1

Muscular System

A. MUSCLE TYPES1. The types of muscle are categorized by

LOCATION, HISTOLOGY, and MODES OF CONTROL.

2. There are three recognizable types of muscle: SKELETAL, CARDIAC and SMOOTH.

2

• SKELETAL MUSCLE tissue is primarily attached to the bones. It is striated and under voluntary neural control.

3

Skeletal Muscles•

4

Smooth Muscle• is located in the major

organs and the walls of the blood vessels. It is non-striated and under involuntary

5

Cardiac Muscle

CARDIAC MUSCLE muscle lining the walls of the heart. It is striated and under involuntary control.

6

B. CHARACTERISTICS AND FUNCTIONS OF MUSCLE TISSUE

1. Muscles are EXCITABLE – they

are capable of receiving and responding to stimuli.

2. Muscles exhibit CONTRACTILITY- the ability to shorten and thicken.

7

• 3. Muscles exhibit EXTENSIBILITY - the ability to be stretched.

• 4. Muscles exhibit ELASTICITY – the ability to return to their original shape after contraction or extension.

8

D. SKELETAL MUSCLE CONTRACTION

1. The mechanics of muscle contraction follow the SLIDING-FILAMENT THEORY.

9

• A muscle ACTION POTENTIAL travels over the SARCOLEMMA and enters the TRANSVERSE TUBULES. This affects the SARCOPLASMIC RETICULUM and causes the release of calcium ions into the SARCOPLASM.

10

• 3. The MUSCLE ACTION POTENTIAL leads to the RELEASE OF CALCIUM ions from the SARCOPLASMIC RETICULUM which triggers the contractile response.

11

D. SKELETAL MUSCLE CONTRACTION

4. Actual contraction is brought about when the thin ACTIN MYOFILAMENTS of the sarcomere SLIDE toward one another and OVER THE THICK MYOSIN MYOFILAMENTS. During this process the myosin cross-bridges pull on the actin myofilaments.

12

• 5. A MOTOR NEURON is responsible for transmitting the nerve impulse (action potential) to a skeletal muscle where it serves as the stimulus for contraction.

13

• 6. The point at which the motor neuron and the muscle sarcolemma meet is referred to as the NEUROMUSCULAR JUNCTION.

14

D. SKELETAL MUSCLE CONTRACTION

7. A chemical NEUROTRANSMITTER released at neuromuscular junctions in skeletal muscle is ACETYLCHOLINE (ACH).8. The MOTOR (EFFERENT ) NEURON and all of the muscle fibers it innervates form a MOTOR UNIT.9. A single motor unit may inervate as few as 10 or as many as 2000 muscle fibers although the average is approximately 150.

15

D. SKELETAL MUSCLE CONTRACTION

10. The number of motor units that fire in a muscle at any one time is the basis for the variability in contraction.11. The process of increasing the number of motor units firing at any one time is called RECRUITMENT.

16

E. PHYSIOLOGY OF SKELETAL MUSCLE CONTRACTION

1. When a nerve impulse, or ACTION POTENTIAL, reaches the AXON TERMINAL, the SYNAPTIIC VESICLES in the axon terminal release AcH.

2. The release of AcH ultimately INITIATES a muscle ACTION POTENTIAL in the muscle fiber SARCOLEMMA.

3.The muscle ACTION POTENTIAL travels into the TRANSVERSE TUBULES and causes the SARCOPLASMIC RECTICULUM to release STORED CALCIUM ions into the SARCOPLASM.

17

E. PHYSIOLOGY OF SKELETAL MUSCLE CONTRACTION

4. The released calcium combines with TROPONIN which to pulls on the tropomyosin filaments and changes its orientation. This exposes the myosin-binding sites on the actin myofilament.

5. Splitting ATP with ATPASE into ADP+ phosphate releases energy which activates the myosin cross-bridges.

18

E. PHYSIOLOGY OF SKELETAL MUSCLE CONTRACTION

6. The ACTIVATED CROSS-BRIGES attach to the actin myofilament, and a change in the orientation of the cross-bridges occurs. This is called a POWER STROKE.

