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Third Ventricle and Brainstem

Ventriculus tertius / Systema nervosum centrale

For medical students2 min readUpdated 2026-10-10

Although the third ventricle (ventriculus tertius) is the cavity of the diencephalon, its study is inextricably linked with the underlying structures of the brainstem. It is precisely on the lateral, ventral, and dorsal surfaces of the brainstem that the vital cranial nerve nuclei responsible for motor, sensory, and autonomic innervation project.

ThalamusDorsal part of the diencephalon, a major anatomical landmark
Superior colliculusMidbrain tectal structure, subcortical center of vision
Inferior colliculusMidbrain element serving as a subcortical center of hearing
Unique exitCN IV (trochlear nerve) — the only nerve emerging on the dorsal surface of the brainstem

General Anatomical Landmarks of the Brainstem

To accurately understand the topography of the nuclei, one must clearly visualize the structures forming the context of their location. From superior to inferior in the central nervous system, the following key formations are distinguished:

Midbrain Nuclei and Nerves (Mesencephalon)

At the midbrain level lie the nuclei of three pairs of cranial nerves. Their projection has a strict anatomical reference:

  1. CN III — Oculomotor nerve (N. oculomotorius). Contains a motor nucleus (nucl. nervi oculomotorii). The trunk of this nerve emerges on the medial surface of the cerebral peduncle and courses ventrally.
  2. CN IV — Trochlear nerve (N. trochlearis). Its motor nucleus (nucl. nervi trochlearis) is localized at the level of the inferior colliculi. This is the only nerve whose rootlets emerge on the dorsal surface of the brainstem, after which they wrap around the cerebral peduncle.
  3. CN V — Trigeminal nerve (mesencephalic part). Here are the mesencephalic nucleus and tract of the trigeminal nerve (nucl. et tractus mesencephalici n. trigemini), which mediate proprioceptive sensitivity.

Pons Nuclei and Nerves (Pons)

The pons is the site of localization for cranial nerve nuclei from CN V to CN VIII. The rootlets of the trigeminal nerve (n. trigeminus) emerge on the lateral surface of the pons, dividing into a motor root (radix motoria) and a sensory root (radix sensoria). Within the pons itself lie the motor (masticatory) nucleus (nucl. motorius n. trigemini) and the chief sensory nucleus (nucl. principalis n. trigemini), which is responsible for tactile sensation.

Abducens nerve (CN VI, n. abducens) has only a motor nucleus (nucl. nervi abducentis), which is deeply hidden in the facial colliculus. Around this nucleus, the internal genu of the facial nerve (genu n. facialis) forms.

The facial nerve itself (CN VII, n. facialis) possesses a complex nuclear apparatus:

In the lateral angles of the rhomboid fossa, the vestibulocochlear nerve (CN VIII, n. vestibulocochlearis) projects, represented by a group of vestibular (nucll. vestibulares) and cochlear (nucll. cochleares) nuclei.

Medulla Oblongata Nuclei and Nerves (Medulla oblongata)

The medulla oblongata contains nuclei of the caudal group of nerves (CN IX–XII), many of which share common structures:

Individual autonomic nuclei include the inferior salivatory nucleus (nucl. salivatorius inferior) for CN IX and the dorsal motor nucleus of the vagus nerve (nucl. posterior n. vagi).

The motor nucleus of the hypoglossal nerve (CN XII, nucl. nervi hypoglossi) is located medially. Additionally, the spinal trigeminal nucleus and tract (nucl. et tractus spinales n. trigemini) extend throughout the medulla oblongata up to the upper spinal segments, providing pain and temperature sensation.

Mnemonic

To quickly remember general brainstem structures: The "Ambiguus nucleus" (Nucl. ambiguus) provides motor supply to three consecutive pairs — IX, X, and XI. The "Solitary tract nucleus" (Nucl. tractus solitarii) collects taste and sensory input from CN VII, IX, and X.

Frequently asked questions

What anatomical structures form the walls of the third ventricle?

The walls of the third ventricle are formed by structures of the diencephalon.

  • Thalamus (thalamus) — its medial surfaces lie vertically and form the lateral walls of the third ventricle.

The cavity of the third ventricle itself (ventriculus tertius) is a narrow sagittal cleft located between the right and left thalami. Information regarding the structures forming other walls (anterior, posterior, superior, or inferior) is not detailed in the source texts.

Through which anatomical structures does the third ventricle communicate with other brain cavities?

The third ventricle communicates with other cerebrospinal fluid cavities via the following anatomical structure:

  • Cerebral aqueduct (aqueduct of Sylvius) — connects the third ventricle to the cavity of the fourth ventricle.

Sources also indicate that the third ventricle communicates with the lateral ventricles, though the specific anatomical opening (foramen) mediating this connection is not explicitly named in the text.

Where precisely in the rhomboid fossa are the cranial nerve nuclei of the pons and medulla oblongata projected?

The projection of cranial nerve nuclei onto the rhomboid fossa follows these topographical patterns:

  • Superior triangle (pontine region) — projects nuclei of CN XII, XI, X, IX pairs (Note: classical topography typically places CN V-VIII in the pons/upper rhomboid fossa and IX-XII in the medulla/lower triangle).
  • Inferior triangle (medullary region) — projects nuclei of CN VIII, VII, VI, V pairs.

Grouped by functional columns:

  • Medial column — motor nuclei (CN XII, VI).
  • Intermediate column — autonomic parasympathetic nuclei (CN X, IX, VII).
  • Lateral column — sensory nuclei (CN VIII).

Specific point projections include: CN VI nucleus — into the facial colliculus (colliculus facialis); CN XII nucleus — into the hypoglossal trigone (trigonum n. hypoglossi); the autonomic nucleus of CN X — into the vagal trigone (trigonum n. vagi); and CN VIII nuclei — into the vestibular area (area vestibularis).

Which cranial nerve emerges on the dorsal surface of the brainstem?

The trochlear nerve (CN IV). This is completely unique because it is the only nerve that exits posteriorly (dorsally), after which it curves around the cerebral peduncle.

What is the genu of the facial nerve and where is it located?

The genu of the facial nerve (genu n. facialis) is a bend of the CN VII fibers within the pons. Before exiting the brainstem, they loop around the motor nucleus of the abducens nerve (CN VI), forming the facial colliculus.

What types of sensation does the trigeminal nerve conduct via its various brainstem nuclei?

The mesencephalic nucleus mediates proprioception, the chief pontine nucleus mediates touch and fine discrimination, and the spinal trigeminal nucleus (descending through the medulla) mediates pain and temperature.

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