Video: Motor unit

From when we wake up and have our morning stretch to when we tuck ourselves in before we go to sleep, the main thing that drags us through our day is our muscles. We perform big and bold movements like running for the bus or working out at the gym, and finer, more delicate movements like messaging your beau to meet for coffee. Our muscles, big and small, are what makes it happen. Today, we're going to look at how nerves and muscles work together to perform these movements in our tutorial about the motor unit.

Before we begin, I'd like to give you a quick overview of what we're going to talk about in this tutorial. A motor unit consists of a motor neuron and all the skeletal muscle fibers that it innervates. So today I'll be taking you through this illustration that we can see here, first talking about the motor neuron and what it's composed of, and then talking about skeletal muscle and its fibers. And because we just love histology, I'll show you what some of these structures look like under the microscope.

In order for our skeletal muscles to perform their functions, they need nerves to tell them what to do and when. So let's start with the motor neuron and its associated structures.

The first structure we're going to look at is the spinal cord, which contains the cell body of our motor neuron. Specifically, the cell body of the motor neuron is found in the anterior or ventral horn of the spinal cord. Next, we should talk about the nerve roots, starting with the anterior or ventral root of the spinal nerve. The anterior root contains efferent fibers, which carry electrical impulses away from the central nervous system towards their target structures such as skeletal muscle. We're particularly interested in this structure as motor neurons controlling skeletal muscles are located in the anterior roots.

We also have a posterior or dorsal root of the spinal nerve. The posterior root contains afferent fibers which return sensory information from the trunk and limbs to their central nervous system. The cell bodies of these nerve fibers are not located in the central gray matter in the spinal cord but instead in a structure called the spinal or the dorsal root ganglion.

The anterior and posterior roots join to form the spinal nerve, which we can see here highlighted in green. The spinal nerve contains a mixture of sensory, motor, and autonomic fibers. Each spinal nerve can be divided into nerve fascicles, which are bundles of nerve fibers surrounded by a connective tissue sheath known as the perineurium.

Okay, time to talk about the man of the hour – the motor neuron. Every motor neuron can be divided into a cell body and an axon. In our illustration, we can see the cell body which, as I mentioned earlier, is located in the anterior horn of the spinal cord. The axon of the motor neuron forms one of the many nerve fibers that we see in each nerve fascicle. The axon transmits electrical impulses from the cell body towards the muscle fibers that it innervates.

Surrounding the axon of the motor neuron, we can see the myelin sheath which is formed by Schwann cells. One of the key functions of myelin is to insulate the axon length ensuring that electrical impulses are transmitted quickly and efficiently to the target structure. Each motor neuron divides into many terminal branches which synapse with muscle fibers at neuromuscular junctions, also known as myoneural junctions. Here, the electrical impulse is transmitted from the motor neuron to the muscle fiber via terminal boutons, to generate muscle contractions. Terminal boutons are also known as neuropodia or axon endfeet, and are characterised as club-shaped enlargements of the terminal branches of the axon. They are specialised to release neurotransmitters which are received by specific parts of the muscle fiber membrane known as motor end plates, signalling muscle contraction.

Larger muscles such as the biceps brachii usually have more muscle fibers per motor unit whereas smaller muscles such as the extraocular muscles usually have fewer muscle fibers per motor unit. Muscles with a higher ratio of motor neurons to muscle fibers or more motor units can carry out finer and more precise motor movements.

Okay, let's move on now and talk about skeletal muscle and its fibers.

Skeletal muscle is also known as voluntary muscle because it is under the voluntary control of the somatic nervous system so all the movements you think about doing are carried out by skeletal muscles. Most skeletal muscles are attached to bone as we can see here in our illustration. As you already know, the bones of the skeleton are connected to one another via joints, and if a muscle crosses a joint, it performs movements of that joint. Muscles are attached to bone via tendons which are predominantly composed of collagen. Tendons are extremely strong allowing them to withstand the stresses generated by muscular contraction. You can find more about the structure of tendons in our dedicated study unit.

