Muscular system anatomy and physiology

Definitions & Key takeaways

The muscular system is composed of specialized cells called muscle fibers, which contain contractile proteins that enable them to contract and relax. The main types of muscle tissue are: skeletal, cardiac and smooth muscles.

Skeletal muscles are voluntary muscles attached to bones, which enable movement of the skeleton, and maintain body temperature by generating heat. The cardiac muscle is present only in the heart and is responsible for cardiac contraction that pumps blood throughout the body. Smooth muscles lie mainly in the walls of hollow viscera, such as the stomach and intestines, where they help to propel substances through these organs.

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Introduction0:00–0:37

The muscular system is made up of three types of muscle tissue: skeletal, smooth, and cardiac muscle tissue. They differ in terms of their location, cell structure, and innervation.
But they also share some characteristics: they’re all excitable, meaning that the cells react to a stimulus, they all contract--meaning that the cells will shorten, they all have extensibility--meaning that the cells can be stretched, and they’re all elastic--meaning that they can recoil or bounce back to their original length.
Let’s start with skeletal muscles. Skeletal muscles usually attach to bones, but in some cases, they attach to the skin, like the muscles in our face that control facial expression.

Skeletal Muscle0:37–1:45

Skeletal muscles are voluntary muscles, meaning that they can be controlled consciously, but some skeletal muscles are also controlled subconsciously.
Your diaphragm, for example, you can contract consciously when you take a big breath, but it also continues to contract and relax without conscious effort when you’re fast asleep or thinking about other things.
Skeletal muscles help you maintain your posture and stabilize joints, and because skeletal muscles use up a lot of energy as they contract and relax, they also generate a lot of heat as a byproduct.
That’s why we shiver to stay warm when it’s really cold. Now let’s take a look at the biceps brachii, a skeletal muscle in your upper arm.
Like most muscles there’s the belly of the muscle and the muscle tendons. The muscle belly is the part that contracts and it’s wrapped in a layer of connective tissue called the epimysium.

Structure1:45–2:43

Now let’s take a look at the cross-section of the muscle belly, there are thin layers of connective tissue called the perimysium that separate the muscle into fascicles.
Each muscle fascicle consists of a bundle of muscle fibers, and each muscle fiber is a muscle cell, or myocyte. Every myocyte is surrounded by a smaller connective tissue sheath called the endomysium.
Together, the endomysium, perimysium and epimysium, bundle together thousands of muscle fibers which give the muscle structure and strength - similar to how it’s easy to snap a single twig, but hard to break a big bundle of sticks.
Together these three layers of connective tissue extend beyond the muscle belly, and become the tough cord-like tendon which attach the muscle to the bone.
When a tendon attaches to a bone that’s not moving it’s called an origin, and when a tendon attaches to a bone that’s moving it’s called an insertion.
Now, let’s zoom in and look at a single myocyte. Myocytes are long cylindrical cells with multiple nuclei located just below the cell membrane which is called the sarcolemma.

Myocytes2:43–3:53

The sarcolemma is unique because it makes these tiny tunnels that project downwards from the surface into the center of the muscle fiber.
These tunnels are called transverse tubules, or T tubules. The cytoplasm of a myocyte is called sarcoplasm, and it contains smooth endoplasmic reticulum which is called sarcoplasmic reticulum.
The sarcoplasmic reticulum stores lots of calcium and runs perpendicular to the T tubules. Now, the sarcoplasm is filled with stacks of long filaments called myofibrils.
Each myofibril has thin actin filaments, and thick myosin filaments that don’t extend through the entire length of the myocyte, but instead they’re arranged into shorter segments called sarcomeres.
Each myocyte is made up of hundreds sarcomeres, and under a microscope, the thick myosin filaments look dark, while the thin actin filaments look light.
This is why skeletal muscles look striated or striped. Okay - so when we want to move, a motor signal is sent from the brain, down the spinal cord and then travels through a motor neuron.

Skeletal Muscle Contraction3:53–4:48

The motor neuron releases the neurotransmitter acetylcholine onto the sarcolemma which has acetylcholine receptors. This causes rapid shifts in ions to occur across the sarcolemma and down the T tubules, which brings some calcium into the myocyte.
Once that happens, the sarcoplasmic reticulum releases its own calcium into the sarcoplasm. When all of that calcium flows into the myocyte, it causes the actin and myosin to bind and pull in on each other - causing thousands of sarcomeres within each myocyte to contract all at once.
That’s how the myocyte contracts. Afterwards, the sarcoplasmic reticulum grabs most of the calcium ions and stores them again, and without calcium, the contraction ends and the muscle relaxes.
Now let’s look at cardiac muscle tissue, which is only found in the walls of the heart. The heart is an incredible muscle that pumps blood into the lungs where it gets oxygenated and then pumps that blood throughout the body.

Cardiac Muscle4:48–6:01

Similar to skeletal muscle fibers, the cardiac muscle fibers are striated and organized into sarcomeres. But cardiac muscle fibers are shorter and contain only one nucleus, which is usually in the center of the cell - called a cardiomyocyte.
Also, unlike skeletal muscle, cardiomyocytes are not consciously controlled, so it’s called involuntary muscle. Cardiomyocytes are covered by endomysium, but they lack perimysium and epimysium.
Now, just like skeletal muscle cells, the cardiomyocytes have T tubules, sarcoplasmic reticulum, and sarcomeres, but they also branch and connect to adjacent cardiomyocytes using intercalated discs.
Intercalated discs are sort of like staples that prevent the cardiomyocytes from pulling apart from one another during contraction.
Intercalated discs also have gap junctions which are like little connections that allow ions to flow from one cardiomyocyte right into another.
Now in addition to cardiomyocytes, there are also some really specialized cardiomyocytes in the heart muscle walls called pacemakers cells.

Pacemaker cells6:01–6:32

Pacemaker cells generate and conduct action potentials, which are basically rapid shifts in ions - including calcium - that enter the cell.
When these ion shifts happen in one cardiomyocyte, they also start happening in the neighboring cardiomyocytes because ions can slip through intercalated discs, and this results in the coordinated contraction of the entire heart.

Smooth muscle6:32–7:46

Now let’s look at smooth muscles. Within the sarcoplasm of the smooth muscle cells there are thin and thick myofilaments, but they’re not organized into sarcomeres.
In fact, that’s why they’re considered “smooth” - it’s because they don’t look striated under a microscope. Smooth muscle cells are fusiform or spindle shaped--wide in the middle and tapered at both ends, and they have one centrally located nucleus.
They’re wrapped within the endomysium, but, like the cardiac muscle, they lack the perimysium and epimysium. In contrast to skeletal and cardiac muscle tissue, smooth muscle cells don’t have T tubules, and instead, they have smaller invaginations of the sarcolemma, called caveolae.
Also, the sarcoplasmic reticulum within the smooth muscle fibers isn’t as extensive as the ones in skeletal or cardiac muscles.
Smooth muscles are usually found in the walls of hollow organs, like the small and large intestines, the bladder, the uterus, and also blood vessels, and like cardiac muscles, smooth muscles are under involuntary control.
Contractions of the smooth muscle is triggered by hormones, nerve stimulation and local factors, like the stretching of the muscle wall.
Alright, as a quick recap. The muscular system is made up of skeletal, cardiac, and smooth muscle tissue.

Review7:46–8:15

Skeletal muscles are muscles attached to the bones, are under voluntary control, and have a striated appearance. Cardiac muscle is found within the walls of the heart, are under involuntary control, and also have a striated appearance.
Finally, smooth muscles are mostly found within walls of hollow organs, are under involuntary control, and do not have a striated appearance.