Lecture 11: Introduction to Muscle Structure and Function 1 — Cellular Scale

Slide 1

• Opens the muscle physiology section of the course, shifting focus from the oxygen supply cascade to the structures that actually generate movement.
• Builds from the subcellular scale (contractile proteins, calcium handling) toward the whole-body scale across the next several lectures.
• Central theme: trade-offs are present at every structural level — between speed, force, fatigue resistance, and economy.

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Slide 2

Learning Objectives

1. Describe the microstructure and functional components of muscle cells (fibers).
2. Outline the events of muscle contraction and relate them to force generation, displacement, and ATP energy use.
3. Define the components of muscle cells that are essential for contractile function (force, work), activation control, and aerobic energy supply.
4. Discuss how these components relate to functional trade-offs in muscle design for force, speed, and endurance.

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Slide 3

The Hierarchical Organization of Skeletal Muscle

• Skeletal muscle is organized hierarchically: whole muscle → fascicles → muscle fibers (myofibers) → myofibrils → sarcomeres (actin + myosin).
• The human body contains over 600 skeletal muscles, accounting for 40–50% of total body weight.
• Skeletal muscle serves multiple functions:
  • Force and power for movement and breathing (e.g., diaphragm — failure is life-threatening).
  • Postural and balance control via small tonic force production.
  • Thermoregulation via shivering and basal heat production.
• Muscle function is also strongly temperature-dependent: outside its physiological temperature range, force, power, and efficiency all decline.

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Slide 4

Muscle Fiber Anatomy and Specialized Terminology

• Skeletal muscle cells (myofibers) are highly specialized for actuating movement, with much of the cell volume devoted to contractile machinery rather than typical organelles.
• Specialized terminology for muscle cells:
  • Sarcolemma — the muscle cell membrane (specialized for transmitting action potentials).
  • Sarcoplasm — the muscle cytoplasm.
  • Sarcoplasmic reticulum (SR) — Ca²⁺ storage organelle (analogous to ER).
• Skeletal muscle is multi-nucleated with nuclei distributed along the fiber length, enabling rapid local regulation of protein synthesis in response to changing mechanical or metabolic demand.
• The sarcoplasm is packed with:
  • Contractile proteins: actin, myosin, plus structural and regulatory proteins (titin, troponin).
  • Mitochondria in two distinct populations (see next slide).
  • Sarcoplasmic reticulum for Ca²⁺ handling.

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Slide 5

Two Populations of Mitochondria

• Skeletal muscle has two morphologically and functionally distinct mitochondrial populations:
  • Intermyofibrillar mitochondria — distributed in arrays between contractile filaments; supply ATP for cross-bridge cycling.
  • Subsarcolemmal mitochondria — packed beneath the sarcolemma; supply ATP for Ca²⁺ uptake into the SR by the Ca²⁺-ATPase pump.
• This spatial partitioning matches ATP production sites to local ATP-consuming machinery.

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Slide 6

Sarcoplasmic Reticulum and Excitation–Contraction Coupling

• The sarcoplasmic reticulum (SR) is a dense network of tubules that stores Ca²⁺ and contains Ca²⁺-ATPase pumps for re-sequestering Ca²⁺ after each contraction.
• T-tubules are invaginations of the sarcolemma that extend into the cell interior, contacting the SR at the triad (T-tubule + two terminal cisternae).
• Excitation–contraction coupling: action potentials originate at the neuromuscular junction, propagate along the sarcolemma, are carried into the cell via T-tubules, depolarize the SR, and trigger Ca²⁺ release into the sarcoplasm.

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Slide 7

Sarcomeres — The Functional Unit of Force and Displacement

• The sarcomere is the functional contractile unit of muscle, defined between two Z-discs with overlapping actin (thin) and myosin (thick) filaments and the structural protein titin.
• Myofibrillar proteins typically occupy 70–80% (up to 90%) of muscle cell volume — leaving little room for organelles.
• Architectural rules linking sarcomere arrangement to function:
  • Sarcomeres in series → determine maximum displacement (range of shortening).
  • Sarcomeres in parallel (cross-section) → determine maximum force.

