Bio 2160 Ch.10

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Muscle Physiology

Types of Muscle

Muscle tissue in the human body is classified into three main types, each with distinct anatomical and physiological properties.

Skeletal Muscle: Voluntary, striated muscle responsible for movement of the skeleton.
Cardiac Muscle: Involuntary, striated muscle found only in the heart.
Smooth Muscle: Involuntary, non-striated muscle found in walls of hollow organs (e.g., intestines, blood vessels).

Skeletal Muscle Structure

Skeletal muscle is organized in a hierarchical structure from the whole muscle down to the molecular level.

Muscle Fiber: Each muscle fiber is a single, multinucleated cell.
Myofibrils: Rod-like structures within muscle fibers containing contractile proteins.
Sarcolemma: The plasma membrane of a muscle fiber.
Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum that stores calcium ions.
Fascicle: Bundle of muscle fibers surrounded by connective tissue.

Skeletal Muscle Fiber Structure

Muscle fibers contain myofibrils, mitochondria, and are innervated by motor neurons at the neuromuscular junction.

Thick Filament: Composed of myosin protein.
Thin Filament: Composed of actin protein.
T-tubules: Invaginations of the sarcolemma that help transmit action potentials.
Neuromuscular Junction: Synapse between a motor neuron and a muscle fiber, where acetylcholine (ACh) is released to initiate contraction.

Sarcomere Components

The sarcomere is the fundamental contractile unit of striated muscle, defined by the region between two Z lines.

Thick Filaments (Myosin): Myosin molecules form crossbridges with actin and have ATPase activity.
Thin Filaments (Actin): F actin forms a double helix; contains binding sites for myosin.
Tropomyosin: Blocks myosin binding sites on actin in the absence of calcium.
Troponin: Binds calcium, actin, and tropomyosin; regulates access to myosin binding sites.
Titin: Provides elasticity and stabilizes thick filaments.

Steps in Muscle Contraction (Excitation-Contraction Coupling)

Muscle contraction is initiated by a sequence of events that link neural stimulation to muscle fiber contraction.

Motor neuron action potential (AP) arrives at the neuromuscular junction.
End plate potential (EPP) is generated by ACh binding to receptors on the muscle fiber.
Increase in intracellular calcium levels, released from the SR.
Calcium binds to troponin, causing tropomyosin to move and expose myosin binding sites on actin.
Crossbridge cycling occurs, resulting in sliding of filaments and muscle contraction.

Muscle Contraction: End Plate Potentials (EPPs)

EPPs are local depolarizations at the neuromuscular junction that trigger muscle action potentials.

Acetylcholine (ACh) is released from the motor neuron and binds to receptors on the muscle fiber, initiating EPP.
Action potential propagates along the sarcolemma and into T-tubules.

Muscle Cell Calcium Elevation

Calcium release from the SR is essential for muscle contraction.

Action potential in T-tubules activates voltage-sensitive receptors, leading to calcium release from the SR.
Elevated calcium enables crossbridge formation between actin and myosin.

Troponin/Tropomyosin Conformational Changes

Calcium binding to troponin induces a conformational change, moving tropomyosin and exposing myosin binding sites on actin.

Allows myosin heads to bind actin and initiate contraction.

Crossbridge Cycling

Crossbridge cycling is the process by which myosin heads bind to actin, perform a power stroke, and release, resulting in muscle contraction.

ATP binds to myosin, causing detachment from actin.
ATP hydrolysis cocks the myosin head.
Myosin binds to actin, releases ADP and Pi, and performs the power stroke.

Termination of Muscle Contraction

Muscle contraction ends when neural stimulation ceases and calcium is removed from the cytoplasm.

Motor neuron input and EPPs terminate.
High calcium concentration closes SR calcium channels.
SERCA pumps actively transport calcium back into the SR.
Calcium dissociates from troponin, and tropomyosin covers myosin binding sites on actin.

