Midbrain - Advanced Anatomy

The Midbrain

Overview

From a clinician's perspective, the midbrain is the neurobiological site of injury that produces movement disorders.

mid-sagittal section

Brainstem levels, from superior to inferior:

Anterior/posterior orientational plane.

Key Neighboring Structures

The hypothalamus, superiorly.
The cerebellum, posteriorly.
The cervical spinal cord, inferiorly.
And the temporal lobe, laterally.

Key Features

The cerebral peduncle, anteriorly.
The posterior commissure, and the colliculi, posteriorly.
The cerebral aqueduct (of Sylvius) is the midbrain portion of the brainstem CSF.

Radiographic Axial Section: General Midbrain Anatomy

Review of an axial section in radiographic (the common clinical) perspective.
Anterior/posterior orientational plane.
Anteriorly: the cerebral peduncles (aka crus cerebri), which look like Mickey Mouse ears.
Posteriorly: the colliculi (and posterior commissure).
From anterior to posterior: the bilateral, thinly-shaped, substantia nigra, which are essential for motor activation.
Then the bilateral, circular-shaped, red nuclei.
Finally, the periaqueductal gray area, which surrounds the cerebral aqueduct.

Anatomic Axial Section: Detailed Midbrain Anatomy

Overview

Now, we're ready for a detailed axial perspective of the midbrain.

Anatomical perspective is opposite that of the radiographic perspective.
First, the core of the midbrain: the tegmentum.
Then, one side of the anterior-lying basis; we'll draw the other side as the cerebral peduncle, soon.
Finally, the posterior-lying tectum.
The cerebral aqueduct is the small cerebrospinal fluid space in the dorsum of the midbrain.

Cerebral Peduncle

Within the basis is the cerebral peduncle (aka crus cerebri).
The center of the crus is divided into the corticonuclear tracts (aka corticobulbar tracts), medially, and the corticospinal tracts, laterally.

Substantia Nigra

Substantia nigra is just posterior to the white matter pathways in the base of the midbrain.
It relies on dopamine, so its melanin-rich.
The substantia nigra divides into:
The pars compacta (posteriorly); loss of pars compacta cells results in Parkinson's disease, which is why dopamine supplementation is used to treat it.
And into the pars reticulata (anteriorly); this iron-rich division of the substantia nigra is fundamental to the direct and indirect pathways.

Red Nucleus

The circular red nucleus is In the anterior aspect of the midbrain tegmentum.

Summary Table

The red nucleus produces upper extremity flexion movements, which are observed in decorticate posturing – we discuss this in detail elsewhere.

Major Sensory Tracts

The lateral midbrain: cluster of major sensory tracts:

Medial lemniscus.
Anterior trigeminothalamic tract (here we use the abbreviation Anterior TTT).
Moving posteriorly, the spinothalamic tract (of the anterolateral system).
Then, the lateral lemniscus, which transmits auditory information.
Further posterior, the posterior trigeminothalamic tract.

Periaqueductal Gray Area

The surrounding periaqueductal gray area, which most notably contains opioids, which help in pain control.

It is also packed with neuropeptides, monoamines, and amino acids.

Medial Longitudinal Fasciculus (MLF)

In the midline is the medial longitudinal fasciculus (MLF) white matter tract, which plays an important role in conjugate horizontal eye movements.

Reticular Formation

Further anterior, label the reticular formation, which most notably helps produce wakefulness.
The raphe nuclei populate its median zone; they are primarily serotinergic and are modulated by psychotropic medications.

Posterior commissure

Finally, within the tectum, the posterior commissure, which is a white matter tract involved in the pupillary light reflex.
The nucleus of the posterior commissure helps control vertical eye movements; compression of this nucleus restricts upgaze gaze.

Superior and Inferior Colliculi

Paired superior and inferior colliculi.
The superior colliculi are involved in visual function.
The inferior colliculi are involved in auditory function.

Cranial Nerve Nuclei (Motor)

Medially, the motor nuclei:
The oculomotor complex of CN 3 in midline.
Edinger-Westphal nucleus of CN 3, which is a key autonomic part of this complex.
The trochlear nucleus of CN 4 is adjacent to them.

Cranial Nerve Nuclei (Sensory)

The mesencephalic trigeminal nucleus of CN 5 (which is a sensory nucleus).

Advanced Details

Now, let's add advanced aspects of midbrain anatomy.

The frontopontine tracts are in the most medial portion of the crus.
The additional corticopontine tracts are in the most lateral portion; they emanate from the occipital, parietal, and temporal cortices.
The red nuclei span the mid and upper midbrain.
Fibers from the superior cerebellar peduncle (the major outflow tract of the cerebellum) decussate in the central midbrain tegmentum. - They lie below the level of the red nuclei in the lower midbrain.

Elsewhere we learn the significance of the different heights of these structures.

The central tegmental tract is in the central, dorsal tegmentum; it carries ascending reticular fibers as part of the ascending arousal system and descending fibers as part of the triangle of Guillain-Mollaret.
The tectospinal tract is just anterior to the medial longitudinal fasciculus, which produces contralateral head turn.

Midbrain - Advanced Anatomy

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