4.5 Motor control and pathways

Motor control and pathways are crucial for understanding how our nervous system orchestrates movement. This topic explores the organization of the motor cortex, descending motor pathways, and the roles of the cerebellum and basal ganglia in coordinating our actions.

We'll dive into motor unit recruitment and how muscles generate graded contractions. This knowledge ties into the broader nervous system chapter, showing how different parts work together to produce smooth, controlled movements in our daily lives.

Motor Cortex: Organization and Function

Somatotopic Organization

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• The motor cortex is located in the frontal lobe of the cerebral cortex and is responsible for planning, initiating, and controlling voluntary movements
• The motor cortex is organized somatotopically, with different areas corresponding to different parts of the body, forming a "motor homunculus" (a distorted representation of the body based on the proportion of cortical area dedicated to each body part)
• The amount of cortical area dedicated to a particular body part is proportional to the precision and complexity of movements required for that body part (the hands and face have larger representations compared to the trunk)
• The somatotopic organization allows for the precise control and coordination of specific muscle groups and movements

Functional Subdivisions

• The primary motor cortex (M1) directly controls the execution of movements by sending signals to the spinal cord and cranial nerve motor nuclei
• M1 contains large pyramidal neurons (Betz cells) that give rise to the corticospinal tract, which is essential for fine, precise, and voluntary movements, particularly in the distal extremities (hands and fingers)
• The premotor cortex (PMC) is involved in planning and preparing movements, as well as in the selection of appropriate motor programs based on sensory input and learned behaviors
• The PMC integrates sensory information (visual, somatosensory) and cognitive input (goals, intentions) to guide the selection and preparation of movements before their execution by M1
• The supplementary motor area (SMA) plays a role in the coordination of complex, sequential, and bilateral movements, as well as in the internal generation of movement sequences
• The SMA is involved in the planning and initiation of self-paced, internally generated movements (without external cues) and in the coordination of bimanual movements (requiring both hands)

Descending Motor Pathways: Functions

Corticospinal and Corticobulbar Tracts

• The corticospinal tract (pyramidal tract) originates from the motor cortex and carries motor commands directly to the spinal cord, controlling fine, precise, and voluntary movements, particularly in the distal extremities
• The corticospinal tract is essential for skilled, dexterous movements (writing, manipulating objects) and is more developed in humans compared to other animals
• The corticobulbar tract carries motor signals from the motor cortex to the cranial nerve motor nuclei in the brainstem, controlling the muscles of the face, head, and neck
• The corticobulbar tract is involved in the control of facial expressions, speech, and swallowing

Brainstem Descending Pathways

• The rubrospinal tract originates in the red nucleus of the midbrain and facilitates flexor muscle activity and inhibits extensor muscle activity, mainly in the upper limbs
• The rubrospinal tract is involved in the control of reaching movements and in the modulation of muscle tone
• The reticulospinal tracts (pontine and medullary) originate in the reticular formation of the brainstem and influence axial and proximal limb muscles, contributing to posture, balance, and locomotion
• The reticulospinal tracts integrate sensory, vestibular, and cortical input to regulate muscle tone, maintain posture, and control automatic and stereotyped movements (walking, running)
• The vestibulospinal tracts (lateral and medial) originate in the vestibular nuclei and are involved in maintaining balance, posture, and head position in response to vestibular input
• The vestibulospinal tracts receive input from the vestibular system (inner ear) and control the extensor muscles of the limbs and trunk to maintain balance and counteract gravitational forces
• The tectospinal tract originates in the superior colliculus and is involved in coordinating head and eye movements in response to visual stimuli
• The tectospinal tract allows for the rapid orientation of the head and eyes towards visual targets, enabling visual tracking and gaze shifts

