Skeletal Muscles

Microscopic Structure of Skeletal Muscle Fibers

Skeletal muscle fibers are specialized cells designed for contraction. Their unique structure enables efficient force generation and movement.

• Sarcolemma: The plasma membrane of a muscle fiber, responsible for conducting electrical signals.

• Sarcoplasm: The cytoplasm of a muscle cell, containing organelles and myofibrils.

• Myoglobin: An oxygen-binding protein found in muscle cells, facilitating oxygen storage.

• Myofibril: Cylindrical structures within muscle fibers, composed of repeating units called sarcomeres.

• Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum that stores and releases calcium ions (Ca++).

• T-tubule: Invaginations of the sarcolemma that transmit action potentials deep into the muscle fiber.

• Actin: Thin filament protein involved in muscle contraction.

• Myosin: Thick filament protein with heads that bind to actin during contraction.

• A band: Dark region containing thick (myosin) filaments.

• I band: Light region containing only thin (actin) filaments.

• Z disc: Boundary of a sarcomere; anchors actin filaments.

• H zone: Central region of the A band where only myosin is present.

• M line: Center of the sarcomere; supports myosin filaments.

• Membranous coverings: Epimysium (surrounds entire muscle), Perimysium (surrounds fascicles), Endomysium (surrounds individual fibers).

Sliding Filament Mechanism of Contraction

Muscle contraction occurs when actin and myosin filaments slide past each other, shortening the sarcomere.

• Troponin: Regulatory protein that binds Ca++ and moves tropomyosin away from actin binding sites.

• Tropomyosin: Covers actin binding sites, preventing myosin attachment in relaxed muscle.

• Ca++: Released from the SR, binds to troponin, initiating contraction.

• ATP: Provides energy for myosin head movement and detachment from actin.

• Muscle relaxation: Occurs when Ca++ is pumped back into the SR, tropomyosin covers actin, and myosin detaches.

Example: During a biceps curl, Ca++ release triggers contraction, and ATP is required for both contraction and relaxation.

Role of T Tubules and Sarcoplasmic Reticulum

T tubules and the SR coordinate the rapid and synchronized contraction of muscle fibers.

• T tubules: Conduct action potentials from the sarcolemma to the interior of the cell.

• Sarcoplasmic Reticulum: Releases Ca++ in response to action potentials, enabling contraction.

Structural Organization of Skeletal Muscles

Skeletal muscles are organized into bundles, each surrounded by connective tissue membranes.

• Epimysium: Surrounds the entire muscle.

• Perimysium: Surrounds fascicles (bundles of fibers).

• Endomysium: Surrounds individual muscle fibers.

• Tendons: Connect muscle to bone; dense regular connective tissue.

• Aponeuroses: Broad, flat tendons connecting muscles to other structures.

Muscle Functions and Characteristics

• Functions: Movement, posture maintenance, joint stabilization, heat generation.

• Functional characteristics: Excitability, contractility, extensibility, elasticity.

Muscle Terminology and Nomenclature

• Agonist: Prime mover responsible for a specific movement.

• Antagonist: Opposes the action of the agonist.

• Synergist: Assists the agonist in performing a movement.

• Fixator: Stabilizes the origin of the agonist.

• Origin: Attachment site that remains stationary during movement.

• Insertion: Attachment site that moves during contraction.

• Nomenclature principles: Muscles are named based on location, shape, size, direction of fibers, number of origins, and action.

Muscle Groups

• Facial expression: Muscles such as orbicularis oculi, zygomaticus, buccinator.

• Eyeball movement: Rectus and oblique muscles.

• Mandible movement: Masseter, temporalis.

• Tongue movement: Genioglossus, hyoglossus.

• Neck muscles: Sternocleidomastoid, scalenes.

• Vertebral column movement: Erector spinae group.

• Respiratory muscles: Diaphragm, intercostals.

• Pelvic floor and perineum: Levator ani, coccygeus.

• Pectoral girdle movement: Trapezius, serratus anterior.

