Nervous System Control of Skeletal Muscles: Role of Motor Units

Key Concepts

• Motor Units: Groups of muscle fibers innervated by a single motor neuron.
• Innervation: Each skeletal muscle fiber must be connected to a motor neuron to contract.

Types of Motor Units

• Small Motor Units:
  • Single motor neuron supplies a small number of muscle fibers.
  • Enables fine motor control (e.g., grasping, texting, typing).
• Large Motor Units:
  • Single motor neuron supplies a large number of muscle fibers.
  • Facilitates simple, powerful movements (e.g., knee extension).
  • Example: Thigh and back muscles, where thousands of fibers are innervated by one neuron.

Function and Importance

• Control Range:
  • Wide variety of motor units allow the nervous system to precisely control muscle action.
• Excitability of Motor Neurons:
  • Smaller, lower threshold motor neurons are more excitable and activate first.
  • Smaller motor units generate less tension.
• Recruitment:
  • Progressive activation of larger motor units as more strength is required.
  • Larger motor units have higher threshold, bigger neurons, and activate larger muscle fibers.
  • Recruitment results in increased muscle tension.
• Force Generation:
  • Largest motor units can produce much greater force (up to 50x smaller units).
  • Allows versatility in muscle usage, from delicate tasks to lifting heavy weights.

Energy and Fatigue

• Maximum Force:
  • Achieved by recruiting all motor units simultaneously.
  • Maximum contraction is energy-intensive and cannot be sustained long.
• Prevention of Fatigue:
  • Motor units alternate between active and resting states.
  • Allows for sustained muscle contractions by distributing energy consumption.

Mechanism of Recruitment

• Recruitment is a strategic method used by the nervous system to optimally use skeletal muscles.