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Paracentral lobule

The paracentral lobule is a U-shaped region located on the medial surface of the cerebral hemisphere, representing the medial continuation of the precentral and postcentral gyri that straddle the central sulcus, and it primarily subserves motor and sensory functions for the contralateral lower limbs, perineum, bladder, and rectum.^[1] This structure is bounded inferiorly by the cingulate sulcus and posteriorly by the marginal ramus of the cingulate sulcus, dividing it into an anterior portion corresponding to Brodmann area 4 (primary motor cortex) and a posterior portion encompassing Brodmann areas 3, 1, and 2 (primary somatosensory cortex).^[2] Anatomically, the paracentral lobule exhibits morphological variations, including continuous (2%), partially segmented (91%), and completely segmented (7%) forms, with the left side typically larger than the right (mean surface areas of 10.22 cm² and 8.36 cm², respectively) and males showing greater dimensions than females.^[1] Functionally, the anterior paracentral lobule encodes voluntary movements of the leg, foot, and pelvic organs through somatotopically organized neurons that fire prior to movement onset, integrating parameters such as force, direction, and speed.^[3] The posterior portion processes somatosensory inputs, including touch, proprioception, and visceral sensations from the lower body, facilitating discrimination and localization of stimuli.^[2] Its blood supply derives predominantly from the paracentral artery, a branch of the anterior cerebral artery, along with contributions from the superior parietal artery, ensuring targeted perfusion to this sensorimotor region.^[4] Clinically, the paracentral lobule's precise anatomy is crucial in neurosurgery for identifying the central sulcus as a cytoarchitectonic landmark, managing lesions that cause paraparesis or sensory deficits, and addressing conditions like epilepsy or tumors affecting lower limb control.^[1]

Anatomy

Location and boundaries

The paracentral lobule is situated on the medial surface of the cerebral hemisphere, representing the medial extension of the precentral and postcentral gyri as they cross the central sulcus. This structure straddles the boundary between the frontal and parietal lobes, with its anterior portion belonging to the frontal lobe and the posterior portion to the parietal lobe; the central sulcus, which indents the medial surface, serves as the cytoarchitectonic divide between these parts.^[5]^[1] The paracentral lobule is delimited by several key sulci. Anteriorly, it is bounded by the paracentral sulcus, which separates it from the medial aspect of the superior frontal gyrus. Posteriorly, the marginal branch (pars marginalis) of the cingulate sulcus forms the border, distinguishing it from the precuneus. Inferiorly, the cingulate sulcus separates the lobule from the cingulate gyrus, while superiorly, it extends to the free margin of the cerebral hemisphere.^[5]^[6]^[1] In terms of dimensions, the paracentral lobule has an average extrasulcal surface area of approximately 9 cm², with the left side typically larger (mean 10.22 cm²) than the right (mean 8.36 cm²). Males exhibit greater dimensions than females (mean left: 10.67 cm², right: 8.80 cm² for males; left 8.80 cm², right 6.99 cm² for females). It is prominently visible in medial sagittal sections of the brain, such as those obtained via MRI, where it appears as a wedge-shaped gyrus above the cingulate sulcus and adjacent to the midline structures like the corpus callosum.^[1]^[5]

Structure and cytoarchitecture

The paracentral lobule comprises the medial extensions of the precentral and postcentral gyri, forming a circumscribed region on the medial hemispheric surface that bridges the frontal and parietal lobes. Its anterior part, corresponding to Brodmann area 4, represents the primary motor cortex, while the posterior part includes Brodmann areas 3, 1, and 2, delineating the primary somatosensory cortex.^[7]^[1] Anatomical variants in the paracentral lobule primarily involve the degree of separation between its precentral and postcentral components. The forms include continuous (fully continuous gyri) in 2%, partially segmented (mostly continuous with lower separation by cingulate sulcus) in 91%, and completely segmented (fully separated gyri) in 7%.^[1] Cytoarchitectonically, the paracentral lobule displays the typical six-layered organization of the neocortex, with regional specialization reflecting its functional divisions. The anterior motor portion features an agranular structure, marked by poorly developed or absent layer IV and prominence of pyramidal cells in layers V and VI, consistent with frontal cortical architecture. In contrast, the posterior sensory portion shows a granular organization, with a well-defined layer IV rich in small granule cells, indicative of parietal koniocortex adapted for sensory processing.^[7]^[1] The paracentral lobule connects to subcortical structures via key white matter tracts, including thalamocortical fibers linking the motor and sensory regions to the ventral lateral (VL) and ventral posterolateral (VPL) thalamic nuclei, respectively, as well as corticostriatal projections to the putamen and caudate nucleus of the basal ganglia, routed through the corona radiata and internal capsule to support sensorimotor integration.

