Muscle Physiology: Structure, Function, and Types of Muscle

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

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 body movement. Characterized by visible stripes (striations) under the microscope.
Cardiac Muscle: Involuntary, striated muscle found only in the heart. Specialized for rhythmic contraction.
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 the contractile machinery.
Sarcolemma: The plasma membrane of a muscle fiber.
Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum for calcium storage and release.
T-tubules: Invaginations of the sarcolemma that help transmit action potentials.

Neuromuscular Junction

The neuromuscular junction is the synapse between a motor neuron and a skeletal muscle fiber, crucial for initiating muscle contraction.

Acetylcholine (ACh) is released from the motor neuron, binds to receptors on the muscle fiber, and generates an End Plate Potential (EPP).
The EPP triggers an action potential in the muscle fiber, leading to contraction.

Sarcomere Components

The sarcomere is the fundamental contractile unit of striated muscle, composed of organized protein filaments.

Thick Filaments (Myosin): Myosin molecules form thick filaments, with heads that bind to actin and possess ATPase activity.
Thin Filaments (Actin): Composed of two strands of F actin in a double helix, with binding sites for myosin.
Tropomyosin: Covers 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 structural support.

Steps in Muscle Contraction (Excitation-Contraction Coupling)

Muscle contraction is initiated by a sequence of events linking neural stimulation to mechanical shortening.

Motor neuron action potential (AP) arrives at the neuromuscular junction.
End plate potential (EPP) is generated in the muscle fiber.
Increase in intracellular calcium levels via release 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.

Crossbridge Cycling

Crossbridge cycling is the process by which myosin heads bind to actin, perform a power stroke, and detach, using ATP.

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.
Cycle repeats as long as calcium and ATP are present.

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 myoplasmic calcium closes SR calcium channels.
SERCA pumps actively transport calcium back into the SR.
Calcium dissociates from troponin, tropomyosin covers myosin binding sites.

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

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 mix of both fiber types.

Glycolytic vs. Oxidative Fibers

Property	Glycolytic (Anaerobic)	Oxidative (Aerobic)
ATP Generation	Glycolysis	Oxidative Phosphorylation
Mitochondria	Few	Many
Diameter	Large	Small
Color	Lighter	Darker (due to myoglobin)
Enzyme Concentration	High glycolysis enzymes	Low glycolysis enzymes
Myoglobin	Low	High

Types of Skeletal Muscle Fibers and Exercise

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

Exercise and Muscle Fatigue

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

Three Types of Muscle: Structural Comparison

Type	Striations	Nuclei	Special Features
Skeletal	Present	Multinucleated	Neuromuscular junctions
Cardiac	Present	Single nucleus	Intercalated disks
Smooth	Absent	Single nucleus	Dense bodies, gap junctions

Smooth Muscle Anatomy and Contraction

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

Contraction occurs via crossbridge cycling, but actin and myosin are anchored to dense bodies rather than Z lines.

Smooth Muscle Contraction Mechanism

Calcium Source: Skeletal muscle uses internal Ca2+ from the SR; smooth muscle uses mostly external Ca2+ entering the cell.
Calcium binds to calmodulin, activating myosin light chain kinase (MLCK), which phosphorylates myosin and enables crossbridge cycling.

Shutting Off Smooth Muscle Contraction

Inactivation of myosin by phosphatases, which remove phosphate groups from myosin light chains, terminating contraction.

Key Equations

ATP Hydrolysis by Myosin: $ \text{ATP} \rightarrow \text{ADP} + \text{P}_i $
Force Generation (Isometric vs. Isotonic): $ \text{Isotonic:} \quad F_{\text{muscle}} \geq F_{\text{load}} $ $ \text{Isometric:} \quad F_{\text{muscle}} < F_{\text{load}} $

Additional info: Muscle physiology is a foundational topic in biological psychology, especially in understanding the biological basis of movement and behavior.