Pons

TS through Lower Part of Pons

At the level of a transverse section through the lower (caudal) pons, passing through the facial colliculi, the following structures are identified:

Tegmentum — Grey Matter

Cranial Nerve Nuclei

• The abducens nucleus (CN VI) is located in the paramedian tegmentum, beneath the facial colliculus in the floor of the fourth ventricle. It provides motor innervation to the lateral rectus muscle, mediating abduction of the eye.
• The facial nerve nucleus (CN VII) lies ventrolateral to the abducens nucleus in the tegmentum. Its axons follow a distinctive intranuclear course, looping dorsally around the abducens nucleus to form the internal genu of the facial nerve, which produces the surface elevation of the facial colliculus, before turning anterolaterally to exit the pons at the pontomedullary junction.
• The vestibular nuclei (medial and inferior at this level) and cochlear nuclei are present, as described at the level of the upper medulla.

Autonomic and Visceral Nuclei

• The superior salivatory nucleus gives rise to preganglionic parasympathetic fibers that travel with the facial nerve (CN VII) to innervate the submandibular and sublingual salivary glands and the lacrimal gland.
• The inferior salivatory nucleus provides preganglionic parasympathetic fibers that travel with the glossopharyngeal nerve (CN IX) to innervate the parotid gland.
• The lacrimal nucleus (sometimes considered part of the superior salivatory nucleus) contributes parasympathetic fibers via the facial nerve to the lacrimal gland, regulating tear secretion.
• The nucleus tractus solitarius (NTS) is present, receiving gustatory and visceral afferent inputs from cranial nerves VII, IX, and X.
• The spinal nucleus of the trigeminal nerve continues at this level, processing pain and temperature information from the face and oral cavity.
• The nuclei of the trapezoid body are present within and adjacent to the trapezoid body fibers, serving as relay stations in the ascending auditory pathway.

Tegmentum — White Matter

• The trapezoid body is a prominent band of transversely oriented fibers crossing the midline in the ventral tegmentum. It carries decussating auditory fibers from the cochlear nuclei and contributes to the formation of the lateral lemniscus.
• The medial lemniscus is present as a vertically oriented band, carrying fine touch, vibration, and conscious proprioception from the contralateral body toward the thalamus.
• The spinal lemniscus (spinothalamic tract) carries pain, temperature, and crude touch sensations from the contralateral body, ascending toward the ventral posterolateral (VPL) nucleus of the thalamus.
• The medial longitudinal fasciculus (MLF) runs in its characteristic paramedian position, coordinating conjugate eye movements and vestibulo-ocular reflexes.
• The tectospinal tract descends from the superior colliculus of the midbrain, mediating reflex head and neck movements in response to visual and auditory stimuli.
• The spinal tract of the trigeminal nerve continues laterally, carrying descending pain and temperature fibers from the face toward the spinal nucleus below.
• The intranuclear course of the abducens and facial nerve fibers is a defining feature of this level, with facial nerve axons looping around the abducens nucleus before exiting the brainstem.
• The anterior spinocerebellar tract is present in the lateral tegmentum, transmitting unconscious proprioceptive information from the lower limb to the cerebellum.

Basilar Part

• The basilar part at this level contains the pontine nuclei, together with the longitudinal fiber tracts — corticospinal, corticonuclear, and corticopontine — and the transversely running pontocerebellar fibers projecting to the contralateral cerebellum via the middle cerebellar peduncle.

Key Description

Abducens Nucleus and Nerve (CN VI)

• The abducens nucleus lies in the floor of the fourth ventricle, just lateral to the medial longitudinal fasciculus (MLF), beneath the facial colliculus. It supplies the lateral rectus muscle, mediating abduction of the eyeball.
• Axons from the abducens nucleus pass ventrally and caudally through the tegmentum and basilar pons, emerging at the pontomedullary junction just superior to the pyramid.
• A lesion of the abducens nucleus or nerve results in ipsilateral lateral rectus palsy, causing the affected eye to deviate medially at rest (convergent squint) and inability to abduct the eye, producing horizontal diplopia (double vision).

Facial Nerve Nucleus and Nerve (CN VII)

• The motor nucleus of the facial nerve lies ventromedial to the abducens nucleus within the pontine tegmentum. It receives bilateral corticonuclear (corticobulbar) input, which explains why an upper motor neuron lesion typically spares the forehead muscles, as they receive input from both hemispheres.
• Axons from the facial nucleus follow a distinctive intranuclear course: they initially pass dorsally toward the floor of the fourth ventricle, loop around the abducens nucleus forming the internal genu of the facial nerve, and then course anterolaterally to exit at the pontomedullary junction, superior to the olive, between the abducens nerve and the nervus intermedius.
• The looping fibers around the abducens nucleus produce the surface elevation in the floor of the fourth ventricle known as the facial colliculus.
• The facial nerve (CN VII) controls the muscles of facial expression and the orbicularis oculi (responsible for eye closure and blinking). It also mediates lacrimation and salivation via parasympathetic fibers, and conveys taste sensation from the anterior two-thirds of the tongue via the chorda tympani.
• Damage to the facial nerve nucleus or its intrapontine fibers produces ipsilateral lower motor neuron facial palsy, characterized by paralysis of all ipsilateral facial muscles, inability to close the eye, drooping of the mouth, loss of the corneal blink reflex, and impaired taste and salivation.