The Twitch

A twitch is the mechanical response of a muscle fiber, motor unit, or whole muscle to a single action potential.

Measured as a change in force or tension.

Isometric vs. Isotonic Twitches

Muscle twitches can be classified based on whether the muscle shortens during contraction.

Isotonic Twitch: Muscle shortens and lifts a load; load must be less than or equal to muscle tension.
Isometric Twitch: Muscle generates force but does not shorten; load exceeds muscle tension.

Slow Twitch vs. Fast Twitch Fibers

Muscle fibers differ in their contraction speed and metabolic properties.

Slow Twitch Fibers: Contain slow myosin; hydrolyze ATP slowly; suited for endurance activities.
Fast Twitch Fibers: Contain fast myosin; hydrolyze ATP quickly; suited for rapid, powerful movements.
Skeletal muscles may contain a mixture of both fiber types.

Glycolytic vs. Oxidative Fibers

Muscle fibers are also classified by their primary metabolic pathway for ATP production.

Property	Glycolytic (Anaerobic)	Oxidative (Aerobic)
ATP Production	Glycolysis	Oxidative Phosphorylation
Glycolysis Enzymes	High	Low
Mitochondria	Few	Many
Myoglobin	Low	High
Diameter	Large	Small
Color	Lighter	Darker

Types of Skeletal Muscle Fibers and Exercise

Exercise can induce changes in muscle fiber properties.

Low Intensity Exercise: Increases oxidative capacity, size and number of mitochondria, decreases fiber diameter, increases capillary density, and can convert glycolytic fibers to oxidative fibers.
High Intensity Exercise: Increases glycolytic capacity, glycolysis enzymes, fiber diameter, number of myofibrils, decreases oxidative capacity, and can convert oxidative fibers to glycolytic fibers.

Exercise and Muscle Fatigue

Muscle fatigue occurs when muscles can no longer sustain contraction.

Low Intensity Exercise (Aerobic): Fatigue due to depletion of energy reserves (glycogen).
High Intensity Exercise (Anaerobic): Fatigue due to lactic acid buildup; very high intensity may cause depletion of acetylcholine (ACh), leading to neuromuscular fatigue.

Three Types of Muscle: Histology and Anatomy

Microscopic and anatomical features distinguish skeletal, cardiac, and smooth muscle.

Skeletal Muscle: Striated, multinucleated, neuromuscular junctions.
Cardiac Muscle: Striated, single nucleus per cell, intercalated disks, gap junctions.
Smooth Muscle: Non-striated, single nucleus, gap junctions, dense bodies.

Smooth Muscle Anatomy

Smooth muscle lacks striations and sarcomeres; actin and myosin are arranged in multiple axes and connect to dense bodies.

Contractions occur via crossbridge cycling.
Dense bodies anchor actin and myosin to the cell membrane and connective tissue.

Smooth Muscle Contraction

Smooth muscle contraction differs from skeletal muscle primarily in the source of calcium.

Skeletal Muscle: Calcium released from internal stores (SR).
Smooth Muscle: Calcium enters from extracellular space.

Smooth Muscle Excitation-Contraction Coupling

Calcium binds to calmodulin, activating myosin light chain kinase (MLCK), which phosphorylates myosin and enables crossbridge cycling.

Phosphorylation of myosin light chain is required for contraction.

Shutting Off Smooth Muscle Contraction

Contraction is terminated by inactivation of myosin via phosphatases, which remove phosphate groups from myosin light chain.

Dephosphorylated myosin cannot interact with actin, ending contraction.

Key Equations

ATP Hydrolysis by Myosin:
Force Generation (Isometric vs. Isotonic):

Example

During a biceps curl, skeletal muscle fibers contract via excitation-contraction coupling, generating force to lift a weight (isotonic contraction). If the weight is too heavy to lift, the muscle contracts without shortening (isometric contraction).

Additional info: Muscle physiology is foundational for understanding movement, motor control, and neuromuscular disorders in psychology and neuroscience.