Functions

Sensory functions

The posterior paracentral lobule serves as the medial extension of the postcentral gyrus, forming part of the primary somatosensory cortex (Brodmann areas 1, 2, and 3) responsible for processing sensory inputs from the contralateral lower body.^[8]^[1] This region, situated posterior to the central sulcus on the medial hemispheric surface, receives and integrates somatosensory signals primarily via thalamocortical projections from the ventral posterolateral nucleus of the thalamus.^[8]^[9] It primarily handles discriminative touch, proprioception, and nociceptive (pain) sensations from the legs, feet, and perineum, contributing to the conscious perception of tactile stimuli, joint position, vibration, and localized pain in these areas.^[8]^[1] These functions are supported by somatotopic organization, where the lower extremity is represented in a distorted map akin to the sensory homunculus, with the leg and foot areas occupying the superomedial portion of the lobule.^[10] Functional neuroimaging studies, such as functional MRI (fMRI), have confirmed this mapping by demonstrating activation in the posterior paracentral lobule during sensory stimulation of the lower limbs, highlighting its role in fine-grained spatial representation of body surfaces.^[10] The posterior paracentral lobule is implicated in the sensory-motor control of pelvic organs, with lesions associated with urinary incontinence and defecation disorders. A 2025 fMRI study observed deactivation in this region during bladder filling sensations, suggesting involvement in a broader network for visceral sensation processing rather than direct activation.^[11]^[12] These pathways, often carried through spinothalamic tracts relaying in the thalamus, contribute to the perception of internal organ states from pelvic organs, distinguishing visceral from somatic sensations while supporting adaptive responses to bodily needs.^[11]

Motor functions

The anterior paracentral lobule serves as the medial extension of the precentral gyrus and functions as part of the primary motor cortex (Brodmann area 4), enabling voluntary control of contralateral lower limb movements.^[3] This region specifically governs actions such as hip and knee flexion/extension, as well as ankle dorsiflexion and plantar flexion for foot control, through its somatotopic organization on the medial cerebral surface.^[13]^[14] Electrical stimulation here elicits discrete leg and foot responses, confirming its role in precise motor execution for the lower extremities.^[3] Adjacent to this primary motor region, the supplementary motor area (Brodmann area 6), located on the medial frontal cortex anterior to the paracentral lobule, contributes to the planning and coordination of complex lower body actions, such as sequential gait patterns or bilateral leg movements.^[2] This area facilitates the initiation and temporal organization of intricate motor sequences involving the lower limbs, integrating higher-order planning before execution by the primary motor cortex.^[2] Beyond voluntary skeletal muscle control, the paracentral lobule participates in autonomic motor functions, including the regulation of micturition through voluntary coordination of bladder and urethral sphincter activity, as well as defecation via pelvic floor muscle control.^[15]^[16] It also supports sexual functions by modulating motor outputs to genital and pelvic musculature.^[17] These autonomic roles are mediated by descending connections from the paracentral lobule to brainstem nuclei and the hypothalamus, influencing visceral efferents.^[15] Activation in the paracentral lobule has been associated with blushing responses, particularly during self-observation, as observed in functional neuroimaging studies.^[18] Motor outputs from the paracentral lobule primarily descend via the corticospinal tract, which originates from pyramidal neurons in layer V of the cortex and synapses in the spinal cord to innervate lower extremity motor neurons.^[3] Sensory inputs from the posterior paracentral lobule briefly integrate with these motor signals to ensure coordinated lower body movements.^[1]

Vascular supply

Arterial supply

The paracentral lobule receives its primary arterial supply from branches of the anterior cerebral artery (ACA), which is responsible for perfusing the medial surfaces of the frontal and parietal lobes.^[19] The key vessel is the paracentral lobule artery (PLA), a terminal branch of the distal ACA that directly irrigates the lobule and adjacent structures such as the cingulate gyrus, precentral gyrus, and postcentral gyrus.^[20] This artery typically originates from the A4 segment of the ACA (in approximately 50% of cases), though variants include origins from the callosomarginal artery (about 30%), the A3 segment (16.7%), or the A5 segment (3.3%).^[20] The pericallosal artery, a continuation of the ACA running along the superior aspect of the corpus callosum, contributes to the supply through its terminal branches, including the paracentral artery in some anatomical configurations.^[21] Collateral contributions may arise from the superior parietal artery, particularly in cases where the primary ACA branches are insufficient, providing additional vascular support to the lobule.^[4] Additionally, the posterior internal frontal artery (an ACA branch) can anastomose medially with the PLA, enhancing regional perfusion.^[20] For territorial distribution, the anterior portion of the paracentral lobule, corresponding to the extension of the precentral gyrus, is primarily vascularized by callosomarginal branches of the ACA, which ascend along the medial frontal lobe.^[22] The posterior portion, extending from the postcentral gyrus, receives supply from parietal branches of the ACA, including precuneal arteries that border the adjacent precuneus.^[23] Redundancy is provided by lateral anastomoses between PLA branches and those of the middle cerebral artery, allowing potential collateral flow in cases of ACA compromise.^[20]

Venous drainage

The venous drainage of the paracentral lobule primarily occurs through the superior cerebral veins, which collect deoxygenated blood from the superolateral and medial surfaces of the cerebral hemisphere and empty into the superior sagittal sinus.^[24] These veins course superiorly and medially, piercing the arachnoid and dura to reach the sinus along the falx cerebri.^[25] Superficial veins in this region follow the contours of the gyri, with the paracentral vein acting as a major tributary that drains the medial aspect of the central region, including the paracentral lobule. In cadaveric studies, the superior two-thirds of the paracentral lobule drained exclusively to the superior sagittal sinus in 19 of 20 hemispheres, often via these paracentral veins or associated anteromedial parietal veins.^[26] Deeper portions of the paracentral lobule contribute to drainage via the subependymal venous system, where medullary veins converge and join the internal cerebral veins, which then unite to form the basal veins of Rosenthal before emptying into the great cerebral vein (of Galen) and straight sinus.^[27] Venous inflow to the superior sagittal sinus exhibits potential asymmetry, with right-sided dominance observed in a subset of individuals, influencing overall parasagittal drainage patterns near the paracentral lobule.

