Lecture 13 - Locomotion

Locomotion and Central Pattern Generators

Course: KIN 255 – Fundamentals of Neuroscience

Institution: University of Waterloo, Department of Kinesiology and Health Sciences

Overview

Understanding the biomechanics of locomotion and the neural substrates involved is paramount for kinesiology and neuroscience studies. This includes exploration of how different body systems interact to facilitate movement and how injuries or disorders can disrupt these processes.

Introduction

An introduction to locomotion covers the basic principles of human movement and the role of central pattern generators (CPGs) in producing rhythmic patterns that are crucial for locomotion.

Key Topics

  • Definitions:

    • Locomotion: The broader ability to move actively from one location to another.

    • Walking: A specific form of locomotion characterized by a unique pattern of foot placement and body posture.

    • Gait: Refers to the particular manner or style in which a person walks. This can be influenced by factors like anatomy, age, and neurological conditions.

  • Human Gait Control: Investigates the neural and biomechanical processes that regulate movement, highlighting the importance of rhythmic control of movements and CPGs.

  • Central Pattern Generator (CPG): A complex neural mechanism that generates rhythmic patterns of motor activation, essential for producing and regulating locomotion through spinal circuits.

  • Illustrative Examples: Research demonstrating the operational mechanisms of CPGs through examples such as walking, running, and more specialized movements.

  • Circuitry involved in CPGs: Understanding how various spinal circuits coordinate to facilitate movement via reciprocal and rhythmic activation of muscles.

  • Modulation of CPG: Exploring how different types of inputs—such as afferent sensory information, interlimb spinal connections, and descending signals from the brain—can alter CPG function and adapt movements to changing tasks or environments.

Innate Locomotion in Infants

  • Newborn Capability: Infants exhibit the ability to produce stepping patterns even without prior walking experience, demonstrating that there exists an innate neural mechanism underlying locomotion that is activated from a very young age.

Definitions of Movement

  • Locomotion: Defined as the ability to navigate spaces and move from one place to another effectively.

  • Walking: Implies a series of alternating foot placements that involve both biomechanical and neurological coordination.

  • Gait: The style of walking, which can be modified both voluntarily (e.g., adjusting to slippery surfaces) and involuntarily due to pathologies (e.g., propulsive gait seen in Parkinson's disease).

Human Gait Characteristics

  • Center of Gravity (COG): The key anatomical point where body weight is effectively distributed; understanding this helps in analyzing movement efficiency and stability.

  • Gait Mechanics: Requires the rhythmic movement of limbs to propel the COG while maintaining dynamic balance; involves various phases of the gait cycle.

    • Gait cycle: A critical measurement reflecting how the body's mass is distributed and managed during walking and running.

Control of Gait

  • Locomotor Rhythm: Identifying the sequence in muscle activation that corresponds with leg movements ensures seamless locomotion.

  • Inter-limb Coordination: The complex interplay between limbs during walking, such as the timing and degree of extension and flexion in each limb.

  • Intra-limb Coordination: The refined control of multiple joints within a single limb enhances movement precision.

  • CNS Involvement: Demonstrates how CPGs enable consistent locomotor patterns across different species, highlighting evolutionary aspects of movement control.

  • Invariant vs Surface Features: Understanding the difference between the consistent elements of movement patterns versus the variable aspects influenced by the environment and individual dynamics.

Locomotor Invariance

  • Consistency Across Species: Even with varied modes of locomotion (e.g., bipedal vs. quadrupedal), there exists a remarkable similarity in rhythmic muscle activity, providing insight into the conservation of movement strategies across evolution.

  • Coordination of Muscle Activity: Analyzed to understand shared functional components between different locomotion styles, revealing underlying patterns in movement.

Walking vs Running

  • Speed and Ground Contact: An increase in locomotion speed correlates with reduced ground contact time, impacting how forces are transferred through the body.

  • Flight Period: Recognizing the phase when both feet are off the ground during running, which presents a unique transition in biomechanical demands.

  • Inquiry into common motor programming: Examining potential similarities in the brain's motor commands for walking versus running, enabling improved performance and rehabilitation strategies.

Central Pattern Generators

  • Definition: A neural substrate that generates and maintains rhythmic motor activity for locomotion independent of sensory feedback.

