Muscular System

I. Muscle types 

A. All muscle cells are elongated and are called fibers.

1. Muscle cells contract because of the presence of two types of muscle filaments - myosin and actin.

2. The prefixes “myo” and “sarco” refer to muscles.

B. Skeletal muscle is the type of muscle found attached to the body’s skeleton.

1. the muscle fibers are cigar shaped and contain multiple nuclei.

2. is also called striated muscle because its fibers appear striped

3. attaches to the skeleton to produce movement

4. is under voluntary control

5. muscle fibers can produce strong contractions because of several layers of connective tissue sheathes that provide strength.

6. contractions are powerful and rapid; the muscle tires quickly and must rest7. 3 key words to remember - skeletal, striated, voluntary

C. Smooth muscle is the type of muscle found in the internal organs and blood vessels.

1. the fibers have no striations2. the fibers are under involuntary control3. found in the walls of the hollow organs4. fibers are large, spindle shaped, and have a single nucleus 5. fibers are arranged in sheets or layers6. often there are two sheets - one running circularly and one running longitudinally7. as the layers contract and relax they change the size of the organ8. contractions are slow and sustained9. 3 key words to remember - visceral, nonstriated, involutary

D. Cardiac muscle is a type of muscle found in the heart.1. muscle fibers are striated2. fibers are under involuntary control3. fibers are arranged in spiral shaped bundles4. fibers are branching cells that are joined by intercalated disks5. 3 key words - cardiac, striated, involuntary

II. Muscles have four roles - produce movement, maintain posture, stabilize joints, and generating heat 

A. Producing movement1. skeletal muscles are responsible for all voluntary movement2. smooth muscles are responsible for the movement of internal organs3. cardiac muscle produces our heart beat

 B. Maintaining posture

1. counteracts that force of gravity2. allows us to maintain a sitting or standing position3. muscles contract almost continuously making tiny adjustments

C. Skeletal muscles and tendons are important for stabilizing some joints such as the shoulder or the knee.

 D. As a by-product of muscle contraction heat is produced.

1. skeletal muscle is the most important source of body heat2. vital for maintaining our constant body temperature

C. Most muscles span across joints attach to bones in at least two places.

1. The bone that moves when a muscle contracts is called the muscle's insertion.

2. The bone that does not move, or is less moveable, is called the muscle's origin.

3. The motion produced by the contraction is called the muscle's action.

V. Microscopic anatomy of skeletal muscle 

A. The muscle cell is called a fiber.1. the cell membrane is called the sarcolemma2. the fiber contains many nuclei that are pushed against the sarcolemma3. the fiber is filled with myofibrils

B. The myofibrils are composed of contractile units called sarcomeres

1. each sarcomere is composed of two types of protein filaments known asmyofilaments - thick and thin filaments3. the protein myosin composes the thick filaments 4. the protein actin composes the thin filaments

B. continued5. striations represent alternating dark and light bands

dark bands = myosin and actinlight bands = actindark bands are called A bandslight bands are called I bands

6. The I bands are interrupted by the Z disk.connects sarcomeresA sarcomere extends from Z disk to the next Z diskanchors the actin filaments of a sarcomere

7. A bands interrupted by the M lineM line represents a protein that anchors the myosin filaments

8. Ultrastructure of myofilamentsa. the thick filament is made of myosin moleculesb. each myosin molecule has a rod-like tail that ends in two headsc. the heads act as cross-bridges that allow temporary connections to form between myosin and actin during contractiond. the thin filaments are made of actin, tropomyosin, and troponine. actin molecules contain active sites where the myosin cross-bridges attachf. tropomyosin - stiffens and thin filamentg. troponin - binds to and blocks the active sites on actin

C. When muscles contraction occurs the actin filaments slide over the myosin filaments and cause the sarcomere to shorten - called the sliding filament theory of muscle contraction

 1. actin molecules are pulled toward the center of the sarcomere2. the distance between Z lines becomes less as each sarcomere shortens3. The combined effect of shortened sarcomeres is the shorten the muscle fiber4. Contraction results from the attachments of the cross-bridges on the myosin molecules to the active sites on the actin molecules. Each cross-bridge attaches and detaches several times during one contraction.

