All About the Muscle
Have you ever experienced a muscle strain? Painful, right?
You might’ve experienced a muscle strain while sleeping, practicing a sport, or even falling in an uncomfortable position. We know that muscle strains occur when a muscle is torn or overstretched, but an important thing to consider is the anatomy behind this “phenomena,” if you will. When it comes to the muscles, they are only building blocks of a much larger system that controls many functions in your body. You’ve guessed it: it’s the muscular system.
What exactly is the muscular system? In short, this organ system contains the body’s muscles and allows movement, as well as circulating blood flow and maintaining posture. While this definition is relatively simple, it doesn’t encapsulate all the aspects of the muscular system. Let’s take a look at each component of the muscular system to better understand it overall, and perhaps learn a few interesting facts!
The Composition of a Muscle
Similar to a bone being part of the skeletal system, muscles are what primarily compose the muscular system. A muscle is a type of soft tissue that allows for greater movement and flexibility in the body. There are three types of muscles in the body: cardiac, smooth, and skeletal. Each of these muscles are located in different parts of the body and have different functionalities, which leads to greater purpose of the muscular system overall.
Cardiac Muscles
In the body, cardiac muscles are what comprise the walls of the heart and are considered involuntary, since they don’t require human conscience to operate. For example, while we might have to consider raising the muscles in our arm, we don’t need to think about telling the muscles in our heart to beat. Cardiac muscle is also a type of striated muscle tissue, which means it appears as a series of long, thin bands moving horizontally along muscle fibers.
In striated tissue, the primary unit is known as a sarcomere. This “building block” is also present in skeletal muscle and is often between two Z bands. In other words, a Z band simply defines the boundaries of a sarcomere. Every sarcomere contains two critical proteins: actin (a globular protein) and myosin (a motor protein). When combined, myosin will slide along actin to generate muscle contractions using ATP (a form of energy). This phenomenon is referred to as the sliding filament theory.
Recently, scientists have speculated that other molecules such as calcium and troponin (a protein) might be involved in motor movements! Sounds cool, right?
A critical thing to note about sarcomeres is their unique cell membrane, which is called a sarcolemma. This cell membrane does contain a lipid bilayer but also has an outer coat of glycocalyx (a component of cell membranes commonly found in bacteria), which connects to a basement membrane. The basement membrane is a type of extracellular matrix that supports signaling between cells.
Skeletal Muscles
Similar to cardiac muscles, skeletal muscles are a type of striated muscle tissue, meaning they occur in a series of lengthy and thin bands. However, skeletal muscles are voluntary and will move when we make the conscious decision to. These muscles neighbor the bones in our skeletal system and attach with collagen fibers called tendons.
Skeletal muscles are also a type of fascicle, which is a bundle of muscle fibers surrounded by a connective tissue called perimysium. Speaking of their properties, when myocytes begin building together, the result is muscle fibers that contain multiple nuclei and mitochondria (for energy purposes).
The banding pattern of skeletal muscles varies compared to the cardiac muscles. Due to certain cytoskeletal elements in skeletal muscle, the resulting bands are much stiffer and more adept for attachment with the bones. This is due to cytoskeleton having certain protein filaments that develop rigidity similar to a vertebrae’s skeleton. The major components of a cytoskeleton are microfilaments (usually containing actin), intermediate filaments, and microtubules; these aspects provide structure for the skeletal muscles and prevents them from damage.
An important component of muscle fibers is the sarcoplasmic reticulum (this applies to cardiac muscle too). This structure is somewhat similar to the endoplasmic reticulum, but also contains a bunch of calcium ions that help with muscle contraction. The endpoints of these reticulums are called terminal cisternae, and they aid in connecting bundles of muscle fiber to each other.
