Chapter 9
Muscle Physiology
Lecture # 23, 24, 25
Objectives: 1. Describe the microscopic structure and functional roles of the myofibrils, sarcoplasmic reticulum, and T
tubules of skeletal muscle fibers (cells).
2. Explain the sliding filament mechanism of skeletal muscle contraction and describe the events
occurring at the neuromuscular junction.
3. Define motor unit and explain how muscle fibers are stimulated to contract.
4. Differentiate between isometric and isotonic contractions.
5. Define muscle twitch and describe the events occurring during its three phases.
6. Explain how smooth, graded contractions of a skeletal muscle are produced.
7. Describe three ways in which ATP is regenerated during skeletal muscle contraction.
8. Define oxygen debt and muscle fatigue. List possible causes of muscle fatigue.
9. Name and describe three types of skeletal muscle fibers. Explain the relative value of each fiber type
in the body.
10. Compare and contrast the effects of aerobic and resistance exercise on skeletal muscles and on other
body systems.
Sarcolemma: cell membrane of muscle fiber
Sarcoplasm: cytoplasm of muscle fiber
Myofibril: contractile elements inside muscle fiber
Sarcomere: contractile unit of myofibril, consists of actin and myosin
I. A Band: dark band, made of actin and myosin
II. I Band: light band, made only of actin w/ Z disc in middle
III. H Zone: in middle of A band, disappears as sarcomere shortens
IV. M Line: found in very middle of H zone (and A band)
V. Z Disc: boundary of sarcomere; hold thick and thin filaments in place
VI. Myofilament: contractile elements of myofibril
A. Thin filaments: lateral elements of sarcomere made of actin. In I band and part of A band
B. Thick filaments: central elements of sarcomere made of myosin. All in A band
C. Elastic filaments: holds thick filaments in place and help muscle spring back after stretching
VII. Myosin: comprise the thick filament; each molecule has a rod-like tail and two globular heads. The heads link thick
and thin filaments during contraction.
VIII. Actin: comprise the thin filaments; globular molecules that form a twisted double strand of “beads”
IX. Tropomyosin: rod-shaped protein that spirals around actin filament and during relaxation it blocks the actin binding
sites.
X. Troponin: a complex that binds to actin, tropomyosin, and calcium. Troponin will move tropomyosin out of the way
during contraction.
XI. Sarcoplasmic reticulum (SR): smooth endoplasmic reticulum around each myofibril that stores calcium
necessary for muscle contraction.
XII. T tubule: tubular extensions of sarcolemma that penetrate into cell to conduct impulses to all myofibrils, and then
to the SR so that calcium is released.
Physiology of Muscle Contraction
I. Neuromuscular Junction and Nerve Stimulus: skeletal muscle cells are stimulated by motor neurons. Their
axons travel from the brain or spinal cord to skeletal muscles they innervate. Each muscle fiber has one neuromuscular
junction, near its middle
A. Motor end plate: a trough-like area on the sarcolemma to increase surface area for ACh receptors. It is
found on the opposite side of the synaptic cleft from the axon.
B. Acetylcholine: a neurotransmitter released from the axon terminal by exocytosis. When the impulse
reaches the axon, calcium channels open, calcium enters, and causes synaptic vesicles to fuse with axonal membrane and
release ACh.
C. Binding of ACh to receptors: ACh diffuses across the synaptic cleft and binds to ACh receptors on the
sarcolemma. After it binds ti is broken down by AChE (acetylcholinesterase) to prevent continued muscle fiber
contraction
II. Excitation contraction coupling: sequence of events from depolarization of sarcolemma to sliding of
myofilaments
A. Latent period: between action potential initiation and beginning of shortening. Following a stimulus there is
a period of seemingly no activity before the sarcomere shortens.
B. Events of contraction: see handout and lab activity
Contraction of Skeletal Muscle
I. Motor Unit: made of one motor neuron and all the muscle fibers it innervates. When a motor neuron fires, all the
muscle cells it innervates contract completely (all or none law). Muscles of fine control have many small motor units (few
fibers per unit). Muscles for weight bearing have few large motor units (many fibers per unit).
II. Isometric contraction: tension (or force) builds in a muscle but the muscle neither shortens nor lengthens, and no
movement occurs at the insertion (ie. Lifting something too heavy, pushing against a wall). Postural muscles and fixators
contract isometrically. Cross bridges form between actin and myosin, but the filaments do not slide.
III. Isotonic contraction: muscle length changes and moves the load (the insertion moves). Once enough tension
develops to move a load, the tension then remains constant.
