Organization of the Membrane Apparatus
To ensure rapid and synchronous signal transmission, the myosymplast utilizes a specialized membrane system that permeates the entire muscle fiber and closely contacts the myofibrils.
T-System (Transverse Tubules) T-tubules are deep, canal-like invaginations formed by the cell membrane (plasmalemma) itself. They run transversely to the longitudinal axis of the fiber, looping around myofibrils, and serve as pathways for conducting excitation deep into the cell.
Sarcoplasmic Reticulum This is a modified smooth (agranular) endoplasmic reticulum. In the muscle fiber, it is represented by L-tubules, which form loops and are oriented longitudinally along the long axis of the fiber. The L-tubules densely wrap around each myofibril.
At contact sites with T-tubules, L-tubules form expansions known as terminal cisternae. Together, these elements form the triad, the most critical structural and functional unit of the membrane apparatus, consisting of one central T-tubule and two adjacent terminal cisternae.
Calcium Transport and Contraction Initiation
The membranes of the terminal cisternae contain two major protein complexes that control intracellular calcium ion ($Ca^{2+}$) levels:
- Calcium pump ($Ca^{2+}$-ATPase). Operates continuously, utilizing ATP energy for active transport. Its role is to pump calcium ions out of the sarcoplasm and into the terminal cisternae. As a result, at rest, a high concentration of $Ca^{2+}$ is maintained inside the store (cisternae), while the surrounding sarcoplasm has an extremely low concentration.
- Calcium channels. Closed at rest, preventing ion flux. They open only upon arrival of a nerve impulse.
The mechanism of contraction initiation follows a strict logical sequence:
- An excitation wave spreads across the plasmalemma and travels deep into the fiber via the T-tubule membranes.
- In the triad region, the electrical signal is transmitted from the T-tubule to the membrane of the terminal cisternae.
- The signal triggers the opening of calcium channels.
- $Ca^{2+}$ ions passively rush out of the terminal cisternae into the sarcoplasm in a massive wave, following their concentration gradient.
- A sharp rise in calcium concentration around the myofibrils stimulates their contraction.
Sarcomeric Organization of Myofibrils
Myofibrils represent the contractile apparatus of the fiber. They are densely packed in the central portion of the myosymplast, pushing the nuclei to the periphery. A characteristic feature of the muscle fiber is striation, which is clearly visible under light microscopy.
This striation results from the orderly alternation of zones with different optical properties:
- I-bands (light): Isotropic zones. Their width in the relaxed state is about 0.8 µm.
- A-bands (dark): Anisotropic zones. Their width is approximately 1.5 µm.
Light and dark bands of adjacent myofibrils align strictly at the same level (synchronously), giving the entire muscle fiber its striated appearance. Electron microscopy reveals this ultrastructure, where the sarcomere serves as the repeating functional unit.
Sarcomere is the segment of a myofibril located between two Z-lines (Z-discs). The Z-line runs precisely through the center of each light I-band. Thus, a single sarcomere entirely contains one dark A-band and two halves of light I-bands at its borders. The total resting length of a sarcomere is about 2.3 µm.
Ultrastructural Topography of the Sarcomere
The dark A-band itself is heterogeneous. Its peripheral parts appear darkest, while a lighter band—the H-zone (about 0.5 µm wide at rest)—lies in the middle. Running precisely through the center of the H-zone is the dark M-line (mesophragma).
Moving from one edge of the sarcomere to the other, we observe the following strict sequence of elements:
- Z-line (sarcomere boundary).
- Half of a light I-band.
- Dark peripheral part of the A-band.
- H-zone with the M-line running through its center.
- Dark peripheral part of the A-band.
- Second half of the light I-band.
- Next Z-line (second boundary).