5. Contraction requires the presence of calcium ions Troponin blocks the active sites on actinCalcium binds with and alters the shape of troponinthe altered shape of troponin opens up the active sites on actin

6. When the active sites on actin are exposed the following four events occur  a. a cross- bridge attaches to an actin active site

b. the myosin head pivots into a bent shape that drags the actin toward the center of the sarcomerec. the cross-bridge de-ttachesd. the cross-bridge re-forms its straighter, "cocked" position

VI. Contraction of a skeletal muscle 

A. Skeletal muscles must be stimulated by nerve impulses to contract.

1. The nerve input is carried by a motor neuron.2. One motor neuron may stimulate several or hundreds of muscle fibers.3. A motor unit is defined as a motor neuron and the muscle fibers it stimulates.4. The neuron branches into a number of axonal

terminals as it nears the fiber. Each terminal forms a junction with a different muscle fiber. These junctions are called neuromuscular junctions.

5. The neuron and the fiber never actually touch. There is a space between the two known as the synaptic cleft.

6. When a nerve impulse reaches the neuromuscular junction, a chemical, called a neurotransmitter is released. The

neurotransmitter for all skeletal muscles is acetylcholine.

7. The nerve impulse causes the muscle fiber to contract.

B. Contraction of a muscle as a whole1. Contraction of a muscle fiber is an “all or none” action.2. An entire muscle can contract in varying degrees. This is called a graded response. 3. A graded response can be produced by: 1) varying the speed with which the muscle is stimulated; and by 2) changing the number of muscle fibers stimulated.4. In most muscle activity, nerve impulses are delivered to the muscle fibers quickly and the combined effect of successive fiber contractions produces a smooth muscle contraction. 5. The force with which a muscle contracts is largely determined by the number of fibers that are stimulated.

C. Energy for muscle contraction1. The energy needed for muscle contraction comes from ATP that is produced by aerobic respiration.2. When a muscle is over worked there is insufficient oxygen for aerobic respiration to occur and the fiber switches to anaerobic respiration.3. Anaerobic respiration produces lactic acid as a by-product. Lactic acid is the cause of muscle soreness and stiffness that may occur after intense activity.

D. Muscle fatigue1. Muscle fatigue occurs if we exercise our muscles continuously for a long time.

2. Muscle fatigue occurs when a muscle is unable to contract even though it is being stimulated.

3. This occurs when a muscle is being contracted continuously without a period of rest. The contractions get shorter and shorter until they stop completely.

4. Muscle fatigue is believed to be due to the body’s inability to supply oxygen to the muscles fast enough. Without oxygen the energy produced by the muscles decreases until there is no

energy for contraction. This is called the “oxygen debt”.

E. Types of muscle contractions1. Twitch contractions are quick, jerky responses to a stimulus. They play a minor role in normal muscle activity.

2. Tetanic contractions are longer, more sustained contractions produced by a series of stimuli reaching the muscle fibers in

rapid succession tetany refers to a smooth, sustained muscle contraction and NOT to the disease known as tetanus caused by C. tetani

3. Isotonic contractions occur when the muscle shortens and movement occurs.

4. Isometric contractions occur when the muscle trys to shorten but it is met with resistance. The muscle does not shorten but the tension in the muscle increases.

5. Muscle tone is a state of continuous, partial muscle contractions that keeps muscles firm and healthy.

F. The effect of exercise on muscle contraction.1. Aerobic exercises produce stronger, more flexible muscles that are more resistant to fatigue. This occurs because the blood supply to the muscle increases as does the number of

mitochondria in each fiber. There is little increase in the size of the muscles.

2. Resistance exercises, or isometric exercises, train the muscles against immovable objects. Consistent isometric exercises cause an increase in the size of muscle cells, not an increase in muscle cells. Increase fiber size causes an increase in the size of

muscles. Increase in size, not number of cells is called hypertrophy.

3. Cross training - alternating between aerobic and resistance exercises provides an effective program for general health.

4. Prolonged inactivity may cause muscle fibers to shrink. This process is called atrophy and the condition is referred to as

“disuse atrophy”. (Is this principle also at work in "our brains" as well?)

VII. Muscle contractions and body movements 

A. . Muscles usually work together to produce smooth, coordinated movement

1. Smooth movement is usually the combination of muscles that produce the movement and muscles that oppose the movement.

2. The muscle has the major responsibility for a movement is called the “prime mover” or the "agonist".

3. A muscle that opposes a movement is called an antagonist.

4. A muscle that helps the prime mover is called a synergist.

5. A fixator is a muscle that assists the prime mover by stabilizing the origin of the prime mover. B. Skeletal muscle mechanics

1. The operation of most skeletal muscles involves leverage and lever systems.

1st class lever2nd class lever3rd class lever