Neuroanatomy of the Pons Study Guide

General Functions of the Brainstem

• Survival Behaviors: The brainstem produces automatic behaviors that are necessary for survival.
• Conduit for Fiber Tracts: It serves as a passageway for all fiber tracts running between the cerebrum and the spinal cord.
• Cranial Nerve Innervation: Important cranial nuclei are located within the brainstem.
  • $10$ of the $12$ pairs of cranial nerves attach to the brainstem.
  • It is heavily involved with the innervation of the face and head.

Major Divisions of the Brainstem

• The brainstem is divided into three major parts:
  • Midbrain.
  • Pons.
  • Medulla.

External Anatomy and Surfaces of the Pons

• General Positioning:
  • Middle part of the brainstem.
  • Upper part of the hindbrain.
  • Connects the medulla to the midbrain.
  • Functions as a bridge between the cerebellar hemispheres.
• Surfaces:
  • Ventral (Anterior) Surface: Convex in shape.
  • Dorsal (Posterior) Surface.
• Borders:
  • Superior Border: Attached to the cerebral peduncles. The superior cerebellar arteries curve along this border, intervening between the oculomotor ($III$) and trochlear ($IV$) nerves.
  • Inferior Border: Continuous with the upper end of the medulla. The anterior inferior cerebellar arteries curve around this border.

Anterior (Ventral) Surface Features

• Pontine Nuclei: This surface is marked by numerous transversely oriented fascicles of pontocerebellar fibers that originate from scattered cell groups called pontine nuclei.
• Pontocerebellar Fibers: These fibers cross the midline and pass to the contralateral (opposite) side of the cerebellum through the massive middle cerebellar peduncle.
• Median Basilar Groove: Located in the midline, providing the location for the Basilar artery.
• Trigeminal ($5^{th}$) Nerve Emergence: The $5^{th}$ nerve emerges with two distinct roots:
  • Lateral Root: Larger, sensory root.
  • Medial Root: Smaller, motor root.
• Middle Cerebellar Peduncle: Positioned lateral to the emergence of the $5^{th}$ nerve.
• Pontomedullary Junction: This is the horizontal sulcus at the junction of the pons and medulla. Three cranial nerves emerge here:
  • $6^{th}$ (Abducent) nerve.
  • $7^{th}$ (Facial) nerve.
  • $8^{th}$ (Vestibulocochlear) nerve.

Posterior (Dorsal) Surface Features

• 4th Ventricle Floor: The dorsal surface forms the upper half of the floor of the $4^{th}$ ventricle (the rhomboid fossa).
• Structural Coverings:
  • It is hidden by the cerebellum, which covers the rostral half of the rhomboid fossa.
  • The superior (anterior) medullary velum forms the roof of the fossa. It is a thin sheet of tissue overlain by the lingula of the vermis of the cerebellum.
  • The velum is attached on each side to the superior cerebellar peduncles and is enclosed by pia mater above and ependyma below.
• Trochlear Nerve Decussation: The trochlear ($IV$) nerves decussate (cross) in the superior medullary velum.
• Rhomboid Fossa Landmarks:
  • Median Sulcus: A longitudinal sulcus that divides the floor into symmetrical right and left halves. It extends from the midbrain aqueduct to the central canal.
  • Stria Medullaris: Glistening white fibers derived from arcuate nuclei that cross the floor transversely at its widest part to enter the inferior cerebellar peduncle.
  • Medial Eminence: A longitudinal elevation on either side of the median sulcus, bounded laterally by the sulcus limitans.
  • Facial Colliculus: An oval swelling in the medial eminence at the level of the superior fovea. It is produced by fibers from the motor nucleus of the facial nerve hooking around the abducent nucleus (internal genu of the facial nerve).
  • Vestibular Area: The region lateral to the sulcus limitans at the lateral angle of the floor, overlying the vestibular nuclei.
• Additional Identified Structures (Rhomboid Fossa Diagram):
  • Locus ceruleus.
  • Superior cerebellar peduncle.
  • Inferior cerebellar peduncle.
  • Superior fovea and Inferior fovea.
  • Hypoglossal triangle.
  • Vagal triangle.
  • Funiculus separans.
  • Area postrema.
  • Obex and Tenia.

Internal Structure of the Pons

• The internal structure is revealed via transverse sections and consists of two main parts:
  • Basilar (Ventral) Part: Positioned anteriorly.
  • Tegmental (Dorsal) Part: Positioned posteriorly; a continuation of the medulla excluding the pyramids.

