Question

Where is the T tubule situated?
A. Sarcolemma
B. Sarcoplasm
C. Sarcotubular system
D. None of the above

Answer 1

Hint: T- tubule or we can say transverse tubules are the extension of the cell membrane that penetrates into the centre of the skeletal and cardiac muscle cells. They are an interconnection of rings each of which is surrounded by myofibrils. It provides an important communication pathway.

Complete answer: Let us first know about these tubules.
T tubules are extensions of the cell membrane which contains a large concentration of ions channels, transporters and pumps and are known for transportations of the action potential into the cell. They play a major role in cellular calcium concentration. Through these mechanisms, these tubules allow heart muscle cells to contract more forcefully by synchronising calcium release throughout the cells.
The structure and function of these cells are affected beat by beat by cardiomyocyte contraction as well as by disease potentially contributing to heart failure and arrhythmias. They were first seen in 1897, and research on them is still going on. They are formed from the same phospholipid bilayers as the surface membrane or sarcolemma of skeletal muscle.
Let us infer about the options first.
Sarcolemma is a specialized membrane which surrounds the striated muscle fibre cells. They are the plasma membrane of the muscle cells and are surrounded by basement membrane and endomysial connective tissues.
Sarcoplasm is the cytoplasm of the muscle cell. It plays a major role in the contraction of the muscle fibres. They contain lots of mitochondria as they release large amounts of energy.
Sarcotubular system is a network of membranes that is found in the striated muscle fibres. T tubules bring the sarcolemma very close to the sarcoplasmic reticulum at all regions throughout the cells.
The T tubules are located in the space between the two SR cisternae and the assembly of the two SR.

So the answer to the above question is D, NONE OF THE ABOVE.

Note: T tubules are an important link in the chain from the electric excitation of a cell to its contraction. When contraction of muscle is needed, stimulation from a nerve causes a flow of charged particles into the cell. As tubules are very thin, they essentially trap the ions. They connect the sarcolemma at one end before travelling deep within the cells forming a network of tubules.