Movement: The Muscular System Study Guide
To move a body part or the whole body, an organism needs to use the muscular system (in conjunction with the skeletal system). The nervous system controls the three types of muscle tissue found in the body. Sometimes, this is done with our conscious control, and sometimes, it is done automatically without our control.
Three Types of Muscle Tissue
All muscle tissue consists of cells that have evolved to specialize in contracting, yet even with this common function, the three types of muscular tissue have different structures. Skeletal (or striated) muscle tissue is consciously controlled by the central nervous system. This type of tissue is attached to the bones, and when it contracts, it moves them. Skeletal tissue also forms the visible muscles and much of the body mass. The second type of muscle tissue is called smooth, and it is usually not under conscious control. Smooth tissue is usually found in the internal organs, especially the intestinal tract and in the walls of blood vessels. The third type of muscle tissue is called cardiac, and it is found only in the heart. This type of muscle tissue is so specialized to contract that it will continue to do so even without stimulation from the nervous system (although it will be a series of uncoordinated contractions that aren't very good at moving blood around the body). Isolated heart cells in a dish will continue to contract on their own until oxygen or nutrient sources are used up.
Muscles only contract; they do not expand. Muscles are grouped in pairs, often in what are called antagonistic pairs, because they are arranged in opposition to each other. Each member of the pair is able to contract, but it doesn't contract at the same time. When one muscle of the pair contracts, the attached joint will move and the bone involved will also move. The other muscle in the pair will lengthen because it is being stretched, not because it is expanding. When the limb gets moved back to its original position, the muscle that contracted will now lengthen as its antagonistic partner contracts.
As an example, when you bring your fist to your shoulder in the classic "making a muscle" bodybuilder pose, you are contracting the biceps muscle on the upper inside of your arm. This muscle moves the elbow joint and raises your lower arm. The muscle opposite the biceps, called the triceps, is on the backside of your arm. While the biceps contracts, the triceps is stretched, which is its relaxed state. When you want to lower your arm, the triceps will contract and the biceps will relax, moving the elbow joint in the opposite direction. The biceps and triceps act as an antagonistic muscle pair.
How Do Muscles Work?
In order for muscles to contract, two muscle proteins are necessary: actin and myosin. Muscle contraction begins when a nerve impulse causes the release of a chemical, called a neurotransmitter, that activates the muscle cells and stimulates them to contract. Muscle contraction is explained as the interaction of thick bands of myosin and thin bands of actin. The thick myosin filaments have small knob-like projections that grab onto the thin actin filaments. As these knobs move slightly, they pull the actin filaments, which slide alongside the myosin filaments. This has the effect of shortening the muscle and thus causing a contraction.
Actin and myosin are not muscles themselves; they are protein molecules that literally grab onto each other and bend or move so that one slides past the other. This is done in roughly the same way you might climb a knotted rope by grabbing a knot and pulling your body up to the next knot, then grabbing onto the next and pulling again.
Muscles are not directly attached to bone. Connective tissue known as tendons forms a link between them (whereas ligaments form a link between two bones). The contraction of a muscle results in the exertion of force upon the tendon, which then pulls the bone to which it is attached. All this movement is coordinated by the central nervous system and results in some very graceful movement, as well as the everyday, but still amazing, ability to walk from one place to another
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