Smooth Muscle
Unlike both cardiac and skeletal muscles, smooth muscle isn’t a form of striated muscle tissue. It is an involuntary type of muscle (meaning it doesn’t require human conscience to use) and forms a wall for many sensitive internal organs, such as the stomach, intestines, and bladder. Smooth muscle also consists of two types: single-unit and multi-unit smooth muscle.
Single-unit smooth muscle is commonly found in the bladder, uterus, and gastro-intestinal tract (the pathway containing all organs in the digestive system). Bundles of SUVSM are connected to surrounding nerves with the help of an autonomic nerve fiber, which is part of the peripheral system. This smooth muscle is also myogenic, meaning it doesn’t require a neuron’s signal to contract (this is unlike multi-unit smooth muscle).
Multi-unit smooth muscles are located in areas like the vasculature (the vascular system) and airways, resulting in different structure compared to SUVSM. These smooth muscles are more independent of each other and thus has less gap junctions, which are spaces that connect the cytoplasm of two cells. While SUVSM cells are myogenic, multi-unit muscle cells are neurogenic, indicating the autonomic nervous system is responsible for contracting these muscles.
Smooth muscle can also be sub-divided based on its frequency of activity: phasic and tonic smooth muscle. While phasic smooth muscle is active for a periodic time (such as the bladder — it’s not used constantly), tonic muscle is almost always active (such as the intestines).
Similar to cardiac muscle, smooth muscle is comprised of cells called myocytes, which are differentiated from other immature cells called myoblasts. Unlike cardiac muscle’s striated nature, smooth muscle is more elastic and can endure more tension, which is why it’s so critical in the digestive system and airways. This is due to the microfilaments in smooth muscle moving in a “staircase” manner to connect with the cell membrane (note that in skeletal and cardiac muscle, the Z bands were used to connect microfilaments).
Divisions of the Muscular System
Due to its involvement with the skeletal system, the skeletal muscles can be divided into the axial and appendicular muscular system. Just as the names imply, the axial muscles support bones in the axial skeleton, whereas the appendicular muscles support the appendicular skeleton. The general responsibility of the axial muscles is to allow movement in the axial skeleton, including positioning the head and spinal column, as well as movement in the ribcage. Similarly, the appendicular muscles allow greater movement in the appendicular skeleton; these sections of the body include our extremities and pelvic region.
Cardiac muscle is only contained in the heart, and smooth muscle is contained in sensitive internal organs, so the muscular system is divided based on types of muscle. Similar to skeletal muscle’s correlation with the skeletal system, cardiac and smooth muscle provide support and protection for the organs they are with.
Key Takeaways
- The muscular system is associated with allowing movement in the body and increasing general functionality.
- The three types of muscle are cardiac, skeletal, and smooth muscle. Though these three types vary drastically, they do share similarities with respect to their fundamental units and (to an extent) structure.
- The skeletal muscles are split into the axial and appendicular muscles, which support those respective parts in the skeletal system.
Glossary
- Muscular System: The organ system primarily responsible for general movement and maintenance of posture.
- Muscle: Considered to be the base unit of the muscular system, muscles allow for greater flexibility throughout the body.
- Cardiac Muscle: One of the three types of muscle that is native to the heart.
- Smooth Muscle: One of the three types of muscles that forms a wall in many sensitive internal organs.
- Skeletal Muscle: One of the three types of muscles that connects to bones and allows for multiple movements within a joint.
- Axial Muscles: The muscles that connect to the axial skeleton, which contains the “vertical” bones of the skeletal systems.
- Appendicular Muscles: The muscles that connect to the appendicular skeleton, which consists of the external limbs.
- Sarcomere: The base unit of striated muscle tissue.
- Sliding Filament Theory: A theory stating that the protein filaments myosin and actin slide along each other to generate ATP and allow muscle contraction.
- Tendon: A piece of strong collagen that attaches a muscle to a bone.
- Fascicle: A bundle of muscle fibers surrounded by a protective connective tissue called perimysium.
- Gap Junction: A space that connects the cytoplasm of two cells.
