Skeletal Muscle: Structure, Function, and Physiology

Overview of Skeletal Muscle

Main Topics in Skeletal Muscle Study

Skeletal muscle is a key tissue in the human body, responsible for movement, posture, and many physiological processes. This section outlines the major topics essential for understanding skeletal muscle in Anatomy & Physiology.

• Introduction to skeletal muscle

• Anatomy of skeletal muscle

• Physiology of skeletal muscle contraction

• Skeletal muscle metabolism

• Skeletal muscle fiber types

• Muscle tension in skeletal muscle

• Factors affecting skeletal muscle

• Effects of exercise and aging on skeletal muscle

• Cardiac muscle tissue

• Smooth muscle tissue

Functions of Skeletal Muscle

Major Roles of Skeletal Muscle in the Body

Skeletal muscle performs several vital functions that are essential for survival and daily activity.

• Move the body: Skeletal muscles attach to bones and enable movement, facial expressions, speaking, breathing, and swallowing.

• Maintain posture: Muscles stabilize joints and maintain body position.

• Protect and support: Muscles package internal organs and hold them in place.

• Regulate elimination of materials: Muscular sphincters control the passage of materials at body orifices.

• Produce heat: Muscle contractions help maintain body temperature.

Characteristics of Skeletal Muscle Tissue

Key Properties of Skeletal Muscle

Skeletal muscle tissue is specialized for contraction and exhibits several important characteristics:

• Excitability: The ability to respond to a stimulus by changing electrical membrane potential.

• Conductivity: The ability to send an electrical change along the length of the cell membrane.

• Contractility: The ability to contract when filaments slide past each other, enabling movement.

• Extensibility: The ability to be stretched without being damaged.

• Elasticity: The ability to return to original length after shortening or lengthening.

Types of Muscle Tissue

Classification of Muscle Tissue

There are three main types of muscle tissue in the body, each with distinct structure and function:

• Skeletal muscle: Voluntary, striated muscle attached to bones.

• Cardiac muscle: Involuntary, striated muscle found in the heart.

• Smooth muscle: Involuntary, non-striated muscle found in walls of hollow organs.

Structural Organization of Skeletal Muscle

Hierarchy of Skeletal Muscle Structure

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

• Muscle organ: Composed of bundles of fascicles, connective tissue, blood vessels, and nerves.

• Fascicle: A bundle of muscle fibers (cells).

• Muscle fiber (myofiber): A single muscle cell, multinucleated and containing many myofibrils.

• Myofibril: A bundle of myofilaments within a muscle fiber.

• Myofilament: Contractile proteins (thick and thin filaments) within myofibrils.

Connective Tissue Components

Layers and Attachments

Connective tissue surrounds and supports skeletal muscle at multiple levels:

• Epimysium: Dense irregular connective tissue wrapping the whole muscle.

• Perimysium: Dense irregular connective tissue wrapping each fascicle; contains blood vessels and nerves.

• Endomysium: Areolar connective tissue wrapping individual muscle fibers; provides electrical insulation and capillary support.

• Tendon: Thick, cordlike structure attaching muscle to bone.

• Aponeurosis: Thin, broad sheet of connective tissue attaching muscle to muscle or bone.

• Deep fascia: Dense irregular connective tissue superficial to epimysium, separating muscles and binding those with similar functions.

• Superficial fascia: Areolar and adipose tissue separating muscles from the skin.

Microscopic Anatomy of Skeletal Muscle

Cellular and Subcellular Features

Skeletal muscle fibers are highly specialized cells with unique structures:

• Sarcolemma: The plasma membrane of a muscle fiber, containing voltage-gated ion channels for electrical signaling.

• Sarcoplasm: The cytoplasm of a muscle fiber, containing organelles and contractile proteins.

• Sarcoplasmic reticulum (SR): Modified smooth ER that stores and releases calcium ions.

• T-tubules: Invaginations of the sarcolemma that conduct electrical signals deep into the cell.

• Myofibrils: Hundreds to thousands per cell, composed of myofilaments.

• Terminal cisternae: Enlarged regions of SR that store calcium; two cisternae and a T-tubule form a triad.

