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MCAT Content / Specialized Cell Muscle Cell / Organization Of Contractile Elements Actin And Myosin Filaments Crossbridges Sliding Filament Model

Organization of Contractile Elements: Actin and Myosin Filaments, Crossbridges, Sliding Filament Model

Topic: Specialized Cell Muscle Cell

In the sliding filament model, the actin and myosin filaments pass each other, forming cross-bridges that shorten the sarcomere.

The mechanism of muscle contraction is the binding of myosin to actin, forming cross-bridges that generate the filament movement. When a muscle contracts, the actin is pulled along myosin toward the center of the sarcomere until the actin completely overlaps with myosin filaments. In other words, for a muscle cell to contract, the sarcomere must shorten. However, the thick and thin filaments, which are components of sarcomeres, do not compress. Instead, they slide by one another, causing the sarcomere to shorten while the filaments remain the same length. 

To better understand the sliding filament model requires knowledge on the sarcomere. When a sarcomere shortens, some regions shorten, whereas others stay the same length.

The sarcomere is the functional contractile region of the myocyte, and defines the region of interaction between a set of thick and thin filaments.

Movement often requires the contraction of a skeletal muscle, as can be observed when the bicep muscle in the arm contracts, drawing the forearm up towards the trunk. The sliding filament model describes the process used by muscles to contract. It is a cycle of repetitive events that causes actin and myosin myofilaments to slide over each other, contracting the sarcomere and generating tension in the muscle.

The sarcomere and the sliding filament model of contraction: During contraction myosin ratchets along actin myofilaments compressing the I and H bands. During stretching this tension is release and the I and H bands expand. The A-band remains constant throughout as the length of the myosin myofilaments does not change.

To understand the sliding filament model requires an understanding of sarcomere structure. A sarcomere is defined as the segment between two neighboring, parallel Z-lines. Z lines are composed of a mixture of actin myofilaments and molecules of the highly elastic protein titin crosslinked by alpha-actinin. Actin myofilaments attach directly to the Z-lines, whereas myosin myofilaments attach via titin molecules.

Surrounding the Z-line is the I-band, the region where actin myofilaments are not superimposed by myosin myofilaments. The I-band is spanned by the titin molecule connecting the Z-line with a myosin filament.

The region between two neighboring, parallel I-bands is known as the A-band and contains the entire length of single myosin myofilaments. Within the A-band is a region known as the H-band, which is the region not superimposed by actin myofilaments. Within the H-band is the M-line, which is composed of myosin myofilaments and titin molecules crosslinked by myomesin.

Titin molecules connect the Z-line with the M-line and provide a scaffold for myosin myofilaments. Their elasticity provides the underpinning of muscle contraction. Titin molecules are thought to play a key role as a molecular ruler maintaining parallel alignment within the sarcomere. Another protein, nebulin, is thought to perform a similar role for actin myofilaments.

The molecular mechanism whereby myosin and acting myofilaments slide over each other is termed the cross-bridge cycle. During muscle contraction, the heads of myosin myofilaments quickly bind and release in a ratcheting fashion, pulling themselves along the actin myofilament.

At the level of the sliding filament model, expansion and contraction only occurs within the I and H-bands. The myofilaments themselves do not contract or expand and so the A-band remains constant.

The amount of force and movement generated by an individual sarcomere is small. However, when multiplied by the number of sarcomeres in a myofibril, myofibrils in a myocyte and myocytes in a muscle, the amount of force and movement generated is significant.

Practice Questions

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Practice Exam 2 B/B Section Passage 9 Question 49

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Key Points

• The sarcomere is the region in which sliding filament contraction occurs.

• During contraction, myosin myofilaments ratchet over actin myofilaments contracting the sarcomere.

• Within the sarcomere, key regions known as the I and H band compress and expand to facilitate this movement.

• The myofilaments themselves do not expand or contract.

• The Sliding Filament Model states that when a sarcomere contracts, the myosin head binds to the actin filament, forming cross-bridges that generate the filament movement.

Key Terms

actin: a protein which interacts with myosin filaments to generate tension

myosin: a motor protein which interacts with actin filaments to generate tension

cross-bridge: a projection from the thick filaments that attach to the thin ones and exert forces on them

sarcomere: a structural unit of a myofibril in striated muscle, consisting of a dark band and the nearer half of each adjacent pale band.

I-band: The area adjacent to the Z-line, where actin myofilaments are not superimposed by myosin myofilaments.

A-band: The length of a myosin myofilament within a sarcomere.

M-line: The line at the center of a sarcomere to which myosin myofilaments bind.

Z-line: Neighbouring, parallel lines that define a sarcomere.

H-band: The area adjacent to the M-line, where myosin myofilaments are not superimposed by actin myofilaments.

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