The motor cortex comprises interconnected fields on the posterior frontal lobe—chiefly Brodmann area 4 (primary motor cortex, M1) and area 6 (premotor cortex and supplementary motor areas)—that plan, select and execute voluntary movements. These regions transform goals into patterned activity in descending pathways to brainstem and spinal motor circuits, enabling dexterous eye, face and limb actions. Modern work shows overlapping, action‑type representations rather than a strictly point‑to‑point "homunculus", and highlights direct cortico‑motoneuronal projections that underwrite fine finger control. Clinically, motor‑cortical organization shapes deficits after stroke and neurodegenerative disease and guides mapping for neurosurgery and neurotechnology.
Subdivisions
thumb|right|upright=1.2|alt=Schematic lateral view of a human brain with a labelled motor homunculus running along the precentral gyrus.|Approximate location of the primary motor cortex on the precentral gyrus of the lateral hemisphere.
Motor cortex is commonly divided into three closely interacting fields:
- the primary motor cortex (M1; Brodmann area 4), which issues descending commands for fine motor control and force production;
- the premotor cortex (lateral area 6), which integrates sensory cues and internal rules to prepare and select actions; and
- the supplementary motor area (SMA; medial area 6), which contributes to internally generated actions, sequential operations, and bimanual coordination.
Nomenclature and boundaries
In classical cytoarchitectonics, Brodmann area 4 (BA4) corresponds to primary motor cortex (M1) occupying the precentral gyrus and the anterior bank of the central sulcus, with medial continuation in the anterior (motor) portion of the paracentral lobule. Its posterior border abuts primary somatosensory cortex (BA3,1,2) along the lip and wall of the central sulcus; its anterior border is the precentral sulcus where area 6 begins. Receptorarchitectonic work subdivides BA4 into a posterior field (4p) concentrated along the sulcal wall and an anterior field (4a) on the gyral crown. Area 6 lies anterior to BA4 across the superior and middle frontal gyri and includes the lateral premotor cortex; on the medial wall it encompasses the supplementary and pre‑supplementary motor areas.
Premotor cortex
thumb|right|upright=1.2|alt=Schematic map of macaque frontal cortex showing PMd and PMv subdivisions (rostral and caudal), SMA, pre‑SMA, SEF and FEF.|Motor representations in non‑human primate frontal cortex identified with microstimulation and recording (schematic).
Premotor cortex is commonly divided into dorsal (PMd) and ventral (PMv) sectors, each with rostral and caudal parts. PMd contributes to reach planning and selection among competing directions, whereas PMv is heavily involved in shaping the hand for grasp and in multisensory guidance of actions in peri‑personal space. These areas are part of a broader parieto‑frontal system linking dorsal visual streams with motor plans, and their boundaries lie within cytoarchitectonic area 6 lateral to BA4.
Eye‑movement motor fields (FEF/SEF)
The frontal eye field (FEF) in the precentral/premotor region and the supplementary eye field (SEF) on the dorsomedial wall form part of the motor network controlling saccades, smooth pursuit and eye–head coordination. FEF receives visual input from occipito‑temporal pathways and projects to the superior colliculus and brainstem gaze centers; SEF participates in internally generated saccade sequences and performance monitoring. Microstimulation of FEF evokes fixed‑vector saccades, whereas SEF stimulation elicits context‑dependent eye movements and sequence effects.
Supplementary motor area (SMA)
Electrical stimulation and functional imaging implicate SMA in initiating internally generated action and in sequencing. SMA also contains a coarse, overlapping body map and sends direct corticospinal projections. Lesions or inactivation can impair movement initiation and transiently abolish bimanual coordination in non‑human primates.
Cytoarchitecture and connectivity
Motor cortex is agranular isocortex with a six‑layered structure; layer IV is reduced or indistinct, whereas layer V contains the large corticospinal neurons. M1 is sometimes termed area gigantopyramidalis because Betz cells are especially prominent there. Premotor and SMA share a similar laminar pattern but lack Betz cells. Afferent input arrives via thalamic relays conveying basal ganglia and cerebellar output; rich corticocortical connections link PMd/PMv with posterior parietal cortex and SMA with prefrontal cortex. Efferents descend via the corticospinal and corticonuclear tracts and via brainstem motor pathways.
Orofacial and speech control
Corticobulbar projections from lateral M1 and ventrolateral premotor cortex target cranial motor nuclei through relay zones in the pontine and medullary reticular formation. Orofacial, laryngeal and tongue representations occupy the inferior precentral gyrus and adjacent opercular cortex. Direct cortico‑motoneuronal influences on nucleus ambiguus (laryngeal) are sparse in most mammals but appear more substantial in humans and great apes, consistent with fine control of phonation and articulation. Lesions produce dysarthria and apraxia of speech; stimulation studies and functional imaging localize laryngeal motor cortex to a dorsal–ventral pair flanking the central sulcus.
Motor maps and coding
Rather than one‑to‑one control of individual muscles, stimulation and single‑unit studies indicate that motor cortex contains heavily overlapping representations and can specify ethologically relevant, multi‑joint actions. Extended‑duration microstimulation in monkeys evokes coordinated movements such as defensive postures or reach‑to‑grasp sequences, suggesting a map of action types arranged across cortex.
Development and plasticity
Motor representations are shaped by development and use. Early corticospinal projections are exuberant; activity‑dependent pruning and myelination refine conduction velocity and terminal specificity through childhood and adolescence. Experience can expand or contract cortical zones devoted to particular movements, and recovery after injury may recruit premotor and somatosensory contributions to descending pathways.
Skill learning and reorganization
Skill acquisition alters representational geometry in M1 and premotor cortex, biases cortico‑motoneuronal drive toward task muscles, and modifies intracortical inhibition/facilitation. Non‑invasive stimulation (e.g., TMS, tDCS) can transiently modulate learning rates and retention.
Lifespan change
Aging is accompanied by altered recruitment of premotor and contralateral homologues during motor tasks and by changes in myelination and thickness gradients across precentral cortex; training can partially normalize these patterns.
Comparative anatomy and evolution
Across mammals, corticospinal organization varies with dexterity. Species with skilled, independent finger movements (e.g., humans, macaques) possess abundant cortico‑motoneuronal projections and a prominent M1 “hand knob”, whereas species with less manual dexterity rely more on propriospinal and brainstem pathways. In non‑primates, corticospinal fibers terminate largely on interneurons, while in higher primates many terminations contact motoneurons directly. The distribution and strength of CM projections correlate with the capacity for independent finger movements and tool manipulation.
Tool use and fine object manipulation in primates rely on parieto‑frontal networks linking anterior intraparietal areas with ventral premotor cortex and M1. Expansion of these circuits in humans is associated with increased CM projection density and greater fractional representation of distal musculature, supporting skilled grasp, tool use and praxis.
Parameters and safety
Intraoperative mapping commonly uses short trains of biphasic or monophasic pulses delivered via bipolar electrodes placed on the cortical surface (typical frequencies ~50–60 Hz; train durations on the order of 1–5 s; currents in the low milliampere range adjusted to evoke responses while avoiding afterdischarges). Mapping proceeds in small spatial steps to delineate essential cortex and white‑matter pathways; electrocorticography is used to monitor afterdischarges, and stimulation is paused or medication given if they arise.
</references>