Now we're going to look at how skeletal muscle is packaged up inside the body. Starting from the outside and working our way inwards, the first connective tissue layer we can see is the deep fascia of the skeletal muscle. This is a layer of dense connective tissue that surrounds individual muscles or groups of muscles to separate them into fascial compartments. For example, if we look at this cross-section, we can see that the thigh is divided into the anterior, posterior, and medial fascial compartments by these layers of deep fascia. The next layer we see is the epimysium which surrounds the entire muscle belly and is composed of dense, irregular connective tissue.

Let's zoom in and see what this looks like under the microscope.

So here we can see the pink muscle fibers with their purple nuclei and surrounding them is this irregular connective tissue highlighted in green which is the epimysium. Deep to the epimysium, we have the perimysium, also known as the interfascicular connective tissue, which divides the muscle belly into fascicles. If we look at our histological slide, we can see how the perimysium is continuous with and essentially an extension of the epimysium.

The last connective tissue layer we're going to talk about is the endomysium, which divides the muscle fascicles into individual muscle fibers. Again, we can see this really nicely under the microscope. Here we have an individual muscle fiber, and surrounding it, we can see the endomysium.

Okay, before we move on, let's quickly summarize what we've just learned. The muscle belly is surrounded by the epimysium. The muscle belly is then divided into muscle fascicles by the perimysium. We can see a nice example of a muscle fascicle on this histological slide here. Muscle fascicles are further divided into individual muscle fibers by the endomysium. Again, we can see what these individual muscle fibers look like under the microscope.

Now that we've seen how skeletal muscle is organized, let's look at our muscle fibers in a bit more detail. Each skeletal muscle fiber is one long multinucleated cell known as a myocyte, which is formed by the fusion of myoblasts. In our previous histological slide, we saw these fibers in cross-section, but in this image, we can get a better appreciation of how long muscle fibers are. Here we can see the nuclei of the skeletal muscle fibers highlighted in green. In this illustration, it might look like each muscle fiber has only one nucleus; however, if we go back to our previous slide, we can see that they are actually multinucleated with purple nuclei dotted along their length.

Our next histological slide shows the fibers in cross-section. I'm showing you this slide again so that we can see how the nuclei of the muscle cells are located on the periphery of the muscle fiber which is characteristic of this type of muscle tissue. Muscle fibers are composed of myofibrils which are made up of contractile units known as sarcomeres. We can see how each muscle fiber is composed of numerous myofibrils in our histological slide.

Within the myofibril, there are various proteins that interact with each other to generate contractions. These proteins are organized into thick and thin myofilaments which give skeletal muscle its striated appearance under the microscope. The thick filaments are composed of myosin and the thin filaments are composed of actin. As I mentioned previously, sarcomeres are the contractile units found in skeletal muscle. Let's look at the components of the sarcomere in more detail.

Highlighted in green, we can see the Z-discs, which divide each myofibril into a series of sarcomeres. So they mark the beginning and end of each sarcomere. The Z-discs also act as an anchoring point for the thin filaments. If we look at this histological slide, we can see these structures quite nicely. Here's a sarcomere and here's another, and between them, we can see the Z-disc.

Next, we're going to look at the A-band, which is marked or measured by the length of a thick myosin filament but also contains overlapping thin actin filaments. If we look at skeletal muscle fibers under the microscope, we can see that they have a striated appearance that distinguishes them from other muscle types. We mentioned earlier that this is due to the organization of the myofilaments. Here we can see the A-bands which stain darker than the alternating lighter bands. The lighter bands are called the I-bands and are formed by the thin myofilament actin. I-bands don't contain the entire length of the thin filaments but are the sections of the thin filament where no thick filament is present. Therefore, there is no myosin in this region. We can also see these bands on our histological slide.

