CHAPTER 11    MUSCULAR SYSTEM

 

Involves skeletal muscles only---nearly 700 named muscles

 

 

1. Skeletal muscles produce movement by exerting force on tendons, which pull on bones or other structures.

2. When a muscle contracts, it usually draws one articulating bone toward the other.

3. The attachment of a tendon to the less movable bone is called the origin.

4. The attachment of a tendon to the more movable bone is called the insertion. 

5. The fleshy portion of the muscle between tendons is called the muscle belly.

6.The origin is usually proximal and the insertion distal.

7. Muscles that move a body part usually do not lie over that part.

8. Most muscles cross at least one joint.

 

 

A lever is a a rigid rod that moves about on a fixed point called a fulcrum. In producing body movement, bones act as levers and joints as fulcrums

 

2 forces act on a lever:

   Resistance (load)--force that opposes movement

   Effort--force exerted to achieve movement

In the body:

   Bones are the levers

   Joints are the fulcrums

   The weight of the body part is the resistance

   Muscle contraction is the effort

 

Relative position of effort, load, and fulcrum on the lever determines whether the system operates at a mechanical advantage or disadvantage.

 

Mechanical advantage—the load is close to the fulcrum and the effort is farther away. Smaller effort can move a heavier load. The effort must move further and faster than the load. Chewing food is an example.

 

Mechanical disadvantage—the effort is close to the fulcrum and the load is farther away. Larger effort to move a lighter load.  The effort moves a shorter distance and slower than the load. Pitching a baseball is an example.

 

 

1. 1st class levers--- fulcrum is between effort and resistance a seesaw is an example---rare in body but raising head when it is bent forward is an example

 

2. 2nd class levers---wheelbarrow—probably don’t exist in body but raising up on tiptoes is an example. Gives strength but not speed and ROM.

 

3. 3rd class levers---common type in body---adduction of thigh, flexion of forearm

 

 

ARRANGEMENT OF FASCICLES

 

Skeletal muscle fibers are arranged in bundles (fascicles or fasciculi) wrapped in perimysium. Fascicles are arranged in several ways in relation to the tendon.

                               See Table 11.1  P. 329

 

 

 

Most movements require several skeletal muscles acting as a group. Most skeletal muscles are arranged in opposing pairs at joints.

 

The muscle or muscle group that produces the desired action is called the prime mover or agonist. As it contracts, another muscle or group, the antagonist, must relax. In flexing the forearm, the biceps brachii is the agonist and the triceps brachii is the antagonist. But turn it around and make extending the forearm the desired movement and the triceps becomes the agonist and the biceps the antagonist.

 

Some muscles cross other joints before reaching the joint at which the primary action occurs. Special muscles called synergists hold these intermediate joints still while the

primary movement occurs.

 

Fixators are muscles that  stabilize the origin of the prime mover (for example, some muscles hold the scapula still while other muscles attached to the scapula move the arm).

 

Compartment—group of skeletal muscles and associated blood vessels and nerves that have a common function. The group is often wrapped together in deep fascia.

 

 

NAMING SKELETAL MUSCLES

TABLE 11.2  P. 332 - 333 lists characteristics used to name muscles, with examples. Study this carefully, since muscle names are much easier to learn when we understand these characteristics. 

 

IDENTIFYING MUSCLES ON TEST

The rest of this chapter has numerous illustrations and descriptions of the location, points of attachment, and action of the muscles of the body. What you will be tested on is much less than what the chapter contains. You will be required to identify 20 muscles on the next hour exam. These will come from the “Muscle Man” diagrams on pages 334 & 335. The diagrams on the test will not have all of the lines—only those going to muscles being asked will be present.

Diagrams with no lines are available in the lab if you would like to test yourself.

 

 

 

 

 

 

 

 

 

 

 

 

CHAPTER 10   MUSCLE TISSUE

 

Muscles produce motion and generate force by contracting

     40-50% of body weight

    

Myology is the study of muscles.

