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Skeletal Muscle:
- Characteristics: Large, multinucleate cells, appear striped under microscope.
- Muscle Configuration: Varies by fiber type.
- Muscle fibers have striations.
Cardiac Muscle:
- Characteristics: Small, uninucleate cells, appear striped under microscope, branched and joined by intercalated disks.
- Striations present.
Smooth Muscle:
- Characteristics: Small, uninucleate cells, lack striations.

Attachment:
- Attached to bones via tendons to form levers.
Origin and Insertion:
- Origin: The muscle attachment closest to the trunk or more stationary bone.
- Insertion: The more distal or more mobile attachment on the bone.
Functions:
- Flexor: Muscle that brings bones closer together.
- Extensor: Muscle that moves bones away from each other.
Antagonistic Muscle Groups:
- Flexor-extensor pairs work in opposition (e.g., Triceps as extensor; Biceps as flexor).

Components:
- Tendon: Connects muscle to bone.
- Nerve and Blood Vessels: Essential for muscle function.
- Connective Tissue: Provides structure and support.
- Muscle Fascicle: Package of muscle fibers.
- Muscle Fiber: The functional unit of muscle tissue.
- Nucleus: Contains genetic material.

Myofibrils:
- Composed of repeating units called sarcomeres, which include:
- A band: Dark band, contains thick myosin filaments.
- I band: Light band, contains thin actin filaments.
- H zone: Central region in the A band that contains only thick filaments.
- Z disk: The boundary of a sarcomere.
- M line: Middle of the sarcomere, holds myosin filaments together.
- Variants: Troponin and tropomyosin are part of the thin filaments that regulate muscle contraction.

Concept: Muscle contraction occurs via the sliding filament theory, where the actin and myosin filaments slide past one another, shortening the sarcomere while their lengths remain unchanged.
Contraction Phases:
- Relaxation: Muscle fibers return to resting state due to calcium ion removal and tropomyosin re-covering the binding sites.
- Action of Calcium: Calcium binds to troponin, causing a conformational change that moves tropomyosin and exposes actin-myosin binding sites.

Mechanism: Signal travels from somatic neurons to the neuromuscular junction, which consists of axon terminals (release ACh), motor end plates on muscle membranes (high concentrations of ACh receptors) and Schwann cell sheaths.
Process:
- Action potential opens voltage-gated calcium channels, leading to calcium influx into muscle fibers.
- Calcium releases from the sarcoplasmic reticulum and integrates with troponin to facilitate contraction.

Mechanism:
- Sarcoplasmic Ca2+-ATPase pumps Ca2+ back into the sarcoplasmic reticulum.
- Decrease in free cytosolic Ca2+ leads to the unbinding of calcium from troponin and ultimately stops contraction as tropomyosin recovers binding sites.

Process:
- Ca2+ binds to troponin, leading to a power stroke where the myosin head pivots, causing actin to move.
- Myosin head releases ADP and inorganic phosphate (Pi), leading to further contraction.

Concept: The tension generated in a muscle fiber is directly proportional to the number of crossbridges between thick and thin filaments.
Optimal Length: Muscle tension is maximized at an optimal resting length (approximately 2.0 µm) and decreased with too much or too little overlap of thick and thin filaments.

Phenomenon: Occurs post-mortem when ATP supplies deplete. Myosin heads remain attached to actin due to lack of ATP, causing muscles to stiffen.

Muscle Twitch: One contraction-relaxation cycle characterized by:
- Latent period: Time between action potential and contraction onset.
- Contraction phase: Development of muscle tension.
- Relaxation phase: Tension diminishes.

ATP Supply: Limited to around 8 muscle twitches.
Phosphocreatine: Provides backup energy, formed during rest and utilized during contraction.

Causes:
- Depletion of glycogen stores.
- Increase in inorganic phosphate levels affecting myosin function.
- Changes in intracellular ion concentrations like potassium, which influence excitability and membrane potential.

Slow-twitch Fibers:
- Rely on oxidative phosphorylation, characterized by more mitochondria and myoglobin (which binds oxygen).
Fast-twitch Fibers:
- Split ATP quickly, operate anaerobically, and have less blood supply.
Plasticity: Muscle fibers can adapt their characteristics based on training and activity levels.

Definition: A motor unit consists of a single somatic neuron and the muscle fibers it controls. Each muscle can have multiple motor units with different fiber types.
Mechanism: Muscle contracts in an all-or-nothing manner, but different motor units can be recruited based on demand.

Isotonic Contraction: Muscle generates force while causing movement (load moves).
Isometric Contraction: Muscle generates force without movement (load does not move).

Functions: Moves materials through internal organs and tubes.
Types:
- Single-unit Smooth Muscle: Cells connected by gap junctions, contracts as a unit.
- Multi-unit Smooth Muscle: Individually stimulated, lack electrical linking.

Triggered by increased cytosolic Ca2+ either from hormones or electrical stimulation. Myosin light chain kinase activates contraction by phosphorylating light chains on myosin heads.
Relaxation Mechanism: Removal of Ca2+ from cytosol; myosin phosphatase reduces myosin activity, leading to decreased muscle tension.

Features: Striated muscle with intercalated disks aiding in synchronized contraction. Cardiac muscle fibers are autorhythmic and electrically coupled.
Control: Regulated by hormones and autonomic nervous signals.

Skeletal Muscle: Fast, strong contractions via somatic motor neuron stimulation.
Smooth Muscle: Slower, more sustained contractions via various signaling pathways.
Cardiac Muscle: Intermediate contraction speed with autorhythmic properties and control via excitation-coupling mechanisms.