Muscle and Muscle Tissue: Structure, Function, and Physiology (Chapter 9 Study Notes)

Muscle and Muscle Tissue: Chapter 9

Overview of Muscle Tissue

Muscle tissue is a fundamental component of the human body, responsible for movement, posture, and various physiological functions. Nearly half of the body's mass is muscle, which can transform chemical energy into mechanical work.

• Types of muscle tissue: Skeletal, cardiac, and smooth

• Characteristics of muscle tissue: Excitability, contractility, extensibility, elasticity

• Functions of muscle tissue: Movement, posture, joint stabilization, heat generation

Terminology and Muscle Fiber Types

Muscle cell terminology often uses prefixes such as myo-, mys-, and sarc- (e.g., sarcoplasm is muscle cell cytoplasm). The basic unit of muscle tissue is the muscle fiber, a long, cylindrical cell.

Types of Muscle Tissue

There are three main types of muscle tissue, each with distinct structure and function.

• Skeletal Muscle

  • Attached to bones, responsible for voluntary movement

  • Striated appearance due to organized myofibrils

• • Key words: skeletal, striated, voluntary

• Cardiac Muscle

  • Found only in the heart

  • Striated, but involuntary

  • Responsible for pumping blood and maintaining circulation

  • Key words: cardiac, striated, involuntary

• Smooth Muscle

  • Located in walls of hollow organs (e.g., intestines, blood vessels)

  • Not striated, involuntary

  • Controls movement of substances through internal passageways

  • Key words: smooth, not striated, involuntary

Characteristics of Muscle Tissue

All muscle types share four main physiological properties:

• Excitability: Ability to receive and respond to stimuli

• Contractility: Ability to shorten forcibly when stimulated

• Extensibility: Ability to be stretched

• Elasticity: Ability to recoil to resting length after stretching

Functions of Muscle Tissue

Muscle tissue performs several essential functions:

• Produce movement

• Maintain posture and body position

• Stabilize joints

• Generate heat as they contract

• Additional functions: Protect organs, form valves, control pupil size, cause 'goosebumps'

Skeletal Muscle Anatomy

Skeletal muscle is an organ composed of muscle fibers, connective tissue sheaths, nerves, and blood vessels.

• Nerve and blood supply: Each muscle receives a nerve, artery, and veins

• Contracting muscle requires significant energy and blood flow

Connective Tissue Sheaths of Skeletal Muscle

Each muscle and muscle fiber is covered by connective tissue sheaths that support and reinforce the muscle.

• Epimysium: Surrounds entire muscle

• Perimysium: Surrounds fascicles (bundles of muscle fibers)

• Endomysium: Surrounds individual muscle fibers

Muscle Attachments

Muscles attach to bones at two places: the origin (immovable or less movable bone) and the insertion (movable bone).

• Attachments can be direct (fleshy) or indirect (via tendon or aponeurosis)

Muscle Fiber Microanatomy and Sliding Filament Model

Skeletal muscle fibers are long, cylindrical cells with specialized structures for contraction.

• Sarcolemma: Plasma membrane of muscle fiber

• Sarcoplasm: Cytoplasm, contains glycosomes and myoglobin

• Myofibrils: Rod-like elements, account for most of muscle cell volume

• Sarcoplasmic reticulum (SR): Specialized endoplasmic reticulum, stores calcium

• T tubules: Invaginations of sarcolemma, transmit electrical impulses

Myofibrils and Striations

Myofibrils are composed of repeating units called sarcomeres, which are the functional units of muscle contraction.

• Striations: Alternating dark (A bands) and light (I bands) regions

• Z disc: Sheet of protein at the midline of I band

• H zone: Lighter region in midsection of A band

Sarcomere Structure

The sarcomere is the smallest contractile unit of muscle fiber, composed of thick and thin myofilaments.

• Thick filaments: Composed of myosin

• Thin filaments: Composed of actin, tropomyosin, and troponin

• Arrangement: Hexagonal pattern, each thick filament surrounded by six thin filaments

Molecular Composition of Myofilaments

• Thick filaments: Myosin molecules with heavy and light chains; myosin heads offset during contraction

• Thin filaments: Actin protein (G actin subunits), tropomyosin, and troponin regulate interaction with myosin

Sarcoplasmic Reticulum and T Tubules

The sarcoplasmic reticulum (SR) and T tubules coordinate muscle contraction by regulating calcium release and electrical signals.

• SR: Runs longitudinally, forms terminal cisterns, stores and releases Ca2+

• T tubules: Invaginations of sarcolemma, transmit action potentials deep into muscle fiber

• Triad: Grouping of a T tubule with two terminal cisterns of SR

Sliding Filament Model of Contraction

Muscle contraction occurs when myosin heads bind to actin, pulling thin filaments toward the center of the sarcomere.

• Shortening occurs as Z discs move closer together, I bands shorten, H zones disappear, and A bands move closer

• Requires nervous system stimulation and calcium release

Steps for Skeletal Muscle Contraction

Four steps are required for skeletal muscle contraction:

1. Nerve stimulation

2. Action potential generation

3. Propagation of action potential

4. Increase in intracellular Ca2+ levels

The Neuromuscular Junction

The neuromuscular junction (NMJ) is the site where a motor neuron stimulates a muscle fiber.

• Axon terminal releases acetylcholine (ACh) into the synaptic cleft

• ACh binds to receptors on the sarcolemma, triggering an action potential

• NMJ consists of axon terminals, synaptic cleft, and junctional folds

• ACh is broken down by acetylcholinesterase to terminate the signal

Clinical Connection: Homeostatic Imbalance

Various toxins, drugs, and diseases can interfere with neuromuscular junction function.

• Example: Myasthenia gravis is an autoimmune disease characterized by drooping eyelids, difficulty swallowing, and muscle weakness

• Caused by shortage of ACh receptors due to immune attack

Table: Comparison of Muscle Tissue Types

Type	Location	Striations	Control	Function
Skeletal	Attached to bones	Striated	Voluntary	Movement, posture
Cardiac	Heart	Striated	Involuntary	Pumping blood
Smooth	Walls of hollow organs	Not striated	Involuntary	Move substances

Key Equations and Concepts

• Sliding Filament Theory: Muscle contraction is driven by the sliding of actin and myosin filaments past each other, powered by ATP hydrolysis.

• Action Potential Propagation: $V_m = V_{rest} + ext{depolarization}$ Where $V_m$ is membrane potential, $V_{rest}$ is resting potential.

• Calcium Release: $[Ca^{2+}]_{cytosol} \uparrow \Rightarrow ext{contraction}$ Increase in cytosolic calcium triggers contraction.

Example: During voluntary movement, a motor neuron releases acetylcholine at the neuromuscular junction, leading to muscle fiber depolarization, calcium release, and contraction via the sliding filament mechanism.

Additional info: Some details, such as the molecular structure of myosin and actin, and the clinical example of myasthenia gravis, were expanded for academic completeness.