 

3 types of muscle tissue:

 

1. Skeletal muscle tissue

   Voluntary

   Striated

Skeletal—attached primarily to bones and moves parts of the skeleton

Striated—alternating light and dark bands are visible upon micro. exam

Voluntary—conscious control

 

2. Cardiac muscle tissue—forms most of the heart

     Striated

     Involuntary—not under conscious control--has a built-in rhythm called autorhythmicity

 

3. Smooth muscle tissue—located in the walls of hollow organs such as BV, stomach, intestine, etc. and also arrector pili muscles.

     Smooth (non-striated)—striations NOT visible under microscope

     Involuntary—NOT under conscious control—some of this muscle displays a degree of wutorhythmicity

 

 

4 functions of muscle tissue:

1. Producing body movements (skeletal muscle)

     Movement of body or parts by skeletal muscle

 

2. Stabilizing body position (skeletal muscle)

     Sitting

     Standing

     Holding head up

 

3. Storing and moving substances within the body—contraction of smooth or skeletal muscle sphincters keeps contents in stomach, parts of intestine, urinary bladder etc.; smooth and cardiac are responsible for heartbeat, digestive movements, etc.

 

4. Generation of heat (thermogenesis) 

     Heat is a by-product of contraction. Used to maintain normal body temp. (85% of all body heat)

 

Characteristics of muscle tissue  (see page 292 for explanation)

 

1. Electrical excitability

2. Contractility

3. Extensibility

4. Elasticity

 

 

 

SKELETAL MUSCLE TISSUE

Each skeletal muscle is an organ in the muscular system, consisting of hundrecs to thousands of muscle fibers (cells). Skeletal muscle cannot function without help from connective tissue, nerves and blood vessels.

 

CONNECTIVE TISSUE COMPONENTS

Connective tissue surrounds, protects, and separates muscle tissue. It also connects skeletal muscles to bones.

 

Fascia---sheet of fibrous CT beneath skin or around muscles

 

   1. Superficial fascia (SubQ layer)---just below skin and consists of adipose and areolar CT. Functions:

       a. Connects skin to underlying muscle

       b. Stores water and fat

       c. Insulation

       d. Mechanical protection

       e. Pathway for nerves and blood vessels

 

    2. Deep fascia—dense irregular CT associated with muscles. Functions:

       a. Wraps around each muscle

       b. Additional deep fascia may wrap around groups of muscles with a common function,

           helping them function as a unit

       c. Allows free movement of muscles (slick)

       d. Carries nerves, blood vessels and lymphatics

       e. Fills space between muscles

       f. Helps attach muscle to bone

 

 

 

Three additional layers of connective tissue are closely associated with each skeletal muscle. These are extensions of the deep fascia.

   1. Epimysium---dense irregular tissue that circles entire muscle

   2. Perimysium---dense irregular that surrounds bundles of muscle fibers called fascicles (10-100 individual muscle fibers per fascicle)

   3. Endomysium---arealar tissue that penetrates fascicles and surrounds individual muscle fibers (muscle cells)

                                 Fig. 10.1 p. 293

 

All three of these wrappings are continuous and contribute collagen fibers to tendons. Tendons are the CT that attaches muscle to bone (dense regular CT). Most tendons are roundish and cordlike, but some are broad and flat and are called aponeuroses. Some tendons are wrapped in tendon sheaths to ease movement in tight spots.

 

BLOOD SUPPLY

Muscle contraction calls for large amounts of ATP, which can only be produced with a generous supply of oxygen and nutrients. Waste and excess heat must be removed. Muscle has large capillary networks to reach each fiber.

 

NERVE SUPPLY

Nerve cells called somatic motor neurons located in the brain and spinal cord are connected by a threadlike axon to skeletal muscle. Signals from the neuron to each muscle cell are necessary for contraction to occur.

 

MICROSCOPIC ANATOMY OF SKELETAL MUSCLE

A skeletal muscle contains hundreds or thousands of long cylindrical muscle cells called muscle fibers. The fibers lie parallel and may be up to 12” long or more (they may run the full length of the muscle). Muscle fibers form during embryonic development as stem cells called myoblasts fuse. Skeletal fibers do not undergo mitosis. Growth occurs by hypertrophy ((fibers get larger), not by hyperplasia (more fibers).