Neurobiotaxis

• The intranuclear course of the facial nerve within the pons is a classic example of neurobiotaxis — the embryological tendency of a nucleus to migrate toward its principal source of afferent stimulation. During development, the facial nucleus migrates ventrally toward the spinal nucleus of the trigeminal nerve, and its axons trail behind, forming the characteristic loop around the abducens nucleus. This migration is thought to facilitate rapid corneal reflex responses.

Superior Salivatory Nucleus, Lacrimal Nucleus, and Nucleus Tractus Solitarius

• The superior salivatory nucleus lies in the lateral reticular formation of the lower pons. It gives rise to preganglionic parasympathetic fibers that travel sequentially via the facial nerve → chorda tympani → lingual nerve → submandibular ganglion, ultimately innervating the submandibular and sublingual salivary glands.
• The lacrimal nucleus is functionally a subdivision of the superior salivatory nucleus. Its preganglionic parasympathetic fibers travel via the facial nerve → greater petrosal nerve → pterygopalatine ganglion → lacrimal gland, regulating tear secretion.
• Damage to these nuclei results in reduced salivation (xerostomia) and reduced tear production (xerophthalmia), leading to difficulty swallowing, impaired speech, and ocular surface irritation.
• The nucleus tractus solitarius (NTS) lies lateral to the superior salivatory nucleus at this level, continuing its role in processing gustatory and visceral afferent inputs.

Vestibular Nuclei

• The vestibular nuclear complex comprises four nuclei — superior, inferior, medial, and lateral — located in the lateral floor of the fourth ventricle. The superior and lateral nuclei lie in the pons, while the inferior and medial nuclei extend into the medulla.
• Afferent inputs arrive from the vestibular division of CN VIII and from the cerebellum.
• Efferent projections include:
  • Vestibulocerebellar fibers, projecting to the cerebellum for balance coordination.
  • The vestibulospinal tracts: the lateral vestibulospinal tract arises from the lateral vestibular nucleus and facilitates extensor muscle tone; the medial vestibulospinal tract arises from the medial vestibular nucleus and coordinates head and neck posture.
  • Projections to the MLF, which coordinates conjugate eye movements and vestibulo-ocular reflexes.
• Lesions of the vestibular nuclei produce vertigo, nausea, postural imbalance, and nystagmus (involuntary rhythmic eye movements).

Cochlear Nuclei, Trapezoid Body, and Lateral Lemniscus

• The dorsal and ventral cochlear nuclei lie on the corresponding surfaces of the inferior cerebellar peduncle and function as the second-order neurons of the ascending auditory pathway, receiving input from the spiral ganglion of the cochlea via the cochlear division of CN VIII.
• The trapezoid body is a band of transversely oriented fibers in the ventral tegmentum, carrying decussating auditory fibers from both cochlear nuclei toward the contralateral side. Embedded within these fibers are the nuclei of the trapezoid body, which receive cochlear input and project ascending fibers as the lateral lemniscus toward the inferior colliculus.
• The superior olivary complex lies in the anterolateral tegmentum. It receives bilateral input from the cochlear nuclei and serves as an important third-order relay in the auditory pathway, playing a central role in binaural sound localization by comparing interaural differences in timing and intensity.
• Damage to these structures may cause sensorineural hearing loss, difficulty localizing sounds, and impaired speech discrimination in noisy environments.

Lemnisci

• The medial lemniscus lies in the ventral tegmentum close to the midline, ascending to carry fine touch, vibration, and conscious proprioception from the contralateral body to the thalamus.
• The trigeminal lemniscus is positioned between the medial and spinal lemnisci, conveying discriminative touch and proprioception from the contralateral face via the principal sensory nucleus of CN V.
• The spinal lemniscus represents the ascending continuation of the lateral spinothalamic tract, carrying pain and temperature sensations from the contralateral body.
• The lateral lemniscus ascends laterally through the tegmentum, transmitting auditory signals from the cochlear nuclei and trapezoid body nuclei toward the inferior colliculus.

Other Tegmental Structures

• The spinal nucleus of the trigeminal nerve lies on the anteromedial aspect of the inferior cerebellar peduncle at this level, continuing to process pain and temperature from the face.
• The reticular formation occupies the central core of the tegmentum as a diffuse network of scattered neurons and interconnecting fibers, contributing to the regulation of consciousness, autonomic function, and motor coordination.

TS through the Upper Part of Pons

At the level of a transverse section through the upper (rostral) pons, passing through the trigeminal nuclei, the section reveals three principal regions: a central cavity, an anterior basilar part, and a posterior tegmentum.

Cavity

• At this level, the cavity of the fourth ventricle has become narrow and is bounded dorsally by the superior medullary velum and laterally by the superior cerebellar peduncles.
• The superior medullary velum is a thin lamina of white matter that bridges the two superior cerebellar peduncles, forming the roof of the upper fourth ventricle. Its anterior (inner) surface is lined by ependymal cells, while its posterior (outer) surface is covered by pia mater.