Clinical significance

Pathological conditions

The paracentral lobule is vulnerable to ischemic stroke due to occlusion of the anterior cerebral artery (ACA), which supplies its medial aspects. Such infarctions typically result in contralateral lower limb paresis, with weakness more pronounced in the distal segments like the foot, due to involvement of the motor representation in the paracentral lobule. Sensory loss in the same region, including deficits in discriminative touch, proprioception, and position sense, accompanies the motor impairment, reflecting damage to the adjacent somatosensory cortex. Additionally, urinary incontinence often emerges as a key symptom, stemming from disruption of cortical control over micturition centers, sometimes accompanied by fecal incontinence in more extensive lesions.^[28]^[29]^[30]^[7] Traumatic lesions to the paracentral lobule, often from head injuries affecting the medial frontal-parietal region, can produce similar deficits through direct cortical damage or secondary effects like contusions. These injuries commonly manifest as contralateral leg weakness or paralysis, impairing gait and fine motor control in the lower extremity. Bladder and bowel dysfunction is a prominent feature, characterized by incontinence due to loss of inhibitory cortical modulation over voiding reflexes, which may persist if the lesion is bilateral or involves the posterior superior frontal gyrus. Sensory disturbances, such as reduced pain and temperature sensation in the affected limb, may also occur, exacerbating functional limitations.^[7]^[7] Tumors, particularly parasagittal meningiomas arising from the falx cerebri, frequently compress the paracentral lobule, leading to progressive neurological deficits. These benign neoplasms can cause contralateral lower limb monoparesis by impinging on the motor cortex, with symptoms often starting subtly and worsening as the tumor enlarges. Pelvic autonomic disturbances, including urinary urgency, retention, or incontinence, arise from compression of bladder control pathways in the region, potentially accompanied by bowel involvement in advanced cases. Sensory deficits in the lower limb and gait instability further contribute to morbidity, highlighting the need for early intervention.^[31]^[32]^[7] Epileptic foci originating in the paracentral lobule typically produce focal seizures with motor manifestations, such as clonic jerks confined to the contralateral lower limb or foot, due to hyperexcitability in the primary motor area. These seizures may begin with tonic posturing before evolving into rhythmic clonic activity, reflecting the region's somatotopic organization. Auras preceding the ictal phase can include perineal sensory phenomena, such as tingling or discomfort in the genital or perianal region, arising from involvement of the medial somatosensory cortex. Such epileptogenic activity is particularly noted in lesions like focal cortical dysplasia within the paracentral lobule, underscoring its high susceptibility to seizure generation.^[33]^[34]^[35]

Diagnostic and surgical considerations

Magnetic resonance imaging (MRI), including diffusion-weighted imaging (DWI), is essential for localizing lesions in the paracentral lobule, particularly in cases of anterior cerebral artery (ACA) territory infarcts, where hyperintense signals on DWI indicate acute ischemia in this region.^[36] Functional MRI (fMRI) further aids in preoperative localization by mapping motor and sensory representations specific to the lower limbs and pelvic organs within the paracentral lobule, helping to delineate eloquent areas adjacent to pathological processes.^[37] These imaging modalities are critical for identifying the extent of involvement and guiding therapeutic decisions without relying on symptomatic presentation alone. Intraoperative direct cortical stimulation (DCS) is employed during awake craniotomy to map and preserve paracentral lobule functions, eliciting responses in lower limb musculature and bladder control to avoid postoperative deficits such as paraparesis or incontinence.^[38] This technique, combined with electrophysiological monitoring, allows real-time identification of essential motor pathways, ensuring maximal safe resection in tumor cases while minimizing neurological morbidity.^[39] The interhemispheric approach provides optimal surgical access to paracentral lobule lesions, such as tumors or vascular malformations, by traversing the fissure and minimizing retraction on the medial cortical surface to reduce injury risk.^[40] Variations like the transfalcine route enhance visualization for parafalcine pathologies, facilitating precise dissection.^[41] Prognostic outcomes in paracentral lobule involvement, particularly from ACA strokes, improve with early intervention such as thrombolysis or thrombectomy within therapeutic windows, which can prevent permanent lower limb weakness, paraparesis, and urinary incontinence by limiting infarct progression.^[42] Factors like infarct size and timely revascularization significantly influence long-term functional recovery in these cases.^[43]

References

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May 28, 2021 · The human paracentral lobule (PCL) is the medial continuation of the precentral and postcentral gyri. It has important functional area related to the lower ...
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