Muscular System

Mark Robertson Professor of Biology

Delta College

Overall Outline• Anatomy of a Skeletal Muscle• Muscle Terminology• Neuromuscular Junction• Steps Used in Muscular

Contraction• Myograms of Twitch vs. Tetonic

Contractions• Aerobic versus Anaerobic Power

( Red vs. Pink vs. White Muscle)• Major Groups in the Body

Anatomy of a Muscle• Structural Design of Muscle Bodies

• Origin (anchoring end with tendon)

• Muscle Body (with fascicles)

• Insertion (mobile end with tendon)

• Fascicles

• Bundles of muscle fibers (linked cells)

• Filled with myofibrils (with layered actin/myosin forming striations)

• Muscle Body Connective Tissue (CT) Layers

• Epimysium (surface layer)

• Perimysium (around fascicles)

• Endomysium (around muscle fibers) Fascicle

Neuromuscular Junction• Fluid-filled synapse formed

between motor neuron and muscle cell sarcolemma

• Pre-synaptic axonal terminal buds are full of storage vesicles (filled with neurotransmitters like acetylcholine -> ACh)

• Action potential (AP) down from our brain causes calcium ion (Ca2+) diffusion into the axonal terminal bud

• Vesicles of ACh drawn to presynaptic membrane by the Ca2+ ions and then released into the synapse

• Acetylcholine binds to post-synaptic receptors to create new AP on sarcolemma

Muscular Terminology

• Striated myofibrils packed inside muscle fibers (linked muscle cells) that are packed inside fascicles within muscle body

• Myofibrils have 2 structural proteins:

• Actin (thin filaments, twisted beads)

• Myosin (thick filaments, bundled golf clubs)

• Myofibrils have 2 control proteins:

• Tropomyosin (double-dutch jumprope)

• Troponin (upside-down snowman)

• T-tubules invaginate sarcolemma and connect to sarcoplasmic reticulum (bags in cell holding Ca2+) down next to the myofilaments (actin/myosin)

Steps of Muscle Contraction1. Brain moves an action potential (AP) down the neuronal axon

2. Calcium ions (Ca2+) enter axon terminal buds from interstitial fluids around the neuron

3. Calcium ions cause binding of acetylcholine-filled vesicles. ACh is released by motor neuron and crosses synapse to bind with skeletal muscle sarcolemma receptors

4. Sarcolemma generates new action potential (AP) when enough ACh received by receptors located there

5. AP travels down T-tubules to sarcoplasmic reticulum (SR)

6. SR releases stored calcium ions (Ca2+) inside muscle cell

7. Calcium ions (Ca2+) bind with troponin, changing it’s shape8. Troponin pulls tropomyosin away from actin’s binding sites

allowing contraction to proceed

9. Myosin heads (primed by ATP) attach to actin’s binding sites

10. Myosin heads pull against actin with constant ATP usage

Myograms

• Types of Exercise• Isometric (same length, change in pressure,

weight does not move)• Isotonic (same pressure, change in length,

weight does move)

• Myogram Parts• Tone (some Ca2+ remains in cell)• Stimulus (ACh release from neuron)• Latent (time needed for AP down T-tubule

and more Ca2+ ions released from SR)• Contraction (myosin moves actin)• Relaxation (Ca2+ uptake into SR w/ATP)• Tone (some Ca2+ remains in cell)

• Twitch vs. Tetonic Myograms

Aerobic/Anaerobic Factors• Cell Size• Power Generated• Creatine Phosphate (CP)

Levels• Myoglobin Concentration• Mitochondria Number• Blood Flow Rate• Fatigue Rate• Contraction Speed

Interconnections of Energy

• Autotrophs• Produce organic molecules and O2

from energy in sunlight or chemicals• Take in CO2 and H2O as building

blocks for organic storage molecules

• Heterotrophs• Give off CO2 and H2O as by-products

of aerobic metabolism of organics• Take in organic molecules from their

diet and O2 from the air to create energy (ATP) using mitochondria

Red vs. White Muscle

• Red Muscle (aerobic)• Lots of blood flow, oxygen,

mitochondria, and myoglobin, but very little glycogen and creatine phosphate (CP)

• Slow contraction with low rate of fatigue, but lower power output

• White Muscle (anaerobic)• Hardly any blood flow, oxygen,

mitochondria, or myoglobin, but loads of glycogen and creatine phosphate (CP)

• Fast contraction with high rate of fatigue, but loads of power

Major Muscle Groups• Group Terminology

• Prime Mover (muscle in the best position for a given action)

• Synergists (muscles in a group that work together)

• Antagonists (muscles in a group with opposite actions)

• Hamstrings (knee flexors: biceps femoris, semimembranosus, semitendinosus)

• Quadriceps Femoris (knee extensors: rectus femoris, vastus medialis/intermedius/lateralis)

• Adductor Triangle of Hip (adductor longus & adductor magnus)

• Flexor Triangle of Hip (iliopsoas & pectineus)• Abdominals (Abs: rectus abdominus, external

oblique, internal oblique, transverse abdominus)• Intercostal Muscles (Internal/External)

Potential Journal Critique Topics

• Myasthenia Gravis?

• Muscular Dystrophy?

• Amyotrophic Lateral Sclerosis (ALS)?

• Duchenne Muscular Dystrophy (DMD)?

• Polymyositis (PM)?