 

Sarcolemma---muscle fiber plasma membrane

Sarcoplasm---cytoplasm of a muscle fiber

Sarcoplasmic reticulum (SR)---network of membranous channels similar to smooth ER

 

Each fiber has many nuclei at the periphery (edge) of the cell. Also there are numerous mitochondria. Muscle cells contain a special protein called myoglobin, which stores oxygen.

 

The sarcoplasm contains long ribbonlike organelles called myofibrils. These are the part that have the light and dark bands that cause visible striations in skeletal muscle. The myofibrils contain three types of even smaller components:

    Thin filaments

    Thick filaments

    Titin (elastic) filaments

These filaments do not extend the full length of a muscle fiber. They are arranged in crosswise compartments called sarcomeres, which are the basic structural and functional units of skeletal and cardiac muscle fibers.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Thick filaments are made of the contractile protein myosin (about 300 myosin molecules per thick filament). They also contain the enzyme ATPase, which splits ATP to provide energy for contraction.

 

 

 

 

 

 

 

Thin filaments are made of the protein actin, the other contractile protein. The molecules form a long strand which twists into a helix. On each actin molecule is a myosin-binding site where crossbridges can form. In relaxed muscle, a protein called tropomyosin covers the myosin-binding sites and blocks this attachment. Another regulatory protein, troponin, is attached to the tropomyosin.

 

Sarcomeres also contain various structural proteins, which contribute to maintaining stability and organization of the myofibril. Some of these are:

     Titin filaments—made of the protein titin and stabilize thick filaments

     Myomesin—forms M lines

     Nebulin—wraps around thin filaments

     Dystrophin—reinforces sarcolemma and helps transmit tension to tendons

 

As you read the underlined sections that follow, be sure to refer to Fig. 10-2 P. 295.

A fluid-filled system of cisterns called the sarcoplasmic reticulum (SR) encircles each myofibril. SR is similar to smooth ER in other cells. The SR of a relaxed muscle stores Ca2+ ions. Release of these ions into the sarcoplasm is a major factor in muscle contraction. The ions are pumped in to the SR by active transport pumps and leave by diffusion through Ca release channels.

 

 

 

 

 

 

 

Transverse tubules (T tubules) are infoldings of the sarcolemma that penetrate the muscle fiber (2 per sarcomere) and are filled with extracellular fluid. On both sides of a T tubule are dilated end sacs of SR called terminal cisterns. A triad is a T tubule and the terminal cisterns on each side of it.

 

 

MUSCULAR ATROPHY AND HYPERTROPHY

Skeletal muscle fibers can change in response to the amount of use they receive:

 

1. Muscular atrophy--wasting away of muscles. Muscle fibers lose myofibrils if the muscle is not used.

        a. Disuse atrophy--normal nerve supply but muscle not used, as in a person who is bedridden or limb in a cast

        b. Denervation atrophy--loss of nerve supply--muscle fibers are gradually replaced by fibrous tissue--not reversible

 

     2. Muscular hypertrophy--increase in the diameter of existing muscle fibers (no new fibers) due to increased stress. Myofibrils, mitochondria, sarcoplasmic reticulum and other components of existing fibers increase, producing larger muscles capable of more forceful contractions. Placing considerable stress on muscles (strenuous workouts, running long distances, hard work, etc.) actually causes microscopic damage to muscle fibers. This is why muscles get sore. However, the fibers not only repair the damage but further respond by becoming stronger.

 

 

 

NEUROMUSCULAR JUNCTION

Skeletal muscle fibers must receive a direct signal from the nervous system in order to contract. Neurons and muscle fibers communicate in specialized regions called neuromuscular junctions. Usually the 2 excitable cells (the somatic motor neuron and the muscle fiber) do not quite actually touch—a microscopic gap called the synaptic cleft separates them. The nerve cell communicates with the muscle cell by releasing a chemical called a neurotransmitter. Sometimes neurons communicate with glands or other neurons—same basic idea.

