Motor cortex

The motor cortex comprises interconnected fields on the posterior frontal lobe—chiefly Brodmann area 4 and area 6 —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

Motor cortex is commonly divided into three closely interacting fields:

the primary motor cortex, which issues descending commands for fine motor control and force production;
the premotor cortex, which integrates sensory cues and internal rules to prepare and select actions; and
the supplementary motor area, which contributes to internally generated actions, sequential operations, and bimanual coordination.
Nomenclature and boundaries

In classical cytoarchitectonics, Brodmann area 4 corresponds to primary motor cortex occupying the precentral gyrus and the anterior bank of the central sulcus, with medial continuation in the anterior portion of the paracentral lobule. Its posterior border abuts primary somatosensory cortex 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 concentrated along the sulcal wall and an anterior field 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.

Nomenclature variants

Human and non‑human primate atlases differ in labeling schemes across anterior agranular cortex. In macaques, premotor fields are often subdivided into F2/F4 and F5/F7, which only partly correspond to human PMd/PMv. In humans, receptorarchitectonic divisions of BA4 into 4a/4p and probabilistic maps derived from imaging produce slightly different borders than gyral/sulcal landmarks, especially near the central sulcus.

Primary motor cortex (M1)

M1 contains large pyramidal neurons in layer V and projects densely to spinal and cranial motor circuits via the corticospinal and corticonuclear tracts. Although Betz cells are distinctive, they form only a small proportion of corticospinal outputs; most corticospinal fibers arise from non‑Betz layer V neurons in M1 and from adjacent motor areas.

Premotor cortex

Premotor cortex is commonly divided into dorsal and ventral 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.

Mirror‑neuron responses

In macaque PMv, some neurons fire both during execution of a grasp and during observation of the same action performed by others; these "mirror" responses have been proposed to contribute to action understanding and imitation. The extent and function of mirror‑like responses in humans remain debated, but convergent EEG/MEG and fMRI evidence shows action‑observation effects in premotor and parietal circuits that project to M1.

Eye‑movement motor fields (FEF/SEF)

The frontal eye field in the precentral/premotor region and the supplementary eye field 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.

Histology

M1, premotor cortex and SMA are agranular isocortex. Layer IV is attenuated or indistinct, while layer V contains large pyramidal neurons including Betz cells in M1. Neuron classes include corticospinal and corticobulbar projection neurons, corticocortical pyramidal cells in layers II/III and V, and diverse GABAergic interneurons. Cortical thickness varies across the precentral gyrus from the gyral crown to the anterior sulcal wall, paralleling shifts in input/output density and myelination. Betz cells constitute a small minority of corticospinal neurons but have exceptionally thick axons and fast conduction velocities.

Descending pathways

Motor cortical output travels in the corticospinal tract and corticobulbar systems. Fibers originate from multiple fields: approximately one quarter from small pyramidal neurons in M1, substantial fractions from premotor and SMA, and a sizable minority from somatosensory cortex; Betz cells account for only a few percent of corticospinal axons. Many corticospinal terminals contact spinal interneurons, whereas direct cortico‑motoneuronal connections are thought to underlie fine finger control.

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 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.

Population coding, dynamics and oscillations

Population vectors, directional tuning and dynamical‑systems descriptions have been used to account for how ensembles in motor cortex evolve during reach and grasp. Beta‑band oscillations increase during hold periods and desynchronize around movement onset; high‑gamma activity scales with force and kinematics in electrocorticography.

Readiness potential and pre‑movement activity

Scalp and intracranial recordings show a slow negative potential, the readiness potential, beginning up to 1–2 s before self‑initiated movement. Sources include SMA, pre‑SMA and M1, with lateralized readiness potentials reflecting effector selection.

Competing frameworks and control principles

Several accounts describe how motor cortex specifies movement: muscle‑based coding, in which neurons correlate with muscle activity; movement‑based coding, emphasizing kinematics/forces of effectors; and dynamical‑systems views, in which population activity flows along low‑dimensional trajectories that generate movement without requiring an explicit set‑point for each muscle. Optimal feedback control frames motor behavior as task‑level goals stabilized by feedback, with motor cortex participating in a distributed controller.

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.

Maturation of corticospinal systems

Transient bilateral projections are common in infancy; progressive myelination, synaptic pruning and strengthening of cortico‑motoneuronal connections accompany the emergence of fine manual dexterity. Diffusion MRI and TMS demonstrate increasing tract integrity and decreasing motor thresholds across childhood and adolescence.

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 can transiently modulate learning rates and retention.