Muscle Twitch: the response of a motor unit to a single action potential of its motor neuron. The fibers quickly
contract and then relax. A twitch has 3 parts:
I. Latent period: between stimulation and contraction, when excitation-contraction coupling is occurring. Muscle
tension begins but there is no tracing on the myogram.
II. Period of contraction: when cross bridges are active, the muscle reaches peak of tension development and tracing
rises to a peak. If tension (pull) exceeds load, movement occurs.
III. Period of relaxation: begins with the reentry of calcium into the sarcoplasmic reticulum. Tension decreases to
zero and tracing returns to baseline and muscle can return to initial length.
Graded Muscle Responses: smooth contractions of varying strength.
I. Wave or Temporal summation: stimuli received in rapid succession at a muscle produce stronger contractions
after each stimulus. More calcium is present since the muscle hasn’t completely relaxed (but if a stimulus comes before
repolarization no summation occurs). If stimulus strength is constant, but rate increases, a sustained but quivering
incomplete tetanus occurs. If all muscle relaxation disappears and the contraction is smooth and sustained, then
complete tetanus occurs.
II. Multiple motor unit summation/recruitment: increasing stimulus voltage calls more and more muscle fibers
into play. Threshold stimuli produce the first observable contraction. As stimulus strength increases to maximal stimulus,
all motor unit become recruited. Stimulus intensity beyond maximal does not produce further contraction.
III. Treppe: beginning contractions in a muscle may only be half as strong as those occurring later on in an activity,
due to more calcium available in the sarcoplasm in later contractions. Heat is also generated and enzymes become more
efficient and muscle more pliable. This is why athletes warm up before a competition or game.
IV. Tone/Tonus: spinal reflexes activate alternating motor units within muscles, even relaxed ones, so that muscles are
almost always slightly contracted. Muscle tone keeps muscles firm, healthy, and ready to respond to stimulation, as well
as stabilize joints and maintain posture. Without it, muscles become flaccid and atrophy.
Muscle Metabolism: ATP provides the energy for cross bridge movement and detachment and calcium pump
I. Creatine phosphate: an almost instant transfer of energy and a phosphate group from CP to ADP. Cells store more
CP than ATP. Supplies last 10-15 seconds. Used in a surge of force.
II. Anaerobic respiration: the conversion of glucose to pyruvic acid and then to lactic acid yields 2 ATP/glucose.
Lactic acid contributes to muscle fatigue and part of muscle soreness. Lasts 30-40 seconds. Used during on/off,
start/stop activities.
III. Aerobic respiration: glucose + oxygen  carbon dioxide + water + ATP (36 ATP/glucose). Supplies during rest
and light to moderate exercise. Requires a continuous delivery of oxygen and nutrient fuels. Lasts hours.
IV. Muscle fatigue (psychological vs. physiological) and oxygen debt: state of physiological inability to
contract even though stimuli may still be present. Psychological fatigue occurs when we think we can’t contract our
muscles anymore. Physiological results from a relative deficit of ATP.
Oxygen debt is the extra amount of oxygen that the body must take in to restore levels to normal. We borrow oxygen
from other sources during exertion and we must replace it, usually by breathing rapidly and deeply after exercise.
V. Physiological contracture (cramp): states of continuous contraction when no ATP is available because cross
bridges cannot detach (like occurs after death in rigor mortis). Can also occur when there is too much calcium.
Velocity and Duration of Contraction: our muscles contain a mixture of three fiber types that determine this.
Also, muscle fibers containing varying amounts of myoglobin, which stores oxygen in muscles and myosin ATPase varies
in the muscle types.
I. Slow oxidative fibers: slow contraction, aerobic respiration, high myoglobin, low glycogen stores, fatigue-resistant,
endurance type activities, red color (due to myoglobin), small diameter fibers, many mitochondria, many capillaries.
II. Fast oxidative fibers: fast contraction, aerobic/anaerobic respiration, high myoglobin, intermediate glycogen
stores, moderately fatigue-resistant, sprinting/walking exercise, red to pink color, medium diameter, many mitochondria,
many capillaries.
III. Fast glycolytic fibers: fast contraction, anaerobic glycolysis, low myoglobin, high glycogen stores, fatigue quickly,
short-term intense or powerful movements. White fibers, large diameter, few mitochondria, few capillaries.
Effect of Exercise: aerobic/endurance exercise: increases capillaries, mitochondria, and myoglobin.
Resistance exercise: muscle hypertrophy, increase size of muscle fibers, more mitochondria, more myofilaments and
myofibrils, store more glycogen, more connective tissue.