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Slide 8

The Sliding Filament Model

• The sliding filament (or swinging lever-arm) model explains how molecular cross-bridge cycling produces sarcomere shortening.
• Cross-bridges form between myosin heads and actin binding sites; the myosin head undergoes a power stroke that ratchets actin past myosin.
• The result is shortening of the sarcomere — the Z-discs are pulled closer to the M-line — without any change in the length of the actin or myosin filaments themselves.
• The molecular flux is enormous: a contracting muscle can cycle ~$8 \times 10^{21}$ molecules of ATP per second across all of its myosin heads.

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Slide 9

Crystalline Order Enables X-ray Diffraction in Living Muscle

• Muscle’s near-crystalline subcellular organization (a regular hexagonal lattice of actin and myosin filaments) makes it uniquely amenable to X-ray diffraction, an experimental technique normally used for crystallized molecules.
• Diffraction patterns from intact muscle (cobra, barnacle leg) show intense reflections corresponding to the regular spacing of contractile proteins.
• This allows direct measurement of molecular dynamics in living, intact muscle tissue — something impossible for almost any other vertebrate tissue.

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Slide 10

In Vivo X-ray Diffraction in Flying Insects

• Tethered insects such as hawkmoths can be flown inside an X-ray beam while their flight muscles (DLMs and DVMs) are simultaneously imaged and EMG-recorded.
• This setup reveals the time course of myofilament lattice dilation and filament stretch during real-time contractions.
• Demonstrates how muscle’s molecular organization allows researchers to integrate molecular dynamics with whole-body movement in a single experiment.

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Slide 11

The Cross-Bridge Cycle (Six Classical Steps)

1. Resting fiber: myosin head bound to ADP + P_i, not attached to actin.
2. In the presence of Ca²⁺, troponin moves tropomyosin off actin’s binding sites; myosin binds to actin and forms a cross-bridge.
3. P_i released from myosin → conformational change.
4. Power stroke: actin slides past myosin, ADP released; myosin remains tightly bound (the rigor state).
5. A new ATP binds to myosin → cross-bridge releases from actin.
6. ATP is hydrolyzed to ADP + P_i, re-energizing the myosin head and returning it to its original orientation. Cycle repeats.

• ATP is required to release the cross-bridge, not to form it. This is why rigor mortis occurs after death — without ATP, cross-bridges remain bound until proteins denature.

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Slide 12

The Activation–Relaxation Cycle (Calcium Cycling)

1. Action potential → Ca²⁺ released from SR into the sarcoplasm.
2. Ca²⁺ binds troponin, displacing tropomyosin and exposing actin binding sites.
3. Cross-bridges form, generate force, and cycle (links to Slide 11).
4. Ca²⁺ is re-sequestered into the SR by Ca²⁺-ATPase pumps (SERCA).
5. As sarcoplasmic [Ca²⁺] falls, Ca²⁺ dissociates from troponin → tropomyosin re-blocks binding sites → no new cross-bridges form.
6. Existing cross-bridges complete their cycles and detach → relaxation.

• Speed of relaxation depends on steps 4–6 — that is, on Ca²⁺ re-sequestration kinetics.
• The Ca²⁺-ATPase pump is a major ATP consumer: an estimated 30–40% of the total energetic cost of isometric contraction is spent on calcium cycling (not cross-bridge cycling itself; Barclay et al. 2007).

