Principles of Human Physiology - Muscle Physiology

Principles of Human Physiology - Chapter 12d: Muscle Physiology

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 their major differences.
• Describe the major characteristics of smooth muscles and cardiac muscles, and compare these muscle types to skeletal muscle.

12.5 Control of Skeletal Muscle Activity

• Muscle activity across joints
  • Origin and Insertion: Points of attachment; origin is typically stationary, while insertion is the moving end of the muscle.
  • Stationary vs. Mobile: Muscles either act to stabilize (stationary) or move (mobile) parts of the body.
  • Muscle Antagonistic Actions: Muscles work in pairs where one muscle contracts (agonist) and the other relaxes (antagonist).
  • Flexion and Extension: Movements facilitated by muscle contractions across joints, with flexion decreasing the angle between body parts and extension increasing it.
  • Muscles Can Only Pull, Not Push: Muscles can only exert force through contraction which results in pulling actions at joints.
  • Combination of Active Contraction & Passive Stretch: Muscles are capable of both active contractions and passive stretching as part of their movement.
  • Motor Unit Stimulation & Inhibition: The balance of activation and deactivation of motor units provides precise control of movement.

Muscle Receptors for Coordinated Activity

• Extrafusal Fibers:

  • Contractile cells of skeletal muscles responsible for voluntary muscle contraction.
  • Innervated by alpha (α) motor neurons.

• Intrafusal Fibers:

  • Specialized muscle spindle fibers that adjust stretch sensitivity.
  • Innervated by gamma (γ) motor neurons.

• Muscle Spindle:

  • Detects changes in muscle length including stretch and contraction, important for proprioception.
  • Sensory endings located in these spindles adjust their sensitivity based on intrafusal fiber action.

• Alpha & Gamma Coactivation:

  • Coordination of extrafusal and intrafusal fiber activity allows for balanced muscle response.
  • Alpha Motor Neuron: Activates extrafusal fibers.
  • Gamma Motor Neuron: Tunes the sensitivity of intrafusal fibers.

• Golgi Tendon Organs (GTOs):

  • Sensory receptors located within tendons that detect passive tension and stretching.
  • When activated by tendon stretch, GTOs induce reflex inhibition of muscle activity to prevent damage.

12.6 Smooth and Cardiac Muscle Comparisons

• Skeletal Muscle: Striated, somatic control, voluntary, fast contractions.

• Smooth Muscle: Found in internal organs; involuntary control through the autonomic nervous system.

  • Characteristics of Smooth Muscle:
  • Spindle-shaped and smaller than skeletal muscle (approx. 1/10 the size).
  • Non-striated appearance due to no sarcomeres, utilizing dense bodies instead.
  • Capable of long-range contractions along multiple axes due to longer actin and myosin filaments.
  • Myosin heads extend the entire length, providing multi-directional tension.

• Mechanism of Smooth Muscle Contraction:

  • Excitation-Contraction Coupling Steps:
  1. Calcium ions ($Ca^{2+}$) primarily sourced from extracellular fluid.
  2. Voltage-gated $Ca^{2+}$ channels open in the plasma membrane.
  3. Release of additional $Ca^{2+}$ from the sarcoplasmic reticulum.
  4. $Ca^{2+}$ binds to calmodulin.
  5. $Ca^{2+}$-calmodulin complex activates myosin light-chain kinase (MLCK).
  6. MLCK phosphorylates myosin, facilitating crossbridge cycling.
  • Relaxation of Smooth Muscle:
  • Myosin light-chain phosphatase removes phosphate from myosin.
  • Active transport of $Ca^{2+}$ removed from the cytoplasm through $Ca^{2+}$-ATPase and $Ca^{2+}$-Na$^{+}$ counter-transport.
  • Myosin ATPase activity in smooth muscle is 10-100 times slower than in skeletal muscle.

• Neural Regulation of Smooth Muscle:

  • Controlled by the autonomic nervous system via sympathetic and parasympathetic responses, which can be either excitatory or inhibitory depending on receptor type and neurotransmitter release.

• Types of Smooth Muscle:

  • Multi-Unit Smooth Muscle:
  • Examples include large airways, arteries, and ocular muscles.
  • Few, if any, gap junctions; each fiber behaves independently with individual innervation.
  • Single-Unit Smooth Muscle:
  • Common in the intestinal tract and blood vessels, where fibers act as a syncytium (contracting together due to gap junctions).
  • Possesses pacemaker cells capable of spontaneous depolarization leading to graded contractions.

Cardiac Muscle

• Intermediate Between Skeletal and Smooth Muscle:

  • Striated structure with sarcomeres, utilizes troponin-tropomyosin regulatory proteins.
  • Contains gap junctions that facilitate electrical coupling between cells within intercalated disks.
  • Pacemaker activity regulated by ion movement and influenced by the autonomic nervous system.

• Action Potential vs. Contraction:

  • The action potential duration is almost equal to the muscle contraction duration, preventing summation due to a lengthy refractory period.

Comparison Table of Muscle Types