The Muscular System

Contraction of Skeletal MuscleMuscle fiber contraction is “all or none”Within a skeletal muscle, not all fibers may

be stimulated during the same intervalDifferent combinations of muscle fiber

contractions may give differing responsesGraded responses—different degrees of

skeletal muscle shortening

Contraction of Skeletal MuscleGraded responses can be produced by

changingThe frequency of muscle stimulationThe number of muscle cells being stimulated

at one time

Types of Graded ResponsesTwitch

Single, brief contractionNot a normal muscle function

Types of Graded Responses

Figure 6.9a

Types of Graded ResponsesTetanus (summing of contractions)

One contraction is immediately followed by another

The muscle does not completely return to a resting state

The effects are added

Types of Graded Responses

Figure 6.9b

Types of Graded ResponsesUnfused (incomplete) tetanus

Some relaxation occurs between contractionsThe results are summed

Types of Graded Responses

Figure 6.9c

Types of Graded ResponsesFused (complete) tetanus

No evidence of relaxation before the following contractions

The result is a sustained muscle contraction

Types of Graded Responses

Figure 6.9d

Muscle Response to Strong StimuliMuscle force depends upon the number of

fibers stimulatedMore fibers contracting results in greater

muscle tensionMuscles can continue to contract unless

they run out of energy

Energy for Muscle ContractionInitially, muscles use stored ATP for energy

ATP bonds are broken to release energyOnly 4–6 seconds worth of ATP is stored by

musclesAfter this initial time, other pathways must

be utilized to produce ATP

Energy for Muscle ContractionDirect phosphorylation of ADP by creatine

phosphate (CP)Muscle cells store CP

CP is a high-energy moleculeAfter ATP is depleted, ADP is leftCP transfers energy to ADP, to regenerate

ATPCP supplies are exhausted in less than 15

seconds

Energy for Muscle Contraction

Figure 6.10a

Energy for Muscle ContractionAerobic respiration

Glucose is broken down to carbon dioxide and water, releasing energy (ATP)

This is a slower reaction that requires continuous oxygen

A series of metabolic pathways occur in the mitochondria

Energy for Muscle Contraction

Figure 6.10b

Energy for Muscle ContractionAnaerobic glycolysis and lactic acid

formationReaction that breaks down glucose without

oxygenGlucose is broken down to pyruvic acid to

produce some ATPPyruvic acid is converted to lactic acid

This reaction is not as efficient, but is fastHuge amounts of glucose are neededLactic acid produces muscle fatigue

Energy for Muscle Contraction

Figure 6.10c

Muscle Fatigue and Oxygen DeficitWhen a muscle is fatigued, it is unable to

contract even with a stimulusCommon cause for muscle fatigue is

oxygen debtOxygen must be “repaid” to tissue to remove

oxygen deficitOxygen is required to get rid of accumulated

lactic acidIncreasing acidity (from lactic acid) and

lack of ATP causes the muscle to contract less

Types of Muscle ContractionsIsotonic contractions

Myofilaments are able to slide past each other during contractions

The muscle shortens and movement occursIsometric contractions

Tension in the muscles increasesThe muscle is unable to shorten or produce

movement

Muscle ToneSome fibers are contracted even in a

relaxed muscleDifferent fibers contract at different times

to provide muscle toneThe process of stimulating various fibers is

under involuntary control

Effect of Exercise on MusclesExercise increases muscle size, strength,

and enduranceAerobic (endurance) exercise (biking,

jogging) results in stronger, more flexible muscles with greater resistance to fatigueMakes body metabolism more efficientImproves digestion, coordination

Resistance (isometric) exercise (weight lifting) increases muscle size and strength