Skeletal muscle 2024 handout 18,19,20

Muscle Physiology

Suggested Reading

• Textbook: Silverthorn 8e, pages 375-395

• Introduction and section 12.1

• Important to focus on figures 12.3, 12.5, 12.8, 12.9, 12.10 for mechanisms description.

Outline of Key Concepts

• Types of Muscle

  • Skeletal muscle: Structure-function aspects

  • Cardiac muscle: Unique features and functions

  • Smooth muscle: Presence in hollow organs and tubes

• Detailed Subtopics

  • Structure and function of skeletal muscle

  • Neuromuscular Junction (NMJ)

  • Fine structure of muscle and myofibrils

  • Calcium (Ca++) binding properties

  • ATP utilization

  • Excitation-contraction coupling

  • Energy use and tension generation

  • Types of muscle fibers in skeletal muscle

Types of Muscle

• Three Major Types:

  • Skeletal Muscle: Makes up the muscular system; handles voluntary movements.

  • Cardiac Muscle: Specialized muscle found only in the heart; involuntary control.

  • Smooth Muscle: Composes hollow organs and tubes; can be striated or unstriated; voluntary or involuntary control.

• Classification of Muscle:

  • Striated: Skeletal and cardiac muscles

  • Unstriated: Smooth muscle

  • Voluntary: Skeletal muscle

  • Involuntary: Cardiac and smooth muscles

Muscle Fiber Structure

• Skeletal Muscle Characteristics:

  • Multinucleated with striations

  • Long fibers arranged in parallel.

• Cardiac Muscle Features:

  • Uninucleated, striated, and features intercalated disks.

• Smooth Muscle:

  • Uninucleated and lacks striations; forms sheets or tubes.

Muscle Contraction Control

• Contraction is regulated by the central nervous system (CNS).

• Neuronal Control Mechanism:

  • Upper Motor Neurons: Originate in the primary motor cortex and synapse with lower motor neurons.

  • Lower Motor Neurons: Located in the spinal cord, project directly to muscle fibers.

• Motor Units: Comprised of a motor neuron and all the muscle fibers it innervates. Can vary in size and recruitment.

Neuromuscular Junction (NMJ)

• Definition: The synapse between lower motor neurons and muscle cells.

• Size Comparison: NMJ is significantly larger (1000-10,000 μm²) than a central synapse (0.05 μm²).

• High density of nicotinic acetylcholine receptors (nAChR) present in the postsynaptic membrane.

Motor Unit Characteristics

• Each muscle fiber receives only one synapse.

• A motor unit may innervate multiple muscle cells, leading to coordinated contraction.

Excitation-Contraction Coupling

• Sequence of events from action potential (AP) in muscle cells leading to contraction:

  1. ACh release at NMJ initiates a muscle action potential.

  2. Action potential propagates along the muscle fiber and into T-tubules.

  3. Voltage changes trigger calcium release from sarcoplasmic reticulum (SR).

  4. Calcium binds to troponin, moving tropomyosin and allowing cross-bridge cycling.

Muscle Fiber Types

• Skeletal Muscle Fiber Types:

  • Type I (Slow Twitch): Fatigue-resistant, high endurance, rich in mitochondria.

  • Type IIa (Fast Oxidative-Glycolytic): Moderate fatigue resistance, larger fiber size, quick but less endurance.

  • Type IIb (Fast Twitch Glycolytic): Largest fibers, fatigue rapidly, primarily use anaerobic mechanisms for ATP.

Tension Development and Energy Use in Muscle

• Muscle tension develops through the sliding filament mechanism: Actin and myosin interact, leading to shortening of the muscle.

• Requires ATP for:

  1. Power stroke in cross-bridge formation.

  2. Calcium transportation back to SR.

  3. Maintaining ion gradients via Na+/K+ ATPase.

• Main energy sources:

  1. Stored ATP.

  2. Creatine phosphate.

  3. Oxidative phosphorylation with oxygen.

  4. Anaerobic glycolysis during intense exercise.

Muscle Fatigue

• Causes can include neurotransmitter release decreases, receptor desensitization, impaired calcium release, and damage to muscle fibers.

Relaxation Mechanism

• Upon cessation of APs, muscle relaxation occurs as follows:

  1. Dissociation of ACh from receptors and degradation.

  2. Active transport of calcium back into the SR.

  3. Tropomyosin returns to block myosin binding sites.

Summary of Key Concepts

• Muscle fibers work in an organized manner, responding to neuronal signals, using ATP efficiently to facilitate movement, and adapting to energy demands during various types of physical activity.