9.4 Motor Units and Muscle Control

• Motor units are a single motor neuron and all muscle fibers it controls
• The smaller the size of the unit, the better the control
• Factors determining the amount of tension produced:

  • frequency of stimulation
  • number of motor units involved
• Single momentary contraction is a muscle twitch
• Muscle twitch is the response to a single stimulus
• All or none principle, each muscle fiber contracts completely or not at all
• Recruitment is the increasing number of active motor units
• Muscle tone is resting tension in a skeletal muscle
• Muscle spindles are specialized muscle cells which are monitored by sensory nerves that control muscle tone in surrounding muscle tissue
• Exercise increases activity of muscle spindles and may enhance muscle tone
• Hypertrophy is the net effect is an enlargement of stimulated muscle
• Atrophy is when muscle size, tone, power is reduced to inactivity
• Atrophy is reversible but dying fibers cannot be replaced
• Why do some motor units control only a few muscle fibers, where others control many fibers?

  The smaller the size of the unit, the better the control
• In a movie, a person is shot with a dart which blocks the action of acetylcholine. The person then falls. Explain why

  Blocking the action of acetylcholine would stop action potentials from crossing, which stops muscle contractions. So the person falls