Question

T-tubules of muscles are the modification of which of the following organelle?
(a) Endoplasmic reticulum
(b) Golgi body
(c) Microtubules
(d) Microfilaments

Answer 1

Hint: Double membraned cell organelle that occupies most of the space inside the cell helps in the synthesis of proteins and metabolism of lipids. It also helps in the storage of calcium ions.

Complete answer:
In many types of muscles, plasma membrane in muscles i.e., sarcolemma invaginates into sarcoplasm and forms transverse tubules(T-tubules). During initial formation, T-tubules have a random, mostly longitudinal disposition between the actin and myosin. They immediately form junctions with sarcoplasmic reticulum elements. The sarcoplasmic reticulum is the endoplasmic reticulum of the muscle cells. T-tubules are extensions of the sarcoplasmic reticulum.

Additional Information: -T-tubules allow action potential transmissions with the help of ion shifts to all parts of the cell. This causes rapid activation of the entire cell. So, in larger cells, T-tubules are one of the main required components.
-Maturation of myocardial cells results in the maturation of T-tubules. In humans, they appear in the first 30 weeks of the gestation period.
- Sarcolemma invaginates into the cytoplasm of muscle cells called sarcoplasm results in the formation of transverse tubules(T-tubules).
-In non-mammalian vertebrates, t-tubules appear at the level of the z line whereas, in mammals, they penetrate into the junction between the A and I bands.
- Transverse tubules and two terminal cisternae result in the formation of the triad system. In terminal cisternae, most of the calcium is stored.
-A unique set of ion channels, supported by membrane structural subdomains is associated with the T-tubule membrane.
-Due to the presence of T-tubules in triads, the sarcoplasmic reticulum appears to be divided into segments.
So, the correct answer is ‘endoplasmic reticulum’.

Note: To help the spread of electrical activity throughout the myocyte, calcium ions are released by T-tubules as they are very close to the sarcoplasmic reticulum organelle that stores calcium.
A honeycomb-like or hexagonal network is formed by the proliferation of T-tubules of striated muscles.