Midbrain

The midbrain, or mesencephalon, is the uppermost part of the brainstem and lies between the diencephalon (above) and the pons (below). It serves as a crucial conduit for motor and sensory pathways and houses nuclei involved in auditory and visual reflexes, eye movements, and consciousness. The midbrain also contains major dopaminergic centers related to motor control and reward processing.

Location

The midbrain is located in the brainstem, superior to the pons and inferior to the thalamus. It passes through the tentorial notch and is surrounded by the cerebral hemispheres. The cerebral aqueduct, a narrow channel connecting the third and fourth ventricles, runs through the midbrain and divides it into dorsal and ventral parts.

External Features

• Anterior surface: Features the cerebral peduncles, which contain descending motor fibers from the cerebral cortex.
• Posterior surface: Shows four rounded elevations called the corpora quadrigemina:
  • Superior colliculi: Involved in visual reflexes and eye tracking.
  • Inferior colliculi: Involved in auditory processing and reflexes.

Internal Structure

The midbrain is divided into three main longitudinal regions:

1. Tectum (Dorsal Midbrain)

• Located posterior to the cerebral aqueduct.
• Includes the superior and inferior colliculi, responsible for visual and auditory reflexes, respectively.

2. Tegmentum (Middle Region)

• Located anterior to the cerebral aqueduct.
• Contains the red nucleus, periaqueductal gray, reticular formation, and cranial nerve nuclei.
• Traversed by ascending sensory tracts such as the medial lemniscus and spinothalamic tract.

3. Basis Pedunculi (Crus Cerebri)

• Most ventral part of the midbrain.
• Contains descending corticospinal, corticobulbar, and corticopontine motor tracts.

Cranial Nerve Nuclei

• Oculomotor nucleus (CN III): Controls most extraocular muscles and levator palpebrae superioris.
• Edinger–Westphal nucleus: Parasympathetic fibers to the eye (pupil constriction and lens accommodation).
• Trochlear nucleus (CN IV): Controls the superior oblique muscle; axons decussate and exit dorsally (unique among cranial nerves).

Other Important Structures

• Red nucleus: Involved in motor coordination, receives input from the cerebellum and projects to the spinal cord via the rubrospinal tract.
• Substantia nigra: Dopaminergic nucleus involved in motor control; degeneration leads to Parkinson’s disease.
• Periaqueductal gray: Involved in pain modulation and autonomic control.
• Reticular formation: Regulates arousal, alertness, and autonomic functions.

Functional Roles

• Visual reflexes: Superior colliculi coordinate head and eye movements in response to visual stimuli.
• Auditory relay: Inferior colliculi are part of the central auditory pathway, relaying signals to the medial geniculate nucleus.
• Eye movement: Oculomotor and trochlear nuclei control voluntary and reflexive eye movement.
• Motor coordination: Red nucleus and substantia nigra contribute to smooth execution of voluntary movement.
• Pain suppression: Periaqueductal gray participates in descending pain modulation pathways.

Blood Supply

• Posterior cerebral artery (PCA): Supplies the tectum and lateral aspects of the midbrain.
• Superior cerebellar artery: Supplies the lateral and dorsal midbrain regions.
• Basilar artery branches: Contribute to the ventral midbrain including the cerebral peduncles.

Venous drainage occurs via the basal vein of Rosenthal and other deep cerebral veins draining into the vein of Galen.

Development

The midbrain originates from the mesencephalon during embryonic development, one of the primary brain vesicles. Its central cavity becomes the cerebral aqueduct. The tectum develops from the alar plate, while the tegmentum and cerebral peduncles arise from the basal plate.

Clinical Significance

• Weber’s syndrome: Caused by infarct of the ventral midbrain; presents with ipsilateral oculomotor palsy and contralateral hemiparesis due to involvement of CN III and corticospinal tract.
• Benedikt’s syndrome: Involves damage to the red nucleus and oculomotor nerve; causes ipsilateral CN III palsy and contralateral tremor/ataxia.
• Parinaud’s syndrome: Caused by dorsal midbrain lesion (e.g., pineal tumor); leads to vertical gaze palsy, light-near dissociation, and eyelid retraction.
• Parkinson’s disease: Degeneration of substantia nigra pars compacta leads to dopamine deficiency and motor symptoms such as rigidity and bradykinesia.
• Aqueductal stenosis: Narrowing of the cerebral aqueduct can obstruct CSF flow and cause hydrocephalus.

Imaging

MRI is the preferred imaging modality for evaluating midbrain anatomy and pathology. T2-weighted and diffusion sequences are used to detect infarcts, demyelination, or compression. Functional imaging (e.g., PET) is used in research for assessing midbrain dopaminergic activity in Parkinson’s disease and other movement disorders.