particular protein is the reason why the sarcoplasmic reticulum and the triad (t-tubules and terminal cisternae) release calcium when an electrical impulse prompts them to do so. As troponin is attached to tropomyosin, when calcium is released into the myofibril, the calcium then attaches to troponin, and due to this action, tropomyosin then shifts, exposing the binding sites for the myosin heads to bind to the actin underneath.
3. Actin: Once troponin has bound with calcium forcing tropomyosin to shift out of the way of the myosin heads, the myosin heads are now given a clear run way to bind themselves at the actin-myosin binding sites located on this protein.
Almost like a love story, evil tropomyosin keeping actin and myosin apart until calcium feeds troponin in a way to distract tropomyosin, allowing actin and myosin to bind together (how romantic!).
But. (When you hear that word, the worst is still to come)
Things aren’t finished. The actin-myosin binding site is now open, but this does not mean the myosin heads will create contraction. For that to happen, another process must occur.
To explain this the best way possible, we will break down each action of the myosin head acting on the actin like a series of snapshots.
Step 1: Binding (Cross Bridge Formation)
The myosin head off of the myosin thick filament reaches out and binds at the aforementioned actin-myosin binding sites located on the actin thin filament; this is called the Cross Bridge. It does this while also having adenosine diphosphate (ADP) and an inorganic phosphate (Pi) attached to it, but as it attaches the inorganic phosphate is released.
Myosin Thick Filament
Step 2: Contraction (Power Stroke)
So, the myosin head is attached to the actin filament, and with that action, the ADP remaining is then released to give energy to be used in the power stroke. The power stroke is best described as the myosin head moving the actin filament toward the center of the sarcomere by snapping forward.
Step 3: Detachment
At this point, the myosin head has used up all of its energy in attaching to the actin filament and then moving the filament toward the middle of the sarcomere, but it needs to detach and recock the head (think of a pistol hammer, the hammer is closed, but to fire it again, you need to cock the hammer toward you) to restart the process. So, to do that, the uncocked, energy-less myosin head has an adenosine triphosphate (ATP) attach to it; now, the head detaches from the actin filament, but is still uncocked.
Step 4: Reloading (Reactivation of myosin head)
So, the final step before the process starts anew at Step 1 is the cocking of the myosin head. To do this, the ATP that attached in Step 3 hydrolyzes (the use of water molecules to separate ATP into ADP + Pi) into ADP and Pi (recall these substrates being in Step 1), the myosin head bends back to its activated state with ADP and Pi attached, essentially putting us back at Step 1.
The process (steps 1-4) repeats over and over again.
It is a bit difficult to imagine how the muscle belly contracts if we only inspect the myosin-actin relationship, so let’s take a step back and look at what is happening here in the sarcomere.
Explaining From the Sarcomere
The reason the myosin thick filament is in the middle of the sarcomere while the actin stretches out from the a-actinin (on the Z-disks) on either side is to create room for movement. This staggered formation allows movement known as contraction. The more the actin-myosin interacts, the closer the Z-disks are pulled together until peak contraction is achieved, as evidenced by the myosin thick filaments crowding the space.
Now, to make things abundantly clear, there are far more than just one myosin head on the myosin thick filament and these myosin heads operate asynchronously. This allows the actin filament to be pulled even when certain myosin heads detach, others along the myosin thick filament are attached to keep the actin from simply ratcheting back to the starting position.
Actin thin filament gets pulled inward, toward center of the sarcomere as myosin heads located on myosin thick filament "walk" toward Z disks. When fully contracted, myosin thick filament can no longer move outward as it reaches the Z-disks on either side of the sarcomere.
To turn off contractionn stimulus, calcium is cleared from the cytosol (the solution inside the muscle fiber) by the sarcoplasmic reticulum calcium pump. This decreases concentration of calcium to bind with troponin, and as there is no calcium to bind with troponin, tropomyosin shifts back over the actin-myosin binding sites on actin once again blocking the myosin heads from binding.
Here we are, we now understand the contraction process (or, at least I hope you do). In recap, our electrical signal from the neuromuscular junction that depolarized the motor end plate spreads throughout the muscle fibers, then enters each muscle fiber through the triad, and then calcium is released that acts on the actin filaments to allow the opening of myosin-actin binding sites at which point the myosin heads from the myosin filament attach and ratchet the actin forward until contraction is maximally achieved. I will add, however, that although we are sure about the majority of these
Writer: Nicolas Verhoeven
Information acquired through the East Carolina University Physiology Department by lecture given by Dr. Ronald Cortright