During which phase of a muscle twitch do cross

Muscle is a specialized contractile tissue that is a distinguishing characteristic of animals. Changes in muscle length support an exquisite array of animal movements, from the dexterity of octopus tentacles and peristaltic waves of Aplysia feet to the precise coordination of linebackers and ballerinas. What molecular mechanisms give rise to muscle contraction? The process of contraction has several key steps, which have been conserved during evolution across the majority of animals

Table of Contents Show

The Sliding Filament Theory
An Analogy for Sliding Filaments in a Sarcomere Shortening Event
What Are Cross Bridges, and How Do They Relate to Sliding Filaments?
During what phase of a muscle twitch are the cross
What are the 3 phases of a twitch?
What is the contraction period of a muscle twitch?

When muscle cells are viewed under the microscope, one can see that they contain a striped pattern (striations). This pattern is formed by a series of basic units called sarcomeres that are arranged in a stacked pattern throughout muscle tissue (Figure 1). There can be thousands of sarcomeres within a single muscle cell. Sarcomeres are highly stereotyped and are repeated throughout muscle cells, and the proteins within them can change in length, which causes the overall length of a muscle to change. An individual sarcomere contains many parallel actin (thin) and myosin (thick) filaments. The interaction of myosin and actin proteins is at the core of our current understanding of sarcomere shortening. How does this shortening happen? It has something to do with a sliding interaction between actin and myosin.

Figure 1: A gastrocnemius muscle (calf) with striped pattern of sarcomeres

The view of a mouse gastrocnemius (calf) muscle under a microscope. The sarcomeres are artifically colored green, and appear as stacked horzontal stripes of similar lengths. (White scale bar = 25 microns.)

The Sliding Filament Theory

In 1954, scientists published two groundbreaking papers describing the molecular basis of muscle contraction. These papers described the position of myosin and actin filaments at various stages of contraction in muscle fibers and proposed how this interaction produced contractile force. Using high-resolution microscopy, A. F. Huxley and R. Niedergerke (1954) and H. E. Huxley and J. Hanson (1954) observed changes in the sarcomeres as muscle tissue shortened. They observed that one zone of the repeated sarcomere arrangement, the "A band," remained relatively constant in length during contraction (Figure 2A). The A band contains thick filaments of myosin, which suggested that the myosin filaments remained central and constant in length while other regions of the sarcomere shortened. The investigators noted that the "I band," rich in thinner filaments made of actin, changed its length along with the sarcomere. These observations led them to propose the sliding filament theory, which states that the sliding of actin past myosin generates muscle tension. Because actin is tethered to structures located at the lateral ends of each sarcomere called z discs or "z bands," any shortening of the actin filament length would result in a shortening of the sarcomere and thus the muscle. This theory has remained impressively intact (Figure 2B).

Figure 2: Comparison of a relaxed and contracted sarcomere

(A) The basic organization of a sarcomere subregion, showing the centralized location of myosin (A band). Actin and the z discs are shown in red. (B) A conceptual diagram representing the connectivity of molecules within a sarcomere. A person standing between two bookcases (z bands) pulls them in via ropes (actin). Myosin (M) is analogous to the person and the pulling arms. (z bands are also called z discs.)

© A) adapted from Huxley, A. F. & Niedergerke, R. Structural Changes in Muscle During Contraction: Interference Microscopy of Living Muscle Fibres. Nature 172, 971-973 (1954). B) © Nature Education All rights reserved.

An Analogy for Sliding Filaments in a Sarcomere Shortening Event

Imagine that you are standing between two large bookcases loaded with books. These large bookcases are several meters apart and are positioned on rails so that they can be easily moved. You are given the task of bringing the two bookcases together, but you are limited to using only your arms and two ropes. Standing centered between the bookcases, you pull on the two ropes — one per arm — which are tied securely to each bookcase. In a repetitive fashion, you pull each rope toward you, regrasp it, and then pull again. Eventually, as you progress through the length of rope, the bookcases move together and approach you. In this example, your arms are similar to the myosin molecules, the ropes are the actin filaments, and the bookcases are the z discs to which the actin is secured, which make up the lateral ends of a sarcomere. Similar to the way you would remain centered between the bookcases, the myosin filaments remain centered during normal muscle contraction (Figure 2B).

What Are Cross Bridges, and How Do They Relate to Sliding Filaments?

One important refinement of the sliding filament theory involved the particular way in which myosin is able to pull upon actin to shorten the sarcomere. Scientists have demonstrated that the globular end of each myosin protein that is nearest actin, called the S1 region, has multiple hinged segments, which can bend and facilitate contraction (Hynes et al. 1987; Spudich 2001). The bending of the myosin S1 region helps explain the way that myosin moves or "walks" along actin. The slimmer and typically longer "tail" region of myosin (S2) also exhibits flexibility, and it rotates in concert with the S1 contraction (Figure 3A).

The movements of myosin appear to be a kind of molecular dance. The myosin reaches forward, binds to actin, contracts, releases actin, and then reaches forward again to bind actin in a new cycle. This process is known as myosin-actin cycling. As the myosin S1 segment binds and releases actin, it forms what are called cross bridges, which extend from the thick myosin filaments to the thin actin filaments. The contraction of myosin's S1 region is called the power stroke (Figure 3). The power stroke requires the hydrolysis of ATP, which breaks a high-energy phosphate bond to release energy.

Figure 3: The power stroke of the swinging cross-bridge model, via myosin-actin cycling

Actin (red) interacts with myosin, shown in globular form (pink) and a filament form (black line). The model shown is that of H. E. Huxley, modified to indicate bending (curved arrow) near the middle of the elongated cross bridge (subfragment 1, or S1) which provides the working stroke. This bending propels actin to the right approximately 10 nanometers (10 nm step). S2 tethers globular myosin to the thick filament (horizontal yellow line), which stays in place while the actin filament moves. Modified from Spudich (2001).

During what phase of a muscle twitch are the cross

The contraction phase is when the muscle is generating tension and is associated with cycling of the cross bridges, and the relaxation phase is the time for the muscle to return to its normal length.

What are the 3 phases of a twitch?

A muscle twitch has a latent period, a contraction phase, and a relaxation phase.

What is the contraction period of a muscle twitch?

The time between the stimulus and the initiation of contraction is termed the latent period, which is followed by the contraction period. At peak contraction the muscle relaxes and returns to its resting position. Taken all together these three periods are termed a twitch.