5: Muscular System
- Page ID
- 99986
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)- 5.1: The Three Types of Muscle Tissue
- Muscle is one of the four primary tissue types of the body and is specialized for contraction and excitability. It exists in three types — skeletal, cardiac, and smooth muscle — each uniquely structured but all designed to produce movement.
- 5.2: Microscopic Anatomy of Skeletal Muscle Fibers
- Skeletal muscle fibers are large, multinucleated cells specialized for contraction and organized into bundles that form muscle tissue. Their cytoplasm is filled with myofibrils composed of repeating sarcomeres made of actin and myosin, which produce force through the sliding filament mechanism. Specialized structures such as the sarcoplasmic reticulum and transverse tubules regulate calcium release, allowing coordinated muscle contraction.
- 5.3: Connective Tissue Wrappings — The Layers That Hold It All Together
- Skeletal muscles are organs composed of muscle fibers, connective tissue, blood vessels, and nerves working together to produce movement. Layers of connective tissue called the mysia (epimysium, perimysium, and endomysium) organize and support muscle fibers and transmit force. This connective tissue continues into tendons or aponeuroses, allowing muscle contractions to be transferred efficiently to bones.
- 5.4: The Sarcomere — Functional Unit of Muscle
- The sarcomere is the functional unit of a skeletal muscle fiber and is responsible for the striated appearance of muscle. It is composed of precisely arranged thin (actin) and thick (myosin) filaments organized between Z-discs, forming repeating units along each myofibril. Shortening of sarcomeres causes myofibrils, muscle fibers, and ultimately the entire muscle to contract.
- 5.5: Skeletal Muscle Motor Units
- A motor unit consists of a single motor neuron and all the muscle fibers it innervates, which contract together when the neuron fires. Motor units vary in size, with small motor units allowing precise movements and large motor units producing powerful contractions. By recruiting motor units and adjusting their firing frequency, the nervous system finely controls muscle strength, precision, and endurance.
- 5.6: Neuromuscular Junction
- The neuromuscular junction is a specialized synapse where a motor neuron communicates with a skeletal muscle fiber to initiate contraction. An electrical signal in the neuron is converted into a chemical signal using acetylcholine, which then triggers an electrical response in the muscle fiber. Proper function of the neuromuscular junction is essential for normal muscle contraction, and disruptions can lead to muscle weakness, as seen in specific diseases.
- 5.7: Sliding Filament Theory of Contraction
- The sliding filament theory explains how muscle contraction occurs when thin (actin) filaments slide past thick (myosin) filaments, shortening the sarcomere and producing force without the filaments themselves changing length.
- 5.8: Detailed Mechanism of Cross-bridge Formation
- The cross-bridge cycle is the molecular mechanism that produces muscle contraction by allowing myosin heads to bind to, pull on, and release actin filaments. Calcium binding to troponin exposes myosin-binding sites on actin, while ATP provides the energy required for repeated cross-bridge formation, power strokes, and detachment. As millions of these cycles occur simultaneously, sarcomeres shorten and generate force without the filaments themselves changing length.
- 5.9: Energy (ATP) for Muscle Contraction
- Muscle contraction requires ATP to power the cross-bridge cycle and to pump calcium back into the sarcoplasmic reticulum. Because muscles store only small amounts of ATP, it must be continuously regenerated through three pathways: creatine phosphate metabolism, anaerobic glycolysis, and aerobic respiration. Together, these systems allow muscles to produce quick bursts of power as well as sustain long-term activity.
- 5.10: Muscle Fatigue
- Muscle fatigue is a decline in a muscle’s ability to generate force during sustained activity and results from multiple interacting factors. It can arise peripherally within the muscle due to ion imbalances, energy depletion, and metabolite waste accumulation, or centrally from reduced neural drive and motivation. Together, these mechanisms limit contraction strength and protect the body from overexertion and injury.
- 5.11: When Muscles Fall Silent — Understanding Paralysis
- Paralysis is the loss of voluntary muscle function, often caused by a breakdown in communication between the nervous system and skeletal muscles. It may be partial or complete and can result from damage to the brain, spinal cord, peripheral nerves, neuromuscular junctions, or muscle fibers. Flaccid paralysis involves limp muscles and spastic paralysis involves stiff muscles.
- 5.12: Exercise and Muscle Performance
- Physical training alters skeletal muscle by increasing muscle fiber size through hypertrophy, while lack of use leads to muscle atrophy and decreased performance. Muscle growth occurs by adding structural proteins to existing fibers rather than forming new muscle cells. These changes reflect the muscle’s ability to adapt structurally and functionally to patterns of use and disuse.