 

Remember, a motor neuron has a long threadlike process called an axon. As the axon approaches the muscle fibers, it forms a number of branches called axon terminals. The portion of the muscle fiber plasma membrane (the sarcolemma) adjacent to the axon terminal is called the motor end plate. Axon terminal plus motor end plate make up the neuromuscular junction.

 

 

 

 

 

 

 

 

 

 

 

 

Synaptic vesicles of neurons connected to skeletal muscle fibers all release the neurotransmitter acetylcholine (ACh). A nerve impulse reaches the axon terminal and causes the release (by exocytosis) of acetylcholine, which diffuses into the synaptic cleft. The motor end plate contains acetylcholine receptors to which the acetylcholine binds. This binding opens channels in the muscle fiber plasma membrane through which Na+ ions rapidly enter and initiate events leading to contraction. In skeletal muscle, each fiber has one neuromuscular junction located at the midpoint of the fiber.

 

 

MOTOR UNITS

A motor neuron (nerve cell) plus all the muscle cells (fibers) it connects to and stimulates is called a motor unit. Motor units vary in the number of muscle fibers, with 150 being the average number and a range of 2 or 3 to 2000 muscle fibers per motor unit. Small delicate muscles such as the larynx muscles and eye muscles have the smallest number of fibers per motor unit to allow precise fine-tuning. Large leg muscles would have the most fibers in each motor unit.

 

 

 

 

 

 

 

 

Whatever the number of muscle fibers involved, when the motor neuron fires every muscle fiber contracts at once. Every muscle has many motor units. The strength of contraction is adjusted by adjusting the number of motor units contracting at once.

 

 

 

CONTRACTION OF SKELETAL MUSCLE

 

Sliding filament mechanism---the thin and thick myofilaments slide past each other during contraction, increasing the amount of overlap and shortening the sarcomere. To bring this about, myosin heads attach to the thin myofilaments, forming crossbridges, and pull on them until the thin myofilaments meet at the middle of the sarcomere or even overlap. Lengths of the myofilaments do not change, but sarcomeres, myofibrils, muscle fibers, and the entire muscle shorten.

 

In a relaxed muscle fiber, the level of calcium in the sarcoplasm is quite low. Active transport pumps in the SR membrane remove most of the calcium and store it in the SR.

 

All body cells normally maintain conditions similar to these found in a resting muscle cell:

 

 

           ECF                           High Na+   Low K+

________________________________________________________________________________________

                                                     Low Na+     High K+

 


SR
 
                                                 Very low Ca2+       

Very high Ca2+

 

10,000 times higher inside than outside in cytosol
 
 

 


                                                                          Net negative charge

 

                                            

                                                        _____________________________________________________________

 

 

STEPS IN MUSCLE CONTRACTION

1. Nerve impulse reaches the neuromuscular junction and causes release of some of the synaptic vesicles by exocytosis.

2. Acetylcholine from these synaptic vesicles diffuses across the synaptic cleft and binds to acetylcholine receptors in the motor end plate.

3. This causes the sarcolemma to briefly become highly permeable to Na+ ions. They rush into the cell in great numbers and change the net charge inside the cell from negative to O to positive (this is called generating a muscle action potential).

4. As a result of the action potential, calcium release channels in the SR membrane open and Ca2+ ions rush out into the sarcoplasm.

5. Calcium binds to the protein troponin and changes the shape of the troponin molecule. This change causes the troponin to pull the tropomyosin off the myosin binding sites on the actin of the thin myofilaments.

6. Energy from ATP is needed at this point. The enzyme ATPase (associated with the myosin heads) splits ATP to provide energy for contraction. ATP is broken down to ADP and a phosphate group, both of which remain attached to the myosin head. The myosin heads are energized by this change.

7. Since myosin binding sites are uncovered, the energized myosin heads of the thick myofilaments are able to grasp the thin myofilaments. When myosin heads are attached to actin they are called crossbridges. The phosphate group is released in this step.