As we'll see later in our tutorial, the thick and thin filaments actually interdigitate with one another. However, the central paler region of the A-band does not contain thin filaments and is called the H-zone. Within the H-zone is a thin M-line, which is the attachment point for the thick myosin filaments.

Okay, now that we're familiar with the structure of the sarcomeres, let's look at how the myofilaments interact with each other to generate muscle contractions.

The sliding filament model describes the mechanism of the skeletal muscle contraction where the thick myosin filaments slide past the thin actin filaments while the two groups of filaments remain at a relatively constant length. So in our diagram, we can see the thick myosin filaments which mark the length of the dark A-band and we can see the paler region of the A-band called the H-zone which doesn't contain thin actin filaments. The actin filaments form the lighter I-bands that we see here. Note the lack of myosin filaments in this region.

The sarcomeres create contractions in the muscle tissue when the myosin filaments slide between the actin filaments pulling the filaments together like so. During contraction, the I-band shorten and the H-zone narrows. This is due to an increase in the level of overlap or interdigitation between the thick and thin filaments, whereas the A-band stays the same as the length of the thick filaments does not change. The contradiction brings the Z-lines closer together and causes the overall length of the sarcomere to decrease. The Z-discs mark the beginning and end of each sarcomere and act as an anchor point for the actin filaments, whereas the M-line is the attachment point for the myosin filaments.

Now that we're familiar with motor units and their neuromuscular compartments, let's get clinical.

In today's clinical notes, we're going to talk about actin accumulation myopathy, which is a rare autosomal dominant disorder. This disorder is caused by a mutation of the gene that helps make actin. This leads to abnormal actin formation and impaired muscle contraction resulting in muscle weakness. Most cases are not inherited and affected individuals have no history of the disorder in their family. People with actin accumulation myopathy present with severe muscle weakness and poor muscle tone. Most individuals do not survive past infancy as the diaphragm, which is the primary muscle of respiration, cannot function effectively. Those who do survive have delayed development of motor skills such as sitting, crawling, standing, and walking.

Before we bring our tutorial to a close, let's quickly summarize what we've learned today.

First, we looked at the motor neuron and its associated structures starting with the spinal cord which contains the cell body of the motor neuron. Then we looked at the anterior root of the spinal nerve, which contains the axon of the motor neuron, and the posterior root of the spinal nerve, which is involved in the transmission of sensory information to the brain. These two roots come together to form the spinal nerve, which is divided into nerve fascicles. We then looked at the motor neuron itself, which is made up of a cell body and an axon. This axon is surrounded by myelin and divides into multiple terminal branches that synapse with multiple fibers at the neuromuscular junctions.

Next, we talked about skeletal muscle and its fibers. We saw that most skeletal muscle is attached to bone via tendons. We then looked at how skeletal muscle is packaged up, starting with the deep fascia of skeletal muscle which separates muscles into fascial compartments. Moving inwards, we saw epimysium, which surrounds the muscle belly; the perimysium, which divides the belly into fascicles; and the endomysium, which splits the fascicles into individual muscle fibers.

Next, we took a closer look at the muscle fibers, seeing that they are multinucleated cells formed by the fusion of myoblasts. Muscle fibers are composed of myofibrils which consists of thick and thin myofilaments. The thick filaments are composed of myosin and the thin filaments are composed of actin. Each myofibril is divided into a series of sarcomeres by the Z-discs. Sarcomeres are the contractile units found in skeletal muscle and consist of A-bands, which are the length of the myosin filament, and I-bands, which are the part of the thin filaments that aren't overlapping with the thick filaments. We then saw the H-zone, which is the region of the A-band not infiltrated by actin, and the M-line, which is the attachment point for myosin.

In our clinical notes, we discussed actin accumulation myopathy, which is an autosomal dominant disorder caused by a mutation of the gene that helps make actin.

And that brings us to the end of our tutorial on the motor unit. I hope you enjoyed it. Thanks for watching and happy studying!