Pons

The pons is a prominent structure of the brainstem located between the midbrain and the medulla oblongata. It serves as a critical relay center for motor and sensory pathways and contains nuclei for multiple cranial nerves. The pons also houses important centers for respiration and is involved in facial sensation, eye movement, hearing, equilibrium, and sleep regulation. It forms the anterior wall of the fourth ventricle.

Location

The pons is situated in the brainstem, anterior to the cerebellum and superior to the medulla. It lies above the medullary pyramids and below the midbrain's cerebral peduncles. Posteriorly, the pons forms the upper part of the floor of the fourth ventricle, while anteriorly it is convex and rests against the clivus of the skull base.

External Features

• Basilar (ventral) surface: Contains the basilar sulcus where the basilar artery lies. Transverse pontine fibers form the visible striations and converge laterally into the middle cerebellar peduncles.
• Posterior (dorsal) surface: Forms part of the floor of the fourth ventricle. It contains facial colliculi and striae medullares and is continuous with the tegmentum of the medulla and midbrain.

Internal Structure

The pons is divided into two main parts:

1. Basilar (ventral) pons

• Contains corticospinal, corticobulbar, and corticopontine tracts (descending motor fibers).
• Houses pontine nuclei that relay signals from the cerebral cortex to the cerebellum via transverse pontocerebellar fibers.

2. Tegmentum (dorsal pons)

• Contains ascending sensory tracts (medial lemniscus, spinothalamic tract) and reticular formation.
• Hosts nuclei of cranial nerves V–VIII.
• Involved in autonomic regulation, arousal, and coordination of eye and facial movements.

Cranial Nerve Nuclei in the Pons

• Trigeminal nerve (CN V): Principal sensory and motor nuclei located in the mid-pons; involved in facial sensation and mastication.
• Abducens nerve (CN VI): Located near the midline in the caudal pons; controls lateral rectus muscle for eye abduction.
• Facial nerve (CN VII): Motor nucleus lies in the caudal pons; innervates muscles of facial expression.
• Vestibulocochlear nerve (CN VIII): Vestibular and cochlear nuclei are located at the pontomedullary junction; involved in balance and hearing.

Connections to the Cerebellum

The pons connects to the cerebellum through three cerebellar peduncles:

• Middle cerebellar peduncles: Largest, carry fibers from pontine nuclei to the contralateral cerebellar hemisphere.
• Superior cerebellar peduncles: Connect pons to midbrain; carry mostly cerebellar output.
• Inferior cerebellar peduncles: Connect pons and medulla to cerebellum; carry proprioceptive and vestibular input.

Functional Roles

• Motor coordination: Through corticopontocerebellar connections and communication with the cerebellum.
• Sensory relay: Ascending tracts transmit fine touch, proprioception, pain, and temperature.
• Facial sensation and movement: Mediated by trigeminal and facial nerve nuclei.
• Hearing and balance: Vestibulocochlear nuclei process auditory and vestibular signals.
• Eye movement: Abducens nucleus controls lateral eye movement.
• Autonomic control: Reticular formation participates in respiratory rhythm generation and sleep-wake cycles.

Blood Supply

• Basilar artery: Primary supplier; gives off paramedian, short circumferential, and long circumferential branches.
• Anterior inferior cerebellar artery (AICA): Supplies lateral inferior pons and part of the cerebellum.
• Superior cerebellar artery: Supplies the rostral pons and adjacent cerebellum.

Venous drainage occurs via transverse pontine veins into the petrosal and basal veins, which drain into the vein of Galen and dural venous sinuses.

Development

The pons originates from the metencephalon, a derivative of the rhombencephalon (hindbrain) during embryonic development. The pontine flexure contributes to its anatomical configuration. Pontine nuclei arise from basal plate neuroblasts and begin forming cerebellar connections early in gestation.

Clinical Significance

• Locked-in syndrome: Bilateral ventral pons infarction leads to quadriplegia and anarthria, with preserved consciousness and vertical eye movement.
• Lateral pontine syndrome: Caused by AICA infarct; involves facial paralysis (CN VII), hearing loss (CN VIII), ataxia, and contralateral pain and temperature loss.
• Medial pontine syndrome (Foville syndrome): Due to paramedian pontine infarct; results in ipsilateral CN VI palsy and contralateral hemiparesis.
• Trigeminal neuralgia: Severe facial pain due to compression or dysfunction of CN V, often at its root entry zone in the pons.
• Pontine glioma: Brainstem tumors affecting pons, especially in children (e.g., diffuse intrinsic pontine glioma); associated with cranial nerve palsies, long tract signs, and poor prognosis.

Imaging

MRI is the modality of choice for evaluating pontine pathology, including infarcts, demyelination, tumors, and compression from adjacent structures. T2-weighted and FLAIR sequences are useful for identifying lesions in the brainstem. Diffusion imaging detects acute ischemia. Brainstem evoked potentials may assess functional integrity in clinical settings.