MUSCLE PHYSIOLOGY

BASIC ANATOMY

FUNCTIONS OF MUSCLE SYSTEM MovementMovement

• Locomotion- bones• Body contents- organs• Stability- postural muscle resist gravity• Communication- facial muscles• Control of body openings- sphincters

Production of heatProduction of heat

ProtectionProtection

CHARACTERISTICS OF MUSCLE TISSUE Responsiveness (excitability)Responsiveness (excitability)

respond to stimuli with electrical change across membrane

ConductivityConductivity

wave of excitation

ContractilityContractility

shorten when stimulated

Extensibility, ElasticityExtensibility, Elasticity

stretch between contractions & recoil to original resting length after stretch

Cells are long & thin and closely packedCells are long & thin and closely packed

Contain Contain contractile proteinscontractile proteins

MUSCLE ANATOMY

This is what we will look at under

the microscope.

ACTION OF MUSCLE = CONTRACTIONMuscle fibers contain many myofibrils

Myofibrils are cylinders of proteins known as:

• Actin- thin, strands of pearls

• Myocin- thick cylinder

During contraction myocin connects with actin and slides the actin past it (power stroke) shortening (contracting) the muscle fiber.

MYOFIBRILS : ACTIN AND MYOSIN

MUSCLE TYPES

Skeletal Muscle• striated, voluntary, body movement

Smooth (Visceral) Muscle• no striations, involuntary, organs

Cardiac Muscle• intercalated discs, involuntary, heart

SKELETAL MUSCLE FIBERSAlso called striated (striped)

Cells are long and thin

Multi-nucleated

Voluntary: you can control it!

Attached to boneStriations

Multiple Nuclei

SMOOTH MUSCLE FIBERS Also called Visceral

Bipolar, or fusiform shape

Mononucleated –usually in the middle of each cell

No striations

Found in walls of visceral organs

Involuntary- you can NOT control them

CARDIAC MUSCLE FIBERS Found ONLY in the HEART

Self exciting

Cells are shorter and branched

Mononucleated

Intercalated discs- connect cells, support synchronised contraction of cardiac tissue

Striations

Cells are called myocytes

IntercalateddiscsNuclei

NEUROMUSCULAR JUNCTIONAction potential travels down the axon of a motor neuron which synapses with a muscle fiber.

Action potential is propagated into muscle fiber which causes influx of Ca++ ions

Motor Unit = one motor nerve fiber and all the muscle fibers innervated by it.

WHEN CA++ IONS HIT TROPONIN AND TROPOMYCIN, THEY MOVE OUT OF THE WAY AND EXPOSE THE BINDING SITE FOR THE MYOSIN-ACTIN CROSS BRIDGE. THE MYOSIN USES POWER STROKE TO SLIDE ACTIN FILAMENT PAST IT CREATING TENSION IN THE MUSCLE FIBER.

SLIDING FILAMENT MECHANISM OF CONTRACTION

THRESHOLD, LATENT PERIOD, TWITCH

• Threshold-minimum voltage necessary to generate action potential & produce contraction. Action potential causes release of Ca++ and activates sliding filament mechanism.

• Twitch- quick cycle of contraction & relaxation

• Latent period- delay between onset of stimulus & onset of twitch ( 2 milli-seconds)

LATENT PERIOD

• Internal tension: force generated during the time of excitation, excitation-contraction coupling, and tensing of muscle. No shortening of the muscle!

• External tension: muscle components are taut and move a load. This is the contraction phase of the twitch.

ISOMETRIC VS ISOTONIC

IsometricIsometric: muscle develops tension but does NOT shorten. (lifting heavy objects)

Isotonic concentric contractionIsotonic concentric contraction: Muscle shortens while maintaining a constant degree of tension. Moves a load.

Isotonic eccentric contractionIsotonic eccentric contraction: Muscle maintains tension while it lengthens. Relaxes without going limp.