Muscle Structure and Function

These animations have been prepared with the objective of serving as teaching tutorials to assist undergraduate students in conceptualizing the complex dynamics of physiological processes -- especially as they relate to insects. I am sharing these animations with my colleagues that wish to link to them, or refer them to their students for the purpose of illustrating course lecture topics.

As a courtesy, to help me evaluate the usefulness of the animations, if you use them in your teaching, please send me an e-mail at: llkeeley@tamu.edu, and let me know if they are helpful. I would also point out that these are works in progress, and will be updated occasionally with more scenes, voice-over, etc. to improve their usefulness for instruction

Instructions for playing the animation

The movie starts on opening and the title page takes several seconds to play.

The menu control allows you to review any scene without viewing the preceding scenes.

play: (4),

stop: (<),

restart the scene: (9)

back up one frame: (7)

advance one frame (8)

The MENU (78) button allows you to load and view any scene without viewing the preceding scenes.

Scene 1 -- Muscle Structure -- Overview

The scene opens with a grasshopper as a model animal, and a fade-in view of the jumping muscle in the femur. Voluntary muscles of animals have a striated appearance because of the sarcomere structures (explained below).  All muscles are striated in insects.

A lens magnifies one of the intact muslces to demonstrate that the muscle is comprised of individual fibers. Fibers are the individual cells of muscles. Each fiber has formed by the merging of of numerous myoblasts to form a syncetium with multiple nuclei. The fiber's plasma membrane is the sarcolemma, and the cytoplasm is the sarcoplasm. The sarcoplasm is packed with contractile elements termed myofibrils. The myofibrils have a striated pattern and the overall muscle has a striated appearance, if it is a voluntary muscle of vertebrate animals. All insect muscles are striated.

Myofibrils are the contractile units of the muscle and are organized into sarcomeres. Sarcomeres are defined at each end by a Z-band. From the Z-bands to the middle of the sarcomere are in sequence: I-bands, A-bands and a middle H-band.

The banding patterns arise from the organization within the sarcomere of two major protein filaments: thick myosin filaments surrounded by thin actin filaments. It is the interaction of the these filaments that results in contraction of the sarcomeres.

Actin filaments attach to the Z-bands. I-bands are regions that contain only actin. A-bands are regions that consist of overlapping actin and myosin. The H-band is a region that contains only myosin.

Scene 2 -- Sarcomere Structure

This scene shows a close-up view of the sarcomere structure at the level of actin and myosin and details of the organization of actin and myosin in the bands.

Scene 3 -- Sarcomere - Fibril Contraction

This scene illustrates the sliding filament rationale for muscle contraction. The actin filaments slide along the myosin filaments thus drawing the Z-bands toward the middle of the sarcomere, thus shortening the sarcomere. This is illustrated first with the sarcomere, then with a series of sarcomeres similar to the situation during contraction by a myofibril.

The scene then goes to the molecular detail of the actin and myosin to demonstrate the molecular events that occur during a sarcomere contraction.

Scene 4 -- Actin - Myosin Crosslinking

Details of actin and myosin are demonstrated relative to the sliding filaments. The scene opens with a view of the molecular relationships of actin and myosin related to the banding pattern.

Crosslinks are identified between the myosin and the actin. The crosslinks are part of the myosin filaments and link to sites on the actin. Myosin crosslinking occurs by a myosin head that is capable of conformational and positional changes. The myosin heads bind with binding-sites on the actin and undergo a conformational rearrangement that rotates the position of the head and slides the actin along the myosin. The head releases from the actin, rotates back to its original position, rebinds to the actin and repeats the cycle.  .

Scene 5 -- Crosslink Detail

This scene shows the crosslinks occuring repetitively.  Note that the myosin heads do not work in unison, but bind independently to the actin.  In this way, the actin cannot return to its "relaxed" conformation when any of the heads are going through their release phase since other myosin head units are still binding and exerting pull on the actin filaments.

Scene 6 -- Crosslink Biochemistry

This scene shows the sequence of biochemical events that occur during crosslinking and sliding of the actin filaments by the myosin heads.

The cycle starts with the myosin head charged with ATP. The myosin head contains ATPase, an enzyme that splits the high energy phosphate bond of adenosine triphosphate (ATP). ATP is bound to the head at the time of release from actin, and the bound ATP is hydrolyzed by ATPase to adenosine diphosphate (ADP) and inorganic phosphate (Pi). The resulting ADP and Pi remain associated with the myosin head.

To start the contraction cycle, the myosin head attaches loosely to actin, then releases Pi which causes tight actin binding.

After tight binding, proteins in the head undergo a conformational change that results in repositioning the angle of attachment of the myosin head to the actin. The shift to the new conformation pulls the actin along the myosin.

After pulling the actin, the head releases ADP, and binds a new molecule of ATP. This results in release of the head from the actin. The myosin head resumes its relaxed conformation and position, and the cycle is ready to repeat.