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 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.
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.
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 parallel 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.
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.