Muscle Physiology

Principles of Human Physiology

Chapter 12d: Muscle Physiology

Overview

• Presentation by Cindy L. Stanfield, Sixth Edition, Pearson Education, Inc.

Chapter Outline

• 12.5 Control of Skeletal Muscle Activity
• 12.6 Smooth and Cardiac Muscle

Learning Outcomes

• Name the three types of skeletal muscle fibers, and describe the major differences among them.
• Describe the major characteristics of smooth muscles and cardiac muscle, and compare these muscle types to skeletal muscle.

12.5 Control of Skeletal Muscle Activity

Muscle Activity Across Joints

• Origin and Insertion: The origin is the stationary attachment of the muscle, while the insertion is the attachment that moves.
• Stationary vs Mobile: Muscles can have a fixed point (origin) and a point that moves (insertion).
• Muscle Antagonistic Actions: Muscles work in opposition to each other.
• Flexion & Extension: Movements that occur at joints to decrease or increase the angle between body parts.
• Muscles Can Only Pull, Not Push: Muscles contract to pull on bones but cannot push.
• Combination of Active Contraction & Passive Stretch: Flexing and extending involves both contraction (active) and the relaxation of opposing muscles (passive).
• Motor Unit Stimulation & Inhibition: Coordination of muscle contraction is controlled by the motor units, which can be stimulated or inhibited.

Muscle Receptors for Coordinated Activity

• Muscle Fibers: Comprise the contractile cells and are essential for muscle function.

  • Extrafusal Fibers: The primary contractile fibers innervated by alpha (α) motor neurons responsible for muscle contraction.
  • Intrafusal Fibers: Specialized muscle fibers within the spindle that adjust the sensitivity of muscle to stretch and are innervated by gamma (γ) motor neurons.

• Muscle Spindle:

  • Detects changes in muscle length via sensory endings.
  • Sensitivity can be adjusted via the action of intrafusal fibers.

• Effects of Muscle Stretch & Contraction on the Muscle Spindle: Understanding how stretch affects sensory outputs from the spindle.

• Alpha & Gamma Coactivation: Coordination mechanism involving both types of neurons to control intrafusal and extrafusal fibers during muscle activities.

• Golgi Tendon Organs (GTOs): Sensory capsules located within tendons that detect passive tension/stretch, leading to reflex inhibition of muscle to prevent damage.

12.6 Smooth and Cardiac Muscle

Comparisons of Muscle Types

• Skeletal Muscle: Striated, voluntary, fast contraction.
• Smooth Muscle: Non-striated, involuntary, slower contractions.
• Cardiac Muscle: Striated, involuntary, intermediate contraction characteristics.

Smooth Muscle Characteristics

• Located in internal organs and blood vessels.
• Controlled by the autonomic nervous system (involuntary).
• Composed of spindle-shaped cells, small in size (approximately 1/10 that of skeletal muscle).
• Features: Non-striated, often characterized by gap junctions that facilitate coordinated contractions.
• Contraction Mechanism:
  • Sliding filament mechanism is utilized, lacking sarcomeres.
  • Dense bodies play a critical role in contraction via longer actin and myosin filaments.

Steps of Excitation-Contraction Coupling in Smooth Muscle

1. Ca2+ Sources: Most calcium ion influx is from the extracellular fluid (ECF).
2. Voltage-Gated Ca2+ Channels: Located in the plasma membrane, facilitate calcium entry upon depolarization.
3. Calcium Triggering: Leads to further release of Ca2+ from the sarcoplasmic reticulum.
4. Calmodulin Activation: Ca2+ binds to calmodulin, forming a calcium-calmodulin complex.
5. Myosin Light-Chain Kinase (MLCK) Activation: The MLCK is activated by the calcium-calmodulin complex, leading to phosphorylation of myosin.
6. Crossbridge Cycling: Activation of myosin ATPase allows crossbridge cycling to occur, leading to contraction.
7. Muscle Relaxation:
   • Myosin phosphatase de-phosphorylates myosin, leading to muscle relaxation.
   • Removal of Ca2+ via Ca2+-ATPase and sodium-calcium (Ca2+-Na+) counter-transport mechanisms.

• Contraction Duration: Actin-myosin interactions are significantly slower in smooth muscle (10-100 times slower than in skeletal muscle).

Neural Regulation of Smooth Muscle Contraction

• Controlled by the autonomic nervous system, affecting contraction either by excitation or inhibition.
• Responses depend on receptor types present, neurotransmitter released from varicosities, and the diffuse nature of binding.

Types of Smooth Muscle

• Multi-Unit Smooth Muscle:

  • Found in large airways/arteries, eyes (i.e., ciliary muscle, iris).
  • Minimal gap junctions, independent action of individual fibers.
  • No inherent tone, requires neural input for contraction.

• Single-Unit Smooth Muscle:

  • Most common type found in intestines, blood vessels, respiratory tract.
  • Muscle fibers are connected via gap junctions and contract as a syncytium.
  • Presence of pacemaker potentials facilitates synchronized contraction.
  • Tone indicates a level of contraction without further stimulation and can respond to stretch reflexes.

Cardiac Muscle Characteristics

• Intermediary between skeletal and smooth muscle: striated with sarcomeres, autoregulated through pacemaker activity.
• Innervated by autonomic nervous system; utilizes calcium from extracellular fluid and sarcoplasmic reticulum for contraction.
• Action Potential vs. Contraction: Cardiac action potentials last almost as long as contractions, preventing summation due to a prolonged refractory period.

Comparison of Muscle Types

Further Features

Types of Smooth Muscle