Muscle PhysiologyBio 219

Dr. Adam RossNapa Valley College

Muscle tissue

• Muscle is an excitable tissue capable of force production

• Three types• Skeletal- striated, voluntary

• Cardiac- non-striated, involuntary

• Smooth- striated, involuntary

Structure of Skeletal Muscle

Neuromuscular Junction

NMJ- Micrograph

Nicotinic Acetylcholine Receptor

nAChR

Events at NMJ

• Action potential causes Ach release

• Ach binds to nAChR• Allows Na+ to enter muscle cell (EPSP)

• Depolarization from Na+ causes opening of voltage-gated Na+ channels

• Leads to AP in muscle which spreads via T-Tubules* • *(will come back to this shortly)

• ACh is brokendown by acetylcholinesterase (AChE)• Also pumped back into presynaptic neuron

Structure of Skeletal Muscle

• Epimysium surrounds entire muscle

• Fascia surrounds individual fascicles• Fasicle is made up of individual muscle fibers

Structure of skeletal muscle fiber

The Sarcomere

• Functional unit of skeletal muscle• Smallest unit that retains all the functional properties

• Easy way to understand what the whole muscle is doing is to focus on one sarcomere• In most cases (all in this class) all sarcomeres are doing the same thing within

a single muscle

The sarcomere

The Sarcomere

• Major Functional Proteins:• Actin: Thin filament

• Myosin: Thick filament

• Troponin + Tropomyosin : Block Actin from interacting with Myosin

• Other important structures:• Sarcoplasmic reticulum

• Contains calcium

• T-Tubule• Carries depolarization (from motor neuron) across entire muscle fiber

Skeletal Muscle Fiber

T-Tubule/ Sarcoplasmic Reticulum

T-Tubule/ Sarcoplasmic Reticulum

• * T-Tubule is depolarized by AP from NMJ (from earlier)

• That depolarization spreads across the muscle fiber and depolarizes the S.R.

• Causes calcium efflux from S.R. into the sarcoplasm

• Conformational change of troponin/tropomyosin complex caused by Ca2+ binding to Troponin

• Opens the myosin binding site on actin *(more on this soon)

Molecular Basis for Contraction

Sliding Filament Theory

Sarcomere micrograph

Basic Events of Muscle Contraction

• Depolarization spreads across T-Tubules

• Causes calcium release from S.R.

• Calcium binds to troponin• Causes conformational change in troponin-tropomyosin complex

• Myosin binds to actin

• ATP is used

• Myosin head pulls Z-lines closer together

Actin and Myosin (Thin and Thick Filaments)

Skeletal Muscle Contraction

Length-Tension Relationship (Sarcomere)

Muscle-Bone-Tendon Systems

• Muscle is attached to bone by tendons.

• Force produced by muscle pulls on tendon, which pulls on bone

• Creates movement around a fixed point (joint)

• Muscles can only pull (not push)

• Muscles are in antagonistic pairs• Ie biceps and triceps OR Quadriceps and hamstring

Types of Contraction

• Isometric• No change in length of muscle

• Eccentric• Muscle is lengthening

• Concentric• Muscle is shortening

Muscle Twitch

• Smallest single contraction possible from a single motor unit

• If muscle is not allowed enough time to relax, twitches can summate

Motor Units

• Single motor neuron and all fibers it innervates• All fibers activated at same time

• Different size motor units:• Small: precise, delicate movement- fingertips

• Large: less delicate, for posture, movement- muscles in trunk, lower back

• Motor units are recruited by size:• Smallest to largest

• Picking up keys vs. picking up 50lb bag of groceries

• Idea is to use as little energy as possible

• Recruitment results in additional force production (more fibers to contract)

Motor Unit

Summation in Skeletal Muscle

• Spatial• Recruitment of motor units

• Temporal• APs fire at faster rate

• Tetanic contraction• Results from summation of a single fiber/ motor unit

Muscle Fiber Types

• Glycolytic• Rely on glycolysis for energy (Glycolysis)

• Oxidative• Rely on aerobic metabolism and need oxygen

• Twitch• Fast

• Slow

• Tonic

Human Twitch Muscle Fiber Types

Regulation of Muscle Size• Balance of hypertrophy and atrophy

• Gene expression plays a large role• Steroids cross membrane and bind to receptors to alter gene expression

• Presence or absence of protein in diet can affect muscle growth

• Aging causes muscle loss• Sarcopenia- 0.5%-1% loss per year after age 50

• Muscles that are used grow• Increase fiber size, not number of fibers

• Muscles that are not used will atrophy• Muscle atrophy can exacerbate symptoms in elderly bed ridden patients

Disorders of Skeletal Muscle

• Amyotrophic Lateral Sclerosis (ALS or Lou Gehrig’s Disease)• Lack of muscle growth due to removal of myelin on motor neurons

• Myasthenia Gravis• “Grave muscle weakness”

• Caused by overactive acetylcholine esterase