Overheads for Chapter 9 - Muscles

Three types of muscle:

Skeletal - striated - voluntary

Cardiac - striated - involuntary

Autorhythmicity

Smooth - nonstriated - involuntary

FUNCTIONS OF MUSCLE TISSUE

1. motion

2. movement of substances within the body

3. stabilizing body positions and regulating organ volume

4. thermogenesis (heat production).

CHARACTERISTICS OF MUSCLE TISSUE

1. Excitability (irritability) is the ability of muscles and nerves to respond to a stimulus by producing electrical signals called impulses or action potentials.

2. Conductivity is the ability of a cell to conduct action potentials (electrical current) along the plasma membrane.

3. Contractility

4. Extensibility

5. Elasticity

Connective Tissues:

Fascia - band or sheet of fibrous connective tissue

superficial fascia

deep fascia

Epimysium surrounds the whole muscle.

Perimysium surrounds bundles of 10-100 or more individual muscle fibers and makes bundles called fascicles.

Endomysium covers each individual fiber

Filaments are arranged into sarcomeres - the basic functional unit of striated muscle.

Tropomyosin

Troponin

Elastic filaments - titin or connectin

Dystrophin

acetylcholinesterase

calsequestrin

rigor mortis

creatine phosphate

phosphagen system

Muscle Fatigue is caused by several factors:

1. depletion of ATP, CP and glycogen stores

2. Insufficient O₂ delivery.

3. Lactic acid buildup - this lowers the pH which interferes with aerobic metabolism and slows down ATP synthesis and causes muscle soreness.

4. Insufficient release of ACh at the NMJ in spite of action potentials.

5. Ion imbalances - K+, Na+ and Ca++ levels have to be maintained.

6. psychological responses ( I’m tired - probably ultimately protective)

Slow oxidative (type I) fibers - these fibers are the smallest in diameter.

1. Also called slow-twitch or fatigue-resistant fibers.

2. They contain large amounts of myoglobin, many mitochondria and many blood capillaries. They look red, and can generate a lot of ATP aerobically.

3. They split ATP at a slow rate, and therefore, have a slow contraction velocity.

4. They are very resistant to fatigue.

5. Found in large #’s in postural muscles.

Fast-twitch muscle fibers are called type II fibers.

Fast oxidative (type II A) fibers -

1. intermediate in diameter and speed of contraction.

2. also called fast-twitch A or fatigue-resistant fibers.

3. contain large amounts of myoglobin, many mitochondria and many blood capillaries. They look red, and can generate a lot of ATP aerobically.

4. They split ATP at a rapid rate, and contraction velocity is very fast.

5. Not quite as resistant to fatigue as type I fibers.

6. Found in large proportions in leg muscles of sprinters.

Fast glycolytic (type II B) fibers

1. Largest in diameter

2. also called fast-twitch B or fatigable fibers.

3. Low myoglobin content, few mitochondria, and relatively few capillaries. (fibers are white.)

4. They do contain large amounts of glycogen. and generate ATP anaerobically (glycolysis).

5. They fatigue easily.

6. They are large and split ATP rapidly, so contractions are strong and rapid.

7. Found in arm muscles. - used intensively for short periods - throwing

Homeostasis of Body Temperature

1. Role of smooth muscle is in opening and closing blood vessels and capillaries near the skin surface.

2. Skeletal muscle - as much as 85% of the energy released in muscles is lost as heat.

        a. Some keeps the body warm

        b. some is lost through skin and lungs

        c. If temp. drops, shivering produces more heat.

        d. Thermoregulation is under the control of the hypothalamus.

There are two types of smooth muscle tissue:

1. Visceral (single-unit) smooth muscle tissue is the more common type.

        b. The fibers form large networks that contract together.

2. Multiunit smooth muscle tissue consists of individual fibers, each with its own motor neuron terminals and few gap junctions.

        a. action potentials stimulate only that motor unit.

        b. in walls of large arteries, large airways, arrector pili, and radial and circular muscles of the iris, and ciliary body which adjusts the focus of the eye.

Smooth muscle is nonstriated and involuntary

    1. Shorter than skeletal muscle fibers.

    2. Single, central nucleus.

    3. Sarcoplasm contains both thick and thin filaments, but they are not arranged in sarcomeres.

    4. There are 10-15 thin filaments for each thick. (skeletal 2:1)

    5. Also contains intermediate filaments.

    6. No transverse tubules, and only scanty SR for storage of Ca++

    7. The intermediate filaments attach to dense bodies which are similar to Z discs.

    8. During contraction, the sliding thick and thin filaments generate tension that is transmitted to the intermediate filaments. The intermediate filaments pull on the dense bodies attached to the sarcolemma, shortening the muscle fiber.

    9. Smooth muscle may contract by twisting into a helix shape.

Physiology of Smooth Muscle