Segmental Motor Control

Presented by: Karen J. Hutchinson, PT, DPT, PhD, Sargent College: Department of Physical Therapy

Describe the 4 key CNS regions for movement control: Identify structures and locations of these regions, including the spinal cord, brainstem, basal ganglia, and cerebellum, and discuss how each region contributes to voluntary and reflexive movements.
Contrast Segmental and Descending Control: Understand the differing functions, including the role of segmental control in local reflexes and motor coordination versus descending control influencing motor initiation and modulation from higher centers of the brain.
Motor unit selection: Explore its role in movement execution, emphasizing the factors that determine which motor units are recruited during different types of physical activity or exercise, such as force requirement and fatigue resistance.
Alpha and Gamma Motor Neurons: Compare their contributions to segmental stretch reflex modulation, detailing how alpha motor neurons innervate extrafusal fibers for muscle contraction, while gamma motor neurons adjust sensitivity of intrafusal fibers within muscle spindles.
Predict deficits in sensory/motor disorders: Understand deficits associated with disorders of segmental circuits, such as spasticity in upper motor neuron lesions versus weakness in lower motor neuron lesions.
Clinical tests for Lower Motor Neuron Syndrome: Discuss outcomes expected from these tests, including electrophysiological evaluations, reflex assessments, and muscle strength tests to characterize lower motor neuron dysfunctions.

Introduce Four Motor Subsystems
Segmental Motor Pathways- A. Motor Neuron Pools Anatomy
- B. Local Circuit Neurons
- C. Muscle Tone, Alpha/Gamma
Motor Units- A. Definition of Motor Unit
- B. Motor Unit Physiology (3 types)
- C. Motor Unit Force Production
Muscle Tone- A. Stretch Reflex
- B. Upper Motor Neuron Impact
- C. Gamma Motor Neuron Gain
- D. Alpha Gamma Coactivation
Golgi Tendon Organs (GTOs)- A. In Series
- B. Autogenic Inhibition, Ib (-) interneurons
Polysynaptic Segmental Motor Responses- A. Flexor Withdrawal
- B. Crossed Extension
- C. Spinal Cord Central Pattern Generators (CPGs)
Lower Motor Neuron Syndrome

Key subsystems: Segmental, Descending, Basal Ganglia, Cerebellar. Each subsystem has a distinct function but works in concert to produce controlled movement.

A. Motor Neuron Pools Anatomy: Motor neuron pools exist in a rod-shaped cluster in the spinal cord, with each pool corresponding to a muscle; precise organization allows for spatial distribution of motor control.
- Retrograde transport from muscle to spinal cord for neuron identification helps understand neuromuscular connections and injury responses.
B. Local Circuit Neurons: Medial (long-distance) and Lateral (short-distance) neurons facilitate coordination across spinal segments, enabling complex movements through synaptic interactions between different segments of the spinal cord.
C. Muscle Tone: Defined as resting tension maintained by the segmental stretch reflex arc preparing for movement; is critical for maintaining posture and is influenced by both sensory input and central nervous system command.
- Normal tone is essential for anti-gravity posture, providing stability against the forces of gravity during static and dynamic tasks.

A. What's a Motor Unit: Comprised of a single alpha motor neuron and the muscle fibers it innervates, motor units serve as the functional units of muscle contraction and movement.
B. Motor Unit Physiology: Different types include Slow (Type I), Fast Fatigable (Type IIb), and Fast Fatigue-Resistant (Type IIa), impacting force production based on recruitment strategies and metabolic properties, which dictate their endurance and capacity for energy utilization.
C. Motor Unit Force Production: The size principle explains how muscle fibers are recruited based on the force required for movement; smaller motor units are activated first, followed by larger units as the demand for force increases, allowing for smooth and controlled contractions.

A. Muscle Stretch Reflex: Involves sensory signaling via muscle spindle afferents (Ia and II), responding to stretch by facilitating contraction of the muscle and inhibition of antagonists, essential for rapid adjustments in posture and movement.
B. UMN Impact on Muscle Tone: Upper motor neuron damage influences muscle tone and stretch reflex sensitivity, leading to exaggerated reflexes or rigidity due to loss of modulation.
C. Gamma Motor Neuron Gain: Impacts muscle spindle response to stretch, modulating stretch reflex intensity and ensuring accurate grading of muscle responses during movement.
D. Alpha-Gamma Coactivation: Coordination between alpha and gamma neurons optimizes muscle spindle sensitivity during movement, allowing for precise control of muscle length and tension.

GTOs sense tension in muscles and facilitate autogenic inhibition, preventing excessive force production to protect against injury by reflexively inhibiting the muscle being contracted and facilitating activation of antagonistic muscles.

A. Flexor Withdrawal Reflex: Withdraws limb from harmful stimuli by activating ipsilateral flexors and inhibiting extensors, serving as a protective mechanism.
B. Crossed Extension Reflex: Provides postural support while flexor withdrawal occurs, allowing stability in the contralateral leg to maintain posture during a threat response.
C. Central Pattern Generators (CPGs): Organize rhythmic movements, such as walking, at the spinal cord level, facilitating alternating flexor and extensor activity independent of sensory feedback.

A. Structures Involved: Includes anterior horn cells and peripheral axons responsible for direct innervation of muscles, impacting movement if compromised.
B. Clinical Presentation: Symptoms include paralysis, hypotonia (decreased muscle tone), and spontaneous muscle twitches (fasciculations), reflecting the loss of innervation and control over muscle contraction.

Assessing sensory and motor testing uncovers neuromuscular system function. Focus on interpreting test results reveals insights into complex motor tasks. Motor control at spinal levels highly influences the selection and coordination of motor units, and understanding these mechanisms is critical for developing rehabilitative strategies for patients with movement disorders.