Sarcoplasmic Reticulum: Structure, Function & Diseases

The sarcoplasmic reticulum is a membrane system of tubes located in the sarcoplasm of muscle fibers. It assists in the transport of substances within the cell and stores calcium ions, the release of which leads to muscle contraction. In various muscle diseases, this task performance is impaired, for example, in malignant hyperthermia or myofascial pain syndrome.

What is the sarcoplasmic reticulum?

Sarcoplasmic reticulum is a tubular membrane system inside muscle fibers. A muscle fiber is equivalent to a muscle cell, but has multiple nuclei formed by cell division (mitosis) that allow the fiber to grow in length during development. Each muscle fiber is subdivided into further fibers called myofibrils. They can be subdivided into transverse sections (sarcomeres) that give striated skeletal muscle its name. The pattern comes from myosin and actin/tropomyosin filaments: very fine filaments that alternately slide into each other according to the zipper principle. Smooth muscle also has a sarcoplasmic reticulum; it works similarly, but its structure is not so clearly divided into individual units. Instead, smooth muscle forms a flat surface. The sarcoplasmic reticulum is similar to the endoplasmic reticulum (ER), which is the inner membrane system in other cell types. Biology distinguishes between smooth ER and rough ER; the latter has numerous ribosomes on its surface. These macromolecules synthesize proteins according to the blueprint provided by the genome. The sarcoplasmic reticulum is a smooth ER. Not only muscles have a smooth ER, but also organs such as the liver or kidney.

Anatomy and structure

In its entirety, the sarcoplasmic reticulum forms a complex tubular system of membranes. It is located in the muscle fiber or muscle cell in the sarcoplasm. The sarcoplasmic reticulum spreads along and surrounds the myofibrils, as it is in their sarcomeres that the actual muscle contraction takes place. Mitochondria, which provide energy for the cell in the form of ATP, are often in close proximity and, like the sarcoplasmic reticulum, lie in the tissue between the individual myofibrils. The membranes of the smooth ER form predominantly tubular structures, but also pouches or cisternae as well as vesicles. They all have an interior space within the membrane, which biology also calls a lumen. The tubular system can adapt to the needs of the tissue by changing its structure and expanding more in certain areas, forming new branches, or joining multiple channels together.

Function and tasks

In the context of muscle contraction, the sarcoplasmic reticulum helps distribute incoming nerve signals throughout the muscle fiber and, with the help of calcium ions, causes the muscle to contract. This is triggered by the signal from a nerve fiber that ends at the muscle. The neuronal information can originate both from the brain and from the spinal cord, through which many reflexes are interconnected. At the end of the nerve fiber is the motor end plate, which, like the end button at the interneuronal synapse, contains vesicles filled with messenger substances (neurotransmitters). The neurotransmitters enter the free when the electrical impulse stimulates the motor end plate. In response, the biochemical molecules transmit the signal to the muscle membrane, where they open ion channels, triggering a change in the cell’s charge. The charge change propagates through the sarcolemma and the T-tubules. The T-tubules are tubes that are perpendicular to the myofibrils; in this case, they are located at the Z-disks of the sarcomeres and are connected to the sarcoplasmic reticulum. When tension reaches the sarcoplasmic reticulum, it releases stored calcium ions. These accumulate on the actin-tropomyosin filament and temporarily alter its structure; as a result, the ends of the myosin filaments can push further between the actin-tropomyosin fibers. In this way, the muscle shortens. The calcium ions do not bind permanently to the actin-tropomyosin complex, but subsequently detach. Subsequently, the sarcoplasmic reticulum reabsorbs the charged particles into its cisterns so that the process can be repeated during the next stimulation.Pumps in the membrane of the tubular system retrieve the calcium ions in the process. In addition, like the endoplasmic reticulum in other cells, the sarcoplasmic reticulum supports the distribution of substances in the sarcoplasm, in a sense serving as a highway for transport molecules.

Diseases

Insufficient functionality of the sarcoplasmic reticulum is associated with various muscular diseases and complications. One example is malignant hyperthermia, which can occur as a result of medical anesthesia. It is characterized by muscle rigidity (rigor), hyperacidity (metabolic acidosis), tachycardia, increased carbon dioxide in the blood or in the breath, oxygen deprivation, and masseter muscle spasm (at the masseter muscle, masseter spasm). The symptoms are due to an uncontrolled release of calcium ions in the muscle fiber, whereupon the tissue contracts as if in voluntary irritation, the cell rapidly suffers from energy deficiency and produces large amounts of heat and carbon dioxide. Various clinical symptoms result, including muscle fiber breakdown (rhabdomyolysis). The cause of malignant hyperthermia is a genetic predisposition that leads to receptor changes. The administration of certain anesthetics triggers an erroneous reaction, which is why medicine also speaks of trigger substances in this context. In myofascial pain syndrome, hardening occurs in muscle tissue, also known as trigger points. The hardening is caused by prolonged muscle contraction: due to insufficient supply to the affected area, the endoplasmic reticulum is unable to pump the released calcium ions back into its interior. The ions are thus still available and ensure a continuation of muscle contraction.