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73 Cards in this Set

  • Front
  • Back
What are the four factors that influence the force of skeletal muscle contraction?
1. number of fibers activated
2. increased muscle size = ↑ muscle fiber diameter
3. ↑ frequency of stimulation
4. length-tension relationship
When whole muscle contracts → ___ do all muscle fibers generate force
motor unit
motor neuron & all muscle fibers it supplies (4 to 1000's)
• when motor neuron fires (action potential) → all muscle fibers it innervates contract
Motor Unit Recruitment
an increase in # of active motor units
• ↑ motor unit recruitment = ↑ force of contraction
↑ motor unit recruitment =
↑ force of contraction
Motor units differ in size
Motor unit X = 5 fibers → less force
Motor unit Y = 7 fibers → more force
Small motor units
fine control, less force per motor unit
Large motor units
course control, greater force per unit area
eye muscles use ___ motor units
hip muscles use ___ motor units
The "bulkier" the muscle (greater cross sectional area) →
• the more tension it can develop
• Resistance exercise increases muscle force by causing muscle cells to hypertrophy (↑ size)

In general: an increase in muscle (organ) size is due to an increase in the diameter of the muscle fibers (cells) → rather than an increase in the number of muscle fibers
Frequency of Stimulation
A _____ neural stimulation produces:
∙ a single contraction or twitch lasts ~ 7-100 msec
Sustained muscular contractions require
many repeated stimuli
What is a myogram?
graph of twitch tension development
What are the 3 phases of a muscle twitch?
Latent period, Contraction phase, and Relaxation phase
What is the Latent period?
before contraction
• AP moves across sarcolemma causing Ca²+ release
What is the Contraction phase?
Ca²+ binds to troponin → tension builds to peak
What is the Relaxation phase?
Ca²+ levels fall → active sites covered; tension falls to resting levels
A _____ stimulus results in a _____ contractile response (muscle twitch)
More rapidly delivered stimuli result in wave summation or incomplete tetanus

If stimuli are quick enough → complete tetanus results
Single stimulus = _____
one muscle twitch

Muscle fiber contracts → then releases
What is wave summation?
• Increasing tension or summation of twitches
• Repeated stimulations before the end of relaxation phase
• Ca²+ remains in sarcoplasm
• Ap frequency exceed time of a single muscle twitch cycle
What is Incomplete Tetanus?
• Twitches reach plateau (of tension)
• If rapid stimulation continues and muscle fibers is not allowed to relax, twitches will reach a maximum level of tension
What is Tetanus?
• If stimulation frequency is high enough, muscle fiber never beings to relax, and is in continuous contraction
The optimal length for muscle fibers is the length at which they can _______
generate maximum force
Within a sarcomere, the ideal ________ is when:
ideal-tension relationship
• muscle is slightly stretched
• thin and thick filaments overlap optimally
What is the Length-Tension Relationship?
sarcomeres have an optimal resting length for developing maximum force
• number of cross bridges formed → depends on overlap of filaments
∙ more crossbridges = more tension (more people pulling in tug of war)

• however → too much or too little reduces efficiency
What is the shortened length?
sarcomeres compressed; Z lines contact thick filaments; thin filaments overlap/interfere with one another
What is the stretched length?
filaments do not overlap; myosin cannot bind to actin
A muscle fiber is either _____ or _____.
"on" (producing tension); "off" (relaxed)

can't vary # of contracting sarcomeres within 1 muscle fiber
Tension at level of individual muscle fiber does vary depending on:
1) fiber's resting length (length-tension relationship)
2) frequency of stimulation
Tension at level of muscle as a whole also varies depending on:
1) tension produced by stimulated muscle fibers
2) total number of muscle fibers stimulated (motor unit recruitment)
What is muscle tone?
some motor units are always active within a skeletal muscle (muscles appear firm and defined)

