Lecture 16 - Lower Motor Neurons

Learning objectives

• Describe the organization of the motor system.

• Determine how the lower motor circuits control movement.

• Differentiate between types of motor units.

• Describe the neural circuitry of motor reflexes.

Organization of the motor system

• Lower motor neuron circuits within brainstem and spinal cord

  • Lower motor neurons are the “final common path” for initiating movement.

  • Local circuit neurons coordinate between muscle groups for organized movement.

• Upper motor neurons from brainstem and cortex initiate voluntary movement and complex sequences of skilled movements

  • Primary motor and premotor cortex plan, initiate, and direct voluntary movements 

• Cerebellum mediates real-time and long-term motor errors to enhance coordination and performance 

• Basal ganglia control and initiate movement 

Motor neuron-muscle relationship

• Tracer studies demonstrate a medial-to-lateral spatial map of motor neurons in relation to musculature

• E.g., calf muscle injection

  • Motor neurons found in the same relative region of ventral horn

• Muscle location on body relates to motor neuron location in spinal cord

  • Somatotopic map 

Local circuit neurons

• Long pathways run medially

  • Postural control and balance

  • Coordinate rhythmic movements of upper and lower limbs 

  • Many segments, axons terminate bilaterally 

• Short pathways run laterally

  • Fine control, skilled behavior (e.g., hands and fingers)

  • >5 segments, axons terminate on same side

Motor unit

• In development, each muscle fiber contacted by many axons

• Terminal branches withdrawn until muscle fiber innervated by a single ɑ motor neuron 

• ɑ motor neuron can synapse with multiple muscle fibers

• Density of motor axons to muscle groups determine precision of control

  • Fine →1 axon, few fibers (e.g., eye)

  • Coarse → 1 axon, many fibers (e.g., leg) 

• ɑ motor neuron and all of the muscle fibers it innervates is called a motor unit

Motor unit types

• 1. Slow (S) motor units

  • Sustained contraction, resistant to fatigue

  • Maintain posture 

  • “Red” muscle fibers

• 2. Fast fatigable (FF) motor units

  • Generate more force

  • Fewer mitochondria, easily fatigued 

  • “White” muscle fiber

• 3. Fast fatigue-resistant (FR) motor units

  • Intermediate, not as fast as FF

  • Twice the force of S, resistant to fatigue

Muscle force

• How can we get more power?

• Increase the number of motor units active at one time 

• As activity to a lower motor neuron pool increases, muscle tension

  • Sequence is S → FR → FF

  • Termed the “size principle”

• Frequency of action potentials in motor neurons also regulates muscle tension

• Increase in force reflects temporal summation of muscle contraction

  • Muscle fibers activated by next action potential before relaxed

Muscle stretch reflex

• Intrafusal muscle fibers contain primary endings (group 1a afferents) that are stimulated when spindle is stretched

  • Rapid adaptation 

  • Detects velocity and direction (limb dynamics)

• Secondary endings (group II afferents) discharge when spindle stretched continuously

  • Slow adaptation 

  • Static position of limbs

• γ motor neurons receive info from primary and secondary endings, and affect resting tension of muscle spindle, modifying sensitivity to stretch

• Reciprocal inhibition → contraction of stretched muscle, relaxation of antagonist muscle

Muscle stretch reflex: modifying the gain

• Level of γ motor neuron activity is referred to as gain

• α motor neuron signals contraction of muscle, AND signals γ motor neuron to contract intrafusal muscle fiber

• Thus, γ motor neuron keeps tension on intrafusal muscle fibers

Regulation of muscle force

• Located at junction of muscle and tendon

• Detect muscular contraction

• Contractions can be strong enough to rip muscle from bone or break bone!

• Muscle contraction excites 1b afferents which excite 1b inhibitory interneurons in spinal cord 

• Inhibits α motor neuron, reducing contraction 

  • Also activates α motor neuron for antagonist muscle

  • Clasp-knife reflex

Flexion-crossed extension reflex

• Stepping on a sharp object results in withdrawal reflex and compensatory support