Muscle Tissue & Physiology – Lecture Review

Connective-tissue sheaths (support, route for nerves/blood):
- Epimysium – surrounds entire muscle (not labeled in slide but foundational).
- Perimysium – covers groups of fibers forming fascicles.
- Endomysium – wraps each individual fiber.
Muscle fiber (cell):
- Encased by sarcolemma (specialized plasma membrane) containing sarcoplasm (cytoplasm).
- Packed with myofibrils → striations.
Sarcomere (functional unit): region from one Z-line to next.
- Filament arrangement yields bands: I (light), A (dark), H zone, M line.
- Contraction brings Z-lines closer, I band narrows, A band width остается неизменной.
T-Tubule/SR System:
- T-tubules conduct action potentials deep into fiber.
- Sarcoplasmic reticulum (SR) stores Ca^{++}.
- Triad = T-tubule + 2 terminal cisternae (SR) ⇒ rapid Ca^{++} release.

Sequence:
1. Motor neuron AP arrives at axon terminal.
2. ACh released → diffuses across synaptic cleft.
3. Binds nicotinic ACh receptors on motor end-plate.
4. Sarcolemma depolarizes → triggers AP propagation along membrane & T-tubules.
Enzymatic breakdown of ACh by acetylcholinesterase terminates signal.

Cross-bridge cycle (per Figure 10.11):
1. Ca^{++} binds troponin → tropomyosin shifts, exposing actin active sites.
2. Energized myosin head (ADP + P_i) binds actin ⇒ cross-bridge.
3. Power stroke: P_i released; myosin pivots pulling thin filament; ADP released.
4. ATP binds → myosin detaches.
5. ATP hydrolysis re-cocks head (returns to high-energy state).
Prerequisites for continued shortening: sustained Ca^{++} in sarcoplasm & adequate ATP.
Relaxation (Fig. 10.9): Ca^{++} pumped back into SR via Ca^{++}-ATPase; tropomyosin re-covers sites; lack of ATP or fatigue also halts contraction.

Three physiological classes:
1. SO (slow oxidative, type I) – high mitochondria, myoglobin, capillaries; fatigue-resistant; marathoners (Fig. 10.18).
2. FO (fast oxidative, type IIa) – intermediate power & endurance.
3. FG (fast glycolytic, type IIb/x) – glycolysis-dependent, powerful, fatigue quickly; hypertrophied in bodybuilders (Fig. 10.19).
Plasticity: resistance training → hypertrophy (↑ fiber diameter, especially FG); disuse → atrophy (Fig. 10.20).

Non-striated; actin & myosin anchored to dense bodies and intermediate filaments across sarcoplasm & membrane → corkscrew contraction.
Excitation: autonomic varicosities release neurotransmitter; can respond to hormones, stretch, pacemaker potentials.
Special attributes: slow, sustained contractions; low energy cost; ability to maintain tone (latch state); hyperplasia possible (e.g., uterine growth).