• Arm and forearm compartments: Biceps brachii, triceps brachii, flexor and extensor groups.

• Thigh movement: Quadriceps, hamstrings.

• Thigh and leg compartments: Adductors, abductors, gastrocnemius, soleus.

Comparison of Muscle Types

The three types of muscle tissue differ in structure, function, and control.

Membrane Transport & Excitable Cells

Structure of the Plasma Membrane

The plasma membrane is a selectively permeable barrier composed of a phospholipid bilayer with embedded proteins.

• Phospholipid bilayer: Provides fluidity and barrier function.

• Proteins: Channels, carriers, receptors, and enzymes.

• Carbohydrates: Glycoproteins and glycolipids for cell recognition.

Types of Transport Across the Plasma Membrane

• Passive transport: No energy required; includes diffusion, facilitated diffusion, and osmosis.

• Active transport: Requires ATP; includes primary and secondary active transport.

• Vesicular transport: Endocytosis and exocytosis for bulk movement.

Osmosis and Fluid Homeostasis

Osmosis is the movement of water across a semipermeable membrane, crucial for maintaining fluid balance.

• Osmosis: Water moves from areas of low solute concentration to high solute concentration.

• Role in homeostasis: Regulates cell volume and extracellular fluid composition.

Example: Red blood cells in hypotonic solution swell due to water influx.

Neuron Structure and Function

Neurons are specialized cells for transmitting electrical signals.

• Dendrites: Receive signals (reception).

• Cell body (soma): Integrates signals.

• Axon: Propagates action potentials (propagation).

• Axon terminals: Transmit signals to other cells (transmission).

Types of Glial Cells

• Astrocytes: Support neurons, maintain blood-brain barrier.

• Oligodendrocytes: Form myelin in CNS.

• Schwann cells: Form myelin in PNS.

• Microglia: Immune defense in CNS.

• Ependymal cells: Line ventricles, produce cerebrospinal fluid.

Structural Classification of Neurons

• Multipolar: Many dendrites, one axon (most common).

• Bipolar: One dendrite, one axon (sensory organs).

• Unipolar: Single process (sensory neurons).

Electrical Activity of Neurons

Neurons generate electrical signals through ion movement and channel activity.

• Resting membrane potential: Maintained by Na+/K+ pump and leak channels.

• Action potential: Rapid depolarization and repolarization due to voltage-gated Na+ and K+ channels.

Equation:

Example: Action potential in a motor neuron triggers muscle contraction.

Propagation of Action Potential

• Myelination: Increases speed via saltatory conduction.

• Axon diameter: Larger diameter increases speed.

• Temperature: Higher temperature increases speed.

Synaptic Transmission

• Synapse: Junction between neurons.

• Postsynaptic potentials: Excitatory or inhibitory changes in membrane potential.

• Synaptic integration: Summation of multiple inputs determines action potential generation.

Neurotransmitters

• Acetylcholine: Excitatory at neuromuscular junction.

• Dopamine: Modulates movement and reward.

• Serotonin: Regulates mood.

• GABA: Inhibitory neurotransmitter.

Neural Circuits

Neuronal Regulation of Muscle Contraction

• Neuromuscular junction: Synapse between motor neuron and muscle fiber; acetylcholine release triggers contraction.

• Motor unit: One motor neuron and all muscle fibers it innervates.

• Isotonic contraction: Muscle changes length (movement).

• Isometric contraction: Muscle tension without length change.

• Spatial summation: Increased force by recruiting more motor units.

• Temporal summation: Increased force by increasing frequency of stimulation.

Types of Skeletal Muscle and Metabolic Properties

• Slow-twitch (Type I): High endurance, aerobic metabolism, slow contraction, resistant to fatigue.

• Fast-twitch (Type II): Rapid contraction, anaerobic metabolism, fatigues quickly.

• Smooth muscle: Non-striated, involuntary, slow contraction, high fatigue resistance.

Comparison of Smooth and Skeletal Muscle

Additional info: Expanded explanations and examples were added for clarity and completeness.