CHAPTER EIGHT

MUSCULARSYSTEM

FUNCTIONS:MOVEMENTPOSTUREHEAT PRODUCTIONPROTECTION

3 TYPES OF MUSCLE TISSUESMOOTH MUSCLE:

InvoluntaryOne nucleus, no striations, tapered at endsDigestive tract, blood vessels

SKELETAL MUSCLE:VoluntaryMultinucleated, striations, cylindrical

CARDIAC MUSCLE:InvoluntaryOne nucleus, striations, intercalated disks

SMOOTH MUSCLE

CharacteristicsConnect to form “syncytia” (similar to cardiac)Impulses and contractions occur much more slowly than

cardiac muscle, howeverCause wave-like movement, called “peristalsis”

Skeletal Muscle Characteristics

Long, multi-nucleated cylindersSeparated by connective tissueEach cell stimulated by motor

neuron

Skeletal Muscle E.M.

Skeletal MUSCLE

Cardiac Muscle CharacteristicsFaintly striated, branchedConnect by intercalated discs to form

a “network”Action potential travels through all

cells connected together in the “syncytium”

Function as a unit

CARDIAC MUSCLE

Type of muscle? Skeletal

Type of muscle? Cardiac

Type of muscle? Smooth

Organization of (skeletal) muscleSkeletal muscle fibers

located in musclesOrganized into fiber bundles

called fascicles (fasciculi)Surrounded by perimysium,

connective tissue layer

MembranesPortions of perimysium

extend into the endomysiumThin layer of CT that covers

each muscle fiberMuscle fiber (bundle)=

multinucleate cellPerimysium also extends to

the CT that surrounds the muscle, the epimysium

Sarcoplasmic ReticulumEach myofibril is surrounded by network of tubes and

storage sacs (Transverse tubules and sarcoplasmic reticulum)

Releases Ca2+ ions when stimulated by motor neuronTriggers contraction (more on this later…)

SARCOMERESarcomere= basic (functional)

contractile unitSeparated by each other by dark Z

lines/discsActin & myosin slide past each other

as the muscle contractsContraction requires Ca2+ and ATP

Other Key Points About Sarcomeres

Z-line/discI Bands

Lighter areas of non-overlap between actin and myosinContain the Z-lines.

Dark Bands = A BandsAreas where some overlap occurs= “Striations” on the slideCoincide with the length of myosin myofilaments.

MICROSCOPIC STRUCTURE

FIBERS: grouped into bundles (fasciculi)

= 1 cell!Fibers contain myofibrils

with:ACTIN: thin myofilamentsMYOSIN: thick

myofilaments

Another look…

Muscular SystemPhysiology of the Muscle

Myosin head “primer”…

Sliding filament theoryIn the absence of Calcium

ions…Tropomyosin blocks

access to the myosin binding site of actin.

When Calcium is released by the Sarcoplasmic reticulumit diffuses into the musclesbinds to the troponinshifting the troponin and tropomyosin

Myosin splits ATP and undergoes a conformational change into a high-energy state.

The head of myosin binds to actin Forms a cross-bridge between the thick and thin

filaments.

The energy stored by myosin is releasedADP and phosphate released from myosin.

The myosin molecule relaxesCauses rotation of the globular headThis leads to the sliding of the filaments.

ATP binds to cross bridge, causing cross bridge to disconnect from actin.

Splitting of ATP leads to re-energizing/ repositioning of the cross bridge.

Ca2+ ions transported back to sarcoplasmic reticulum (req. ATP)

When the calcium level decreasestroponin locks tropomyosin back into the blocking position thin filament (actin) slides back to the resting state (when

ATP binds to myosin head)

Review of the Role of ATP1. ATP transfers its energy to the myosin cross bridge,

which in turn energizes the power stroke. 2. ATP disconnects the myosin cross bridge from the

binding site on actin. 3. ATP fuels the pump that actively transports calcium

ions back into the sarcoplasmic reticulum.

Motor UnitStimulation of a muscle by a nerve impulse (motor

nerve) is required before a muscle can shortenNeuromuscular junction: point of contact b/w nerve

ending and the muscle fiber it innervates.Motor unit: motor neuron + muscle cell

Neuromuscular Junction

Motor Unit

Muscle Stimulus

Threshold stimulus: minimal level of stimulation required to cause a muscle fiber to contract

All-or-none response: once stimulated, a muscle fiber will contract completely

Types of ContractionsTwitch contractions: a single contraction of a muscle fiber

caused by a single threshold stimulusTetanic contractions: sustained and steady muscular

contractions caused by a series of stimuli bombarding a muscle in rapid succession

Isotonic vs. Isometric ContractionIsotonic Contraction

Produces movement at the jointMuscle shortens

IsometricDoes not produce movement at

the jointMuscle does not shorten but

tension within the muscle increases

Antagonist vs. Agonist MusclesAgonistic muscles have the same action

Adductor magnus, adductor brevisVastus intermedius, rectus femoris

Antagonistic muscles have opposites actions (when one is fully contracted, the other must be relaxed)Biceps brachii, triceps brachiiForearm flexors, forearm extensorsRectus abdominus, erector spinae

Strength vs. EnduranceStrength training…

Increases number of muscle fibers’ myofilaments (causes increase in diameter)

Increases bulk of muscle (hypertrophy)Endurance training…

Increases number of mitochondria in muscle cell