• Multinucleation: Muscle fibers are multinucleated due to fusion of myoblasts during development.

• Satellite cells: Support and repair muscle fibers.

Myofilaments and Sarcomere Structure

Contractile Proteins and Organization

Myofilaments are the contractile proteins within myofibrils, organized into repeating units called sarcomeres.

• Thick filaments: Composed of myosin protein molecules; myosin heads have actin and ATP binding sites.

• Thin filaments: Twisted strands of actin (F-actin made of G-actin monomers); includes regulatory proteins tropomyosin and troponin.

• Sarcomere: The functional unit of muscle contraction, defined by Z discs at each end.

• I band: Light region containing only thin filaments; shortens during contraction.

• A band: Dark region containing thick filaments and overlapping thin filaments; includes H zone and M line.

• H zone: Central region of A band with only thick filaments; disappears during maximal contraction.

• M line: Middle of H zone; attachment site for thick filaments.

• Connectin (titin): Stabilizes thick filaments and provides elasticity.

• Dystrophin: Anchors myofibrils to sarcolemma; abnormalities cause muscular dystrophy.

Energy and Metabolism in Muscle Fibers

ATP Production and Storage

Muscle fibers require large amounts of energy for contraction:

• Mitochondria: Abundant in muscle fibers for aerobic ATP production.

• Myoglobin: Oxygen-binding protein that stores oxygen for aerobic metabolism.

• Glycogen: Stored in muscle fibers for rapid energy needs.

Equation for Cellular Respiration:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

Innervation of Skeletal Muscle Fibers

Motor Units and Neuromuscular Junction

Muscle contraction is controlled by motor neurons:

• Motor unit: A motor neuron and all the muscle fibers it controls; small units allow precise control, large units allow powerful contractions.

• Neuromuscular junction: The site where a motor neuron innervates a muscle fiber, typically at the mid-region.

• Synaptic knob: Contains synaptic vesicles filled with acetylcholine (ACh).

• Motor end plate: Specialized region of sarcolemma with ACh receptors.

• Synaptic cleft: Narrow space separating neuron and muscle cell.

Physiology of Skeletal Muscle Contraction

Events Leading to Contraction

Muscle contraction involves a series of electrical and chemical events:

1. Excitation: Nerve signal triggers release of ACh, which binds to receptors on the motor end plate, generating an end-plate potential (EPP).

2. Excitation-Contraction Coupling: EPP leads to action potential propagation along sarcolemma and T-tubules, triggering calcium release from the SR.

3. Crossbridge Cycling: Calcium binds to troponin, exposing myosin-binding sites on actin. Myosin heads form crossbridges, perform power strokes, release, and reset using ATP.

Steps of Crossbridge Cycling:

1. Crossbridge formation: Myosin head attaches to actin.

2. Power stroke: Myosin head pulls actin toward center of sarcomere.

3. Release: ATP binds to myosin, causing release from actin.

4. Reset: ATP hydrolysis "cocks" the myosin head for another cycle.

Equation for ATP Hydrolysis:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Muscle Relaxation

Events Leading to Relaxation

Muscle relaxation occurs when stimulation ceases:

• Termination of nerve signal and ACh release.

• Hydrolysis of ACh by acetylcholinesterase.

• Closure of ACh receptors and cessation of EPP.

• Closure of calcium channels and return of calcium to SR by pumps.

• Troponin returns to original shape, tropomyosin blocks myosin-binding sites, and muscle returns to resting length due to elasticity.

Summary Table: Key Features of Skeletal Muscle

Feature	Description
Excitability	Responds to stimuli with electrical changes
Contractility	Shortens to produce movement
Extensibility	Can be stretched without damage
Elasticity	Returns to original length after stretching
Multinucleation	Multiple nuclei per muscle fiber
Energy supply	Abundant mitochondria, myoglobin, and glycogen

Additional info: These notes are based on textbook slides and lecture outlines for a college-level Anatomy & Physiology course, focusing on skeletal muscle structure and function. For more detail on muscle metabolism, fiber types, and clinical applications, refer to subsequent chapters or sections.