Muscle Tone - normal tension & firmness of muscle at rest

Muscle units actively maintain body position, without motion

Asynchronous (motor units rotate) to avoid fatigue
Nervous system exerts most of its control over muscle force by ...
varying # of active motor units
• during sustained contracted → motor units are activated on rotating basis
∙ some rest & recover while others actively contract
After contraction, a muscle fiber returns to resting length by:
1) elastic forces
2) opposing muscle contraction
3) gravity
What is elastic force?
energy "spent" in contraction stretching tendons is recovered as they recoil or rebound to original length
What is opposing muscle contractions?
reverse direction of original motion; antagonist skeletal muscle pairs
What is gravity?
can take the place of opposing muscle contraction to return a muscle to its resting state
Muscle Metabolism: Energy for Contraction
• ATP is only energy source used directly for contractile activity
• Muscles have limited ATP storage (enough for ~ 4-6 seconds of activity)
• ATP is regenerated within milliseconds by one or more of the following:
1) direct phosphorylation of ADP by creatine phosphate (CP)
2) anaerobic respiration (glycolysis)
3) aerobic respiration
Direct phosphorylation [coupled reaction of creatine phosphate (CP) and ADP]
Energy Source: CP
Oxygen use: None
Products: 1 ATP per CP, creatine
Duration of energy provision: 15 seconds
Anaerobic mechanism (glycolysis and lactic acid fermentation)
Energy Source: glucose
Oxygen use: None
Products: 2 ATP per glucose, lactic acid
Duration of energy provision: 30-60 seconds
Aerobic mechanism (aerobic cellular respiration)
Energy Source: glucose; pyruvic acid; free fatty acids from adipose tissue; amino acids from protein catabolism
Oxygen use: Required
Products: 38 ATP per glucose, CO₂, H₂O
Duration of energy provision: Hours
Role of Creatine/Creatine Phosphate System
• ~ 4-6 seconds of vigorous exercise → creatine phosphate (CP) is primary source of ATP
∙ creatine kinase → catalyzes reaction
• Stored ATP + CP provide maximum muscle power for ~ 16 seconds (100m dash)
• Reaction is reversible → CP is replenished after exercise
What occurs during Short-duration exercise?
6 seconds: ATP stored in muscles is used first
10 seconds: ATP is formed from creatine phosphate and ADP.
30-40 seconds to End of exercise: Glycogen stored in muscles is broken down to glucose, which is oxidized to generate ATP.
What occurs during Prolonged-duration exercise?
Hours: ATP is generated by breakdown of several nutrient energy fuels by aerobic pathway. This pathway uses oxygen released from myoglobin or delivered in the blood by hemoglobin. When it ends, the oxygen deficit is paid back.
Anaerobic Respiration (Glycolysis)

After stored ATP + CP are used →
more ATP is made via:
• breakdown (catabolism) of glucose
∙ glucose from blood or glycogen stored in muscle
initial phase of glucose breakdown
Anaerobic Respiration (Glycolysis)

1 glucose →
broken down into 2 pyretic acid
• O₂ not used
• 2 ATP produced
Anaerobic Respiration (Glycolysis)

If muscles are contracting vigorously:
• bulging muscles compress BV = ↓ blood flow & O₂ delivery
• pyretic acid converted into lactic acid (= anaerobic respiration)
Anaerobic Respiration (Glycolysis)

Most lactic acid →
diffuses into blood
• converted into pyretic acid by liver or recycled for energy
Anaerobic pathway:
• produces less ATP than aerobic
∙ but produces ATP 2 ½ times faster
• therefore used for short periods of strenuous exercise
Aerobic Respiration

If muscles are at rest or prolonged, moderate exercise:
ATP comes from aerobic respiration
• occurs in mitochondria
• requires oxygen

