Pons

Pons

Also known as: pons Varolii

The middle part of the brainstem, a bulging "bridge" of transverse fibres to the cerebellum; its tegmentum holds the nuclei of CN V, VI, VII and VIII, the horizontal gaze centre and respiratory centres.

Overview

Length≈2.5 cm

Cranial nerve nucleiV (motor, principal sensory), VI, VII, VIII (part)

The pons is ≈2.5 cm long and rests on the clivus, with the basilar artery in a midline groove on its anterior surface. Its dorsal surface forms the upper half of the floor of the fourth ventricle. It has two parts.

The ventral basilar pons contains the pontine nuclei. Corticopontine fibres end on these nuclei, and their axons cross as transverse pontine fibres to form the massive middle cerebellar peduncle. This cortico-ponto-cerebellar route lets the cerebellum match ongoing movement to the cortical plan. The corticospinal fibres pass through the basilar pons in scattered bundles.

The dorsal tegmentum contains:
• the motor and principal sensory nuclei of V, with V emerging from the lateral pons;
• the abducens nucleus, encircled by the genu of the facial nerve to form the facial colliculus;
• the facial motor nucleus;
• the superior salivatory nucleus;
• the vestibular and cochlear nuclei (shared with the medulla);
• the paramedian pontine reticular formation (PPRF), the horizontal gaze centre;
• the locus coeruleus, the main noradrenergic nucleus;
• the pontine respiratory group, which smooths the respiratory rhythm and terminates inspiration;
• the medial lemniscus, spinothalamic tract and MLF.
CN VI, VII and VIII emerge at the pontomedullary junction. VII and VIII emerge at the cerebellopontine angle.

Location

Middle brainstem, between the midbrain and medulla, anterior to the cerebellum and fourth ventricle, on the clivus.

Parts & subdivisions

Basilar pons

Pontine nuclei, transverse pontocerebellar fibres → middle cerebellar peduncle, corticospinal bundles.

Pontine tegmentum

Nuclei of V, VI, VII (and VIII), PPRF, locus coeruleus, medial lemniscus, spinothalamic tract, MLF.

Facial colliculus

Swelling in the floor of the fourth ventricle formed by the facial nerve looping over the abducens nucleus.

Locus coeruleus

Blue-pigmented noradrenergic nucleus projecting throughout the CNS; vigilance, attention, stress responses; affected early in Parkinson and Alzheimer disease.

Pontine respiratory group

Parabrachial and Kölliker–Fuse nuclei ("pneumotaxic centre"): limits inspiration and fine-tunes rate; lower pons "apneustic" activity prolongs inspiration.

Function

Relays cortical motor plans to the cerebellum (corticopontocerebellar pathway)

Facial sensation and mastication (V); facial expression, lacrimation, salivation, taste (VII)

Lateral gaze (VI nucleus and PPRF)

Hearing and balance (cochlear and vestibular nuclei)

Regulation of the respiratory rhythm

Arousal and REM sleep (pontine tegmentum, locus coeruleus)

Clinical significance

Internuclear ophthalmoplegia (INO)

A lesion of the MLF, typically demyelination in young adults with MS (often bilateral) or a stroke in older adults, disconnects the abducens nucleus from the contralateral medial rectus subnucleus. On lateral gaze the ipsilateral eye fails to adduct and the abducting eye shows nystagmus. Convergence is preserved. A combined PPRF/VI-nucleus + MLF lesion gives the one-and-a-half syndrome.

Pontine haemorrhage and osmotic demyelination

• Pontine haemorrhage: sudden coma, quadriplegia, pinpoint pupils (sympathetic fibres interrupted) and hyperthermia.
• Osmotic demyelination (central pontine myelinolysis): too-rapid correction of chronic hyponatraemia (above ≈8–10 mmol/L in 24 h) demyelinates the basilar pons, causing spastic quadriparesis, pseudobulbar palsy and locked-in syndrome.