Motor units and muscle fiber types

Question 1

alpha (a) motorneurons

Answer 1

a-motorneuron = a neuron innervating power-producing extrafusal muscle fibers

larger than y-motorneurons

muscle spindles are stretch receptors that signal the length and changes in length of muscles

→ intrafusal fibers stretches, muscle spindle detects

→ CNS detects stretch and send info back to spinal cord

→ excites alpha motorneuron associated with muscle being stretched

→ contraction of muscle to stably hold object in place

Question 2

dorsal and ventral root ganglion

Answer 2

dorsal root ganglion - sends sensory neural signals to the CNS (to the spine)

ventral root ganglion - info from alpha and gamma motorneurons leaves the CNS and innervates with extrafusal muscle fibers

Question 3

motor unit

Answer 3

• motor unit = a-motorneuron and all the muscle fibers it innervates; a unit of force production
• much larger than y-motorneurons
• # of muscle fibers associated with a motor unit ranges from ~10 (eye) to thousands (back musculature)
• innervation ratio, lower innervation ratio for more precise movements
• at 50 years old 1% annual decrease in their total number of motor units

Question 4

gamma (y) motorneurons

Answer 4

• y-motorneurons = small neurons innervating intrafusal muscle fibers
• determine sensitivity of intrafusal muscle fibers to stretch
• smaller than y-motorneurons
• oriented in parallel to extrafusal muscle fibers
• polar-to-polar connections

Question 5

extrafusal muscle fiber types

Answer 5

1. fast twitch, fatigable (FF) (Fast Motor Unit)
   - characterized by highest conduction velocity (100 m/s)
   - large fiber diameter
   - innervate fast twitch muscle fibers (Type II)
2. fast twitch, fatigue resistant (FR)
   - characterized by medium conduction velocity (60 m/s)
   - medium fiber diameter
   - innervate fast and/or slow twitch muscle fibers
3. slow twitch, fatigue resistant (SR) (Slow Motor Unit)
   - characterized by slow conduction velocity (40 m/s)
   - small fiber diameter
   - innervate slow twitch muscle fibers (Type I)

Question 6

motor unit types

Answer 6

• sprinter: 80% fast and 20% slow motor units
• marathoner: 20% fast and 80% slow motor units
• average person: 50% fast and 50% slow motor units
• couch potato: 60% fast and 40% slow motor units
• spinal injury: 96% fast and 4% slow motor units

Question 7

Henneman Size Principle

Answer 7

• motor unit is invariant from smallest to largest
• size of motor units increase with size of force production

Question 8

clinical relevance

Answer 8

• falls are the leading cause of injury in Canadians over 65 years of age
• falls account for 85% of hospitalization costs (about $2 billion/year health care costs)
• 40% of falls due to incorrect weight shifting

Question 9

why older adults fall

Answer 9

• individuals 30 years onward lose ~1%/year of total number of motor units with more rapid decrease after 60
• most young individuals have a random arrangement of motor-unit and muscle fiber types
• older adults exhibit selective loss of fast motor units
• large diameter (i.e. fast) motor units exhibit signs of demyelinization and thus reduced conduction times

Question 10

limiting age-related decline

Answer 10

square stepping exercise, adults step on a pattern and recruits more fast muscle fibers and prevent big loss of muscle fibers

wobble board, recruit fast twitch muscle fibers