Cerebellum and Basal Ganglia: Motor Control

Cerebellum

• The cerebellum receives input from the motor cortex, sensory systems, and other parts of the brain, and integrates this information to fine-tune and coordinate motor output
• The cerebellum compares the intended movement (efference copy) with the actual sensory feedback (reafference) to detect and correct errors in motor performance
• The cerebellum is involved in motor learning, adaptation, and the smooth execution of movements by comparing intended movements with actual performance and making necessary adjustments
• The cerebellum plays a crucial role in the acquisition and storage of motor skills and in the adaptation of movements to changing environmental conditions (learning to ride a bicycle, adapting to walking on uneven terrain)
• The cerebellum contributes to the maintenance of balance, posture, and muscle tone by processing vestibular, proprioceptive, and visual information
• The cerebellum integrates sensory input from the vestibular system, proprioceptors (muscle spindles, Golgi tendon organs), and vision to control balance, posture, and smooth, coordinated movements

Basal Ganglia

• The basal ganglia, which include the striatum (caudate nucleus and putamen), globus pallidus, substantia nigra, and subthalamic nucleus, are involved in the selection, initiation, and execution of voluntary movements
• The basal ganglia receive input from the cerebral cortex and send output to the motor cortex via the thalamus, forming a cortico-basal ganglia-thalamo-cortical loop that modulates motor activity
• The basal ganglia play a role in motor planning, the control of automatic and repetitive movements, and the regulation of muscle tone
• The basal ganglia are involved in the selection and initiation of appropriate motor programs, the suppression of unwanted movements, and the smooth transition between different motor patterns (walking to running)
• The basal ganglia contribute to the regulation of muscle tone by balancing the excitatory and inhibitory influences on the motor cortex and brainstem motor centers
• Dysfunction of the basal ganglia can lead to movement disorders such as Parkinson's disease (characterized by bradykinesia, tremor, and rigidity due to dopamine depletion in the substantia nigra) and Huntington's disease (characterized by chorea and cognitive impairment due to degeneration of the striatum)

Motor Unit Recruitment: Graded Muscle Contractions

Motor Units

• A motor unit consists of a single alpha motor neuron and all the muscle fibers it innervates, and is the functional unit of muscle contraction
• The number of muscle fibers innervated by a single motor neuron varies depending on the muscle and the precision required for its control (extraocular muscles have small motor units for fine control, while postural muscles have large motor units for powerful contractions)
• Motor units are classified based on their contractile and metabolic properties into slow-twitch, fatigue-resistant (type I), fast-twitch, fatigue-resistant (type IIa), and fast-twitch, fatigable (type IIb) units
• The proportion of different motor unit types within a muscle determines its functional characteristics and resistance to fatigue (postural muscles have a higher proportion of slow-twitch fibers, while sprinter muscles have a higher proportion of fast-twitch fibers)

Recruitment and Size Principle

• Motor unit recruitment refers to the progressive activation of motor units within a muscle to increase the force of contraction
• The size principle governs motor unit recruitment, stating that motor units are recruited in order of increasing size (and force output) as the demand for muscle force increases
• Smaller motor units, which innervate slow-twitch, fatigue-resistant muscle fibers, are recruited first for low-force, precise, and sustained contractions (maintaining posture, fine motor control)
• Larger motor units, which innervate fast-twitch, fatigable muscle fibers, are recruited later for high-force, rapid, and powerful contractions (sprinting, jumping)
• The orderly recruitment of motor units enables smooth, precise, and efficient control of muscle force and movement, allowing for a wide range of force output and contraction speeds

Spatial and Temporal Summation

• The recruitment of additional motor units (spatial summation) and the increase in firing frequency of active motor units (temporal summation) allow for the gradation of muscle force output
• Spatial summation refers to the activation of more motor units within a muscle as the force demand increases, resulting in a greater number of muscle fibers contributing to the contraction
• Temporal summation refers to the increase in the firing rate of active motor units, leading to a higher frequency of muscle fiber stimulation and a greater force of contraction
• The combination of spatial and temporal summation enables the fine control of muscle force output over a wide range, from delicate, precise movements to powerful, explosive contractions (writing with a pen to throwing a ball)
• The nervous system adjusts the recruitment and firing rate of motor units based on the desired force, speed, and duration of the movement, as well as the feedback received from sensory receptors (muscle spindles, Golgi tendon organs) and other sources (vision, balance)