The Ventral (Basilar) Part of the Pons

• This region is similar in structure at all levels and contains:
  1. Pontine Nuclei: Scattered groups of cells separated by nerve fibers.
  2. Transverse Fibers: Ponto-cerebellar fibers crossing the midline to enter the opposite middle cerebellar peduncle.
  3. Vertical Fibers:
  • Corticospinal fibers.
  • Corticonuclear fibers: These end in the motor nuclei of cranial nerves, mainly on the opposite side.
  • Corticopontine fibers.

The Tegmental (Dorsal) Part of the Pons

• The tegmental portion is traversed by ascending and descending tracts and contains a decussation of transverse fibers called the trapezoid body.
• It contains nuclei for cranial nerves $V$ (trigeminal), $VI$ (abducent), $VII$ (facial), and $VIII$ (vestibulocochlear).
• The tegmentum is studied at two distinct levels due to structural differences:
  1. Level of the Facial Colliculus (Lower pons).
  2. Level of the Trigeminal Nucleus (Upper pons).

Transverse Section at the Lower Pons (Level of Facial Colliculi)

• Nuclei:
  • Abducent Nucleus: Located beneath the facial colliculus in the floor of the $4^{th}$ ventricle.
  • Motor Nucleus of the Facial Nerve: Located ventrolateral to the abducent nucleus. Its fibers wind around the abducent nucleus (forming the facial colliculus) then pass anteriorly between the facial nucleus and the spinal tract of the trigeminal.
  • Superior and Inferior Salivatory and Lacrimatory Nuclei: Located medial to the motor nucleus of the facial nerve.
  • Vestibular Nuclei: Located beneath the vestibular area.
  • Dorsal and Ventral Cochlear Nuclei: Situated dorsal and ventral to the inferior cerebellar peduncle, respectively.
  • Spinal Nucleus of the Trigeminal Nerve and its Tract: Located on the anteromedial aspect of the inferior cerebellar peduncle.
• Tracts and White Matter:
  • Trapezoid Body: A trapezium-shaped mass of white fibers in the anterior tegmentum, formed by decussating fibers from the cochlear nuclei of both sides.
  • Medial Lemniscus: Most anterior part of the tegmentum, with its long axis running transversely.
  • Spinal Lemniscus: Located lateral to the medial lemniscus.
  • Medial Longitudinal Bundle (MLB): Occupies a paramedian position in the most posterior part.
  • Tectospinal Tracts: Located ventral to the medial longitudinal bundles.
  • Spinal Tract of Trigeminal: Located lateral and dorsal to the motor facial nucleus.

Transverse Section at the Upper Pons (Trigeminal Chief Sensory Nucleus)

• Nuclei:
  • Motor Nucleus of Trigeminal Nerve: Situated in the dorsolateral part beneath the lateral part of the $4^{th}$ ventricle. Motor fibers travel anteriorly and exit on the anterior surface.
  • Principal (Main) Sensory Nucleus of Trigeminal Nerve: Situated lateral to the motor nucleus.
• White Matter and Tracts:
  • Lateral Lemniscus: Well formed at this level.
  • Spinal Lemniscus: Positioned between the medial and lateral lemnisci.
  • Trigeminal Lemniscus: Consists of trigeminothalamic fibers, located between the medial and spinal lemnisci.
  • Medial Lemniscus.
  • Trapezoid Body.
  • Superior Cerebellar Peduncle and Superior Medullary Velum.

Blood Supply of the Pons

• Basilar Artery: Supplies the pons through numerous pontine branches.
• Anterior Inferior Cerebellar Artery (AICA).

Clinical Correlations

• Millard–Gubler Syndrome:
  • Etiology: A lesion in the lower part of the pons involving the pyramidal tract and the emerging fibers of the abducent ($VI$) and facial ($VII$) nerves.
  • Clinical Features:
  • Ipsilateral Medial Squint: Due to involvement of the abducent nerve.
  • Ipsilateral Facial Palsy: Due to involvement of the facial nerve fibers.
  • Contralateral Hemiplegia: Due to involvement of the corticospinal tract.
• Cerebellopontine Angle Tumor (Acoustic Neuroma):
  • Anatomy: The angle contains cranial nerves $VII$ and $VIII$. The tumor usually arises from the $VIII^{th}$ nerve.
  • Clinical Features:
  • Nerve VII Damage: Ipsilateral facial paralysis, loss of taste in the anterior two-thirds of the tongue, and hyperacusis.
  • Nerve VIII Damage: Deafness and vertigo.