A sarcomere is composed of a thick and thin filaments, H-zone, A-, Z-, and I-bands. During contraction, the sarcomere gets shorter due to the shortening of the H-zone and the I-band. The other structures remain do not change.
Sarcomere
In the sarcomere, we can identify the following components,
- A-band. This band reflects the length of the thick filament, including a small portion of the thin filament.
- I-band. This is the area located between the ends of the adjacent thick filaments. It is composed only of thin filaments.
- H-zone. This is the area located between the ends of the thin filaments. It is only composed of a portion of the thick filaments.
- Z-band is the vertical line placed at the end of each sarcomere. In adjacent sarcomeres, Z-band can be found in the middle of the I band.
Muscle
- Muscle fiber contractions are based on physiological and biochemical events that occur at a cellular level.
- These phenomena are due to stimulation produced by somatic motor neurons, in which axons get in touch with muscle fibers through a neuromuscular synapse.
In rest
- Attraction strengths between myosin and actin filaments are inhibited by the tropomyosin.
- When an action potential is originated in the central nervous system, it travels to the somatic motor neuron membrane: the muscle fiber, and activates the calcium channels releasing it in the neuron.
- Calcium makes vesicles fuse with the membrane and releases the neurotransmitter named acetylcholine (Ach) into the synaptic space in the juncture.
- Then, Ach binds to its receptors on the skeletal muscle fiber.
- Ion channels open, and positively charged sodium ions cross the membrane to get into the muscle fiber (sarcoplasm), and potassium gets out.
- The difference in charges caused by the migration of sodium and potassium makes the muscle fiber membrane becomes more positively charged (depolarized).
- The action potential caused by the depolarization enters the t-tubules. Consequently, it depolarizes the inner portion of the muscle fiber.
- This depolarization activates calcium channels in the T tubules membrane and releases the calcium into the sarcolemma.
- At this point, tropomyosin is obstructing binding sites for myosin on the thin filament.
Contraction
- When calcium binds the troponin C, the troponin T alters the structure of tropomyosin by moving it, getting to unblock the binding sites.
- Myosin binds to the uncovered actin-binding sites, and while doing it ATP is transformed into ADP and inorganic phosphate.
- Z-bands are then pulled toward each other
- The sarcomere, the H-zone, and the I-band get shorter
- All these events produce muscle fiber contraction.
By measuring the lengths of all the structures that compose the sarcomere -before and after contraction-, we can see that the changing values are the ones of the H-zone, the I-band, and the sarcomere length.
Here I show my measurements, but you should take the measures according to your image.
Before contraction After contraction Structure
26 mm 14 mm I-band
29 mm 15 mm H-zone
58 mm 46 mm Sarcomere
So, during contraction, the whole sarcomere gets shorter due to the shortening of the H-zone and the I-band.
You can learn more about sarcomeres at
https://brainly.com/question/7209548
