Medulla oblongata

The medulla oblongata is the most caudal portion of the brainstem, continuous inferiorly with the spinal cord and superiorly with the pons. It houses essential autonomic centers that regulate cardiovascular, respiratory, and reflexive activities vital for survival. The medulla also contains nuclei of multiple cranial nerves and serves as a conduit for ascending sensory and descending motor tracts between the spinal cord and brain.

Location

The medulla lies in the posterior cranial fossa, anterior to the cerebellum and directly atop the foramen magnum. It extends from the inferior border of the pons (at the pontomedullary junction) down to the level of the first cervical spinal nerve root. It is approximately 3 cm in length and is divided into rostral (open) and caudal (closed) parts, based on the relationship to the fourth ventricle.

External Features

Anterior (Ventral) Surface

• Pyramids: Longitudinal ridges formed by the corticospinal tracts; site of the pyramidal decussation where most motor fibers cross to the contralateral side.
• Olives: Oval prominences lateral to the pyramids; formed by the inferior olivary nuclei, involved in motor coordination via cerebellar connections.
• Rootlets of CN XII (Hypoglossal): Exit between the pyramid and olive.

Posterior (Dorsal) Surface

• Gracile and cuneate tubercles: Elevations containing the gracile and cuneate nuclei, which relay fine touch and proprioception from the lower and upper body, respectively.
• Posterior median sulcus: Continuation from the spinal cord separating the dorsal columns.
• Obex: The inferior tip of the fourth ventricle where it narrows to become the central canal of the spinal cord.

Internal Structure

The medulla contains both gray matter nuclei and white matter tracts. It is organized into:

• Gray matter: Includes cranial nerve nuclei, sensory relay nuclei, and autonomic centers.
• White matter: Includes ascending tracts (e.g., dorsal columns, spinothalamic tract) and descending tracts (e.g., corticospinal, corticobulbar tracts).

Cranial Nerve Nuclei in the Medulla

• Glossopharyngeal nerve (CN IX): Sensory and motor nuclei in the upper medulla; involved in taste, salivation, and baroreceptor reflexes.
• Vagus nerve (CN X): Dorsal motor nucleus and nucleus ambiguus; regulates parasympathetic output to thoracic and abdominal viscera.
• Accessory nerve (CN XI): Nucleus ambiguus contributes to motor fibers controlling sternocleidomastoid and trapezius muscles.
• Hypoglossal nerve (CN XII): Motor nucleus in the lower medulla; innervates intrinsic and extrinsic muscles of the tongue.
• Nucleus solitarius: Receives visceral sensory input from CN VII, IX, and X.
• Nucleus ambiguus: Motor fibers of CN IX, X, and XI controlling muscles of the pharynx and larynx.

Autonomic Centers

• Cardiovascular center: Regulates heart rate and blood pressure via sympathetic and parasympathetic output.
• Respiratory center: Coordinates with the pons to control respiratory rhythm and depth.
• Vomiting, coughing, sneezing, and swallowing centers: Mediate reflex responses essential for airway protection and digestion.

Ascending and Descending Tracts

• Corticospinal tract: Descending motor pathway from cerebral cortex to spinal cord; decussates at the pyramids.
• Dorsal column–medial lemniscus pathway: Fine touch, vibration, and proprioception fibers from gracile and cuneate nuclei ascend to the thalamus.
• Spinothalamic tract: Carries pain and temperature from the contralateral body to the thalamus.
• Spinocerebellar tracts: Convey unconscious proprioception to the cerebellum.

Blood Supply

• Anterior spinal artery: Supplies the medial medulla, including pyramids and hypoglossal nucleus.
• Posterior inferior cerebellar artery (PICA): Supplies the lateral medulla and parts of the inferior cerebellum.
• Vertebral arteries: Contribute to both medial and lateral medullary regions.

Venous drainage is into the medullary veins, which drain into the vertebral and cerebellar venous systems.

Development

The medulla develops from the myelencephalon, the most caudal part of the embryonic hindbrain (rhombencephalon). The basal plate gives rise to motor nuclei, while the alar plate forms sensory nuclei. The roof plate thins to become part of the fourth ventricle.

Clinical Significance

• Medial medullary syndrome (Dejerine syndrome): Caused by infarction of the anterior spinal artery; presents with contralateral hemiparesis, contralateral proprioception loss, and ipsilateral tongue weakness.
• Lateral medullary syndrome (Wallenberg syndrome): Due to PICA infarct; results in loss of pain/temp sensation, dysphagia, hoarseness, vertigo, and Horner’s syndrome.
• Central hypoventilation: Bilateral damage to the medullary respiratory centers can cause loss of automatic breathing.
• Dysphagia and aspiration: Lesions affecting the nucleus ambiguus impair swallowing reflexes and laryngeal function.
• Syringobulbia: A cavitary lesion in the medulla may disrupt cranial nerve function and respiratory centers.
• Brainstem herniation: Increased intracranial pressure can force the medulla through the foramen magnum (tonsillar herniation), compressing vital centers and leading to death.

Imaging

MRI is the gold standard for evaluating medullary pathology, including infarction, demyelination, tumors, and congenital malformations. Diffusion-weighted imaging is useful for detecting acute infarcts. Brainstem evoked potentials may be used to assess function in comatose patients or during intraoperative monitoring.