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Slide 13

Twitch Kinetics Vary By Fiber Type

• Single-fiber experiments allow simultaneous measurement of force and free [Ca²⁺] following an action potential.
• Across toadfish red, white, and superfast fibers, twitch duration spans more than an order of magnitude:
  • Red (slow oxidative): long, slow twitch.
  • White (fast glycolytic): shorter twitch.
  • Superfast / sonic (toadfish swim bladder, rattlesnake tail-shaker): extraordinarily rapid Ca²⁺ release and reuptake — twitches complete in ~10–20 ms.
• Three molecular determinants of contraction speed:
  1. SR Ca²⁺-ATPase isoforms → Ca²⁺ cycling dynamics, activation/relaxation rate.
  2. Troponin isoforms → actin–myosin binding regulation.
  3. Myosin isoforms → cross-bridge cycling and detachment rate.
• Historically, myosin isoforms were the focus of fiber-type classification; modern work shows that all three vary together and contribute to fiber speed.

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Slide 14

The “Zero-Sum Game” of Muscle Volume Fractions

• A given muscle fiber volume must be partitioned among three competing components:
  • Myofibrils → force capacity
  • Sarcoplasmic reticulum (SR) → activation/relaxation speed
  • Mitochondria → aerobic energy supply
• Increasing one component necessarily reduces space available for the others — the “zero-sum game” (Rome & Lindstedt 1998).
• Approximate ranges:
  • SR: ~5% (slower fibers) up to ~30% (superfast specialists).
  • Mitochondria: ~5% (anaerobic fibers) up to ~30% (highly aerobic fibers).
  • The remainder is myofibrils (70–90%).
• Result: muscle fibers occupy a continuum on this trade-off surface depending on their functional specialization.

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Slide 15

Three Cellular Designs Visualized

Muscle type	Myofibrils	Mitochondria	SR	Functional profile
Anaerobic, high-force (e.g., white fiber)	~85%	~5%	~10%	High force, fast, rapid fatigue
Aerobic (e.g., type I red fiber)	~70%	~25%	~5%	Lower force, slow, fatigue-resistant
Super fast (e.g., rattlesnake tail-shaker)	~50%	~20%	~30%	Super fast, very low force, fatigue-resistant

• Each design reflects a different solution to the volume-fraction trade-off, optimized for a different functional role.
• Most vertebrate muscles are mixed fiber types — fish are an exception (red and white muscle in distinct anatomical compartments).
• Fiber type is highly plastic — cross-reinnervation experiments (transplanting nerves between fast and slow muscles) demonstrate that fiber type can switch in response to neural activation pattern.

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Slide 16

Traditional Fiber Type Classifications

• Three textbook categories, based on histochemical staining:
  • Type I — slow oxidative
  • Type IIa — fast oxidative/glycolytic
  • Type IIx (also IIb in rodents) — fast glycolytic
• Maximum shortening velocity scales: Type IIx > IIa > I.

Characteristic	Type IIx (fast glycolytic)	Type IIa (fast oxidative)	Type I (slow oxidative)
Number of mitochondria	Low	High/moderate	High
Resistance to fatigue	Low	High/moderate	High
Predominant energy system	Anaerobic	Combination	Aerobic
ATPase activity	Highest	High	Low
Vmax (speed of shortening)	Highest	High	Low
Efficiency	Low	Moderate	High
Specific tension	High	High	Moderate

• Categories are useful but represent points on a continuum — within-fiber-type variation in mitochondrial, SR, and protein-isoform composition is substantial.

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Slide 17

Fiber Type Shifts With Training

• Type II fibers have higher specific force and higher maximum power than Type I, but at the cost of fatigue resistance.
• Training-induced fiber type shifts:
  • Endurance training (low intensity, high reps) → shift toward oxidative fibers (lower force, fatigue-resistant).
  • High-intensity, low-rep strength/power training → shift toward fast glycolytic fibers (higher force, more fatigable).
• These shifts demonstrate the plasticity of fiber type within an individual.

