Membrane systems of the muscle fiber

Organization of the membrane system

For rapid and synchronous signal transmission, the myosymplast uses a specialized membrane system. It permeates the entire muscle fiber and is in close contact with the myofibrils.

T-system (transverse tubules) T-tubules are deep channel-like invaginations formed by the cell membrane itself — the plasmalemma. They run transversely to the long axis of the fiber, encircling the 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 appear as loops oriented longitudinally along the long axis of the fiber. L-tubules densely wrap around each myofibril.

At contact sites with T-tubules, L-tubules form expansions called terminal cisterns. Together, these elements form the triad — the critical structural and functional unit of the membrane system, consisting of one central T-tubule and two adjacent terminal cisterns flanking it.

Calcium transport and contraction initiation

The membranes of the terminal cisterns contain two vital protein complexes that control calcium ion ($Ca^{2+}$) levels in the cell:

1. Calcium pump ($Ca^{2+}$ pump). Operates continuously, utilizing ATP energy for active transport. Its role is to pump calcium ions out of the sarcoplasm into the terminal cisterns. As a result, a high concentration of $Ca^{2+}$ is maintained inside the storage depot (cisterns) at rest, while the surrounding sarcoplasm maintains an extremely low concentration.
2. Calcium channels. At rest, they are closed and impermeable to ions. They open only upon the arrival of a nerve impulse.

The mechanism of contraction initiation represents a strict logical sequence:

• The excitation wave propagates along the plasmalemma and travels deep into the fiber via the membranes of the T-tubules.
• In the region of the triads, the electrical signal is transmitted from the T-tubule to the membrane of the terminal cisterns.
• The signal triggers the opening of calcium channels.
• $Ca^{2+}$ ions passively surge out of the terminal cisterns into the sarcoplasm in a cascading fashion, obeying the concentration gradient.
• A sharp increase in calcium concentration around the myofibrils stimulates their contraction.

Sarcomeric organization of myofibrils

Myofibrils are the contractile apparatus of the fiber. They are densely packed in the central part of the myosymplast, pushing the nuclei to the periphery. A characteristic feature of the muscle fiber is transverse striation, which is clearly visible under light microscopy.

This striation arises from the ordered alternation of regions 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.

The bands of all adjacent myofibrils lie strictly at the same level (synchronously), giving the entire muscle fiber a striated appearance. Electron microscopy allows this structure to be examined at the ultrastructural level, where the sarcomere serves as the basic unit.

The sarcomere is the segment of a myofibril located between two Z lines (telophragms). The Z line itself passes precisely through the center of each light I-band. Thus, a single sarcomere entirely includes one dark A-band and two halves of light I-bands at the edges. 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. Exactly through the center of the H-zone runs the dark M-line (mesophragm).

Moving from one edge of the sarcomere to the other, we observe the following strict sequence of elements:

1. Z-line (sarcomere boundary).
2. Half of the light I-band.
3. Dark peripheral part of the A-band.
4. H-zone with the M-line passing through its center.
5. Dark peripheral part of the A-band.
6. Second half of the light I-band.
7. Next Z-line (second boundary).

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Optical properties of isotropic and anisotropic bands
• Mechanism of the calcium pump and ATP expenditure
• Ultrastructure of myofilaments: elements forming dark and light bands
• Synchronization of striation at the level of the entire myosymplast

Other topics in the «Muscle Tissues» section

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