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• Bilateral Symmetry, Skeletal Muscle Structure, Regulation of Contraction, Specialized Muscles, Force Production, Invertebrate Muscles, Energetics of Contraction, and Ecophysiology of Locomotion are all important concepts.

Active & Non-Active Animals

• Movement was initially thought to define life, but this is false
• Sessile animals are fixed in one place and don't move freely, such as sponges, corals, and barnacles
• Planktonic animals are free-floating organisms carried by currents with limited swimming ability, like jellyfish and krill
• Active animals can move independently by swimming, crawling, or flying and work against gravity
• Corals can use motor proteins to move their arms via ATP usage.

Symmetry in Organisms

• Asymmetry means the body has no symmetry and an irregular shape, found in sponges, which are usually sessile
• Circular Symmetry means body parts are arranged in a circle around a center point, common in microscopic and sessile or planktonic organisms
• Radial Symmetry means body parts are arranged around a central axis and can be divided into multiple similar sections, evident in sea anemones and starfish, often sessile or planktonic
• Bilateral Symmetry means the body can be divided into two equal halves along a single axis, with a right and left side, a front and back, and a head where the main processing organs are often located
• Streamlining creates a smooth, aerodynamic/hydrodynamic body shape for directional movement.
• Radial symmetric organisms like sea stars can move equally well in multiple directions
• Vertebrates and insects are bilaterally symmetrical and found in almost all groups
• Active movement is linked to evolution of bilateral symmetry
• Bilateral symmetry is ancient but has been lost at least 4 times, such as in Cnidarians, thought to be secondarily radial symmetric

Striated vs. Smooth Muscles

• Striated muscles are long, cylindrical, multinucleated cells with a striated appearance under a microscope due to organized sarcomere structure
• Striated muscles are responsible for voluntary movements (somatic NS) and generate force by contracting and pulling bones using tendons
• Cardiac muscle is a specialized striated muscle that is branched, has a single nucleus per cell, is connected via intercalated discs, and exhibits involuntary, rhythmic movement (autonomic NS)
• Smooth muscles are spindle-shaped, uninucleate cells without sarcomere organization, arranged in layers around organs and blood vessels

Vertebrate Skeletal Muscles

• Skeletal and cardiac muscles are striated and attached to bones via different mechanisms depending on the organism
• Muscles are organized into multiple levels: muscle (organ level), fascicles (tissue level), and muscle fibers (cellular level)
• Muscles consist of bundles (fascicles) surrounded by connective tissue (epimysium)
• Fascicles are bundles of individual muscle fibers surrounded by connective tissue (perimysium)
• Muscle fibers are long, cylindrical, multinucleated cells surrounded by connective tissue (endomysium) and contain myofibrils
• Myofibrils are subcellular, rod-like structures made of repeating contractile units called sarcomeres
• Sarcomeres are the basic contractile unit, composed of thick (myosin) and thin (actin) filaments, with Z-lines marking the boundaries
• T-tubules, extensions of the sarcoplasmic reticulum storing Ca2+, help transmit nerve impulses deep into the muscle

Sarcomere Components

• The sarcomere is the functional unit composed of thick and thin filaments
• Thick filaments are made of myosin, featuring two head-groups with only one biologically active
• Thin filaments are made of actin
• G-actin units form F-actin, where two F-actin structures intertwine in a helical structure containing troponin and tropomyosin
• The Z-line/disc marks the sarcomere boundaries
• The M-line is where myosin filaments are anchored
• The I-band is the light band containing only actin
• The A-band is the dark band containing both myosin and thin filaments
• The H-zone is the central region of the A-band where only myosin is present and disappears during contraction

Sarcomere Function and Sliding Filament Theory

• The sarcomere structure is highly organized and looks different at different stages
• How the sarcomere works is defined by the Sliding Filament Theory by Huxley & Huxley
• Nerve impulse arrival leads to ACh release at the neuromuscular junction, triggering Ca2+ release from the sarcoplasmic reticulum
• Dihydropyridine receptors activate and open Ryanodine receptors in the SR membrane, releasing Ca2+ into the Moplasm
• Ca2+ binds to troponin, shifting tropomyosin and freeing actin binding sites
• Myosin starts in a low-energy state (45°) without ATP bound
• ATP binding leads to a cocked state (90°) of the myosin head, positioning it to bind to actin
• ATP hydrolysis to ADP+Pi enables new binding, requiring Mg2+
• The process forms Actomyosin (strong bond)
• Myosin releases Pi and tilts back to 45°, pulling the actin filament during filament sliding
• A new ATP molecule binds to myosin, releasing actin and restarts the cycle upon nerve signal cessation and Ca2+ being pumped back

