The Midbrain

Overview

Here, we'll learn about the midbrain.

• Start a table.
• Denote that, from a clinician's perspective, the midbrain is the neurobiological site of injury that produces movement disorders.

mid-sagittal section

• Start a mid-sagittal section.
• First, draw the different brainstem levels, from superior to inferior:
• Midbrain
• Pons
• Medulla
• Label the anterior/posterior orientational plane.

Key Neighboring Structures

Include the key structures that border the brainstem:

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

Key Features

Now, let's point out some key features within the midbrain.

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

Radiographic Axial Section: General Midbrain Anatomy

Now, show the anterior/posterior plane of our upcoming axial sections.

• Before we draw a detailed anatomical section, let's review an axial section in radiographic (the common clinical) perspective.
• Show its anterior/posterior orientational plane.
• Draw the core of the midbrain.
• Then, anteriorly, attach the cerebral peduncles (aka crus cerebri), which look like Mickey Mouse ears.
• Posteriorly, attach the colliculi (and posterior commissure).
• Now, from anterior to posterior, demarcate the bilateral, thinly-shaped, substantia nigra, which are essential for motor activation.
• Then the bilateral, circular-shaped, red nuclei.
• Finally, demarcate 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.

• Here, let's use an anatomical perspective, which is opposite that of the radiographic perspective.
• Show the anterior/posterior orientation.
• First, draw 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.
• Show that the cerebral aqueduct is the small cerebrospinal fluid space in the dorsum of the midbrain.
• Show that we'll draw the tracts on its right side and nuclei on the left.

Cerebral Peduncle

• Now, anteriorly, within the basis, draw the cerebral peduncle (aka crus cerebri).
• Divide the center of the crus into the corticonuclear tracts (aka corticobulbar tracts), medially, and the corticospinal tracts, laterally.

Substantia Nigra

• Now, draw the substantia nigra just posterior to the white matter pathways in the base of the midbrain.
• Indicate that it relies on dopamine, so its melanin-rich.
• Show that 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

• Now, in the anterior aspect of the midbrain tegmentum, draw the circular red nucleus.

Summary Table

• Start a table, so we can keep tract of the many midbrain structures and their functions.
• To what extent we can, let's maintain the anterior-posterior orientation we established in the midsaggital section.
• Indicate that the red nucleus produces upper extremity flexion movements, which are observed in decorticate posturing – we discuss this in detail elsewhere.

Major Sensory Tracts

Next, in the lateral midbrain, draw the cluster of major sensory tracts:

• Start with the medial lemniscus.
• Then, draw the draw the 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, draw the posterior trigeminothalamic tract.

Periaqueductal Gray Area

• First, label 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)

• Then, just in front of it, in midline, label 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.
• Show that the raphe nuclei populate its median zone, and write that they are primarily serotinergic and are modulated by psychotropic medications.

Posterior commissure

• Finally, within the tectum, label 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

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

Cranial Nerve Nuclei (Sensory)

Laterally, draw the sensory nucleus:

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

Advanced Details

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

• In the most medial portion of the crus, draw the frontopontine tracts.
• Then, in the most lateral portion, draw the additional corticopontine tracts; they emanate from the occipital, parietal, and temporal cortices.
• Make a notation that the red nuclei span the mid and upper midbrain.
• And then show that fibers from the superior cerebellar peduncle (the major outflow tract of the cerebellum) decussate in the central midbrain tegmentum. - Indicate that 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.

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