Chapter 55: Cortical and Brain Stem Control of Motor Function

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Cortical and Brain Stem Motor Control

Most voluntary movements initiated by the cerebral cortex are achieved when the cortex activates functional motor patterns stored in lower brain structures. These lower integrated centers include the spinal cord, brain stem, basal ganglia, and cerebellum. These structures process incoming commands and send specific executive signals directly to the peripheral skeletal muscles. However, for specialized movements, the cerebral cortex possesses a direct pathway to anterior motor neurons, bypassing subcortical relays. Direct cortical pathways are especially essential for controlling fine, highly skilled manual tasks executed by fingers and hands.  

The primary motor cortex resides in the posterior third of the frontal lobes anterior to the central sulcus. Posterior to the central sulcus lies the somatosensory cortex, which supplies sensory input to drive motor responses. The motor cortex itself is divided into three distinct functional subareas: the primary motor cortex, the premotor area, and the supplementary motor area. Each subdivision maintains its own distinct topographical map of body muscle groups and executes specialized motor functions.  

Primary Motor Cortex and Corticospinal Tract

The primary motor cortex lies in the first frontal convolution directly anterior to the central sulcus. It spans laterally from the sylvian fissure, extends superiorly over the cerebral apex, and dips deep into the longitudinal fissure.  

• Primary motor cortex corresponds to Brodmann’s area 4.  
• Topographical representation maps body muscles systematically across the cortical surface.  
• Face and mouth regions lie laterally near the sylvian fissure.  
• Hand and arm areas occupy the midportion of primary motor cortex.  
• Trunk representations locate near the apex of the cerebral hemisphere.  
• Leg and foot areas dip deep into the longitudinal fissure.  
• Over half of the primary motor cortex controls hand muscles and speech.  
• Point stimulation in hand regions excites muscle group movement patterns.  
• Excitation of single neurons coordinates directional movements across multiple muscles.  
• Premotor area lies one to three centimeters anterior to primary motor cortex.  
• Premotor signals generate complex movement patterns involving multiple body joints.  
• Anterior premotor cortex creates a sensory motor image of intended movements.  
• Posterior premotor cortex translates motor images into sequential muscle contractions.  
• Signals pass directly to primary cortex or via basal ganglia and thalamus.  
• Specialized mirror neurons fire during task execution or observation of others.  
• Mirror neurons located in premotor and parietal cortex transform sensory acts into motor plans.  
• Supplementary motor area extends along the upper border of the longitudinal fissure.  
• Supplementary stimulation frequently elicits bilateral movements such as simultaneous hand grasping.  
• Supplementary cortex provides background attitudinal movements and positional fixation of body segments.  
• Specialized Broca’s area controls word formation and associated respiratory speech mechanics.  
• Frontal eye field area located above Broca’s region manages voluntary eye movements.  
• Head rotation area coordinates head orientation in conjunction with eye movements.  
• Hand skill area destruction produces motor apraxia characterized by uncoordinated movement.  
• Corticospinal tract forms the primary direct output pathway from the cortex.  
• Pyramidal tract originates 30 percent from primary motor cortex.  
• Premotor and supplementary areas contribute 30 percent of corticospinal fibers.  
• Somatosensory areas posterior to central sulcus contribute remaining 40 percent.  
• Fibers descend through posterior limb of internal capsule and medullary pyramids.  
• Most pyramidal fibers decussate in lower medulla into lateral corticospinal tracts.  
• Uncrossed fibers descend ipsilaterally via ventral corticospinal tracts for bilateral postural control.  
• Giant Betz cells produce large 16 micrometer myelinated nerve fibers.  
• Betz cell axons transmit impulses rapidly at 70 meters per second.  
• Betz fibers comprise roughly 3 percent of over one million total pyramidal tract fibers.  
• Remaining 97 percent smaller fibers transmit tonic background signals to spinal motor centers.  

Subcortical Collaterals and Cortical Columnar Organization

The motor cortex gives off extensive accessory fibers to subcortical structures while maintaining a organized internal columnar structure.  