Energetics of Muscle Contraction

• Tropomyosin covers actin's binding sites again to return the sarcomere to its original length
• Sarcomere length at rest and force generation is similar across vertebrates
• Max force generation occurs at 2 – 2.2 µm sarcomere length
• A dose dependency of Ca2+ can be observed
• Actin and myosin can bind to each other if Ca2+ frees binding sites by redirect tropomyosin to form strong bond between filaments
• Rigor mortis is when no new ATP results in stiff muscles due to the irreversible actin-myosin bond
• Different AP frequencies lead to different muscle tensions.
• A single AP leads to a twitch
• Low-frequency APs = continuous twitching
• High-frequency APs = muscle tension quickly reaches maximum (= tetanus)
• Red muscles are slow-twitch, Type I fibers & use aerobic/oxidative metabolism for sustained contraction
• White muscles are fast-twitch, Type II fibers, use anaerobic respiration (glycolysis) for quick, powerful movement

Muscle Types & Potential

• Fast oxidative Type Ila fibers are a mix between red and white muscles, using both aerobic and anaerobic metabolism
• Aerobic/endurance training (running, swimming) increases red muscle efficiency
• Strength training (lifting weights, sprinting) enhances white muscle power
• There is huge potential in plasticity for muscles
• In vertebrates, a single AP leads to a single twitch of uniform size, with force summoned through APs
• In invertebrates, AP size differs, which means contraction is modulated in shape/size

Adaptive Movement

• Muscles adapt to maximize efficiency in various environments
• Frogs always jump in a standardized way
• Muscles operate at optimal activation level during jumping
• The skeletal system is specially adapted for jumping which includes having elongated hind limbs for a bigger lever effect
• The pelvis is highly flexible
• Last vertebrae are fused together, building a rod-like structure (urostyle) providing a rigid base for better force
• Frogs jump at the perfect sarcomere length (~2µm) and velocity and prioritize power
• Toadfish make high noises using superfast muscles (SFM) contracted 100-250 Hz.
• Toadfish have specialized Ca2+ handling, with a highly developed sarcoplasmic reticulum (SR), allowing rapid Ca2+ cycling with high ATP turnover rate
• Neurons may use a strong ACh-esterase clearing neuromuscular junction
• Toadfish swimming involves two types of muscles: Type I (slow-twitch oxidative), and Type IIb = fast-twitch glycolytic spiraled inside the body mainly bending at a strong angle to quickly get away.
• During escape response the muscles use excessive overlap between myosin & actin which blocks some binding-sites
• Contraction speed but not efficiency prioritized, they are compensating for the lower force on the contracting site

Insect Flight Muscles

• Insect flight muscles have high power & high frequency
• Synchronous flight muscles, where there are muscles that attach directly to the wings under neurogenic control (one nerve impulse = one contraction) which leads to a slower wingbeat (100-200 Hz)
• Asynchronous flight muscles don't attach to the wings. The wings move by deforming the thorax, which increases the wingbeat frequency
• Contraction of one stretches activates the other, creating a cyclic repetition
• These muscles avoid rate limitations & decoupling neurons from muscle contraction which are needed in a certain range
• A solution is flying insects need to pee often.
• Clams use two adductor muscles to open and close the shell and is connected by a specialized catch muscle mechanism

Physical Factors Affecting Locomotion

• Clam muscles can induce rigor mortis, requiring serotonin to exit that state and avoid predators
• Clam energy =0.3% energy of vertebrate
• Animal movement follows scaling principles
• Small animals move quickly for their size but have lower absolute speed
• Larger animals have greater inertia & greater momentum: hard to start movement
• Running the ground which gravity affects
• Swimming has more drag forces where streamlining affects
• Flying needs balanced lift and drag
• Medium has a big influence on movement.
• Shape/mass & locomotion determine the cost of transport across the whole animal kingdom.
• There is less transport as the mass is increasing.
• Swimming is > flying > running