8. Myosin heads rotate and pull the thin myofilaments a little way toward the center of the sarcomere (M line)—this is called the power stroke. The ADP is released in this step.

9. Some of the myosin heads release as a new ATP binds to them. They break down this new ATP and get a new grip on the thin myofilament further down it—the power stroke is rapidly repeated over and over, each time pulling the thin myofilaments further toward the center.

10. When maximum overlap has been achieved, repeated power strokes must still go on to maintain the contraction.

11. Power strokes can continue as long as ATP is available and calcium levels remain high.

               Steps  6 – 9 above are known as the contraction cycle

 

 

RELAXATION

1. Acety;choline is rapidly broken down by an enzyme named acetylcholinesterase in the synaptic cleft, so unless nerve impulses continue to cause the release of more acetylcholine, the acetycholine quickly disappears.

2. Without acetylcholine bound to the receptors, the sarcolemma becomes impermeable to Na+. The extra Na+ ions are pumped out of the cell and the action potential is over.

3. Calcium ions are pumped out of the sarcoplasm back into the SR. The Ca2+ ions bind to a protein called calsequestrin inside the SR, so that a large amount of calcium can be stored.

4. As ccalcium levels in the sarcoplasm drop, the troponin no longer has calcium bound to it, so it no longer pulls on the tropomyosin. This means that the tropomyosin covers the myosin binding sites again.

5. Myosin heads can no longer grasp the thin myofilaments, so they slip back to the original resting position.

6. Sarcomere returns to its original length.

 

 

ADDITIONAL FACTORS INVOLVED IN CONTRACTIONS

 

1. Length-tension relationship--the length of the sarcomeres in a muscle before a contraction begins can influence the tension (force) the muscle can generate. The greatest tension can occur when the overlap between thick filaments and thin filaments goes from the edge of the H zone to the end of a thick filament. This optimum overlap places a maximum number of myosin heads in contact with thin myofilaments.

   Too much stretch--few myosin heads in contact

   Shortened sarcomere--compression of thick myofilaments causes them to crumple so there are also fewer myosin heads in contact.

 

2. Active and passive tension---muscle tension is mostly active tension, which is generated by the thick and thin filaments (contractile elements). However, the elastic components of muscle, such as titin filaments, connective tissue wrappings, and tendons stretch a little and pull back as the contraction begins. This is passive tension.

 

 

 

 

 

 

MUSCLE METABOLISM

 

Large amounts of ATP are required for muscle activity. This ATP comes from several sources:

     1. Stored ATP—mu scle fibers do not store much ATP—only a few seconds worth, but this is enough for some activities.

     2. Creatine phosphate—muscle fibers contain a special molecule, creatine phosphate, which is synthesized while the muscle is resting as an enzyme, creatine kinase, takes a phosphate from ATP and transfers it to creatine.  The resulting creatine phosphate can quickly transfer its high-energy phosphate group to ADP (created as ATP is broken down) when more ATP is needed. This is called the phosphagen system and provides energy for short-term maximal contractions (about 15 seconds).

   3. Anaerobic cellular respiration—If  muscle activity continues longer than this, glucose must be broken down to generate ATP. Muscle cells may obtain glucose by facilitated diffusion from the bloodstream or from the conversion of glycogen stored in the muscle fiber to glucose.

 

A series of ten reactions called glycolysis occurs in the cytosol and begins the breakdown of the glucose molecule. In these reactions, one glucose molecule is converted into 2 pyruvic acid molecules and releases 2 ATPs in the process. This does not require oxygen (anaerobic). Relatively small amounts of ATP are produced, but this can provide energy for about 30-40 seconds of activity.

 

Nature intends for this pyruvic acid to then enter the mitochondria and be further broken down there. This requires oxygen. If there is not enough oxygen to allow all the pyruvic acid to be used this way, some of it is converted to lactic acid. This produces the burning sensation we feel in muscles during strenuous activity.

 

Not all the lactic acid produced remains in the muscle. Most of it diffuses out into the blood. Liver cells can convert some of this back to glucose.

