Skeletal Muscle Structure and Function Quiz

Skeletal Muscle: Structure and Function

• Skeletal muscle is an organ, comprising 40-60% of total body weight. There are approximately 600 muscles in the human body.
• Skeletal muscle's main functions include producing force, transmitting force to the skeleton via tendons, facilitating movement, stabilizing joints, and maintaining posture.

Learning Objectives

• Describe muscle structure
• Identify differences in muscle characteristics of fast-twitch and slow-twitch fibers
• Define the sliding filament mechanism
• Explain the process of the cross-bridge cycle

Muscle Anatomy (Gross)

• Force produced by the muscle is transmitted to the skeleton via tendons.
• Muscles cause movement or stabilize joints or maintain posture.

Muscle Structure (Detailed)

• Muscle: The whole muscle
• Epimysium (Deep fascia): Outer layer of the muscle
• Fasciculus: Bundles of muscle fibers
• Perimysium: Surrounds individual fasciculi
• Endomysium: Connective tissue layer surrounding individual muscle fibers
• Single muscle fiber: Individual muscle cell
• Sarcoplasm: Cytoplasm of muscle fiber
• Myofibril: Contractile fibers within the muscle fiber
• Sarcolemma: Cell membrane of the muscle fiber
• Nuclei: Multiple nuclei per muscle fiber
• Tendon: Attaches muscle to bone
• Muscle belly: The main body of the muscle

Muscle Fiber

• Nucleus: Located in the muscle fiber
• Light I band: Light band within the myofibril
• Dark A band: Dark band within the myofibril
• Myofibril: Contractile filament
• Mitochondrion: Powerhouse of the cell, generates energy
• Sarcolemma: Membrane surrounding the muscle fiber
• Myofibrils: Contractile proteins
• Each myofibril is comprised of thousands of sarcomeres.

Sarcomere Structure

• Sarcomeres are the functional units of a muscle.
• Sarcomeres are joined in series and parallel to one another.
• Myofilaments make up sarcomeres; these include actin and myosin proteins.
• Myosin (thick filament)
  • Composed of around 300 myosin molecules per thick filament.
  • Contains two subunits: S1 (globular head) and S2 (flexible region and tail).
• Actin (thin filament)
  • Subunits in double helical strands.
  • Tropomyosin interacts with actin and covers binding sites allowing thick filaments to bind.
• Other structural proteins include Titin, Nebulin, and Desmin.

Sliding Filament Mechanism

• Huxley 1954
• During muscle contraction, filaments slide past each other.
• The filaments themselves remain relatively unchanged in length despite changes in muscle length.

Sarcoplasmic Reticulum

• Interconnecting tubules surrounding myofibrils
• Regulates intracellular calcium levels
• Stores calcium and releases calcium on stimulation to allow muscle contraction.

The Cross-Bridge Cycle

• Myosin heads attach to actin, forming cross-bridges.
• Myosin heads undergo a conformational change, pulling the actin filaments inward.
• ATP is necessary for the detachment of myosin heads from actin.
• The process repeats to shorten the sarcomere.

Length-Tension Relationship

• Filaments that don't overlap don't generate tension.
• Optimal tension is generated when filaments can butt against the Z-lines.
• As the muscle stretches further, the overlap between myosin and actin decreases resulting in less tension.

Force-Velocity Relationship

• During shortening, force is less than isometric force (the faster the movement the less time myosin heads have to attach).
• During lengthening, force is greater than isometric force (the myosin is stretched further and then a forceful detachment).

Fibre Type Characteristics

• Different fiber types (Type I, Type IIA, and Type IIX) have varying characteristics.
  • Speed of contraction
  • Force production
  • Metabolic profile
  • Appearance
  • Oxidative capacity
  • Glycolytic capacity

Study Questions

• Explain how nerve impulses are propagated down the axon towards the neuromuscular junction.
• How does excitation of a neuron result in excitation of a muscle cell? Describe the process with reference to the appropriate neurotransmitter.
• Describe how feedback from sensory receptors can regulate muscle contraction.

Skeletal Muscle II: Innervation and Control

• Objectives:
  • Name the basic structures of a motor neuron
  • Explain how nerve impulses are transmitted from the spine to the muscle to cause contraction
  • Describe how feedback from sensory receptors can regulate muscle contraction

The Nervous System

• Central nervous system (CNS)
• Peripheral nervous system (PNS)
  • Afferent (sensory) division
  • Efferent (motor) division
    • Somatic nervous system
    • Autonomic nervous system
      • Sympathetic division
      • Parasympathetic division

The Motor Neuron

• Motor neuron: a single motor neuron and all the fibers it innervates
• Gross movements involve 2,000-3,000 fibers per motor unit.
• Fine movements involve 1-2(or 3) fibers per motor unit.
• Motor neuron components:
  • Cell body
  • Dendrites
  • Axon
  • Myelin sheath
  • Nodes of Ranvier
  • Terminal branches
  • Synaptic vesicles

Transmission of Nerve Impulses (Action Potential)

• The electrical signal that propagates down a neuron's axon
• Depolarization and repolarization occur as Na+ and K+ ions flow in and out of the axon
• Action potentials propagate down the axon through saltatory conduction along the myelinated segments.

Neuromuscular Junction

• The synapse between a motor neuron and a muscle fiber
• Acetylcholine (neurotransmitter) is released into the synaptic cleft
• Acetylcholine binds to receptors on the muscle fiber, initiating muscle contraction.

Golgi Tendon Organs (GTO)

• Sensory receptors located in the tendons
• Detects muscle tension and send inhibitory signals to reduce muscle contraction when tension is excessive.

Muscle Spindles

• Sensory receptors located within muscles
• Detects changes in muscle length
• Sends signals to the central nervous system to regulate muscle contraction to prevent overstretching.

Test your knowledge on skeletal muscle anatomy and functionality. This quiz covers muscle structure, characteristics of muscle fibers, and key mechanisms such as the sliding filament theory and the cross-bridge cycle. Perfect for students of anatomy and physiology.