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Slide 18

Fiber Type Distribution Across Athletes

Sport	% Slow Fibers (Type I)	% Fast Fibers (Types IIx + IIa)
Distance runners	70–80	20–30
Track sprinters	25–30	70–75
Nonathletes	47–53	47–53

• Differences reflect both training adaptations and genetic predisposition — these are difficult to disentangle in cross-sectional comparisons.
• Self-selection into sport is also a factor: individuals with naturally faster muscles may gravitate toward sprint events and vice versa.

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Slide 19

Endurance Training Increases Mitochondrial Volume in All Fiber Types

• Endurance training increases mitochondrial volume density in all three fiber types, not just slow fibers.
• Baseline differs: type I fibers start with higher mitochondrial density than IIA or IIB.
• Time course differs:
  • Type IIB: rapid increase over 6 weeks, then plateau.
  • Type I: continues to increase progressively through 6 months.
• Reinforces that fiber type is a continuum with substantial within-type plasticity — even “fast glycolytic” fibers acquire more mitochondria with endurance training.

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Slide 20

Strength Training Shifts SR Calcium Handling

• Eight weeks of strength training in elite rowers (under either 10-set or 3-set protocols) increased both:
  • SR Ca²⁺ uptake rate (P = 0.003)
  • SR Ca²⁺ release rate (P = 0.0001)
• Faster Ca²⁺ cycling enables faster contraction–relaxation — important when speed and force must be combined (as in rowing).
• Mechanistically, this likely reflects changes in SR amount and Ca²⁺-ATPase isoform expression, providing more rapid activation control.

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Slide 21

Hummingbirds — An Apparent Exception to the Zero-Sum Game

• Hummingbirds are physiological extremes:
  • Highest mass-specific metabolic rates among vertebrates.
  • Capable of sustained hovering flight (highly demanding aerobically and mechanically).
  • Wingbeat frequencies of ~40–80 Hz — in the audible range when one is nearby.
• Their flight muscles seem to combine high force, high speed, and high endurance simultaneously — apparently challenging the zero-sum game framework.

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Slide 22

Hovering Aerodynamics Demand Lift on Both Strokes

• Particle-image-velocimetry (PIV) studies of hovering hummingbirds reveal downward momentum jets during both the downstroke and the upstroke.
• Most birds generate lift only on the downstroke; hummingbirds generate substantial lift on the upstroke as well, allowing true hovering.
• This places extreme power demands on both the downstroke and upstroke flight muscles.

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Slide 23

Hummingbird Flight Muscles Are A Single Specialized Type

• The pectoralis (downstroke) and supracoracoideus (upstroke) are large flight muscles that together comprise much of the hummingbird’s body mass.
• They are made up exclusively of fast oxidative–glycolytic fibers (type IIa) — unusual for vertebrate muscle, which is typically a mosaic of fiber types.
• EMG recordings show alternating, precisely timed activation of pectoralis and supracoracoideus across each wingbeat cycle (~80 ms shown).

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Slide 24

Hummingbird Mitochondria — Giant and Densely Packed

• Hummingbird flight-muscle mitochondria are giant and occupy ~35–50% of cell volume — far above typical vertebrate muscle (~5–25%).
• Mitochondria contain densely “double-packed” cristae, providing extra inner membrane surface area for oxidative phosphorylation.
• Hummingbirds also maintain higher body temperature than most vertebrates, which accelerates ATP synthesis and Ca²⁺-pump kinetics.
• These specializations expand the cellular envelope — hummingbirds shift the entire trade-off surface, rather than violating the zero-sum game.

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Slide 25

Trade-offs Persist: Low Force and High Temperature Sensitivity

• Hummingbird flight muscle has:
  • Very low specific tension (force per cross-sectional area) — a direct consequence of mitochondria taking up so much cell volume that less is available for myofibrils.
  • High temperature sensitivity — force falls steeply at lower temperatures.
• Functional consequences:
  • Hummingbirds enter torpor in cold weather because their muscles lose function as temperature drops.
  • The “broken zero-sum game” comes with its own trade-offs: extreme aerobic capacity at the cost of low force and a narrow operating temperature range.

