Muscle Cell Structure & Excitation–Contraction Coupling

Instructor’s goal: connect sarcomere‐level events (previous lecture) to whole-cell architecture and physiology.
Textbook cross-references: p.327 (myofibril micrograph), p.337 (cell anatomy), p.342 (neuromuscular junction), p.348 (filament interaction diagrams).
Study tips the instructor stressed:
- Re-draw structures and “tell the story yourself.”
- Use both the outline and the textbook; their different organizations force deeper thinking.
- Review prior chapters on cell membrane/ion channels before tackling today’s gate discussion.

Whole muscle → fascicles → individual muscle fibers (= muscle cells).
Each muscle fiber is
- Multinucleated.
- Packed with myofibrils (long protein cylinders) that run the cell’s length.
Myofibrils contain repeating sarcomeres:
- Z-line to Z-line: basic contractile unit drawn in the prior lecture.
- Thick filaments = myosin; thin filaments = actin + regulatory proteins (troponin, tropomyosin).
Eating “muscle/meat” = ingesting these protein filaments.

myo- : general prefix for muscle.
sarco- : also muscle-related, often refers to cellular components specific to muscle fibers.
- Sarcolemma = muscle-cell plasma membrane (structurally identical to any cell membrane).
- Sarcoplasm = cytoplasm of the muscle cell.

Endomysium surrounds each individual muscle fiber, lying external to the sarcolemma.

Sarcolemma is not a flat sheath; it dives into the cell as numerous tiny tunnels called transverse (T) tubules.
- Visualized as little “holes” on the surface images.
- Function: carry the surface action potential deep into the cell, wrapping around every myofibril so all sarcomeres receive the electrical signal simultaneously.

Every T tubule is flanked by two enlarged sacs of sarcoplasmic reticulum (SR) → terminal cisterns.
Arrangement = Terminal Cistern – T Tubule – Terminal Cistern (always three → “triad”).
Color-coding in various textbook figures:
- Middle T tubule sometimes yellow/pink/purple; outer cisterns blue.
Physiological importance:
1. Action potential travels down sarcolemma → dives into T tubule.
2. Voltage change in T-tubule membrane opens Ca2+ release channels in both adjacent terminal cisterns.
3. Stored Ca2+ floods the sarcoplasm.

SR = network of membranous tubes analogous to smooth ER in other cells; specialized for Ca2+ handling.
Terminal cistern = SR region storing large Ca2+ reserve.
- Calcium held by protein calsequestrin (acts like a water cistern → holding tank).
At rest: Ca2+ kept inside cisterns.
During excitation: gates open → Ca2+ diffuses out along gradient into sarcoplasm and toward filaments.

Blood homeostasis: $[Ca^{2+}]_{blood}=9.2\text{ to }10.4\,\text{mg/dL}$ must be maintained for nerve and muscle function.
Intracellular muscle use:
- Source during contraction: terminal cisterns, not the bloodstream (although bloodstream ultimately replenishes).

Neuronal action potential arrives at neuromuscular junction (NMJ).
Voltage change opens voltage-gated Ca2+ channels in axon terminal; extracellular Ca2+ enters, triggering vesicle exocytosis of acetylcholine (ACh).
ACh crosses synaptic cleft → binds ligand-gated Na+ channels on motor end plate; Na+ influx starts a new action potential on sarcolemma.
Action potential spreads over sarcolemma → dives into every T tubule.
Depolarization of T-tubule membrane mechanically/chemically opens Ca2+ release channels in both terminal cisterns of each triad.
Ca2+ floods sarcoplasm → binds troponin.
Troponin undergoes conformational change → pushes tropomyosin off actin’s myosin-binding sites.
Myosin heads (pre-energized by partial ATP hydrolysis) attach to exposed sites → cross bridge forms.
Power stroke:
- Stored energy in myosin head unleashes → head pivots, pulling thin filament toward sarcomere center.
- Described artistically as “swivel” or “drag”—basis of the Sliding Filament Mechanism.
ATP binds myosin, causing detachment; ATP hydrolysis re-cocks head → Cross-bridge Cycling continues as long as Ca2+ and ATP remain.
When neural stimulation ceases, Ca2+ actively pumped back into SR (requires ATP); troponin releases Ca2+, tropomyosin re-covers sites, muscle relaxes.

Two functional classes mentioned:
1. Ligand-gated channels (e.g., ACh receptor/Na+ channel at NMJ).
2. Voltage-gated channels (propagate action potentials along sarcolemma & T tubules).
Instructor will create a follow-up video + worksheet reviewing charge changes and gate mechanics; suggested review: Cell Membrane chapter.

Components: motor neuron axon terminal, synaptic cleft, motor end plate (specialized sarcolemma region).
Synaptic cleft = microscopic gap.
Calcium-triggered ACh release initiates all downstream events.

Maintaining proper blood Ca2+ is not only skeletal but also cardiac and neural safety issue; dietary, hormonal (parathyroid) regulation implied.
Real-world relevance: meat as dietary protein source is literally bundles of these protein filaments.

Re-draw: muscle fiber cross-section showing triads wrapped around myofibrils.
Practice tracing an action potential from neuron → sarcolemma → T-tubule → Ca2+ release.
Review cell-membrane chapter for ion channel mechanics.
Anticipate next lecture: deeper dive into ligand vs voltage gates and quantitative membrane potential shifts.