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.