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Pyramid of medulla oblongata

The pyramid of the medulla oblongata is a paired, elongated longitudinal elevation located on the anterior surface of the medulla. It contains densely packed descending motor fibers of the corticospinal tract, making it a critical structure for voluntary motor control. The pyramids are a defining surface landmark of the medulla and play a central role in motor signal transmission from the cerebral cortex to the spinal cord.

Lesions involving the pyramids result in characteristic motor deficits and are commonly evaluated in brainstem vascular, demyelinating, degenerative, and traumatic conditions.

Synonyms

  • Medullary pyramid

  • Pyramidal tract of the medulla

Location

  • Located on the anterior (ventral) surface of the medulla oblongata

  • Positioned on either side of the anterior median fissure

  • Extends from the pontomedullary junction superiorly to the pyramidal decussation inferiorly

  • Anterior to the inferior olivary nuclei

  • Posterior to the premedullary cistern

  • Inferiorly continuous with the corticospinal tracts of the spinal cord

Anatomical components

  • Corticospinal fibers:

    • Descending upper motor neuron axons

    • Originate from the primary motor cortex

  • Pyramidal decussation (caudal end):

    • Crossing of approximately 85–90% of corticospinal fibers

    • Forms the lateral corticospinal tracts

  • Uncrossed fibers:

    • Continue as anterior corticospinal tracts

Relations

Anteriorly:

  • Anterior median fissure

  • Premedullary cistern

Posteriorly:

  • Medullary tegmentum

Laterally:

  • Inferior olivary nucleus

  • Hypoglossal nerve rootlets (CN XII) emerging between pyramid and olive

Superiorly:

  • Pons at the pontomedullary junction

Inferiorly:

  • Pyramidal decussation

  • Upper cervical spinal cord

Nerves related to the pyramids

  • Hypoglossal nerve (CN XII):

    • Rootlets emerge from the preolivary sulcus between the pyramid and olive

  • Functional relevance:

    • Close anatomical relationship makes CN XII vulnerable in medial medullary lesions

Function

  • Voluntary motor control: Conduction of motor impulses from cortex to spinal motor neurons

  • Fine motor movement: Especially important for distal limb musculature

  • Motor pathway integration: Acts as a major conduit for pyramidal (upper motor neuron) signals

Clinical significance

  • Medial medullary (Dejerine) syndrome:

    • Ipsilateral tongue weakness (CN XII involvement)

    • Contralateral spastic paralysis (pyramidal tract damage)

  • Pyramidal tract lesions: Produce upper motor neuron signs (spasticity, hyperreflexia, Babinski sign)

  • Infarction: Often due to anterior spinal artery occlusion

  • Neurodegenerative disease: Amyotrophic lateral sclerosis may involve corticospinal tracts

  • Trauma or compression: Can disrupt descending motor pathways

MRI appearance

T1-weighted images:

  • Normal pyramid: Low-to-intermediate signal intensity reflecting dense white matter tracts

  • Symmetry: Bilateral symmetric appearance

  • Chronic pathology: Volume loss or altered signal in long-standing degeneration

T2-weighted images:

  • Normal: Low-to-intermediate signal intensity, typically darker than adjacent gray matter

  • Normal pattern: Well-defined linear structures on the ventral medulla

  • Pathology: Hyperintense signal in infarction, demyelination, or gliosis

FLAIR:

  • Normal: Minimal conspicuity with preserved medullary architecture

  • Pathology: Hyperintensity in chronic infarcts or demyelinating lesions

Diffusion-weighted imaging (DWI):

  • Normal: No diffusion restriction

  • Acute infarction: Bright restricted diffusion with corresponding low ADC values

  • Early detection: Sensitive for medial medullary stroke

Post-contrast T1-weighted images:

  • Normal: No enhancement

  • Subacute infarction: Mild enhancement possible

  • Tumor or inflammation: Abnormal focal or diffuse enhancement

CT appearance

Non-contrast CT:

  • Normal pyramids: Usually indistinguishable from surrounding medulla

Post-contrast CT:

  • Normal: No enhancement

MRI image

MRI Pyramid of medulla oblongata axial anatomy image -img-00000-00000

MRI image

MRI Pyramid of medulla oblongata coronal anatomy image -img-00000-00000