Glucose + O₂ → CO₂ + H₂O + ATP
Aerobic Respiration

During prolonged light-moderate exercise:
• First 30 minutes - energy comes from:
∙ muscle glycogen, plasma glucose, free fatty acids
• After ~ 30 minutes
∙ fatty acids are a major energy source
Aerobic respiration of glucose yields ______
~ 32 ATP (vs. anaerobic respiration = 2 ATP)
What is muscle fatigue?
muscle is physiologically unable to contract
Many factors have been associated with fatigue:
• Ionic imbalance (K+, Pi, Ca²+) interfere with E-C coupling
• SR damage - interferes with Ca²+ regulation & release
Most studies show a total lack of ATP rarely occurs
possibly locally occurs (writer's cramp)
Lactic acid →
long been assumed to cause fatigue
• likely more related to "psychological fatigue" (muscle can still "go", but we feel too tired)
• lactic acid = ↑ [H+] ICF causes muscle ache (alters contractile proteins)
• but likely not cause of "physiological fatigue"
What is the Recovery Period?
Time required after exertion for muscles to return to normal
• Oxygen becomes available
• Mitochondrial activity resumes
• The Cori Cycle - removal and recycling of lactic acid by the liver
• Liver converts lactic acid to pyretic acid
• Glucose is released to recharge muscle glycogen reserves
What is the Oxygen Debt?
Vigorous exercise causes dramatic changes in muscle chemistry
For a muscle to return to a resting state:
• oxygen reserves must be replenished
• lactic acid must be converted to pyretic acid
• glycogen stores must be replaced
• ATP and CP reserves must be re-synthesized
Define te Oxygen Debt
extra amount of O₂ needed for above restorative processes resulting in heavy breathing
Muscle Fiber Type can be classified based on:
1) speed of contraction - speed of shortening (how fast myosin ATPases split ATP + electrical activity of motor neurons)
• slow fibers
• fast fibers
2) major pathway of ATP formation
• oxidative fibers - rely on aerobic pathway
• glycolytic fibers - rely on anaerobic glycolysis
oxidative fibers
rely on aerobic pathway
glycolytic fibers
rely on anaerobic glycolysis
What are the 3 types of Skeletal Muscle Fibers
1. Slow oxidative (SO)
2. Fast oxidative (FO)
3. Fast glycolytic (FG)
What are slow oxidative muscle fibers?
("red muscle") - slow to contract, flow to fatigue
• small diameter
• many mitochondria (aerobic respiration)
• high oxygen supply
• contain large quantities of myoglobin (red pigment, binds oxygen)
What is myoglobin?
structurally related to hemoglobin; reversibly binds O₂ acts as a storage reserve of O₂
What is fast oxidative muscle fibers?
• mid-sized, mid-speed
• low myoglobin
• more capillaries than fast fiber (aerobic), slower to fatigue
What is fast glycolytic muscle fibers?
("white muscle") - contracts quickly
• large diameter (densely packed myofibrils)
• large glycogen reserves
• few mitochondria (mostly anaerobic)
• strong/high powered contractions, fatigue quickly
Aerobic (Endurance) Exercise
Leads to increased:
• Muscle capillaries
• Number of mitochondria
• Myoglobin synthesis

Results in greater endurance, strength, and resistance to fatigue
-May convert fast glycolytic fibers into fast oxidative fibers
Resistance Exercise (Typically Anaerobic)
Results in:
• Muscle hypertrophy (due to increase in muscle fiber size)
• Increased mitochondria, myofilaments, glycogen stores, and CT
Smooth Muscle Physiology
• Lacks straitions and sarcomeres (unlike skeletal & cardiac tissue)
∙ thick & thin filaments arranged obliquely
• contraction occurs by cross bridge formation between thick & thin filaments
∙ activated by graded potential or action potential
Excitation-Contraction Coupling in Smooth Muscle
1. Smooth muscle cell depolarizes
2. Ca²+ enters cell from ECF via V-G Ca²+ channels &/or sarcoplasmic reticulum (via ryanodine receptor Ca²+ channels)
3. Ca²+ binds to calmodulin (Ca-calmodulin)
4. Ca-calmodulin activates myosin light chain kinase
5. Myosin light chain kinase is phosphorylated
6. Crossbridge cycling/contraction
7. Myosin light chain kinase dephosphorylated → relaxation