Muscular System Overview
Histology and Physiology of Muscles
- Overview
- Studies the structure and function of muscle types
- Focuses on skeletal muscle fibers, cardiac and smooth muscle, and related disorders such as muscular dystrophy and myasthenia gravis
- Includes:
- Types and characteristics of muscles
- Skeletal muscle cell anatomy and physiology
- Nerve-muscle interactions
- Whole muscle behavior
- Muscle metabolism
Functions of the Muscular System
- Body Movements
- Skeletal muscles facilitate movement
- Posture Maintenance
- Stabilizes the body position
- Respiration
- Aids in the breathing process via skeletal muscles
- Heat Production
- Generates heat through muscle contractions
- Communication
- Enables facial expressions and speech via skeletal muscles
- Smooth Muscle Functions
- Constriction of organs and blood vessels
- Cardiac Muscle Functions
- Responsible for heartbeat and blood circulation
Functional Characteristics of Muscle
- Contractility
- The ability of muscles to shorten forcibly when stimulated
- Excitability
- The capability to receive and respond to external stimuli
- Extensibility
- The capacity to be stretched or extended beyond resting length
- Elasticity
- The ability to recoil to original length after being stretched
- Conductivity
- Local electrical excitation creates a wave that travels along the muscle fiber
Types of Muscle Tissue
- Three types of muscle tissue exist:
- Skeletal Muscle:
- Attached to bones; responsible for body movements and posture
- Smooth Muscle:
- Found in walls of hollow organs and vessels; facilitates substance movement
- Cardiac Muscle:
- Located in the heart; responsible for pumping blood
- Differences in:
- Structure
- Location
- Function
- Activation Method
- Each muscle is classified as an organ consisting of muscle tissue, blood vessels, nerve fibers, and connective tissue
Comparison of Muscle Types
| Features | Skeletal Muscle | Smooth Muscle | Cardiac Muscle |
|---|---|---|---|
| Location | Attached to bones | Walls of hollow organs and blood vessels | Heart |
| Appearance | Long and cylindrical | Spindle-shaped | Cylindrical and branched |
| Nucleus | Multiple, peripherally located | Single, centrally located | Single, centrally located |
| Cell-cell Attachments | None | Gap junctions in some visceral cells | Intercalated disks |
| Control | Voluntary (and involuntary via reflexes) | Involuntary | Involuntary |
| Striations | Yes | No | Yes |
| Function | Body movement | Food movement, blood pressure regulation | Pumps blood throughout the body |
Skeletal Muscle Structure
- Skeletal muscle fibers are:
- Elongated cells often designated as skeletal muscle cells
- Contain several nuclei located near the plasma membrane
- Striated appearance attributed to actin and myosin myofilaments
- Fibers can extend the entire length of a muscle
- Capable of rapid contraction but prone to fatigue
- Under voluntary control
Connective Tissue in Muscle
- Fascia: Connective tissue sheets providing structure
- Epimysium: Surrounds entire muscle, a dense collagenous connective layer
- Perimysium: Encloses fascicles (bundles of muscle fibers)
- Endomysium: Thin connective tissue surrounding individual muscle fibers
- Connective tissue pathways allow for blood vessels and nerves to access muscle fibers
Skeletal Muscle Fiber Terminology
- Sarcolemma: Plasma membrane of muscle cells
- Sarcoplasm: Cytoplasm inside muscle cells
- Prefixes for Muscle Cells:
- "Myo", "mys", and "sarco" denote muscle
- Myofilaments: Essential for muscle contraction (actin and myosin)
- Myofibrils: Densely packed contractile elements making up most muscle volume
Sarcomeres and Myofilaments
- Sarcomeres: Smallest contractile units in muscle surrounded by Z disks
- Six actin myofilaments surround each myosin myofilament
- Appearance:
- Striated due to A bands (thick filaments) and I bands (thin filaments)
- Thick Filaments:
- Extend through A bands
- Thin Filaments:
- Extend across I bands, partway into A bands
- Z-disk: Anchors thin filaments, connects myofibrils
- M-lines: Darker areas due to desmin proteins
Structural Hierarchy of Skeletal Muscle
- Muscle: A contractile organ typically attached to bones; composed of muscle fiber bundles
- Fascicle: Bundle of muscle fibers, surrounded by perimysium
- Muscle Fiber: Single elongated muscle cell; contains myofibrils and is enclosed in endomysium
- Myofibril: Bundle of myofilaments within a muscle fiber; appears striated
- Sarcomere: Segment of a myofibril between two Z discs; functional unit responsible for contraction
- Myofilaments: Fibrous protein strands (thick - myosin, thin - actin) responsible for contraction process; slide over each other to shorten the sarcomere and muscle