Comprehensive Notes on Animal Movement, Locomotion, and Muscle Physiology

Muscle Structure and the Role of Microfilaments

• Muscle Cell Specialization: Muscle cells are biological units specialized to actively contract and produce tension. They serve as transducers, converting the chemical energy stored in adenosine triphosphate (ATP) into mechanical energy.
• Microfilaments and Actin:
  • Microfilaments are polymers composed of protein subunits.
  • Actin: Consists of two protofilaments comprised of $100$ globular subunits. Actin is found in particularly high concentrations within skeletal and cardiac muscle cells.
• Myosin Structure and Types:
  • Myosin is a long, rod-shaped molecule consisting of two protein chains.
  • Myosin Head: This is the globular part of the heavy meromyosin end.
  • Myosin-1: Refers to monomeric myosin.
  • Myosin-2: Refers to polymerized myosin molecules.
  • Structural components include the C-terminus, a coiled-coil of two $\alpha$ helices, a hinge region, light chains, and a bare zone. Dimensions mentioned include a width of $21415^{15015022222222222^{}}150\,nm$ for the length and $2\,nm$ for width segments.
• Regulatory Proteins:
  • Troponin and Tropomyosin: These proteins regulate the interaction between actin and myosin.
  • Tropomyosin: Forms a tube-like structure that covers the binding sites on the actin helix.
  • Troponin: Attaches to tropomyosin; it contains three subunits. It is the component that interacts with calcium ions ($Ca^{2+}$).

Classification and Anatomy of Muscle Types

• Primary Divisions:
  • Striated Muscle: Divided into Skeletal muscle and Cardiac muscle.
  • Smooth Muscle: Also referred to as unstriated muscle.
• Control Mechanisms: Muscles are further categorized as Voluntary (under conscious control) or Involuntary (not under conscious control).
• Functional Structures:
  • Muscle Fiber: A single muscle cell.
  • Myofibril: Sub-units within a muscle fiber containing the contractile apparatus.
  • Sarcomere: The basic functional unit of striated muscle, characterized by a highly ordered array of thick myosin and thin actin filaments.

Sarcomere Architecture and Membrane Systems

• Filament Arrangement:
  • Thin Filaments (Actin): Attached to the Z line at each end of the sarcomere.
  • Thick Filaments (Myosin): Extend in parallel between the ends of the sarcomere; they are held in place at the M band (or M line).
  • A Band: The dark band representing the full length of the thick myosin filaments.
  • I Band: The light band consisting only of thin actin filaments.
  • H Zone: The region within the A band where only thick filaments are present (no actin overlap).
• Internal Membrane Systems:
  • Sarcotubules (T-tubules): Form a ring around each individual sarcomere, facilitating the spread of electrical impulses.
  • Sarcoplasmic Reticulum (SR): An intracellular membrane system located between adjacent t-tubules that forms a sheath around each sarcomere. The SR is responsible for sequestering $Ca^{2+}$ and releasing it to initiate contraction.
  • Triad: A specialized complex consisting of the SR of one sarcomere, a t-tubule, and the SR of the adjacent sarcomere.

The Sliding Filament Model and Contraction Mechanics

• Mechanism of Shortening: Sarcomere contraction occurs through the sliding movement of adjacent thin actin and thick myosin filaments past one another.
• Filament Length Constancy: Crucially, neither the actin nor the myosin filaments change their individual lengths during the shortening of the sarcomere.
• Band Changes:
  • The A band always maintains the same length.
  • The I and H bands vary in length, becoming shorter during contraction and longer during relaxation.
• Cross-Bridge Cycle:
  1. Binding: The myosin cross-bridge (head) binds to a specific molecule on the actin filament.
  2. Power Stroke: The cross-bridge undergoes a conformational change (bends), pulling the thin myofilament inward toward the center of the sarcomere.
  3. Detachment: The cross-bridge detaches from the actin at the end of the power stroke and returns to its original conformation.
  4. Re-binding: The cross-bridge binds to a more distal actin molecule to repeat the cycle.
• Enzymatic Activity: The myosin head possesses ATPase activity, which provides the energy required for the power stroke.

