What is the Difference Between Sarcolemma and Sarcoplasmic Reticulum

The main difference between the sarcolemma and sarcoplasmic reticulum is that sarcolemma is the plasma membrane of a striated muscle fiber. But, the sarcoplasmic reticulum is the smooth endoplasmic reticulum in muscle fibers. Furthermore, sarcolemma consists of a lipid bilayer and a thin, outer coat of polysaccharide material (glycocalyx), which contacts the basement membrane while the membranous tubules of the sarcoplasmic reticulum are made up of sarcolemma. 

Sarcolemma and sarcoplasmic reticulum are two anatomical structures of the plasma membrane of muscle cells. However, although they are nearby structures, they perform different functions. 

Key Areas Covered 

1. What is Sarcolemma
     – Definition, Structure, Function
2. What is Sarcoplasmic Reticulum
     – Definition, Structure, Function
3. What are the Similarities Between Sarcolemma and Sarcoplasmic Reticulum
     – Outline of Common Features
4. What is the Difference Between Sarcolemma and Sarcoplasmic Reticulum
     – Comparison of Key Differences

Key Terms 

Muscle Fibers, Plasma Membrane, Sarcolemma, Sarcoplasmic Reticulum, Striated Muscle Fibers

What is Sarcolemma 

Sarcolemma or myolemma is the plasma membrane of the muscle cells. Generally, it contains a lipid bilayer and a glycocalyx, a thin outer coat of polysaccharide material, contacting the basement membrane. Here, numerous, thin collagen fibrils, as well as specialized proteins such as laminin, occurs in the basement membrane, providing a scaffold, which adheres to the muscle fiber. Also, the actin skeleton of the muscle cell connects to the basement membrane through the transmembrane proteins of the plasma membrane. Ultimately, at each end of the muscle fiber, sarcolemma connects to the tendon fiber.

Figure 1: Skeletal Muscle Fiber

Furthermore, the function of the sarcolemma is similar to the typical plasma membrane. Therefore, it acts as a barrier between intracellular and extracellular compartments. Here, it separates fluids from the two different compartments and is selectively permeable to water through aquaporin channels. In the meanwhile, ion pumps of the sarcolemma create an ion gradient either to drive co-transporters on the membrane or to generate action potentials.  

What is Sarcoplasmic Reticulum 

Sarcoplasmic reticulum is the smooth endoplasmic reticulum, invaginating the plasma membrane of the muscle cells. Basically, the sarcolemma forms membranous tubules radially and longitudinally within the fiber called transverse tubules (T-tubules).  Especially, the terminal cisternal enlargements of the transverse tubules are known as the sarcoplasmic reticulum. Besides, the general functions of the endoplasmic reticulum include the folding of protein molecules in sacs called cisternae and the transport of synthesized proteins in vesicles to the Golgi apparatus. However, the main function of the sarcoplasmic reticulum is to store calcium ions, pumping them out into the sarcoplasm. Moreover, the function of calcium ions is to interact with contractile proteins. 

Similarities Between Sarcolemma and Sarcoplasmic Reticulum  

• Sarcolemma and sarcoplasmic reticulum are two anatomical structures of the striated muscle fibers.  
• Both take part in the formation of the outer membrane.  
• However, they perform different functions. 

Difference Between Sarcolemma and Sarcoplasmic Reticulum 

Definition 

Sarcolemma refers to the fine transparent tubular sheath, which envelops the fibers of skeletal muscles. But, sarcoplasmic reticulum refers to the specialized endoplasmic reticulum of cardiac muscle and skeletal striated muscle, functioning as a storage and release area for calcium. 

Type of Structure 

The sarcolemma is a cell membrane structure, but the sarcoplasmic reticulum is an endoplasmic reticulum. 

Made up of 

Sarcolemma consists of a lipid bilayer and a thin, outer coat of polysaccharide material (glycocalyx), which contacts the basement membrane. Meanwhile, the membranous tubules of the sarcoplasmic reticulum are made up of sarcolemma. 

Occurrence 

Sarcolemma occurs surrounding the striated muscle fiber while sarcoplasmic reticulum occurs invaginating the cell membrane of the muscle cell. 

Function 

Sarcolemma acts as a barrier between intracellular and extracellular compartments while sarcoplasmic reticulum synthesizes molecules and stores calcium ions. 

Conclusion 

In brief, the sarcolemma is the plasma membrane, which surrounds the cytoplasm of striated muscle fibers. It consists of a lipid bilayer and a glycocalyx, contacting the basement membrane. In addition to that, it serves as a barrier between intracellular and extracellular compartments. On the other hand, sarcoplasmic reticulum is the endoplasmic reticulum, occurring as invaginations of the cell membrane. Normally, the tubules of it are made up of sarcolemma. And, its function is to synthesize molecules while storing calcium ions. Therefore, the main difference between the sarcolemma and sarcoplasmic reticulum is their structure and function. 

References:

1. “Muscle Lab.” Histology @ Yale, Available Here.

Image Courtesy:

1. “Blausen 0801 SkeletalMuscle” By BruceBlaus. Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. – Own work (CC BY 3.0) via Commons Wikimedia