  • Examples of CPG-mediated Behaviors: Include not only locomotion but also key behaviors like breathing and mastication, illustrating CPGs' broad influence across motor systems in the central nervous system.

  • CPGs: Capable of sustaining rhythmic operations even once sensory nerves are severed, reflecting their intrinsic functionality.

CPG Mechanisms

  • Initiation of Patterns: CPGs facilitate the excitation of flexor muscles while simultaneously inhibiting extensors, showcasing complex neural control through intermediary interneurons.

  • Rhythm Generators: These timing components adjust the coordination of muscle activity, fundamental to producing consistent and effective locomotion.

CPG Patterns Across Species

  • Spinal Cord Stimulation Effects: Different responses to stimulation at varying spinal cord levels underscore that multiple, distinct CPGs likely exist within the neural architecture.

  • Rhythmic Behavior: Surfaces as a product of shared functional capacities across diverse CPGs, enhancing the adaptability of locomotor systems.

Propriospinal Connections

  • Definition: Nerve tracts that connect various levels of the spinal cord, facilitating integrated locomotor control across vertebrates.

  • Function: These connections help modulate CPG activity, allowing for refined and coordinated limb movements that are adaptive during locomotion.

CPG Modulation

  • Sources of Control: Emphasizing the role of sensory inputs, spinal interlimb modulation, and descending pathways from the brain in adjusting CPG outputs.

  • Adaptive Mechanisms: CPGs constantly adjust to sensory inputs that convey information about muscle status, while sustaining natural rhythmic patterns necessary for movement.

Reflex Pathways

  • Identification of Reflex Pathways: Analyzing how different sensory fibers contribute to stretch reflex pathways crucial for modifying CPG functions during locomotion.

Descending Commands to CPGs

  • Role of MLR: The mesencephalic locomotor region (MLR) is critical for providing excitatory inputs that help regulate spinal CPGs and their patterns of locomotion.

MLR Adjustments

  • Tonic to Phasic Input Changes: This phenomenon facilitates rapid changes in locomotor rhythm by manipulating the rates and patterns of muscle alternation, acting akin to a “gas pedal” for coordinated movement execution.

Effect of Sensory Inputs on CPG

  • Experimental Observations: Research indicating how simulated sensory feedback can alter muscle firing patterns within the CPG, demonstrating the dynamic nature of motor control.

Impact of Treadmill Speeds on Gait

  • Sensory Feedback Mechanisms: Studies show that cats can maintain altered gait speeds without direct MLR inputs, illustrating the effectiveness of proprioceptive feedback in adapting movements.

Reflex Pathways Identification

  • Scope: Critical examination of reflex pathways that relate to myotatic and inverse myotatic reflexes, establishing their significance in the functionality of CPGs during locomotion.

Sensory Inputs to CPG

  • Role of Sensory Feedback: Essential for signaling phase transitions throughout the gait cycle, reinforcing extensor activity, and adapting to various environmental conditions.

Phase Transitions in CPG

  • Influencing Factors: Position of body joints, particularly the hip, significantly alters the firing patterns of knee extensors and flexors, illustrating how sensory inputs guide transitions in movement phases.

Weight Bearing and CPG Modulation

  • Stance Heightened by Sensory Input: An increase in sensory feedback is associated with CPG adjustments that enhance weight bearing on the stance leg, particularly beneficial during unforeseen challenges to stability.

Phase-Dependent Reflex Modulation

  • CPG and Reflex Interaction: Spinal CPGs can adjust reflex responses based on the current phase of the gait cycle, showcasing the sophisticated adaptability required in dynamic environments.

Human CPG Research Challenges

  • CPG Investigation in Humans: Challenges arise from ethical concerns regarding electrode implantation, complicating studies aimed at directly analyzing human CPG involvement in locomotion.

Species Variability in CPG Control

  • Relationship with Nervous System Complexity: As species evolve, increased complexity in their nervous systems correlates with enhanced sensory feedback mechanisms and descending modulation capabilities over purely CPG-based movements.

Summary of Key Concepts

  • Main Points Recap: Definitions of locomotion, walking, and gait; understanding of human gait control and rhythmic patterns; an overview of CPG circuitry and examples; and insights into the modulatory influences acting on CPGs from both internal (biological, structural) and external (environmental, sensory) sources.