Actin-Myosin Regulation and Excitation-Contraction Coupling

• Ionic Requirements: Contraction requires both $Ca^{2+}$ and magnesium ions ($Mg^{2+}+$).
  • $Mg^{2+}$: Acts as an enzymatic cofactor for ATPase activity.
  • $Ca^{2+}$: Serves a complex regulatory role. In vertebrate striated muscle, the system is actin-regulated.
• Regulation Steps:
  • In a relaxed state, tropomyosin physically covers the cross-bridge binding sites on actin.
  • When the muscle is excited, $Ca^{2+}$ is released and binds to troponin.
  • This binding causes a structural change in troponin I, which pulls the troponin-tropomyosin complex aside, exposing the binding sites.
• Excitation Process:
  1. Acetylcholine (ACh) is released by the motor neuron axon into the neuromuscular junction.
  2. ACh binds to receptors on the motor end plate, generating an Action Potential (AP).
  3. The AP propagates down the T-tubules, triggering the release of $Ca^{2+}$ from the lateral sacs of the SR.
  4. Elevated $Ca^{2+}$ levels elicit localized sarcomeric contraction.
• Relaxation Process:
  • Electrical stimulation ceases.
  • Acetylcholinesterase removes ACh from the junction.
  • $Ca^{2+}$ pumps in the SR actively take up the released ions.
  • The troponin-tropomyosin complex returns to its original blocking position.

Mechanical Properties, Stimulation, and Time Course

• Stimulation Thresholds:
  • Rheobase: The threshold voltage required for a very long duration impulse to elicit a muscle response.
  • Utilisation Time: The shortest duration of a rheobase-level stimulus that can elicit a contraction (noted as difficult to measure).
  • Chronaxie: The minimum duration required for a voltage that is $2 \times$ the rheobase to elicit a response; this serves as a measure of cell excitability.
• Contraction Types:
  • Isometric: Muscle develops tension without changing length.
  • Isotonic: Muscle changes length while maintaining constant tension.
  • Note: Most animal movements involve a combination of both.
• Twitch Dynamics:
  • Latency Period: The delay between electrical stimulation and the first detectable increase in tension.
  • Maximum Isometric Tension: Typically occurs $150\,msec$ after the stimulus.
  • Relaxation Completion: Usually finished after approximately $900\,msec$.
  • Active State: The period of force generation where intracellular $Ca^{2+}$ is high, cross-bridges are cycling, and ATP is being hydrolyzed.
• Muscle Categorization by Speed:
  • Fastest mammalian muscle: Eye oculomotor muscle.
  • Exceptionally fast non-mammal: Puffer fish sound production muscle.
  • Slow muscle: Sloth claw retractor muscle.
  • Generally very slow: many smooth muscles.
• Summation and Tetany:
  • Individual twitches are "all-or-none."
  • Summation: Occurs if a second twitch is initiated before the first has relaxed.
  • Incomplete Tetany: Successive contractions with discernible individual twitches.
  • Complete Tetany: Occurs when stimulation frequency reaches the fusion frequency, resulting in a smooth, continuous force generation.
  • Maximum Force: Striated muscles generate a relatively constant relative force per unit area of $10 - 30\,N\,cm^{-2}$.

Terrestrial Locomotion: Crawling, Walking, and Running

• Crawling Methods (e.g., snakes):
  • Serpentine crawling.
  • Concertina crawling.
  • Rectilinear locomotion.
  • Side winding.
• Walking and Running Mechanics:
  • Body mass is supported on specific points of contact with the substrate.
  • Body equilibrium must be maintained as legs are alternately raised and lowered.
• Evolutionary Trends:
  • Reptiles/Amphibians: Exhibit a sprawled posture with limbs placed laterally; the vertebral column undulates laterally.
  • Cursorial Locomotion: Evolution toward limb re-alignment (vertical flexion of column). Torsion brings digits forward in line with travel, increasing oscillation efficiency.
• Gait and Phase:
  • Gait: The pattern of limb movement (e.g., Pace/amble, Trot, Bound/pronk, Half-bound, Gallop).
  • Relative Phase (RF): The cycle offset between feet.
  • Duty Factor (DF): The fraction of the total cycle time that a specific foot is on the ground.
  • Examples:
    • Walking Biped: Left foot $RF = 0$, Right foot $RF = 0.5$, $DF = 0.6$ (on ground $60\%$ of cycle).
    • Walking Quadruped: Legs work in opposite phase; hind legs offset by $0.25$ from forelegs; $DF > 0.5$.
    • Trotting Horse: $DF < 0.5$.
• Speed Enhancement Strategies:
  • Fast running involves low duty factors.
  • Speed increases primarily by increasing stride length, and to a lesser extent, stride rate.
  • Limb Lengthening: Long distal limb elements and foot postures (Plantigrade: sole on ground; Digitigrade: digits support weight; Unguligrade: tips of toes).

