Muscular System 2

Muscular System 2

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Muscular System 2...

Muscular System 2:

There are three types of muscular tissue, skeletal, visceral, and cardiac. For our purposes, we will be looking at skeletal muscle, which makes up about 40% of an individual's body weight.

Skeletal muscles are under conscious control and when they contract they move the bones. Skeletal muscles also allow us to smile, frown, pout, show surprise, and exhibit other forms of facial expressions.

Skeletal muscle has four primary characteristics that relate to it's functions:

excitability; is the ability to receive and respond to a stimulus. In order to function properly, muscles have to respond to a stimulus from the nervous system.

contractility; is the ability to shorten or contract. When a muscle responds to a stimulus, it shortens to produce movement.

extensibility; means that a muscle can be stretched or extended. Skeletal muscles are often arranged in opposing pairs. When one muscle contracts, the other muscle is relaxed and is stretched.

elasticity; is the capacity to recoil or return to the original shape and length after contraction or extension.

Muscle contraction fulfills four important functions in the body:

movement- integrated action of muscles, joints, and bones.

posture- such as sitting and standing is maintained as a result of muscle contraction.

joint stability- muscle tendons are the major factor in stabilizing such joints as the knee and the shoulder.

heat production- to maintain body temperature is an important by product of muscle metabolism. Nearly 85% of the heat produced in the body is the result of muscle contraction.

Nearly all movement in the body is a result of muscle contraction. Exceptions to this are the action of our cilia in the lungs, and the flagellum on sperm cells.

STRUCTURE OF THE SKELETAL MUSCLE

A whole skeletal muscle is considered an organ of the muscular system. Each organ or muscle consists of skeletal muscle tissue, connective tissue, nerve tissue, and blood or vascular tissue.

Each fine thread is called a muscle fiber. This type of muscle tissue has three names; skeletal muscle because it attaches to bone, striated muscle because it has cross stripes or striations, and voluntary muscle because its contractions can be controlled voluntary.

Most skeletal muscles attach to two bones that have a movable joint between them. Most muscles extend from one bone across a joint to another bone. The muscle's attachment to this more stationary bone is called its origin. Its attachment to a more movable bone is called its insertion. The rest of the muscle is called the body of the muscle.

Tendons anchor firmly to bone. They are a dense fibrous connective tissue in the shape of heavy cords and they have great strength. They do not tear or pull away from the bone easily.

The basic structural and functional unit of the skeletal muscle is called the sarcomere. The microscopic structure of the sarcomere consists of many myofilaments.

Muscle fibers are arranged in a highly organized way. Each muscle fiber has two kinds of very fine and threadlike structures called thick and thin myofilaments.

The thick myofilaments are formed by the protein myosin. The thin filaments are formed by the protein actin. When seen under a microscope, dark and light striations are seen. Dark bands called Z bands separate the repeating units of the sarcomeres from each other.

Contraction of a muscle causes the two types of myofilaments to slide toward each other shortening the sarcomere thus shortening the muscle. When the muscle relaxes, the sarcomeres return to their resting length, and the filaments resume their resting positions. The typical myofibril consists of 10,000 or more sarcomeres that are strung together in long chains.

SLIDING FILAMENT THEORY

In a relaxed muscle fiber, the receptor sites on the actin thin filaments are covered and inactivated. Heads on the myosin thick filaments are also inactivated and are bound or attached to ATP or (adenosine triphosphate). Ah... ATP you say. I remember that. That is a chemical compound that stores chemical energy within the cell for use by body cells. All if this lays in waiting for the correct signal. Calcium also is stored in the channels of the muscle cell endoplasmic reticulum.

Because the muscle cell needs energy for contraction, there are multiple nuclei and numerous mitochondria (power plants of the cells....coming back to you now??)

When an impulse travels from the nerve cell to the cell membrane of the muscle cell, calcium ions are released. This rapid influx of calcium ions into the cytoplasm of the muscle cell causes a change in the shape of the actin thin filaments and exposes the myosin thick binding sites.

At the same time, ATP is broken down to become ADP and the previous inactive myosin heads become energized and begin to interact with the actin thin filaments. They form a “cross bridge” so that the actin slides toward the center of the myosin. This action shortens the length of the sarcomere.

This is not as confusing as one would think. Just take it step by step and it begins to make sense.

When there are no more nerve impulses, muscle impulses stop and calcium is actively transported from the cytoplasm of the muscle cell back into the endoplasmic reticulum to be stored for the next round. Without the calcium, the actin and myosin are reconfigured or redesigned into their noncontracting state and the muscle fiber relaxes

Individual muscle fibers contract according to the all or none principle. When the muscle fibers receive sufficient stimulus to contract, all the sarcomeres shorten at the same time. A greater stimulus will not produce a greater contraction. If there is insufficient stimulus, then none of the sarcomeres contract. Therefore, it is all or none.

The minimal stimulus to cause muscle fiber contraction is called a threshold; a lesser stimulus that is insufficient to cause contraction is called a subthreshold.

OK...I know that you are dying to know this.... What exactly is rigor mortis?

Rigor mortis means the “stiffness of death”. Within a short time of death, the ATP in muscle breaks down so there is no ATP available to detach the cross bridges between the actin and the myosin. The myofilaments remain locked in a contracted position and the body becomes rigid. Day or so later, muscle proteins begin to break down and the rigor mortis disappears.

A muscle's response to a single threshold stimulus is called a twitch. This is not the way a muscle in the body normally functions

A muscle twitch, the response to a single stimulus shows a lag phase when initially there is no response to the stimulus. Then there is a contraction phase where the tension of the muscle increases to a peak. If the tension is great enough to overcome the weight load, then movement occurs. This is followed by a relaxation phase, when the tension is decreased and a relaxed state is achieved.

If a second stronger stimulus is applied during the relaxation phase, the second twitch is stronger than the first. If the muscle is stimulated at an increasingly faster rate the relaxation disappears and the contractions merge into a smooth, sustained contraction called tetany.

Treppe is a staircase effect and it is an increase in the force of muscle contraction in response to a successive threshold of stimuli of the same intensity.

The word tetanus is often confusing because it means different things to different people. In reference to muscle contraction, it produces a steady contraction of a muscle fiber without a relaxation phase. The term also refers to a disease commonly called “lockjaw”, that is caused by the bacterium Clostridium tetani. The toxin causes the nerves to become highly excitable, which in turn causes uncontrollable muscle contractions, or spasms.

Muscle tone refers to the continued state of partial contraction that is present in muscles. This produces a constant tension in the muscles and keeps them ready for activity.

MOVEMENTS

Muscles move bones by pulling on them. When the muscle contracts, the insertion moves toward the origin.