Did you know that skeletal muscles are organized into tightly packed groups? This lesson identifies the connective tissue layers that organize skeletal muscle into fascicles, muscle fibers and myofibrils. Additionally, the microscopic structure of the sarcomere is described.
Organization of Skeletal Muscle
Our bodies contain numerous skeletal muscle organs – for example, the biceps brachii of the arm and the gastrocnemius of the leg. The muscles that move and support our body are attached to bones at each end by what we call tendons.These tendons are composed of connective tissue, which not only attaches the bone to the muscle, but also wraps around and within the muscle.
That separates the organ into smaller components. In this sense, you can think of a skeletal muscle organ as being like a nesting egg in which the smaller eggs are similar to, and contained within, the larger eggs. In this lesson, we will describe how a skeletal muscle organ is arranged into progressively smaller parts.
A skeletal muscle is arranged much like a can of Vienna sausage. If you’ve ever opened a can of Vienna sausage, you’ve noticed the links are packed tightly together, and you can see the ends of the links.Similarly, when you look at muscle, you can see several fascicles that are packed tightly together in a transverse section.
Each fascicle is basically a package of muscle fibers.While each sausage is surrounded by a jelly-like substance, each fascicle is surrounded by a connective tissue layer that’s referred to as the perimysium, as in ‘perimeter.’ The perimysium is important, as it contains lots of blood vessels and nerves that provide nutrients and regulate contraction, respectively.
The epimysium is the connective tissue that surrounds the entire muscle organ.
That’s easy to remember, as ‘epi-‘ means ‘above or on top.’ Now, let’s take a look at an individual fascicle within the muscle organ.
From this perspective, you can see the endomysium is a connective tissue that surrounds individual muscle fibers (also referred to as muscle cells). The prefix ‘endo-‘ means ‘below or within.’ The endomysium contains tiny capillaries and individual neurons providing nutrients and innervation of the muscle fiber.It’s important to note that each layer of connective tissue contains collagen fibers, and they come together to form the tendon on each end of the muscle organ. Again, the tendon attaches the muscle to the bone.
If we look at a skeletal muscle under a microscope, it looks, once again, like a can of Vienna sausages.
At the microscopic level, the sausages can be thought of as individual muscle fibers packed within a fascicle. Remember, there are several fascicles packed within the muscle organ.Now, let’s take a look at an individual muscle fiber. The muscle fiber membrane is called the sarcolemma. Skeletal muscle cells or fibers are composed of hundreds to thousands of contractile organs that we call myofibrils.
Each myofibril runs the length of the muscle fiber. Remember, muscle fibers run the length of the muscle organ. The myofibrils are packed tightly together, with lots of mitochondria and sarcoplasmic reticulum in between the myofibrils.
The mitochondria synthesize ATP that’s needed for contraction, and the sarcoplasmic reticulum stores the calcium that is needed for contraction as well. Additionally, you can see the T-tubules, or transverse tubules, that carry the excitatory nervous impulse from the sarcolemma to the sarcoplasmic reticulum, where calcium is released when the cell is stimulated.
Now, let’s take a look at an individual myofibril within the muscle cell. At this level, we can see what we call the sarcomere, which is the functional unit of skeletal muscle, as each sarcomere contains all the tools needed for contraction.
Like a long train, sarcomeres are butted up end-to-end, running the length of the myofibril. Each sarcomere consists of bundles of proteins referred to as myofilaments.
Thick filaments (or thick myofilaments) contain myosin, while thin filaments contain actin. Actin and myosin collectively are referred to as the contractile proteins, which cause muscle shortening when they interact with each other.
In summary, skeletal muscle organs are organized into progressively smaller compartments. A skeletal muscle organ is arranged into several fascicles. They’re packed side-by-side and surrounded by the perimysium, which contains blood vessels and nerves.The epimysium is the connective tissue surrounding the entire muscle organ.
The endomysium is the connective tissue surrounding individual muscle fibers, and they’re packed within a fascicle.Myofibrils are organelles within the muscle cell that contain thick and thin myofilaments. The myofilaments are organized into sarcomeres, and they serve as the functional units of skeletal muscle. The sarcomeres are butted up end-to-end along the myofibril. The proteins actin and myosin are contained within the myofilaments and are referred to as contractile proteins.
After viewing this video, you’ll be able to describe the progressively smaller structures contained in skeletal muscle organs.