5.1 Principles of motor control
4 min read•august 14, 2024
Motor control is a complex system that orchestrates our movements. It's organized hierarchically, with higher brain areas planning and lower areas executing. This system relies on sensory feedback to fine-tune movements and adapt to changing conditions.
The motor control system balances open-loop and closed-loop control. It also deals with motor redundancy, finding efficient solutions among many possible movement options. Understanding these principles helps explain how we move so smoothly and adapt to new situations.
Motor Control System Hierarchy
Organization and Function of Motor Control Centers
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- The motor control system has a hierarchical organization, with higher-level areas (cerebral cortex, ) planning and coordinating movements and lower-level areas (brainstem, spinal cord) executing specific motor commands
- The primary (M1) is the main output area for voluntary movements, with neurons projecting directly to the spinal cord and brainstem motor neurons
- The premotor cortex and supplementary motor area plan and prepare movements and coordinate complex motor sequences
- The cerebellum fine-tunes and coordinates movements and is involved in and adaptation (smooth, accurate, well-timed movements)
- The basal ganglia select and initiate voluntary movements and control automatic and repetitive movements
Brainstem and Spinal Cord Motor Control Centers
- The brainstem contains important motor control centers
- The reticular formation regulates muscle tone and postural control
- The vestibular nuclei contribute to balance and eye movements
- The spinal cord contains local motor circuits, such as (CPGs)
- CPGs can produce rhythmic motor patterns (walking, swimming) even without descending input from higher centers
Sensory Feedback in Motor Control
Proprioceptive and Cutaneous Feedback
- Sensory feedback, particularly from proprioceptors (muscle spindles, Golgi tendon organs) and cutaneous receptors, is essential for accurate and adaptive motor control
- Proprioceptive feedback provides information about the position, movement, and force of body parts, allowing for continuous monitoring and adjustment of motor commands
- Muscle spindles detect changes in muscle length and provide feedback for maintaining posture and controlling movement velocity
- Golgi tendon organs detect changes in muscle tension and provide feedback for regulating muscle force and preventing injury
- Cutaneous receptors in the skin provide information about touch, pressure, and vibration, which is important for fine motor control and object manipulation (grasping a pen, typing on a keyboard)
Integration of Sensory Feedback
- Visual and vestibular feedback also contribute to motor control, particularly for maintaining balance and coordinating eye and head movements
- The cerebellum integrates sensory feedback with motor commands to ensure smooth, accurate, and well-timed movements and to adapt to changing environmental conditions
- Sensory feedback allows for error detection and correction in real-time, enabling the motor system to make adjustments and maintain accuracy (reaching for a moving target, walking on uneven terrain)
Open-Loop vs Closed-Loop Control
Open-Loop Control Systems
- Open-loop control systems operate without sensory feedback, relying on predetermined motor commands to execute a movement
- Typically used for fast, ballistic movements where there is insufficient time for sensory feedback to influence the ongoing movement (throwing a ball, swinging a bat)
- More susceptible to errors and variability, as there is no mechanism to correct for deviations from the intended movement trajectory
Closed-Loop Control Systems
- Closed-loop control systems incorporate sensory feedback to continuously monitor and adjust motor commands during the execution of a movement
- More common in slower, precision-based movements where accuracy is crucial (threading a needle, tracing a line)
- Allows for greater accuracy and adaptability, as sensory feedback can be used to detect and correct errors in real-time
- Most complex movements involve a combination of open-loop and closed-loop control
- Open-loop control initiates the movement
- Closed-loop control refines it based on sensory feedback (reaching for a cup, then adjusting grip based on tactile feedback)
Motor Redundancy and Movement Control
Degrees of Freedom and Motor Redundancy
- Motor redundancy refers to the multiple ways to achieve a given motor goal due to the large number of degrees of freedom in the human body
- The central nervous system (CNS) must select a particular solution from the many possible combinations of joint angles, muscle activations, and movement trajectories that could accomplish a task
- Motor redundancy allows for flexibility and adaptability in movement control, as the CNS can choose different solutions based on factors such as energy efficiency, task constraints, or environmental conditions (reaching for an object on a high shelf vs. a low shelf)
Strategies for Simplifying Motor Control
- The CNS may use strategies such as synergies (co-activation of muscle groups) or optimal to simplify the control problem and produce consistent, well-coordinated movements
- Motor learning and adaptation can involve the exploration and refinement of different solutions to a motor task, taking advantage of the flexibility provided by motor redundancy (learning a new dance move, adapting to a new tennis racket)
- Injuries or neurological disorders that reduce the available degrees of freedom can limit motor redundancy and may require the development of compensatory strategies for effective movement control (using a cane after a leg injury, adapting to a prosthetic limb)
Key Terms to Review (18)
Amyotrophic lateral sclerosis: Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease that affects motor neurons in the brain and spinal cord, leading to muscle weakness, paralysis, and ultimately respiratory failure. The condition disrupts the principles of motor control by impairing voluntary muscle movements, severely impacting an individual’s ability to perform basic functions. As it progresses, ALS significantly affects the motor cortex and corticospinal tract, contributing to the overall understanding of neurodegenerative diseases.
Basal Ganglia: The basal ganglia is a group of subcortical nuclei in the brain that play a crucial role in the regulation of voluntary motor control, procedural learning, habit formation, and various cognitive functions. It connects with the cerebral cortex, thalamus, and brainstem, forming complex circuits that influence movement and behavior. Understanding its function is essential for grasping how motor control is executed and how learning and memory processes are integrated.