7. The POWER STROKE RESULTS in the SLIDING OF THE THIN ACTIN MYPFILAMENTS.

19

E. PHYSIOLOGY OF SKELETAL MUSCEL CONTRACTION

8.Repeated detachment and reattachment of the cross-bridges results in the shortening (ISOTONIC CONTRACTION) or increased tension without shortening (ISOMETRIC CONTRACTION) of the muscle.

9. The enzyme, acetylcholinesterase (AcHE), into the neuromuscular junction, destroys acetylcholine and stops the generation of a muscle action potential.

20

E. PHYSIOLOGY OF SKELETAL MUSCLE CONTRACTION

10. Calcium ions are then resorbed into the sarcoplasmic reticulum, exposing the troponin. This causes the cross-bridges to separate and the muscle fober resumes its resting state.

21

F. ENERGY NEEDED FOR CONTRACTION

1. The immediate and direct source of energy ofr muscle

contraction is ATP.2. PHOSPHOCREATINE and

the metabolism of GLYCOGEN and fats are

necessary for the continuous generation of

ATP by muscle fibers.

22

F. ENERGY NEEDED FOR CONTRACTION

3. PHOSPHOCREATINE and ATP comprise the PHOSPHAGEN SYSTEM, which allows for initial contraction and short bursts of maximal contraction lasting up to fifteen seconds.

4. The GLYCOGEN-LACTIC ACID SYSTEM provides energy via glycolysis. This is an anaerobic process which will allow thirty to fourty seconds of maximum muscular contraction after the phosphocreatine supply is exhausted.

23

F. ENERGY NEEDED FOR CONTRACTION

5. An AEROBIC RESPIRATION and GLYCOLYSIS are used for prolonged muscular contraction and will function efficiently as long as oxygen and nutrients are present in adequate amounts.

24

G. THE ALL-OR-NONE PRINCIPLE

1. The weakest stimulus capable of causing contraction in a motor unit is called a LIMINAL or THRESHOLD STIMULUS.

2. A stimulus not capable of inducing contraction is called a SUBLIMINAL OR SUBTHRESHOLD STIMULUS.

25

G. THE ALL-OR-NONE PRINCIPLE

3.The ALL-OR-NONE PRINCIPLE states that once a threshold or liminal stimulus is applied, all of the individual muscle fibers of a motor unit will contract to their greatest extent. Any further increase in the degree of stimulation will not cause a corresponding increase in muscle contraction.

26

H. TYPES OF SKELETAL MUSCLE CONTRACTION

1. Skeletal muscles are capable of producing several different types of contractions, depending upon the strength and frequency of stimulation.

2. The various kinds of contractions are TWITCH, TETANUS, TREPPE, ISOTONIC and ISOMETRIC.

27

H. TYPES OF SKELETAL MUSCLE CONTRACTION

3. A MYOGRAM is a record of muscular contraction that shows the various phases involved in the various phases of muscular contraction.

4. The three major phases arte the LATENT PERIOD, CONTRACTION PERIOD and RELAXATION OR REFRACTORY PERIOD.

28

H. TYPES OF SKELETAL MUSCLE CONTRACTION

5. The LATENT PERIOD is the time lapse between the action potential and the response of the muscle fibers.

6. The CONTRACTION PERIOD it’s the time frame in which the muscle fibers are undergoing shortening.

7. The REFRACTORY PERIOD is that time frame when a muscle has temporarily lost its excitability.

29

H. TYPES OF SKELETAL MUSCLE CONTRACTION

8. Skeletal muscles have short refractory periods.

9. WAVE SUMMATION results in an increase in the strength of muscle contraction. It occurs when a second stimulus is applied to the muscle before it has completely relaxed after the previous stimulus.

30

H. TYPES OF SKELETAL MUSCLE CONTRACTION

10. A sustained partial contraction of portions of skeletal muscle results in muscle tone and occurs even in relaxed or uncontracted muscle.

11. MUSCLE TONE is essential for the maintenance of posture.

12. HYPOTONIA is a decrease in muscle tone. HYPERTONIA is an ncrease in muscle tone.

31

H. TYPES OF SKELETAL MUSCLE CONTRACTION

13. MUSCULAR ATROPHY refers to the wasting away of muscle. Muscular hypertrophy refers to the increase in the diameter of muscle fibers, resulting from enlargement or overgrowth.

