Muscle

striated

long parallel arrangement

multiple peripheral nuclei

striated w/ intercalated discs

branched cells

central nucleus

non-striated

fusiform (tapering at both ends)

central "cork screw" nucleus

skeletal muscle

• Immature myoblasts eventually fuse to form multinucleated muscle fibers (muscle cell)
• - each muscle cell is surrounded by a basal lamina

• Myoblasts that do not fuse are called satellite cells
• - play a minor role in muscle regeneration

• Endomysium – CT surrounding individual muscle cells
• - composed of reticular fibers and external lamina (basal lamina)

• Perimysium – thicker CT that surrounds and defines fascicles
• - functional bundles of muscle fibers; visible to naked eye

• Epimysium – dense CT that surrounds the entire muscle (collection of fascicles)
• - Investing fascia of gross anatomy

Involuntary contractions

slight contraction that gives muscle firmness (assisting stability of joints and posture)

Muscle length does not change

Muscle length changes

Contracting muscle shortens during movement

Muscle length changes

• Contracting muscle lengthens during movement
• (lowering an object / squatting down)

Speed of contraction / metabolic activity

• ATPase enzymatic velocity
• Myoglobin content
• Mitochondrial number

• Red / slow twitch / slow oxidative
• not easily fatigued (postural / endurance muscles)

• High myoglobin content
• Many mitochondria (high in oxidative enzymes)
• Slow myosin ATPase reaction velocity

• Predominate in the deep back muscles and other postural muscles
• (maintain upright posture)

Soleus muscle of the calf

• White / fast twitch / fast glycolytic
• powerful rapid contraction (easily fatigued)

• Low myoglobin content
• Few mitochondria (low in oxidative enzymes)
• Very fast myosin ATPase reaction velocity

Extraoccular eye muscles

• Intermediate / fast oxidative glycolytic
• Make up fast twitch fatigue resistant motor units

• Fascicles are collections of fibers bound by a perimysium
• Fibers are collections of myofibrils (extend the entire length of cell)
• Myofibrils are composed of myofilaments
• Myofilaments are subdivided into thick (myosin) and thin (actin) filaments

• Thick filaments - Myosin (2 Heavy chains + 2 Pairs of Light chains)
• Heavy chain tails aggregate forming thick filaments
• Heavy chain globular head has binding sites for actin & ATP)

• Thin filaments - Actin (F-actin)
• F-actin = double helix of globular monomers (G-actin)

Long thin molecule that wraps around the actin filaments (bind in F-actin groove)

Troponin is complex of three subunits

• TnT - binds to tropomyosin
• TnC - binds to calcium ions
• TnI - inhibits the actin-myosin interaction

• Sarcomere
• (region between two Z lines)

Actin (thin) filaments only

Overlapping myosin (thick) and actin (thin) filaments

Myosin (thick) filaments only

Z-line: where actin (thin) filaments are bound together by alpha-actin (found in smooth muscle)

M-line: where adjacent myosin (thick) filaments are linked (center of H-band)

• 1. Attachment (rigor): myosin is bound to actin
• 2. Release: myosin is uncoupled from actin and binds ATP
• 3. Bending: ATP hydrolysis causes myosin head to bend and advance

4. Force Generation: myosin binds to actin and returns to its original conformation – forcing movement of the thin filament along the thick filament (ADP is lost)

5. Reattachment: cycle can repeat

Thin filaments (actin) slide past thick filaments (myosin)

• I-bands / H-bands shorten
• Sarcomere shortens (Z-lines are drawn closer)
• A-band does not change in length

• Low [Ca+] (Calcium is being pumped back into SR)
• - myosin binding site on actin is blocked

• High [Ca+]
• - Calcium binds to TnC
• - Conformational change breaks TnI-actin bond
• - Tropomyosin moves (no longer blocking binding site on actin)

Specialized network of endoplasmic reticulum around myofibrils

Actively transports Ca²⁺ back into cisternae after contraction

Triad = SR-TT-SR @ the A-I junction

Rapidly spread depolarization thru muscle fiber (widespread release of Ca²⁺ from the SR)

• This allows for uniform contraction of the muscle fiber
• - voltage gated Ca⁺ channels open

Stretch receptor (rate and duration sensitive)

• Has a CT capsule and intrafusal muscle fibers
• (skeletal m. fibers producing force for movement are extrafusal fibers)

Has both afferent and efferent innervation

Anchored to perimysium and endomysium of extrafusal fibers

• Rapid tap of the tendon causes a rapid stretch of the muscle spindle
• (Initiates monosynaptic deep tendon reflex)

Form end-to-end attachment between cardiac myocytes

• Macula adherens
• Zonula adherens
• Gap junctions

Diad = SR & T-tubule

• T-tubules:
• - larger than in skeletal m.
• - located at Z-line (not the A-I junction)
• - large mitochondria span distance between T-tubules

Purkinje Fibers

• few myofibrils
• form gap junctions with cardiac m. cells
• no intercalated disks
• large pale cells (abundant mitochondria)

Desmin

Calmodulin!

(no troponin)

Slow prolonged contraction (initiated by ANS)

caldesmon / calponin are actin binding site proteins that block the binding site on myosin