Hopping, Jumping, and Metabolic Costs

• Hopping: Movement of both legs is in phase ($RF = 0$) with a low $DF$.
• Jumping: Characterized by extreme disequilibrium and very low $DF$.
  • Height is limited by body mass and velocity at takeoff.
  • Aerodynamic drag acts as a decelerating force decreasing jump height.
• Metabolic Cost of Transport ($COT_{net}$):
  • Metabolic rate usually increases linearly with velocity in terrestrial locomotion.
  • The relationship becomes non-linear if the animal changes gait.
  • Cost calculation: $VO_2\,locomotion - VO_2\,rest$.
  • Small animals expend relatively more energy on locomotion than large animals.
• Burrowing:
  • Exhibits a significantly higher metabolic cost than walking or running.
  • Compaction Effects: Cost increases with soil density. For the species N. kunapalari, the regression for net cost of transport is $y = 0.0107x + 0.004$ ($R^2 = 0.9937$), where $x$ is compaction level in $kg/cm^{22}$ and $y$ is $cm^{33}\,O_2/g/cm$.

Aquatic Locomotion: Principles and Reynold's Number

• Environmental Constraints: Fluid density provides buoyancy; movement creates drag and lift forces.
• Thrust Generation: Depends on medium properties, organism size, and swimming mechanism.
• Reynold’s Number ($Re$):
  • The ratio of inertial forces to viscous forces.
  • Small $Re$: Viscous forces predominate (e.g., small organisms).
  • Large $Re$: Inertial forces predominate (e.g., large swimmers).
• Drag Characteristics:
  • At low $Re$: Drag is proportional ($\propto$) to velocity ($v$).
  • At high $Re$: Drag is proportional to square of velocity ($v^2$).
  • Total drag = Friction drag (surface) + Pressure drag.
• Lift vs. Drag in Swimming:
  • Undulatory movements push against the medium generating drag-based thrust.
  • Hydrodynamic force of the tail = Drag (parallel) + Lift (perpendicular).
  • Hydrofoils: Stiff, wing-shaped tails (used by larger animals like penguins, whales, and turtles) emphasize lift forces to produce thrust.

Energetics of Swimming and Flight

• Metabolic Cost of Swimming:
  • Power requirement ($P$) = $Drag \times velocity$.
  • At high $Re$, $P \propto v^3$.
  • Swimming at the surface is more expensive due to energy dissipation in the wake.
• Principles of Aerial Locomotion:
  • Air has lower density and higher kinematic viscosity than water.
  • Water Viscosity ($\eta$): $1.002 \times 10^{-6}\,kg\,m^{-1}s^{-1}$; Air Viscosity: $0.813 \times 10^{-5}\,kg\,m^{-1}s^{-1}$.
  • Water Density (p): $998.2\,kg\,m^{-3}$; Air Density: $1.205\,kg\,m^{-3}$.
• Gliding Dynamics:
  • Relies on an aerofoil to produce lift ($L$) significantly greater than drag ($D$).
  • Glide Ratio: Calculated as $L/D$ or the cotangent of the glide angle.
  • Gliding gecko ($45^{\circ}$, ratio $1$); Flying lemur ($5^{\circ}$, ratio $11$); Albatross ($3^{\circ}$, ratio $19$).
  • No direct metabolic cost for the glide itself, but energy is used to keep membranes/wings rigid.
• Flapping Flight:
  • Total Aerodynamic Power ($P_{total}$) = Parasite power ($P_{par}$) + Induced drag ($P_{in}$) + Profile power ($P_{pro}$).
  • There is a specific velocity for minimum power expenditure.
  • The cost of hovering is notably high.

Comparative Energetic Analysis

• Order of Economy (Most to Least):
  1. Swimming: The most economical mode because of buoyancy.
  2. Flying: More economical than terrestrial travel.
  3. Walking/Running: Relatively expensive.
  4. Burrowing: Most expensive; involves moving substrate and overcoming soil compaction.
• Note: All comparisons utilize mass-specific rates to normalize data across different animal sizes.