32

I. SKELETAL MUSCLES AND MOVEMENT

1. Skeletal muscles produce movements by exerting force on tendons, which in turn pull bones or other structures, such as skin.

2. Most muscles cross at least one Joint and are attached to the articulating bones that form the Joint. The bones serve as levers while the Joints serve as fulcrums.

3. When a muscle contracts it draws a movable bone toward a stationary one.

33

I. SKELETAL MUSCLES AND MOVEMENT

4. The ORIGIN is the point of attachment on the stationary bones while the INSERTION is the point of attachment on movable bones.

5. Skeletal muscle fibers (cells) are arranged in a parallel fashion, in bundles called

FASCICULI.6. The FASCICULI are arranged relative to the

tendons in one of four arrangements: parallel, convergent, pennate or circular.

34

I. SKELETAL MUSCLES AND MOVEMENT

7. Fascicular arrangement can be correlated to muscle strength and range of movement.

35

J. CARDIAC MUSCLE1. CARDIAC MUSCLE is found only in the

HEART. It is striated and neurologically involuntary.

2. The muscle fibers are quadrangular and generally contain a single, centrally located nucleus.

36

J. CARDIAC MUSCLE3. Compared to skeletal muscle, cardiac

muscle contains more sarcoplasm, mitochondria, larger transverse tubules and a less developed sarcoplasmic reticulum.

4. The myofilaments are not arranged in discrete myofibrils.

5. Muscle fibers branch freely and are connected via gap junctions

37

J. CARDIAC MUSCLE6. Cardiac muscle cells are connected via

INTERCALATED DISCS which aid in the conduction of muscle action potentials by way of the gap junctions located at the discs.

7. Cardiac muscle contracts and relaxes rapidly, continuously, and rhythmically.

8. Energy for contraction is supplied by large, numerous mitochondria and through the catabolism of fats and glycogen.

38

J. CARDIAC MUSCLE9. Cardiac muscle is MYOGENIC, it can

contract without extrinsic stimulation and can remain contracted for sustained periods of time.

10. Cardiac muscle exhibits a long refractory period, referred to as a PLATEAU POTENTIAL which prevents tetanus of the muscle.

39

K. SMOOTH MUSCLE

1. SMOOTH MUSCLE is non-striated and involuntary. It is also referred to as VISCERAL muscle.

2. The smooth muscle fibers are elongated, tapered fibers with a single, centrally located nucleus. They contain actin and myosin myofilaments, but unlike skeletal muscle, the filaments are not arranged in an orderly fashion

40

K. SMOOTH MUSCLE

3. VISCERAL or single unit smooth muscle, is found in the walls of the viscera as well as the walls of both small arteries and veins.

4. MULTIUNIT SMOOTH MUSCLE is found in large blood vessels, large airways and in the eyes. The muscle fibers operate singly, rather than as a unit.

41

K. SMOOTH MUSCLE

5. Compared to skeletal muscle, smooth muscle displays a longer duration of contraction and relaxation.

6. Smooth muscle fibers respond to nerve impulses, hormones and local chemical factors.

7. Smooth muscle fibers are very extensible without developing tension.

42

L. GROUP ACTIONS OF SKELETAL MUSCLE

1.Most movements are coordinated by several skeletal muscles acting in groups rather than individually.

2.Most skeletal muscles are arranged in opposing pairs at joints.

3.The muscle that causes a desired movement is called the AGONIST; the ANTAGONIST produces the opposite action.

43

L. GROUP ACTIONS OF SKELETAL MUSCLE

4. Most movements also involve muscles called SYNERGISTS. These function to steady movements and to aid the agonist in functioning more efficiently.

5. Muscles can also be FIXATORS which stabilize the origin of the agonist so that it can operate more efficiently.

6. Most muscles can act as AGONISTS, ANTAGONISTS, SYNERGISTS or FIXATORS depending on the movement.

44

M. NAMING SKELETAL MUSCLES

1. There are nearly 700 skeletal muscles. Their names are based on several characteristics. The muscle names may indicate the DIRECTION of MUSCLE FIBERS or their SIZE, SHAPE, ACTION, SITE of ORIGIN or ORIGIN and INSERTION POINTS.

REFER TO THE TABLES IN THE CHAPTER AS A STUDY GUIDE.