 

   4. Aerobic cellular respiration---the most efficient way to produce large amounts of ATP is a series of reactions called aerobic cellular respiration. These reactions occur in the mitochondria and require large amounts of oxygen. It takes about a full minute for the respiratory and cardiovascular systems to get going and begin to deliver this extra oxygen, so cellular respiration (aerobic energy production) cannot operate the instant muscle activity begins.     

 

Some of the large amount of oxygen needed diffuses into the muscle from the blood and

blood flow to skeletal muscles increases greatly during exercise. Also, in an effort to have extra oxygen available to muscles as soon as possible, muscle fibers contain myoglobin, which stores some oxygen when the muscle is at rest and can release it when needed. In exercise lasting more than 10 minutes, 90% of the energy is produced aerobically.

 

Summary: In aerobic cellular respiration, the pyruvic acid left at the end of glycolysis is further broken down in the mitochondria. With plenty of oxygen available, it ends up as carbon dioxide and water, with large amounts of ATP produced along the way. If there is not enough oxygen to allow complete breakdown of all the pyruvic acid, some of it is converted to lactic acid. Most of the lactic acid diffuses into the blood and is carried to the liver, which converts most of it back to glucose.

 

 

MUSCLE FATIGUE

This is the inability of muscle to maintain its strength of contraction or tension when pushed beyond its limit. Factors involved include inadequate release of Ca2+ ions from the SR, depletion of creatine phosphate, insufficient oxygen, depletion of glycogen, buildup of lactic acid, decreased release of acetylcholine, etc. Surprisingly, ATP levels may remain near normal.

     Central fatigue—feel tired and want to rest but can force ourselves to go on

     True muscle fatigue—involuntary relaxation

 

After exercise is stopped, heavy breathing continues--this is called oxygen debt or recovery oxygen uptake. Extra oxygen is needed to “undo” what occurred during muscular activity:

   1. Convert lactic acid to glycogen, which is stored in liver and muscle cells.

   2. Resynthesize creatine phosphate

   3. Replace stored oxygen in muscle (myoglobin)

   4. Maintain the higher rate of metabolic reactions that occurs throughout the body for a period of time following exercise

 

 

 

TWITCH CONTRACTIONS--a twitch contraction is a brief contraction of all muscle fibers in a motor unit in response to a single nerve impulse. It can also be produced in the lab by electrical stimulation, which is the way they are studied. A graph called a myogram shows events in the contraction:

 

 

                                                                                  Time is measured in

                                                                                  milliseconds (1 millisecond

                                                                                  equals 1/1000 of a second)

 

 

 

 

        1) Latent period--2 milliseconds—Ca2+ ions are being released and filaments begin to

            exert force 

         2) Contraction period--10-100 milliseconds

         3)  Relaxation period--10-100 milliseconds--active transport of Ca2+ ions back into

             the SR

                       (Durations are average--there is some variation among fibers)

 

If 2 stimuli are applied very close together, the muscle will respond to the first but not to the second. This period of lost excitability is known as the refractory period. It lasts about 5 milliseconds in skeletal muscle and ends long before the fiber relaxes. The refractory period is longer in cardiac muscle (300 milliseconds).

 

 

CONTROL OF MUSCLE TENSION

A single action potential in a motor neuron produces a single contraction in all the muscle fibers in that neuron's motor unit. The contraction is all-or-none because muscle fibers when stimulated to contract at all always contract to their fullest extent. There are no partial contractions of individual fibers. However, partial contractions (contractions of varying strength) of an entire muscle are possible by variations in:

   1. The frequency (timing) of stimulation

   2. The number of motor units involved

 

FREQUENCY OF STIMULATION

When two stimuli are applied so that the second arrives after the refractory period ends, the skeletal muscle fiber will respond to both stimuli. If the second stimulus arrives after

the refractory period but before the muscle has completely relaxed, the second contraction

will be stronger than the first. This is called wave (temporal) summation--stimuli arrive at

different times and cause larger contractions. This may be due to a buildup of Ca2+ ions in

the sarcoplasm.

