The I bands contain only thin (actin) filaments, whereas the A bands contain thick (myosin) filaments. The myosin and actin filaments overlap in peripheral regions of the A band, whereas a middle region (called the H zone) contains only myosin.

H-band is the zone of the thick filaments that has no actin. Within the H-zone is a thin M-line (from the German “Mittelscheibe”, the disc in the middle of the sarcomere) formed of cross-connecting elements of the cytoskeleton.

In infrared astronomy, the H band refers to an atmospheric transmission window centred on 1.65 micrometres with a Full width at half maximum of 0.35 micrometres (in the near-infrared).

The H-zone consists of myosin only, the I-band consists of actin only and the A-band contains both actin and myosin. The M-line holds together the thick myosin filaments.

The myofibrils are made up of thick and thin myofilaments, which help give the muscle its striped appearance. The thick filaments are composed of myosin, and the thin filaments are predominantly actin, along with two other muscle proteins, tropomyosin and troponin.

The sarcomeres of striated muscle are composed of parallel thick and thin filaments. Thin filaments from neighbouring sarcomeres are crosslinked in the Z-disc, and thick filaments within a sarcomere are crosslinked in the M-line.

Z-line – forms periphery of sarcomere where thin actin filaments attach. M-line – found inside H-zone and forms the middle of the sarcomere.

The Z-line defines the lateral boundaries of the sarcomere and anchores thin, titin and nebulin filaments. Because of these anchoring properties, Z-lines are responsible for force transmission, generated by the actin–myosin cross-bridge cycling.

alpha-actinin homodimers

: any of the dark thin bands across a striated muscle fiber that mark the junction of actin filaments in adjacent sarcomeres.

An irregular Z line is characterized by < 1 cm columnar tongues that extend proximal to the gastroesophageal junction, a finding that has been reported in approximately 10–15% of the population undergoing upper endoscopy [1, 2].

On each myofibril, regularly occurring dark bands, called Z lines, can be seen where actin and myosin filaments overlap. The region between two Z lines is called a sarcomere; sarcomeres can be considered the primary structural and functional unit of muscle tissue.

A sarcomere is defined as the distance between the Z-lines. The Z-lines are pulled closer together during contraction and move further apart during relaxation. The Z-lines are closer during contraction because actin and myosin interaction generates cross-bridges, which slide the myofilaments over each other.

Smooth muscle contains tropomyosin, but lacks troponin on its thin filaments. Instead, smooth muscle thin filaments bind another potential regulatory protein, termed caldesmon. In place of well-defined Z-lines, smooth muscles have dense bodies which contain a smooth muscle-specific isoform of α-actinin.

Connectin filaments link thick filaments and Z lines in frog skeletal muscle as revealed by immunoelectron microscopy. J Cell Biol.

The H zone—the central region of the A zone—contains only thick filaments and is shortened during contraction. The I band contains only thin filaments and also shortens. The zone of overlap, in which thin filaments and thick filaments occupy the same area, increases as the thin filaments move inward.

A sarcomere is defined as the distance between two consecutive Z discs or Z lines; when a muscle contracts, the distance between the Z discs is reduced. The A band does not shorten—it remains the same length—but A bands of different sarcomeres move closer together during contraction, eventually disappearing.

During a muscle contraction, every sarcomere will shorten (1) bringing the Z-lines closer together (2). The myofibrils shorten (3) too, as does the whole muscle cell. Yet the myofilaments (the thin and thick filaments) do not get shorter (4).

Which of the following is the correct sequence of events for muscle contraction? Motor neuron action potential, neurotransmitter release, muscle cell action potential, release of calcium ions from SR, ATP-driven power stroke, sliding of myofilaments.

So, Correct answer is ‘Stimuli Neurotransmitter secretion →→ Excitation of T-system →→ Release of Ca2+ →→ Cross-bridges formation →→ Sliding of actin filaments →→ H band diminishes’

acetylcholine