Animal Physiology Unit 6 ReviewMuscle Physiology and Locomotion

Key Concepts and Terminology

• Sarcomere: the basic functional unit of a muscle fiber consists of thick and thin filaments arranged in a repeating pattern
• Myosin: a motor protein that forms the thick filaments in sarcomeres interacts with actin to generate muscle contraction
• Actin: a protein that forms the thin filaments in sarcomeres binds to myosin during muscle contraction
• Neuromuscular junction: the synapse between a motor neuron and a muscle fiber where neurotransmitters are released to initiate muscle contraction
• Sliding filament theory: explains muscle contraction through the sliding of myosin and actin filaments past each other shortening the sarcomere
• Excitation-contraction coupling: the process by which an action potential in a motor neuron leads to muscle contraction
• Adenosine triphosphate (ATP): the primary energy source for muscle contraction
  • Hydrolysis of ATP provides the energy needed for myosin to bind to actin and generate force

Muscle Structure and Types

• Skeletal muscle: voluntary muscle attached to bones via tendons controlled by the somatic nervous system (biceps, quadriceps)
  • Composed of long, multinucleated muscle fibers arranged in parallel
  • Exhibits a striated appearance due to the organized arrangement of sarcomeres
• Cardiac muscle: involuntary muscle found in the heart wall controlled by the autonomic nervous system
  • Consists of branched, uninucleated muscle cells connected by intercalated discs
  • Exhibits a striated appearance similar to skeletal muscle
• Smooth muscle: involuntary muscle found in the walls of blood vessels and internal organs (intestines, uterus) controlled by the autonomic nervous system
  • Composed of spindle-shaped, uninucleated muscle cells
  • Lacks the striated appearance of skeletal and cardiac muscle due to the irregular arrangement of sarcomeres
• Connective tissue: surrounds and supports muscle fibers
  • Endomysium: surrounds individual muscle fibers
  • Perimysium: surrounds bundles of muscle fibers (fascicles)
  • Epimysium: surrounds the entire muscle

Muscle Contraction Mechanism

• Sliding filament theory: myosin and actin filaments slide past each other shortening the sarcomere and generating muscle contraction
• Cross-bridge cycling: the cyclic attachment and detachment of myosin heads to actin filaments
  1. Myosin head binds to actin forming a cross-bridge
  2. Myosin head pivots pulling the actin filament towards the center of the sarcomere (power stroke)
  3. ATP binds to the myosin head causing it to detach from actin
  4. ATP hydrolysis resets the myosin head for the next cycle
• Calcium regulation: the release and uptake of calcium ions (Ca2+) by the sarcoplasmic reticulum controls muscle contraction
  • Troponin and tropomyosin: regulatory proteins associated with actin filaments
    • In the absence of Ca2+, tropomyosin blocks the myosin-binding sites on actin preventing contraction
    • Ca2+ binds to troponin causing a conformational change that moves tropomyosin exposing the myosin-binding sites on actin allowing contraction to occur
• Excitation-contraction coupling: the process linking an action potential in a motor neuron to muscle contraction
  1. Action potential arrives at the neuromuscular junction releasing acetylcholine
  2. Acetylcholine binds to receptors on the muscle fiber membrane depolarizing it
  3. Depolarization spreads through the T-tubules triggering Ca2+ release from the sarcoplasmic reticulum
  4. Ca2+ binds to troponin initiating muscle contraction

Energy Systems in Muscle Function

• ATP: the immediate energy source for muscle contraction
  • Stored in limited quantities within muscle fibers
  • Quickly depleted during high-intensity exercise
• Creatine phosphate (CP): a high-energy phosphate compound that serves as a rapid source of ATP regeneration
  • Creatine kinase catalyzes the transfer of a phosphate group from CP to ADP forming ATP
  • Provides energy for short-duration, high-intensity activities (weightlifting, sprinting)
• Glycolysis: the anaerobic breakdown of glucose to pyruvate producing ATP
  • Occurs in the cytoplasm of muscle cells
  • Produces a net gain of 2 ATP per glucose molecule
  • Pyruvate can be further metabolized aerobically in the mitochondria or converted to lactate anaerobically
• Oxidative phosphorylation: the aerobic production of ATP in the mitochondria
  • Utilizes the electron transport chain and chemiosmosis to generate a proton gradient driving ATP synthesis
  • Requires oxygen and yields a high amount of ATP (up to 38 ATP per glucose molecule)
  • Predominant energy system for low-intensity, long-duration activities (marathon running, cycling)
• Lactate fermentation: the anaerobic conversion of pyruvate to lactate regenerating NAD+ for continued glycolysis
  • Occurs when oxygen supply is limited during high-intensity exercise
  • Allows for continued ATP production through glycolysis but can lead to muscle fatigue and soreness

