Lecture 13: Motor neurons, units, cortex 

  1. What are the characteristics of muscle fibers?

Characteristics of muscle fibers: Responsiveness (excitability), conductivity, contractibility, extensibility, elasticity

Responsiveness (excitability): to chemical signals, stretch, and electrical charges across plasma membrane

Conductivity: Local electrical change triggers wave of excitation that travels along the muscle fiber

Contractibility: Shortens when stimulated

Extensibility: Capable of being stretched

Elasticity: Returns to its original resting state after being stretched

  1. What is meant by the size principle when it comes to motor neuron pools?

Size principle: Neurons with a smaller motor unit usually have smaller axon diameters and cell bodies (i.e. motor unit size generally correlates with the size of the neuron’s axon diameter and cell body size)

  1. What is a motor unit?

Motor unit: A motor neuron and the muscle it innervates

  1. What is the difference between alpha and gamma motor neurons?

Alpha motor neurons:

  • Innervate extrafusal muscle fibers
  • Control muscle contraction
  • Voluntary movement

Gamma motor neurons:

  • Innervate intrafusal muscle fibers
  • Control external forces acting on the muscle
  • Proprioception
  1. Describe the descending axonal tracts

Lateral pathways (how many and what are they?): (2) Corticospinal, rubrospinal

Ventromedial pathways (how many and what are they?): (4) Vestibulospinal, Tectospinal, Pontine reticulospinal, Medullary reticulospinal

Corticospinal: Lateral pathway, [what it controls]

Rubrospinal: Lateral pathway, [what it controls]

Vestibulospinal: Ventromedial pathway, [controls]

Tectospinal: Ventromedial pathway, [controls]

Pontine reticulospinal: Ventromedial pathway; enhances antigravity reflexes

Medullary reticulospinal: Ventromedial pathway; liberates antigravity muscle from reflex control

  1. Describe the role of areas 4 and 6 in motor control

Area 4: Primary motor cortex, M1

Area 6: “higher” motor area

Lateral region of area 6: premotor area (PMA)

Medial region of area 6: supplementary motor area (SMA)

  1. How is the somatotopic map of the motor cortex organized?

Proportional to how much control we have