Question

In Figures 1 and 2 above, label the A-bands, I-bands, and H-zones. Measure

Answer 1

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

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