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Slide 26

Regional Endothermy and Heater Organs

• Some fish — tunas and mackerel sharks — maintain elevated red-muscle temperature above ambient water using a vascular countercurrent heat exchanger (the rete mirabile).
• The rete is anatomically associated with the slow-twitch aerobic red muscle used for sustained swimming, with viscera, and (in some species) with cranial/orbital regions.
• Heater organs: in some tunas, extraocular muscles have lost their contractile function and instead perform futile Ca²⁺ cycling between cytoplasm and SR — generating large amounts of heat to warm the eye and brain.
• Demonstrates that muscle’s molecular machinery can be co-opted purely for thermogenesis, separating the calcium-cycling and force-generating functions.

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Slide 27

Thermoregulatory “Breath-Holding” in Hammerhead Sharks

• The scalloped hammerhead (Sphyrna lewini) makes rapid deep dives from ~26°C surface waters to depths >800 m at <5°C.
• It maintains muscle temperature during these excursions by suspending gill heat exchange (a kind of thermoregulatory “breath-holding”) rather than by endothermy.
• Proposed mechanisms:
  • Shunting blood away from the gills.
  • Closing gill slits and mouth.
  • Reducing ram-ventilation to minimize convective heat loss across the gill epithelium.
• Reinforces the importance of maintaining muscle temperature for muscle function — these deep-diving predators must keep their muscle warm enough to function effectively at depth.

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Slide 28

Learning Objectives — Recap

1. Microstructure and functional components of muscle fibers — sarcolemma, sarcoplasm, SR, T-tubules, two mitochondrial populations, and the sarcomere as the contractile unit.
2. Events of muscle contraction — the cross-bridge cycle (six steps), the role of ATP in releasing cross-bridges, and the relationship between cross-bridge cycling and force/displacement.
3. Components essential for contractile function, activation control, and aerobic energy supply — myofibrils, SR + Ca²⁺-ATPase, and intermyofibrillar mitochondria, respectively.
4. Functional trade-offs — the zero-sum game of volume fractions and the resulting fiber-type continuum from high-force/anaerobic to slow/aerobic to super-fast/low-force, plus extreme cases (hummingbirds, tunas) that expand or shift the trade-off surface.

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Key Equations

Lecture 11 covers cellular structure, the cross-bridge cycle, calcium handling, and qualitative volume-fraction trade-offs — no formal equations are introduced. Quantitative relationships for muscle force, length, velocity, and power are formalized in Lecture 12.

