Brain Control of Movement (Chapters $$17$$, $$18$$, and $$19$$)

Introduction to Upper Motor Neuron Systems

  • Foundational Movement: While spinal circuits provide the basic foundation for movement, complex behaviors require extensive brain control to coordinate posture, balance, locomotion, and the suppression of inappropriate actions.

  • Upper Motor Neurons: These are neurons located in the brain that send descending signals to the spinal cord. Most voluntary movement is organized through these neurons rather than independent spinal activity.

  • Two Major Descending Systems:

    • Lateral Pathways: These originate primarily from the cerebral cortex. They are responsible for skilled voluntary motion, specifically the distal muscles of the limbs (e.g., precise movements of the hands and fingers).

    • Ventromedial Pathways: These originate primarily from the brainstem motor centers. They regulate posture, balance, head position, and locomotion, primarily controlling axial and proximal muscles to maintain stability.

Cortical Regions for Motor Control

  • Location: Upper motor neurons responsible for voluntary movement are located in the posterior frontal lobe.

  • Primary Motor Cortex (M1M1):

    • Corresponds to Broadman Area 4 and occupies the precentral gyrus.

    • Somatotopic Organization: The cortex is organized into a motor map called the motor homunculus.

    • Cortical Representation: Territory is not proportional to physical size but to the degree of motor control required. Areas requiring high precision (fingers, face, tongue) have disproportionately large representations.

    • Function: Neurons represent coordinated movements and patterns of muscle activation rather than isolated single muscle contractions.

  • Premotor Cortex:

    • Located immediately rostral to the primary motor cortex and involved in movement planning and preparation.

    • Receives input from frontal and parietal association areas regarding behavior goals, motivation, and intention.

    • Lateral Premotor Cortex: Important for movements guided by external sensory cues.

      • Mirror Neurons: These cells become active both when an individual performs an action and when they observe another person performing that same action.

    • Medial Premotor Cortex (Supplementary Motor Area): Primarily involved in movements guided by internal cues, such as learned motor sequences and movements generated from memory.

    • Example (Yoga): Following an instructor's motions in class utilizes mirror neurons (external cues); practicing those same motions at home from memory utilizes the medial premotor circuits (internal cues).

  • Neural Structure: Most corticospinal neurons are pyramidal neurons located in layer 5 of the cerebral cortex.

    • Betz Cells: The largest neurons in the central nervous system. Despite their fame for size and rapid conduction, they represent only a small fraction of the total corticospinal neurons.

Information Flow and the Corticospinal Tract

  • Integration Pathway: Visual info (occipital/temporal) →\rightarrow Posterior parietal cortex (transferred to movement representations) →\rightarrow Premotor regions (development of plans) →\rightarrow Primary motor cortex (generation of commands).

  • The Corticospinal Tract: The major descending pathway. Axons descend through the internal capsule and continue through the brainstem.

  • Pyramidal Decussation: Occurs at the caudal medulla. This is a defining feature where the majority of fibers cross the midline.

    • Effect: The right motor cortex controls the left side of the body, and the left motor cortex controls the right side.

  • Lateral Corticospinal Tract: Specifically targets distal limb musculature for skilled motion.

Brainstem Motor Systems

  • Vestibulospinal Tract:

    • Originates from vestibular nuclei (receiving input from the inner ear).

    • Lateral Vestibulospinal Tract: Influences muscles of the trunk and proximal limbs.

    • Medial Vestibulospinal Tract: Controls the head, neck, and eyes.

    • Function: Link between sensory vestibular input and motor systems to maintain balance and stability.

  • Reticular Formation and Reticulospinal Pathways:

    • Pontine (Medial) Reticular Formation: Facilitates extensor muscle activity and antigravity reflexes; supports upright posture.

    • Medullary (Lateral) Reticular Formation: Generally reduces extensor tone and modifies reflexes.

  • Direct vs. Indirect Pathways to Spinal Cord:

    • Direct: Corticospinal projections to lateral motor circuits for skilled motion.