Basilar Part

• The basilar part at this level has the same organization as at lower pontine levels, containing the pontine nuclei and the four groups of longitudinal and transverse nerve fiber tracts: corticospinal, corticonuclear, corticopontine, and pontocerebellar fibers.

Tegmentum — Grey Matter

• The motor nucleus of the trigeminal nerve (CN V) is present at this level. It is a special visceral efferent (SVE) nucleus that supplies the muscles of mastication (temporalis, masseter, medial and lateral pterygoids), along with the mylohyoid, anterior belly of the digastric, tensor veli palatini, and tensor tympani — all derivatives of the first pharyngeal arch.
• The principal (main) sensory nucleus of the trigeminal nerve lies lateral to the motor nucleus. It receives discriminative touch and pressure sensations from the ipsilateral face and oral cavity, functioning as the pontine equivalent of the dorsal column nuclei for facial sensation. Its axons cross the midline and ascend as the trigeminal lemniscus to the ventral posteromedial (VPM) nucleus of the thalamus.

Trigeminal Nuclei

• The trigeminal nuclear complex consists of one motor nucleus and three sensory nuclei:
  • The motor nucleus lies in the pons and innervates the muscles of the first pharyngeal arch derivatives.
  • The mesencephalic nucleus extends into the midbrain and is unique in containing primary afferent (first-order) neuron cell bodies. It mediates proprioception from the muscles of mastication, the extraocular muscles, and the periodontium.
  • The principal sensory nucleus lies in the pons and processes fine touch and pressure from the face.
  • The spinal nucleus extends from the lower pons through the medulla oblongata to the level of the second cervical spinal segment (C2). It processes pain and temperature from the face, scalp, and oral cavity, and is functionally analogous to the dorsal horn of the spinal cord.
• Lesions of the trigeminal nuclei may produce ipsilateral facial sensory loss, weakness of the masticatory muscles, or, in cases of abnormal neural activity within the system, trigeminal neuralgia — a condition characterized by severe, paroxysmal facial pain.

Tegmentum — White Matter

• The four lemnisci — medial, trigeminal, spinal, and lateral — are all present in the tegmentum at this level, ascending toward their respective thalamic relay nuclei as described at lower pontine levels.
• The medial longitudinal fasciculus (MLF) continues in its paramedian position, coordinating conjugate eye movements and vestibulo-ocular reflexes.
• The superior cerebellar peduncles (brachia conjunctiva) are prominent at this level, lying dorsolateral to the tegmentum. They represent the principal efferent pathway of the cerebellum, carrying dentatorubrothalamic fibers from the dentate nucleus to the contralateral red nucleus and ventral lateral nucleus of the thalamus, and decussating in the caudal midbrain tegmentum.
• The reticular formation continues as a diffuse network in the central tegmentum, contributing to the regulation of arousal, autonomic function, and descending motor control.

Lemnisci

In the tegmentum of the upper pons, four lemnisci are arranged in a consistent sequence from medial to lateral:

• The medial lemniscus (band of Reil) is the most medially positioned. It carries second-order neurons originating from the nucleus gracilis (tract of Goll) and nucleus cuneatus (tract of Burdach), conveying conscious proprioception, discriminative (fine) touch, and vibration sense from the contralateral body to the ventral posterolateral (VPL) nucleus of the thalamus.
• The trigeminal lemniscus lies lateral to the medial lemniscus. It carries pain, temperature, and discriminative touch sensations from the contralateral face. Its fibers originate from the spinal nucleus (pain and temperature) and the principal sensory nucleus (fine touch) of the trigeminal nerve, and ascend to the ventral posteromedial (VPM) nucleus of the thalamus.
• The spinal lemniscus lies lateral to the trigeminal lemniscus and represents the ascending continuation of two spinothalamic tracts: the lateral spinothalamic tract, carrying pain and temperature from the contralateral body, and the anterior spinothalamic tract, carrying crude touch and pressure from the contralateral body. Both project to the VPL nucleus of the thalamus.
• The lateral lemniscus is the most laterally positioned and constitutes the principal ascending auditory pathway in the brainstem. It carries third-order auditory neurons derived from the superior olivary complex, which itself receives input sequentially from the cochlear nuclei → trapezoid body nuclei → superior olivary nucleus. The lateral lemniscus ascends to terminate in the inferior colliculus of the midbrain.

Blood Supply of Pons

• The pons receives its arterial supply exclusively from branches of the basilar artery and its derivatives:
  • Pontine arteries (paramedian and circumferential branches) arise directly from the basilar artery and represent the primary arterial supply to the pons. They are further divided into short circumferential branches, supplying the anteromedial and anterolateral tegmentum, and long circumferential branches, supplying the lateral and dorsal tegmentum.
  • Branches of the anterior inferior cerebellar artery (AICA), which arises from the lower basilar artery, supply the caudal and lateral pons as well as the anterior surface of the cerebellum.

The superior cerebellar artery, arising from the rostral basilar artery just proximal to its bifurcation, supplies the upper (rostral) pons and the superior surface of the cerebellum.

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