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Glossary of Key Terms

Term	Definition
Myofiber (muscle fiber)	A single multi-nucleated skeletal muscle cell, specialized for force generation and movement.
Sarcolemma	The plasma membrane of a muscle cell; specialized for action potential propagation.
Sarcoplasm	The cytoplasm of a muscle cell; densely packed with myofibrils, mitochondria, and SR.
Sarcoplasmic reticulum (SR)	A specialized intracellular network of tubules that stores Ca2+ and contains Ca2+-ATPase pumps; functionally analogous to the endoplasmic reticulum.
T-tubule	An invagination of the sarcolemma that conducts the action potential into the cell interior, contacting the SR at the triad.
Triad	The structural unit formed by one T-tubule and two flanking SR terminal cisternae; the site of excitation–contraction coupling.
Intermyofibrillar mitochondria	Mitochondria distributed in arrays between myofibrils; supply ATP for cross-bridge cycling.
Subsarcolemmal mitochondria	Mitochondria packed beneath the sarcolemma; supply ATP for SR Ca2+-ATPase activity.
Myofibril	A long, cylindrical chain of sarcomeres within a muscle fiber; many myofibrils together fill most of the fiber volume.
Sarcomere	The functional contractile unit of striated muscle, defined between two Z-discs; contains overlapping actin and myosin filaments.
Z-disc (Z-line)	The structural boundary of a sarcomere; anchors actin (thin) filaments.
Actin (thin filament)	The thin contractile filament; provides binding sites for myosin heads. Binding sites are blocked by tropomyosin in the absence of Ca2+.
Myosin (thick filament)	The thick contractile filament; its globular heads form cross-bridges with actin and undergo the power stroke.
Titin	A giant elastic structural protein that maintains sarcomere alignment and contributes to passive tension.
Troponin	A regulatory protein bound to actin/tropomyosin; binds Ca2+ and exposes actin binding sites for cross-bridge formation.
Cross-bridge cycle	The six-step molecular cycle of myosin head attachment, power stroke, ADP release, ATP binding, and hydrolysis-driven re-cocking that produces sarcomere shortening.
Power stroke	The conformational change in the myosin head, triggered by Pi release, that ratchets the actin filament past myosin.
Rigor	The state in which actin and myosin remain tightly bound because no ATP is available to release them; underlies rigor mortis.
Sliding filament model	The model that explains sarcomere shortening as the result of cross-bridge cycling sliding actin past myosin without changes in filament length.
Excitation–contraction coupling	The sequence linking sarcolemmal action potentials to Ca2+ release from the SR and, ultimately, cross-bridge activation.
SR Ca2+-ATPase (SERCA)	The pump that re-sequesters Ca2+ into the SR after activation; a major ATP consumer (~30–40% of isometric ATP cost).
Twitch	The brief mechanical response to a single action potential; its time course depends on Ca2+ release/uptake and cross-bridge kinetics.
Superfast (sonic) muscle	Highly specialized muscle (e.g., toadfish swim bladder, rattlesnake tail-shaker) capable of contraction rates >100 Hz; high SR fraction with rapid Ca2+ handling.
Zero-sum game	The principle that a fixed muscle-cell volume must be partitioned among myofibrils, SR, and mitochondria, so increases in one come at the expense of the others (Rome & Lindstedt 1998).
Type I fiber (slow oxidative)	Slow, fatigue-resistant fiber type with high mitochondrial density, low ATPase activity, and high efficiency.
Type IIa fiber (fast oxidative–glycolytic)	Fast fiber type with high mitochondrial density and intermediate fatigue resistance.
Type IIx fiber (fast glycolytic)	Fast fiber type with low mitochondrial density, high ATPase activity, high specific tension, and rapid fatigue.
Specific tension	Force generated per unit cross-sectional area of a muscle fiber; reflects myofibrillar volume fraction and isoform composition.
Vmax (maximum shortening velocity)	The maximum unloaded velocity at which a muscle fiber can shorten; primarily determined by myosin isoform.
Cross-reinnervation	Experimental swap of nerves between fast and slow muscles, demonstrating that fiber type can shift in response to the pattern of neural activation.
Mitochondrial volume density	The fraction of muscle cell volume occupied by mitochondria; increases with endurance training in all fiber types.
Hovering flight	Sustained flight in place; aerodynamically requires lift on both upstroke and downstroke and is among the most metabolically demanding forms of locomotion.
Pectoralis	The downstroke flight muscle in birds; in hummingbirds, composed exclusively of type IIa fibers and packed with giant mitochondria.
Supracoracoideus	The upstroke flight muscle in birds; functions via a tendon that loops over the shoulder.
Regional endothermy	The maintenance of elevated temperature in selected tissues (e.g., red swimming muscle, brain) by use of vascular countercurrent heat exchangers.
Rete mirabile	A vascular countercurrent heat exchanger that traps metabolic heat in tissues such as red swimming muscle in tunas and mackerel sharks.
Heater organ	A muscle (e.g., extraocular in some tunas) that has lost contractile function and dedicates its calcium cycling machinery to thermogenesis via futile Ca2+ cycling.
Torpor	A state of greatly reduced metabolic rate and body temperature; used by hummingbirds when ambient temperatures fall too low for muscle function.