Neuromuscular Junction and Motor Unit Recruitment

Muscle Physiology

• We are 1.3-1.5 times stronger eccentrically than concentrically
• Eccentric contractions rip myosin from actin, leading to muscle soreness and repair, which increases strength
• The force of contraction is less than the force of the load during eccentric contractions

Muscle Tension

• The amount of muscle tension is dependent on two factors:
  • Recruitment: the number of motor units (MUs) activated
  • Rate of recruitment (coding frequency): how rapidly the nervous system stimulates the muscle

Muscle Fiber Types

• Slow-twitch (Type I, SO): aerobic pathways, dark in color, small motor units, slower, high endurance, high myoglobin, and dense capillary beds
• Fast-oxidative (Type IIa, FOG): intermediate between Type I and Type IIb, similar to fast-twitch but with more mitochondria
• Fast-twitch (Type IIb/x, FG): anaerobic pathways, light in color, large motor units, fast, fewer mitochondria, and little/no myoglobin

Muscle Fiber Distribution

• Skeletal muscles typically have a mix of all three fiber types, but the proportion varies from muscle to muscle
• Single motor units have single types of fibers
• Postural muscles tend to have slow-twitch fibers (e.g., soleus)
• Eye and hand muscles tend to have fast-twitch fibers only

Muscle Contractions

• Isotonic contractions:
  • Concentric: muscle contracts as it shortens (force of contraction > force of load)
  • Eccentric: muscle contracts as it lengthens (force of load > force of contraction)

Muscle Function and Aging

• Between ages 30-50, there is a 10% loss of muscle mass
• By age 80, there is a 40% loss of muscle mass

Muscle Structure

• Thick filaments:
  • Made up of myosin, with a binding site for actin and ATP
  • Elongated tail with a golf club head
• Thin filaments:
  • Made up of actin, tropomyosin, and troponin
  • Double-stranded helix with binding sites for myosin
• Sarcomere structure:
  • A-bands: isotropic, area of thick filaments
  • I-bands: area of no overlap between thin filaments
  • H-zone: area of non-overlap between thick filaments
  • M-line: proteins in the middle of the sarcomere, anchoring myosin tails
  • Z-disc: proteins separating sarcomeres, anchoring thin filaments

Motor Units and Neuromuscular Junction

• Motor unit: motor neuron and all the muscle fibers it innervates
• Motor neuron can stimulate a few to 2,000 muscle fibers
• All or none response: all muscle fibers of a motor unit contract at the same time
• Recruitment: smallest motor units are recruited first, and as demand increases, larger motor units are recruited
• Fatigue sets in when motor units can no longer be recruited

Neuromuscular Junction

• Contact site of motor neuron on muscle fiber
• Components:
  • Synaptic knob: end of axon, secretes neurotransmitter
  • Synaptic vesicles: contain neurotransmitter
  • Synaptic cleft: space between synaptic knob and post-synaptic membrane
  • Motor end plate: post-synaptic membrane on muscle fiber
  • Acetylcholine receptors: on motor end plate, receives neurotransmitter
  • Acetylcholinesterase: breaks down neurotransmitter

Explore key concepts related to neuromuscular junctions and motor unit recruitment in the nervous system. Learn about the structure of the neuromuscular junction, the process of recruitment of motor units, and the connection between muscle fibers and the nervous system.