• Betz cell collaterals return to cortex to sharpen signal boundaries via lateral inhibition.  
• Corticostriate fibers project from motor cortex into caudate nucleus and putamen.  
• Corticorubral fibers send motor commands into midbrain red nucleus.  
• Motor fibers branch into reticular formation and vestibular nuclei of brain stem.  
• Pontile fibers synapse in pontine nuclei to form pontocerebellar inputs.  
• Collateral fibers project to inferior olivary nuclei to feed olivocerebellar pathways.  
• Incoming somatosensory fibers project directly from ventrobasal complex of thalamus.  
• Thalamic ventrolateral and ventroanterior nuclei relay coordination signals from cerebellum and basal ganglia.  
• Red nucleus in mesencephalon receives direct fibers via corticorubral tract.  
• Magnocellular red nucleus contains large neurons giving rise to rubrospinal tract.  
• Rubrospinal fibers decussate in lower brain stem into lateral columns.  
• Rubrospinal tract terminates on spinal interneurons alongside lateral corticospinal tract.  
• Corticospinal and rubrospinal tracts together constitute the lateral motor system.  
• Extrapyramidal system encompasses all non-pyramidal motor pathways including basal ganglia and reticular centers.  
• Motor cortex cells organize into vertical functional columns displaying six layers.  
• Pyramidal output neurons reside within cortical layer five.  
• Incoming sensory signals terminate primarily within cortical layers two through four.  
• Cortical columns function as processing and amplification units for muscle activation.  
• Dynamic neurons fire rapidly at onset to generate initial contraction force.  
• Static neurons maintain continuous slower firing to sustain prolonged muscle contraction.  
• Somatosensory feedback from spindles and tactile receptors provides positive feedback loop.  
• Stroke damage to internal capsule causes loss of voluntary fine finger control.  
• Lesions damaging adjacent motor pathways disinhibit brain stem nuclei, causing spasticity.  

Brain Stem Function and Antigravity Muscle Regulation

The brain stem consists of the medulla, pons, and mesencephalon, serving as an extension of the spinal cord while providing master control functions.  

• Brain stem controls respiration, cardiovascular function, equilibrium, and eye movements.  
• Pontine reticular nuclei locate posteriorly and laterally in pons and mesencephalon.  
• Medullary reticular nuclei extend through entire medulla near the midline.  
• Pontine system transmits excitatory signals via pontine reticulospinal tract.  
• Pontine reticular fibers terminate on medial motor neurons supporting axial antigravity muscles.  
• Pontine reticular nuclei possess high intrinsic excitability augmented by vestibular inputs.  
• Medullary reticular nuclei send inhibitory signals via medullary reticulospinal tract.  
• Medullary system receives inputs from corticospinal and rubrospinal pathways to balance pontine excitation.  
• Cortical control manipulates reticular systems to allow standing or local muscle inhibition.  
• Vestibular nuclei cooperate with pontine reticular system via lateral and medial vestibulospinal tracts.  
• Vestibular signals selectively excite antigravity muscles to maintain active body equilibrium.  
• Sectioning brain stem below mid-mesencephalon level produces clinical decerebrate rigidity.  
• Decerebrate rigidity results from uninhibited pontine reticular and vestibular system overactivity.  

Vestibular Apparatus and Equilibrium Control

The vestibular apparatus is the sensory organ for detecting equilibrium, encased in the temporal bone's bony labyrinth.  

• Membranous labyrinth contains ductus cochlearis, three semicircular canals, utricle, and saccule.  
• Maculae are sensory regions in utricle and saccule detecting head orientation.  
• Utricle macula lies horizontally, detecting head orientation when upright.  
• Saccule macula lies vertically, detecting head orientation when lying down.  
• Gelatinous macular layer contains heavy calcium carbonate crystals called statoconia.  
• Hair cells project stereocilia and single large kinocilium into gelatinous layer.  
• Bending cilia toward kinocilium depolarizes receptor cell by opening ion channels.  
• Bending cilia away from kinocilium hyperpolarizes cell, reducing resting discharge.  
• Resting vestibular hair cells fire continuous impulses at 100 per second.  
• Macular hair cell directional orientation creates specific neural patterns for head positions.  
• Utricle and saccule maculae detect linear acceleration during sudden forward movement.  
• Statoconia mass inertia bends cilia backward during acceleration, triggering forward leaning reflexes.  
• Semicircular ducts locate at right angles representing three planes of space.  
• Ampulla enlargement in each duct contains crista ampullaris covered by gelatinous cupula.  
• Angular acceleration causes endolymph fluid inertia to bend cupula opposite head movement.  
• Semicircular ducts predict impending dysequilibrium during rapid turns to allow anticipatory adjustments.  
• Vestibular signals traverse medial longitudinal fasciculus to coordinate corrective eye movements.  
• Neck joint proprioceptors transmit opposing signals to distinguish head bending from whole-body leaning.  
• Footpad pressure and air resistance exteroceptors provide supplemental equilibrium information.  
• Visual input compensates for vestibular loss by detecting visual image shifts.  
• Vestibular nerve fibers project to vestibular nuclei, cerebellum, and brain stem reticular formation.  
• Cerebellar flocculonodular lobes process dynamic equilibrium signals from semicircular ducts.  
• Cortical equilibrium center locates deep within parietal lobe in sylvian fissure.  
• Anencephalic infants demonstrate brain stem integration of stereotyped feeding, yawning, and righting movements.