Neuromuscular Junction and Control

• Motor neuron: a nerve cell that innervates muscle fibers controlling their contraction
  • Cell body located in the spinal cord or brainstem
  • Axon extends to the muscle forming the neuromuscular junction
• Neuromuscular junction: the synapse between a motor neuron and a muscle fiber
  • Presynaptic terminal: contains synaptic vesicles filled with acetylcholine
  • Synaptic cleft: the space between the presynaptic terminal and the muscle fiber membrane
  • Postsynaptic membrane: contains acetylcholine receptors on the muscle fiber
• Neurotransmitter release: the process by which acetylcholine is released from the presynaptic terminal
  1. Action potential arrives at the presynaptic terminal
  2. Voltage-gated calcium channels open allowing Ca2+ influx
  3. Ca2+ triggers the fusion of synaptic vesicles with the presynaptic membrane releasing acetylcholine into the synaptic cleft
• Acetylcholine receptor activation: the binding of acetylcholine to receptors on the postsynaptic membrane
  • Causes the opening of ligand-gated sodium channels depolarizing the muscle fiber membrane
  • Acetylcholinesterase rapidly degrades acetylcholine terminating its action
• Motor unit: a motor neuron and all the muscle fibers it innervates
  • All muscle fibers within a motor unit are of the same type and contract simultaneously
  • Smaller motor units (fewer muscle fibers) allow for fine motor control
  • Larger motor units (more muscle fibers) generate greater force but with less precision

Muscle Fiber Types and Adaptations

• Type I (slow-twitch) fibers: adapted for low-intensity, long-duration activities (endurance running, cycling)
  • High mitochondrial density and capillary supply for efficient aerobic metabolism
  • High myoglobin content for oxygen storage
  • Slow contraction velocity but resistant to fatigue
• Type II (fast-twitch) fibers: adapted for high-intensity, short-duration activities (sprinting, weightlifting)
  • Type IIa fibers: intermediate properties between Type I and Type IIx fibers
    • Moderate mitochondrial density and capillary supply
    • Faster contraction velocity than Type I fibers but more fatigue-resistant than Type IIx fibers
  • Type IIx fibers: optimized for maximal force production and rapid contraction
    • Low mitochondrial density and capillary supply
    • High glycolytic enzyme activity for anaerobic metabolism
    • Fastest contraction velocity but rapidly fatigued
• Fiber type composition: the relative proportion of Type I, Type IIa, and Type IIx fibers within a muscle
  • Varies among individuals and can be influenced by genetics and training
  • Muscles involved in postural control (soleus) have a higher proportion of Type I fibers
  • Muscles involved in explosive movements (gastrocnemius) have a higher proportion of Type II fibers
• Muscle fiber adaptations to training: specific types of exercise can induce changes in muscle fiber properties
  • Endurance training: increases mitochondrial density, capillary supply, and myoglobin content in Type I and Type IIa fibers improving aerobic capacity
  • Resistance training: increases muscle fiber size (hypertrophy) and force production primarily in Type II fibers

Locomotion Principles and Biomechanics

• Lever systems: the arrangement of bones, joints, and muscles that allows for movement and force generation
  • First-class lever: fulcrum between the effort and the load (rare in the human body)
  • Second-class lever: load between the fulcrum and the effort (e.g., ankle joint during standing)
  • Third-class lever: effort between the fulcrum and the load (e.g., elbow joint during biceps curl)
• Force-velocity relationship: the inverse relationship between the force a muscle can generate and the velocity of its contraction
  • Maximum force is produced during isometric contractions (no change in muscle length)
  • As contraction velocity increases, the force-generating capacity of the muscle decreases
• Length-tension relationship: the relationship between a muscle's length and its force-generating capacity
  • Optimal force production occurs at the muscle's resting length where there is maximal overlap between myosin and actin filaments
  • As the muscle is stretched or shortened from its resting length, force production decreases
• Elastic energy storage: the storage and release of energy in the elastic components of muscle and tendon
  • During eccentric contractions, elastic energy is stored in the muscle-tendon unit
  • During subsequent concentric contractions, the stored elastic energy is released enhancing force production and efficiency (e.g., running, jumping)
• Gait patterns: the cyclic patterns of limb movement during locomotion
  • Walking: characterized by alternating swing and stance phases for each limb with at least one foot always in contact with the ground
  • Running: characterized by alternating swing and stance phases for each limb with a brief aerial phase where both feet are off the ground
  • Galloping: an asymmetrical gait pattern used by quadrupedal animals involving a leading limb and a trailing limb on each side of the body

Comparative Muscle Physiology Across Species

• Scaling of muscle properties: the relationship between an animal's size and its muscle properties
  • Smaller animals tend to have higher mass-specific metabolic rates and faster contraction velocities
  • Larger animals tend to have lower mass-specific metabolic rates and slower contraction velocities but generate more absolute force
• Insect flight muscles: specialized muscles that power the high-frequency wing movements in insects
  • Asynchronous flight muscles: contract multiple times per nerve impulse allowing for high-frequency contractions (up to 1000 Hz in some species)
  • Synchronous flight muscles: contract once per nerve impulse providing greater control but at lower frequencies
• Fish swimming muscles: adapted for efficient locomotion in an aquatic environment
  • Red muscle: slow-twitch, aerobic muscle used for sustained swimming
  • White muscle: fast-twitch, anaerobic muscle used for burst swimming and rapid acceleration
• Avian flight muscles: optimized for the demands of powered flight
  • Pectoralis major: the primary downstroke muscle responsible for generating lift
  • Supracoracoideus: the primary upstroke muscle that elevates the wing
  • High mitochondrial density and capillary supply for efficient aerobic metabolism
• Mammalian skeletal muscles: diverse in their properties and functions depending on the species and its locomotor adaptations
  • Cursorial mammals (e.g., horses, cheetahs): long, slender limbs with muscles optimized for efficient running
  • Fossorial mammals (e.g., moles, badgers): short, powerful limbs with muscles adapted for digging
  • Arboreal mammals (e.g., monkeys, squirrels): flexible limbs and grasping muscles for climbing and maneuvering in trees