 

TETANUS--sustained muscle contraction. If a muscle is stimulated 20-30 times per second, it partly relaxes between stimuli and the result is incomplete (unfused) tetanus. Stimulation 80-100 times per second causes complete (fused) tetanus. Relaxation

is either incomplete or does not occur at all. Most of our useful voluntary contractions are

tetanic contractions.

 

 

MOTOR UNIT RECRUITMENT

This is a major factor in producing contractions of varying strength. Under normal conditions, some motor units of a muscle are active while others are inactive. This ability to adjust the number of motor units firing at once is called recruitment (multiple motor unit summation). Varying the number of motor units recruited at once allows:

   a. Contractions of variable strength

   b. Sustained contractions with less fatigue--some fibers rest while others contract

and then they can switch places

   c. Smooth movements

 

MUSCLE TONE---Even relaxed skeletal muscles usually have a few muscle fibers contracted (different groups of fibers at different times). This firms up the muscle without producing movement. Muscle tone is regulated by the number of motor units involved and is essential for posture.

 

 

ISOTONIC & ISOMETRIC CONTRACTIONS

      Isotonic contractions--occur when muscle contraction produces shortening of the muscle and movement. Tension remains almost constant.

                Concentric phase---muscle shortens—pick up book

                Eccentric phase---muscle lengthens while contraction is still occurring---putting

                          the book down slowly

       Isometric contractions--muscle does not shorten but tension increases greatly--no movement but energy is expended. This type of contraction is used to maintain posture and support objects in fixed positions.

 

 

 

TYPES OF SKELETAL MUSCLE FIBERS

 

All skeletal muscle fibers are not exactly alike. The types are:

   1. Slow oxidative (SO) fibers—have the smallest diameter and are the least powerful type. These contain large amounts of myoglobin and many mitochondria and capillaries. Have a red appearance (because of the myoglobin) and the capacity to slowly generate large amounts of ATP by aerobic cellular respiration. Once produced, this ATP is broken down relatively slowly. Fibers contract more slowly and are more resistant to fatigue. These are found in large numbers in postural muscles that hold up the head and neck, for example. They are also called slow twitch fibers.

 

2. Fast glycolytic  (FG) fibers---large fibers with the most myofibrils. They are capable of rapid powerful contractions but relatively quick to fatigue. These contain large amounts of the enzymes needed for glycolysis and fewer mitochondria and capillaries. (Most of their energy is produced by glycolysis.) Color is more pale--sometimes called white muscle. Also known as fast twitch fibers.

 

3. Fast oxidative-glycolytic (FOG) fibers—these are a sort of intermediate fiber. They are also called fatigue-resistant fast twitch fibers. They are red and are equipped to produce ATP by cellular respiration, like SO fibers,  but then break down the ATP faster than SO fibers could.  They also have the ability to produce considerable amounts of ATP by glycolysis.

 

Most skeletal muscles contain a mix of all three types of fibers. Often about half are SO fibers. The percentage of SO fibers increases in postural muscles of the neck, back, etc. Muscles such as those of the arms and shoulders tend to have more FG fibers. Leg muscles have mostly SO and FOG fibers. The exact proportion of these fiber types may vary among individuals, and this partly explains why certain athletes excel in certain events. It appears that changes in fiber type can occur to a limited extent. For example, regular endurance exercises can apparently cause some FG fibers to convert to FOGs.

 

             TABLE 10.1  P.  313 COMPARES THE 3 TYPES OF FIBERS

 

 

CARDIAC MUSCLE TISSUE

This makes up most of the heart wall. It is striated but involuntary and displays autorhythmicity (the ability to contract without nerve stimulation). Fibers are almost square and usually have a single centrally-located nucleus. They have abundant sarcoplasm and large, numerous mitochondria. Cardiac muscle fibers have sarcomeres with the same structure as skeletal, and contraction occurs by the same processes, except that nerve impulses and acetylcholine are not required.