    • Indirect: Cortical projections to brainstem nuclei (e.g., reticular formation), which then communicate via reticulospinal pathways to medial motor circuits for posture.

The Basal Ganglia

  • Function: Primarily involved in movement selection (facilitating desired programs while suppressing competing programs).

  • Structure:

    • Striatum: Composed of the caudate nucleus and the putamen. The primary entry point for cortical input.

    • Other Nuclei: Globus pallidus, substantia nigra, and subthalamic nuclei.

  • Medium Spiny Neurons: Neurons in the striatum that integrate information from the cortex, thalamus, and brainstem.

  • Inhibitory Organization: Under resting conditions, the output nuclei (Internal segment of the Globus Pallidus [GPI] and Substantia Nigra) are GABAergic and tonically active, meaning they continuously suppress the thalamus.

  • Disinhibition: To move, the basal ganglia must reduce this tonic inhibition. Striatal activation inhibits the GPI/Substantia Nigra, which in turn stops inhibiting the thalamus, allowing it to excite the motor cortex.

  • Direct and Indirect Pathways:

    • Direct Pathway: Cortex →\rightarrow Striatum →\rightarrow GPi/SN (-). Facilitates movement initiation by reducing thalamic suppression.

    • Indirect Pathway: Cortex →\rightarrow Striatum →\rightarrow External segment of the Globus Pallidus (GPE) (-) →\rightarrow Subthalamic Nucleus (STN) (+) →\rightarrow GPi/SN (+). This increases inhibitory output to the thalamus, suppressing movement.

  • Center-Surround Model: The direct pathway acts as the "center" (focused disinhibition of a specific program), while the indirect pathway acts as the "surround" (broad inhibition of competing actions).

  • Basal Ganglia Loops:

    • Motor Loop: Involves the putamen; focused on movement selection.

    • Cognitive Loop: Involves the caudate nucleus; focused on planning and executive control.

    • Limbic Loop: Involves the ventral striatum; focused on motivation and reward.

  • Disorders:

    • Huntington's Disease: Degeneration of striatal medium spiny neurons (specifically the indirect pathway), leading to involuntary movements because inhibition of competing programs fails.

    • Parkinson's Disease: Degeneration of dopaminergic neurons in the substantia nigra. Dopamine usually promotes the direct pathway; its loss leads to excessive inhibition of the thalamus, causing bradykinesia, rigidity, and tremors.

The Cerebellum

  • Function: Acts as a comparator, error corrector, and coordinator. It compares intended movement with actual sensory feedback.

  • Functional Regions:

    • Cerebrocerebellum: Lateral hemispheres. Involved in planning, timing, and execution of complex sequences. Input via pontine nuclei.

    • Spinocerebellum: Includes the vermis (posture/eyes) and intermediate zones (distal limb control). Monitors continuous body position.

    • Vestibulocerebellum: Flocculonodular lobe. Controls balance and the vestibulo-ocular reflex (VOR).

  • Circuitry and Mapping:

    • Ipsilateral Control: The right cerebellum controls the right side of the body.

    • Fragmented Maps: Unlike the continuous homunculus in the cortex, cerebellar somatotopic maps are discontinuous.

  • Cerebellar Outputs (Deep Cerebellar Nuclei):

    • Dentate Nucleus: Receives output from cerebrocerebellum; projects to premotor cortex via thalamus.

    • Interposed and Fastigial Nuclei: Receive output from spinocerebellum; fastigial influences posture, interposed influences limb coordination.

  • Cellular Organization:

    • Purkinje Cells: The sole output of the cerebellar cortex; they are inhibitory to the deep cerebellar nuclei.

    • Mossy Fibers: Carry info from cortex/brainstem/spinal cord. They excite granule cells, which give rise to parallel fibers that synapse on Purkinje cells.

    • Climbing Fibers: Originate from the inferior olive. They form powerful synapses with Purkinje cells and convey movement error signals for motor learning.