Lecture 16 - Lower Motor Neurons

Organization of the motor system

The structure and arrangement of neural components that control muscle movement.

Lower motor neurons

The final common path for initiating movement from the brainstem and spinal cord.

Local circuit neurons

Neurons that coordinate between muscle groups for organized movement.

Upper motor neurons

Neurons from the brainstem and cortex that initiate voluntary movement and complex sequences of skilled movements.

Primary motor cortex

The region of the brain responsible for planning, initiating, and directing voluntary movements.

Cerebellum

Brain structure that mediates real-time and long-term motor errors to enhance coordination and performance.

Basal ganglia

Brain regions that control and initiate movement.

Somatotopic map

The spatial arrangement of motor neurons in the spinal cord reflecting their corresponding muscle location.

Local circuit neurons medial pathways

Long pathways that run medially in the spinal cord and are involved in postural control and balance.

Local circuit neurons lateral pathways

Short pathways that run laterally and are involved in fine control and skilled behavior.

Motor unit

A motor neuron and all of the muscle fibers it innervates.

Slow (S) motor units

Motor units that are resistant to fatigue and responsible for sustained contraction and posture.

Fast fatigable (FF) motor units

Motor units that generate more force but have fewer mitochondria and are easily fatigued.

Fast fatigue-resistant (FR) motor units

Intermediate motor units that can generate twice the force of S units and are resistant to fatigue.

Size principle

The recruitment order of motor units based on their size: Small (S) → Fatigue-Resistant (FR) → Fast Fatigable (FF).

Gain in muscle stretch reflex

The level of γ motor neuron activity that modifies the sensitivity of muscle spindles to stretch.

Muscle stretch reflex

A reflex that helps maintain muscle tension and responds to stretching of intrafusal muscle fibers.

Clasp-knife reflex

A reflex where a muscle suddenly relaxes when a load is applied.

Flexion-crossed extension reflex

A reflex that results in withdrawal from a painful stimulus and compensatory extension on the opposite side.

1b afferents

Sensory neurons that detect muscle contraction and provide feedback for regulating muscle force.