Biomechanical Perspective: The biomechanical perspective refers to the study of the mechanical aspects of biological systems, particularly in how forces interact with the body to produce movement. This viewpoint combines principles from physics and biology to analyze how muscles, bones, and joints work together during physical activities, emphasizing the importance of understanding the mechanics involved in motor control and movement execution.
Central Pattern Generators: Central pattern generators (CPGs) are neural circuits located in the central nervous system that produce rhythmic outputs in the absence of sensory feedback. They are crucial for controlling repetitive movements such as walking, swimming, and flying. These circuits can coordinate complex motor patterns and allow for smooth execution of movement, serving as a foundational mechanism for motor control and reflexes.
Cognitive motor control: Cognitive motor control refers to the processes by which the brain plans, executes, and adjusts movements based on cognitive functions such as attention, decision-making, and spatial awareness. This control mechanism integrates sensory information and higher-level cognitive processes to facilitate smooth and coordinated physical actions, ensuring that our movements are not just automatic but also contextually appropriate and adaptable.
Electromyography: Electromyography (EMG) is a diagnostic procedure that assesses the electrical activity of skeletal muscles. It measures the electrical signals produced by muscles when they contract and relax, providing insights into muscle function and the integrity of the neuromuscular system. This technique is crucial for understanding how muscles work in coordination with the nervous system, especially in the context of motor control and reflexes.
Extrapyramidal system: The extrapyramidal system is a network of pathways and structures in the brain that are responsible for the regulation and coordination of involuntary movements and reflexes. This system works alongside the pyramidal system, but it primarily focuses on muscle tone, posture, and automatic movements rather than voluntary motor control. It includes various structures like the basal ganglia, substantia nigra, and cerebellum, all of which play critical roles in maintaining smooth and balanced motor function.
Feedback Control: Feedback control refers to a process in which the system adjusts its actions based on the output it generates, allowing for corrections and adaptations in response to deviations from desired performance. This dynamic process is essential in motor control, where sensory information is continuously monitored to refine and enhance movements, ensuring accuracy and efficiency. Through feedback mechanisms, the brain can evaluate the success of motor actions and make necessary adjustments to improve future performance.
Feedforward control: Feedforward control is a proactive mechanism in motor control that anticipates the required motor commands based on sensory information to optimize movement performance. This type of control enables the nervous system to predict and prepare for the demands of an action before it occurs, effectively reducing the lag time that might come with feedback processes.
Functional MRI: Functional MRI (fMRI) is a neuroimaging technique that measures and maps brain activity by detecting changes in blood flow and oxygen levels. This non-invasive method allows researchers and clinicians to observe brain functions in real-time, making it essential for understanding various neural processes related to cognition, emotion, and motor control.
Hugo Münsterberg: Hugo Münsterberg was a German psychologist and philosopher who made significant contributions to the fields of psychology, especially in relation to motor control and applied psychology. He is known for his work on the psychology of movement and the application of psychological principles to practical problems, emphasizing how cognitive processes influence motor skills and performance.
Motor cortex: The motor cortex is a critical region of the brain responsible for planning, controlling, and executing voluntary movements. It plays a key role in the motor control system, where it integrates sensory information and sends signals to various muscles throughout the body to produce coordinated movements. This area is essential for both fine motor skills and larger movements, making it a fundamental part of how we interact with our environment.
Motor Hierarchy: Motor hierarchy refers to the organized structure of motor control systems in the brain and spinal cord that governs voluntary movement. This system allows for the coordination of motor tasks by breaking them down into smaller, manageable components, enabling complex movements to be executed efficiently and effectively.
Motor learning: Motor learning is the process of acquiring and refining motor skills through practice and experience, leading to a relatively permanent change in the ability to perform these skills. This process involves the brain and nervous system's adaptation to various factors such as feedback, task complexity, and environmental conditions, ultimately allowing for improved coordination and execution of movements.
Parkinson's Disease: Parkinson's disease is a progressive neurodegenerative disorder that primarily affects movement and is characterized by the degeneration of dopamine-producing neurons in the substantia nigra. This condition leads to motor symptoms such as tremors, stiffness, and bradykinesia, and it also has significant implications for neurotransmitter balance and intracellular signaling pathways.
Pyramidal tract: The pyramidal tract is a major pathway in the central nervous system that originates in the motor cortex and is primarily responsible for the voluntary control of skeletal muscles. This tract consists of two main components: the corticospinal tract, which extends to the spinal cord, and the corticobulbar tract, which connects to brainstem motor nuclei. It plays a critical role in the principles of motor control by facilitating precise and coordinated movements.
Sensorimotor integration: Sensorimotor integration is the process by which sensory information is combined with motor commands to produce coordinated movements and behaviors. This complex interaction occurs in the brain, where sensory input from various modalities is interpreted and used to guide motor responses. Effective sensorimotor integration is crucial for tasks ranging from simple reflexes to complex actions like playing sports or driving.
Wilder Penfield: Wilder Penfield was a renowned neurosurgeon and neuroscientist known for his groundbreaking work in mapping the brain's functional areas, particularly in relation to motor control and sensory perception. His pioneering techniques, especially the development of the Montreal Procedure for epilepsy treatment, allowed him to stimulate specific brain regions while patients were awake, leading to significant advancements in understanding how different parts of the brain contribute to motor functions and movement coordination.