What is the sarcoplasmic reticulum?
The sarcoplasmic reticulum is a tubular membrane system in the interior of muscle fibers. A muscle fiber corresponds to a muscle cell, but has several cell nuclei, which arise through cell division (mitosis) and allow the fiber in the development to grow in length.
Each muscle fiber is subdivided into further fibers, the so-called myofibrils. They can be divided into transverse sections (sarcomeres), which give the striated skeletal muscle their name. The pattern is caused by myosin and actin / tropomyosin filaments: very fine threads, which slide into each other by the zipper principle alternately. The smooth muscles also have a sarcoplasmic reticulum; it works similarly, but its structure is not so clearly divided into individual units. Instead, smooth muscles form a flat surface.
The sarcoplasmic reticulum resembles the endoplasmic reticulum (ER), which represents the internal membrane system in other cell types. The biology distinguishes between smooth ER and rough ER; The latter has numerous ribosomes on the surface. These macromolecules synthesize proteins according to the blueprint that the genetic material dictates. The sarcoplasmic reticulum is a smooth ER. Not only muscles have a smooth ER but also organs like the liver or kidney.
Anatomy & Construction
In its entirety, the sarcoplasmic reticulum forms a complex tube system of membranes. It is located in the muscle fiber or muscle cell in the sarcoplasm. The sarcoplasmic reticulum spreads along the myofibrils and surrounds them, since the actual muscle contraction takes place in their sarcomeres. Mitochondria, which provide energy to the cell in the form of ATP, are often in the immediate vicinity and, like the sarcoplasmic reticulum, are located in the tissue between the individual myofibrils.
The membranes of the smooth ER form predominantly tubular structures, but also bags or cisterns and vesicles. They all have within the membrane an interior that biology also calls lumen. The tube system can adapt to the needs of the tissue by changing its structure and expanding more in certain areas, forming new branches or joining several channels together.
Function & Tasks
As part of the muscle contraction, the sarcoplasmic reticulum helps to distribute incoming nerve signals in the muscle fiber and cause the contraction of the muscle with the help of calcium ions. The reason for this is the signal of a nerve fiber that ends in the muscle. The neuronal information can come from the brain as well as from the spinal cord, over which many reflexes are connected.
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 are released when the electrical impulse excites the motor end plate. The biochemical molecules then transmit the signal to the muscle membrane, where they open up ion channels, triggering a change in the cell's charge. The charge change spreads over the sarcolemma and T-tubules.
The T-tubules are tubes that are perpendicular to the myofibrils; they lie on the Z-slices of sarcomeres and are in communication with the sarcoplasmic reticulum. When tension reaches the sarcoplasmic reticulum, it releases stored calcium ions. These attach themselves to the actin-tropomyosin filament and temporarily change its structure; as a result, the ends of the myosin filaments may continue to slide between the actin-tropomyosin fibers. In this way, the muscle shortens.
The calcium ions do not bind permanently to the actin-tropomyosin complex, but then dissolve again. Subsequently, the sarcoplasmic reticulum resumes the charged particles in its cisterns so that the process can be repeated at the next irritation. Pumps in the membrane of the tube system bring back the calcium ions. In addition, the sarcoplasmic reticulum, like the endoplasmic reticulum in other cells, promotes the distribution of substances in the sarcoplasm, effectively acting as a roadway for transport molecules.
Inadequate functionality of the sarcoplasmic reticulum is associated with various muscular diseases and complications. An example of this is malignant hyperthermia, which can occur as a result of medical anesthesia.
It is characterized by muscle stiffness (rigor), hyperacidity (metabolic acidosis), tachycardia, increased levels of carbon dioxide in the blood or in the air, lack of oxygen and muscle masses (masseter muscle, masseter spasm). The symptoms are due to an uncontrolled release of calcium ions in the muscle fiber, whereupon the tissue contracts like a random irritation, the cell rapidly suffers from lack of energy and produces large amounts of heat and carbon dioxide.
Various clinical symptoms are the result, including muscle fiber decay (rhabdomyolysis). The cause of malignant hyperthermia is a genetic predisposition that leads to receptor changes. The administration of certain anesthetics triggers a false reaction, which is why the medicine speaks in this context of trigger substances.
The myofascial pain syndrome causes hardening in the muscle tissue, which are also known as trigger points. The hardening is caused by a prolonged muscle contraction: Due to inadequate care of the affected area, the endoplasmic reticulum is not able to pump the liberated calcium ions back into its interior. The ions are thus still available and ensure a continuation of muscle contraction.