Motor Units and All-or-None Principle

Motor Units

• A motor unit consists of an anterior motor neuron and the specific muscle fibers it innervates.
• Each muscle fiber typically receives input from only one neuron, but one neuron can innervate several muscle fibers.
• The number of muscle fibers per motor neuron correlates with a muscle's movement function. Less precise movements may involve hundreds of fibers per neuron, while highly precise movements can use as few as one fiber per neuron.
• Muscles that require less precision may have several hundred fibers served by one motor neuron.
• Muscles that function with great precision may have as few as one muscle fiber per motor neuron.

All-or-None Principle

• A stimulus strong enough to trigger an action potential in the motor neuron activates all fibers in the motor unit, causing them to contract synchronously.
• There is no gradual activation; the entire unit contracts or does not.
• A stronger action potential does not result in a stronger contraction.

Gradation of Force (Acute)

• Force generation in a muscle depends on several factors:
  • Number of motor units recruited—more units mean more force.
  • Frequency of motor unit discharge—repeated stimuli before relaxation increases total tension—this is known as summation.
  • Type of motor unit recruited—larger motor units, like Type II, create more force.
  • Preloading—activating the stretch reflex.
  • Speed of contraction

Summation of Force

• Normal body movements involve sustained contractions, not simple twitches.
• Increased frequency of stimuli doesn't allow fibers to relax completely, resulting in summation of force, (increasing tension) until tetanus is achieved.
• Tetanus, a continuous stimulation, results in peak tension leading to sustained contractions.

Muscle Actions

• Static (isometric): Muscle produces force without changing length; joint angle stays the same (e.g., holding a weight).
• Dynamic: Muscle action involves changes in joint angle:
  • Concentric: Muscle shortens as it produces force (e.g., lifting a weight).
  • Eccentric: Muscle lengthens as it produces force (e.g., lowering a weight).
  • Isotonic: Contractions maintain constant force, change in length. (e.g., doing bicep curls).
  • Isokinetic: Maintain constant speed while changing length. (e.g., using isokinetic machines)

Force-Velocity Curve

• The maximum force a muscle can generate varies with contraction speed.
• Maximum isometric force is generated at zero velocity; as speed increases, force decreases.
• Greatest power is generated at intermediate velocities.

Size Principle

• Low-threshold motor units are recruited first because they need lower electrical stimulation to activate.
• Higher threshold motor units are recruited later, when greater force is required.
• The body recruits motor units in an orderly manner to produce smooth muscle action; this is known as selective recruitment.
• Selective recruitment is important during explosive movements because it quickly recruits high-threshold motor units for maximum force and power production.

Review Questions

• Discuss the five ways muscle fiber can acutely increase force output.
• Distinguish major muscle action types and provide examples.
• Explain the relationship between force and velocity of contraction.
• Describe general concentric speeds for generating maximum force and power.
• Compare ability to generate force in concentric, eccentric, and isometric actions.

Explore the fundamental concepts of motor units and the all-or-none principle in muscle physiology. Understand how motor neurons innervate muscle fibers and the relationship between precision movements and motor unit composition. Test your knowledge on these essential topics in muscular function.