The Muscular System
I. Muscle tissue
A. There are 3 types of muscle tissue in the body
1. Skeletal muscle
a. "red meat" attached to bones -- it is 40-50% of body weight
b. striated and multinucleate
c. voluntary - we control the contractions
2. Cardiac muscle
a. composes bulk of heart
b. striated and branched, one nucleus
c. involuntary
3. Smooth or Visceral muscle
a. found in walls of hollow visceral structures (organs) such as digestive tract, blood vessels, and ureters
b. nonstriated (appears smooth), one nucleus
c. involuntary
B. All muscle cells specialize in contraction
II. Characteristics of muscles
A. Contractibility - the ability to shorten, unique to muscle tissue***
B. Excitability- the ability to respond to certain stimuli
C. Extensibility- the ability to be stretched
D. Elasticity- the ability to return to its original length when relaxing
III. Functions of skeletal muscle
A. Movement
1. Muscles must be attached to bones (by tendons) for leverage in order to have something to pull against
2. Muscles only pull, never push
3. The muscles of the body are arranged in antagonist pairs.
a. one produces mvmt in a single direction called the prime mover
b. the other in the opposite direction called the antagonist
4. Another group help steady or stabililze and are called synergists.
B. Posture or muscle tone
1. A specialized type of m contraction, called tonic contraction, enables us to maintain body position
a. only a few of a muscle's fibers shorten at one time
b. produce no mvmt of body parts
c. maintain m tone called posture
2. Good vs bad posture
C. Heat production
1. Survival depends on the body's ability to maintain a constant body temp
2. Contraction of m fibers produces most of the heat required to maintain normal body temp
IV. Structure of Skeletal muscles
A. Macroscopic structure
1. Each skeletal muscle is an organ composed mainly of skeletal muscle cells and connective tissue
2. Most skeletal muscles extend from one bone across a joint to another bone
3. Parts of a skeletal muscle
a. Origin-attachment to the bone that doesn't move (much)
b. Insertion-attachment to the bone that moves during contraction
c. Body-main part of the muscle
4. Muscles attach to bone by tendons-strong cords of fibrous connective tissue
a. This fibrous tissue is called fascia (aka epimysium)
b. If the tissue forms sheets the tendons are called aponeuroses
c. Tendons attach to periosteum of the bone
B. Microstructure
1. Muscle cells, called muscle fibers, are long, cylindrical cells
a. sometimes run the entire length of the muscle
b. are about the diameter of a human hair
2. Muscle fibers are bundled together in groups of up to 150 fibers
a. the bundles are called fasciculi
b. fasciculi are surrounded by perimysium
3. Each fiber is surrounded by endomysium
4. The parts inside a m. cell have different names
a. M. cell membrane is called the sarcolemma
b. cytoplasm is called sarcoplasm
c. endoplasmic reticulum is the sarcoplasmic reticulum
5. Inside the sarcoplasm are hundreds to thousands of myofibrils
a. each myofibril is 1/100 the diameter of human hair
b. contain the apparatus that contracts the m. cell
c. myofibrils hold two types of myofilaments: actin and myosin
d. myosin are thick filaments (1/10,000 diameter of hair) and have cross-bridges.
e. actin are thin but arranged in a double helix (like DNA)
f. myosin and actin are arranged longitudinally in the smallest contractile unit of skeletal muscle called the sarcomere
C. Motor unit
1. Stimulation of a muscle by a nerve impulse is required before a muscle can shorten and produce mvmt.
2. A motor neuron is this type of nerve
3. A neuromuscular junction is the point of contact between a nerve ending and the m fiber it innervates
4. A motor unit is the combination of a motor neuron with the muscle cell or cells that it innervates
5. All or none law
V. Sliding Filament Theory (SFT) of muscle contraction
A. When the myofilaments myosin and actin slide toward each other the muscle shortens and when the filaments slide apart the muscle relaxes
B. This occurs in the sarcomere
C. Contraction requires calcium and energy-rich ATP molecules
D. refer to handout "how muscles contract" and highlight
VI. Muscle fatigue
A. If muscle cells are stimulated repeatedly w/o adequate rest, the strength of m contraction decreases, resulting in fatigue.
B. If stimulation continues the muscle eventually loses its ability to contract
C. During exercise, stored ATP becomes depleted but more can be made with rapid consumption of oxygen taken from the blood cells
D. When no more oxygen is available, the muscles switch to anaerobic energy but this produces lactic acid and creates muscle soreness
E. When exercise stops, metabolism and labored breathing remain high to "pay the debt" for using all the oxygen -- this is oxygen debt
VII. Types of skeletal muscle contractions
A. Twitch contraction
1. quick, jerky response to a stimulus
2. seen in isolated muscles during research
3. normalmvmt requires smooth and sustained contraction, not jerky
B. Tetanic contraction
1. steady and sustained contraction
2. produced by a series of stimuli bombarding the muscle
3. contractions "melt" together to cause tetanus
C. Isotonic contraction
1. contraction of a muscle that produces mvmt at a joint
2. if the muscle lengthens it is called an eccentric contraction
3. if the muscle shortens it is called a concentric contraction
4. most types of body mvmts are caused by isotonic contractions
D. Isometric contraction
1. contraction of a muscle that does not produce mvmt
2. tension within the muscle does increase
3. important for rehabilitation if immobilized
VIII. Effects of exercise on skeletal muscles
A. Exercise should be done on a regular schedule and done correctly
1. improves muscle tone and posture
2. results in more efficient heart and lung functioning
3. reduces fatigue
4. regular exercise increases muscle size, called hypertrophy
5. prolonged inactivity causes disuse atrophy
B. Strength training is exercise involving contraction of muscle against heavy resistance
1. increases the size of the muscle fiber
2. cannot change the number of muscle fibers
3. sometimes called anaerobic training
C. Endurance training is exercise that increases a muscle's ability to sustain moderate exercise over a long period
1. sometimes called aerobic training
2. does not usually increase the size of the muscle fiber
3. makes a muscle more efficient in the delivery of oxygen and nutrients