What is Muscle Action?

A change in the body or a bodily organ as a result of its functioning is referred to as an action. Muscle action can be defined as the specific movement of a bodily part generated by the contraction of a muscle. The muscle proteins actin and myosin are the important contractile proteins involved in muscle action. Muscle contraction, in addition to movement, serves several other important functions in the body, including posture, heat generation, and joint stability. Muscle contraction maintains posture, which includes standing and sitting.

How do Muscle System work?

Muscle fibers are specialized cells that make up the muscular system of our body. Movement is controlled by muscles, which are attached to bones, internal organs, and blood vessels. Muscle contraction is the source of nearly all movement in the body. Normally, a voluntary muscle operates throughout a joint. It is linked to both bones by tendons (the connective tissue that attaches muscle to bone).  When the muscles contract, only one bone normally moves; the other is a static object.

Each movement at the synovial joint is the consequence of the contracting or relaxing muscles joined to the bones on each side of the articulation. The structural nature of a synovial joint determines the degree and type of movement it may produce.

Functional group of a Muscle

A functional group of muscles is a collection of skeletal muscles controlled by somatic motor neurons and located in the same general area, all of which contribute to the same general set of actions in the movement of a specific body part or parts; each muscle in the set is separated from the next by its epimysium, and the entire set is surrounded by a layer of deep fascia. A skeletal muscle can play four different functional roles within its group, based on the action or movement of the group: an agonist or prime mover, an antagonist, a fixator, or a synergist. When a joint angle changes, the apparent contraction or shortening of a muscle refers to an action that shortens the space between the two attachments of the ends of a muscle group.

Prime mover or agonist

The prime mover is the main muscle in a muscle functional group that causes a specific or unique movement. They keep the body in place efficiently and contract to create the usual range of motion in a joint. Because they are the muscles, largely responsible for creating movement in the body, they are called prime movers. The muscle generates the majority of the force in a specific movement.

Antagonists

Muscles that reverse or oppose a certain movement in body parts are known as antagonists. This group of muscles reverse or oppose the movement caused by agonists in the body.

Fixator

A skeletal muscle that is responsible for movement by contracting to immobilize the origin of the prime mover muscle. The action of the fixator stabilizes a joint that supports the prime mover/agonist when it contracts. For example, the abdominals act as fixators to stabilize the body for knee and hip movements.

Synergist

Any skeletal muscle that assists the prime mover muscle in its movement is referred to as a synergist.

The sliding filament theory of muscle contraction

A set of fundamental units called sarcomeres are placed in a stacked arrangement throughout muscle tissue to create a striped pattern (striations). A single muscle cell can have thousands of sarcomeres. The proteins within them can alter in length, resulting in changes in the overall length of a muscle. Much parallel actin (thin) and thick myosin filaments make up a single sarcomere. Sarcomere shortening relies on the interaction of actin and myosin proteins.

During contraction, one zone of the repeating sarcomere arrangement, the "A band," stands relatively constant in length. The presence of thick myosin filaments in the A band suggests that myosin filaments stand central and constant in length while other sections of the sarcomere shrink. The "I band," which is made up of weaker actin filaments, altered the length of the sarcomere. Muscle tension is generated by actin sliding past myosin. Because actin is connected to z discs or "z bands" at the lateral ends of each sarcomere, any shortening of the actin filament causes the sarcomere to shorten.

Muscle action of three different types of muscle tissues

Muscle tissue is divided into three categories in the muscular system: cardiac, smooth muscle, and skeletal muscle. In the human body, each kind of muscle tissue has its own structure and function.

Skeletal muscle

Skeletal muscle is a kind of muscle that moves bones and other tissues. There are over 600 skeletal muscles in the human body. Skeletal muscles contract and relax in accordance with nervous system signals to move bones.

Cardiac muscle

The heart's cardiac muscle contracts to pump blood. There are three layers in the heart wall. The myocardium, the middle layer, is in charge of the heart's pumping action. To make the heartbeat, cardiac muscle, which is located in the myocardium, contracts in response to signals from the cardiac conduction system. Cardiac muscle is striated in appearance. It has a nucleus and is branched.

Smooth muscle

The smooth muscle that makes up organs like the stomach and bladder adjusts its shape to make biological activities easier. It is made up of muscle cell fibers that are enveloped by protective tissue. It is made up of narrow spindle-shaped cells with a centrally placed nucleus. Smooth muscle contractions are involuntary movements caused by signals sent from the autonomic nervous system to the smooth muscle tissue. Smooth muscular tissue's cell arrangement provides for a high degree of flexibility in contraction and relaxation. The smooth muscle of the alimentary canal aids in the movement of swallowed food and nutrients via peristaltic waves.

Different muscle actions or contractions

A muscle contraction does not always imply that the muscle shortens; it just indicates that stress has been generated. Muscles can contract in a variety of ways:

Isometric contraction

Isometric contraction is more precisely described as muscle contraction that occurs without external movement or change in joint angle or space between origin and insertion. It occurs when a muscle's force perfectly balances the resistance put on it, resulting in no movement. For example, when a muscle tries to push or pull an immovable object.

Isotonic contraction

This is a movement that occurs when the tension created by a contracting muscle is more than the load placed on the muscle. This occurs when muscles successfully push or pull an object. Isotonic contractions are further classified as follows:

Concentric contraction

This is a muscle activity that provides a force to overcome the weight being operated upon the muscle. This is a kind of contraction in which the length of the muscle shortens in response to an opposing stimulus, for example, lifting a weight up. During a concentric contraction, the shortened muscles operate as agonists and perform the work.

Eccentric contraction

This is a kind of muscle action or contraction during which the muscles of body parts lengthen in response to resistance to a load, such as lowering a weight slowly and gradually. During an eccentric contraction, the lengthening muscles operate as agonists and perform the work.

Eccentric contractions are more powerful than concentric contractions and isometric in part because more cross-bridges are linked to actin at the same time in eccentric contractions within each muscle fiber.

Context and Applications

This topic is significant in the exams of school, graduate, and post-graduate levels especially for Bachelors in Zoology and Masters in Zoology.

Practice Problems

Question 1: Which is true about sarcomere?

1. Part between two A lines
2. Part between two Z-lines
3. Part between two I bands
4. Part between two H lines

Answer: Option 2 is correct.

Explanation: Actin is connected to z bands at the ends of each sarcomere.

Question 2: Which of the following act as a muscle's contractile protein?

1. Myosin
2. Tubulin
3. Both 1 and 2
4. None

Answer: Option 1 is correct.

Explanation: Sarcomere shortening is based on the interaction of myosin and actin proteins.

Question 3: In striated muscle, what is the functional unit of the contractile system?

1. Actin
2. Myosin
3. Sarcomere
4. A band

Answer: Option 3 is correct.

Explanation: A set of fundamental units called sarcomeres are placed in a stacked arrangement throughout muscle tissue to create a striped pattern. A single muscle cell can have thousands of sarcomeres.

Question 4: Shortening of sarcomeres is caused by filament sliding, according to the sliding filament model of muscle contraction.

1. True
2. False
3. Maybe true
4. None of the above

Answer: Option 1 is correct.

Explanation: Shortening of sarcomeres is due to the filaments sliding one over the other, based on the "Sliding filament model of muscle contraction.

Question 5: Eccentric contractions are powerful than isometric and concentric contractions, because of _______.

1. There is a smaller number of cross-bridges bound to actin at the same time
2. More cross-bridges are linked to actin at the same time
3. Both1 and 2
4. None

Answer: Option 2 is correct.

Explanation: More cross-bridges are linked to actin simultaneously in eccentric contractions within each muscle fiber.