 

Fibers branch and interconnect with each other, forming TWO separate networks, an upper (both atria) and a lower (both ventricles). Each fiber in a network is connected to neighboring fibers by thickenings of the sarcolemma called intercalated discs. The discs contain gap junctions, which allow muscle action potentials to spread from one fiber to another. When one fiber of the network is stimulated, all become stimulated and contract as a unit.

 

Under normal conditions, cardiac muscle contracts about 75 times per minute without resting. ATP is generated almost entirely aerobically. Cardiac muscle contracts due to a conduction system of specialized muscle tissue within the heart. Nerve impulses can increase or decrease the rate, but do not set the basic rhythm.

 

Cardiac muscle remains contracted 10-15 times longer than skeletal. The refractory period is longer to be sure to allow time for the heart to relax between beats (for rest and filling with blood). Heart muscle cannot undergo tetanus due to this prolonged refractory period.

 

 

 

SMOOTH MUSCLE TISSUE

Two types of smooth muscle:

     1. Visceral (single-unit) smooth muscle tissue forms sheets in the walls of small arteries and veins and hollow viscera. Gap junctions between fibers form networks through which muscle action potentials can spread, so the network contracts as a unit. This type shows autorhythmicity, but to a lesser extent than cardiac muscle.

 

     2. Multiunit smooth muscle tissue is found in the walls of large arteries, large air passages in the lungs, arrector pili muscles and eye muscles. Each individual fiber has its own nerve supply and must be stimulated individually.

 

Both types of smooth muscle are nonstriated and involuntary. Fibers are smaller than skeletal and spindle-shaped with a single centrally-located nucleus. Thick and thin myofilaments are present but not in the orderly sarcomere arrangement of skeletal muscle. Also present are intermediate filaments, which attach to structures called dense bodies (skeletal and cardiac don’t have either of these.). During contraction, thick and thin myofilaments generate tension that is transmitted to the intermediate filaments, which pull on the dense bodies and shorten the fiber. Current thinking is that fibers twist into a helix as they shorten.

 

Compared to skeletal muscle, smooth muscle fibers contract more slowly and contractions last much longer. Smooth muscle can shorten and stretch to a greater extent. Basic mechanisms of contraction are similar and involve an increase in Ca2+ levels in the sarcoplasm, although this occurs more slowly. Ca2+ ions bind to the regulatory protein calmodulin, which in turn activates an enzyme, myosin light chain kinase.  The enzyme splits ATP to activate the myosin heads of the thick filaments.

 

Removal of the Ca2+ from the sarcoplasm is also slower, which slows relaxation and provides for smooth muscle tone, a state of sustained partial contraction. Smooth muscle fibers can stretch considerably without developing tension (the stress-relaxation response). This allows the stomach, urinary bladder, etc. to stretch without putting pressure on the contents.

 

 

REGENERATION OF MUSCLE TISSUE

   1. Cardiac—until recently believed that there was no regeneration at all but we now know that under certain conditions cardiac muscle can regenerate to some extent

   2. Skeletal—limited powers of regeneration—fibers do not divide after 6 mos - 1 year of age. Limited numbers of stem cells called satellite cells can develop into new fibers after an injury. Most of the healing occurs by scar tissue formation (fibrosis).

   3. Smooth—mre regeneration than other muscle. Some smooth muscle cells retain the ability to divide. Stem cells called pericytes can also develop into smooth muscle fibers.

 

SUMMARY OF FEATURES OF VARIOUS TYPES OF MUSCLE

 

 

SKELETAL

CARDIAC

SMOOTH

MICROSCOPIC APPEARANCE & FEATURES

LONG CYLINDRICAL WITH MULTIPLE NUCELI

BRANCHED WITH INTERCALATED DISCS—ONE NUCLEUS PER FIBER

SPINDLE-SHAPED, ONE NUCLEUS PER FIBER

STRIATIONS

YES

YES

NO

THICK & THIN FILAMENTS

YES

YES

YES

LOCATION

MOSTLY ATTACHED TO BONES BY TENDONS

HEART

INTERNAL—MOSTLY IN THE WALLS OF HOLLOW ORGANS