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  • P O N S


  • At the end of lecture the student will able

  • To demonstrate the gross features of pons.

  • To describe the internal gray and whit matters of pons.

  • To clinically correlates the anatomy of pons.

  • P O N S

  • The PONS is the part of Brain-Stem, lying above the medulla oblongata.

  • Below Mid Brain, anterior to Cerebellum.

  • As the name indicates Pons (Bridge) connecting right & left Cerebellar hemispheres.

  • It consists of 2 distinctive parts;

a. Dorsal Portion ; Pontine Tegmentum

b. Ventral portion ; PONS proper is convex & transversely striated

The dorsal part is hidden by Cerebellum & forms the rostral part of

4th ventricle.

  • Anterior to cerebellum.

  • Connects medulla to mid brain.

  • 1 inch (2.5 cm) long.

  • The anterior surface is convex from side to side & shows many transverse fibers that converges on each side to form middle Cerebellar peduncle.

  • There is shallow groove in the midline on its ventral surface is the Basiliar groove, which lodges the Basiliar artery.

  • Contains Cranial nerve nuclei of V, VI, VII & VIII.

  • Trigeminal nerve.

  • Abducent nerve.

  • Facial nerve.

  • Vestibulocochlear nerve.


  • On anterolateral side on each side, the tri-germinal nerve emerges.

  • The 5th nerve has small motor root & large sensory root.

  • In the groove between Pons & Medulla oblongata they emerge from medial to lateral side the abducent, facial & vestibular-cochlear nerve.

  1. The Posterior surface is hidden from view by Cerebellum.

  2. Forms upper half of floor of 4th ventricle & its triangular in shape.

  3. This surface is limited laterally by superior Cerebellar Peduncles.

  4. Lateral to Selcus is an elongated elevation bounded laterally by Sulcus Limitans.

  • The inferior end of the sulcus is slightly expanded to form facial colliculus, which is produced by root of facial nerve , winding around the abducent nerve.

  • The floor of superior part of sulcus limitans is blurish grey in colour & is called the Substantia Feruigenea.

  • This colour is due to deeply pigmented nerve cells.

  • Lateral to Sulcus Limitans is the area vestibuli, produced by underlying vestibular nucleus.

  • PONS Internal Structure

  • PONS is divided by transversaries running fibers of trapezoid body.

  • Tegmenteum posterior part.

  • Basal part anteriorly.

  • PONS structure may be studied at 2 levels;

  1. Transverse section through caudle part, passing through facial colliculus.

  2. T/S through cranial part, passing through the tegmental nuclei.

  • T/S through the caudal part.

  • Medial leminiscus; rotates as it passes from medulla to PONS.

  • It is situated in most anterior part of tegmenteum with its long axis running transversely.

  • The medial leminiscus is accompanied by spinal & lateral leminiscus.

  • T/S Through The Caudal Part


  1. Contains cranial nerve nuclei.

  2. Facial Nucleus; lies posterior to reticular formation of PONS.

  3. Abducent nerve nuclei; the facial nerve fiber winds around it forming Facial Colliculus.

  4. The nuclei & tract of 5th Cranial nerve is attached ventral & medial to surface at the junction of PONS with middle Cerebellar Peduncle.

  5. Medial Vestibular nuclei are present in the floor of 4th Ventricle.


  • Principle sensory nuclei of trigeminal nerve- motor nuclei is medial to superior sensory nucleus beneath the lateral part of fourth ventricle within the trigeminal nerve.


  • Lies posterior to lateral part of medial liminscus.

  • The fibers of facial nerve winds around nucleus of abducent nerve producing facial colliculus.

  • The fibers of facial nerve then pass anteriorly between facial & superior end of the nucleus of spinal cord of trigerminal nerve.


  • The medial vertibular nucleus is situated lateral to abducent nucleus has close relationship to ICP.

  • The superior part of lateral & inferior part of superior vertibular nucleus are found at this level.

  • The posterior & anterior cochlear nucleus lie at this level.

  • T/S Through The Caudal Part


  1. The medial leminiscus occupies a different position & its configuration is changed, this bundle present on each side of median raphe & lies anterior & just above the ventral portion of PONS.

  2. The Medial Longitudnal Fasiculus are situated dorsally on each side of median raphe is a main pathway which connect cochlear nerve to nerve of ocular muscles in 3rd, 4th & 6th Cranial Nerve.

  3. Pontine reticular formation is more extensive than medullary reticular formation but occupies a similar region is represented by PONS reticular nuclei.


  • Immediately behind ventral part of Pons transverse fibers made up trapezoid body, medial leminiscus lateral & spinal leminiscus.

  • Superior Cerebellar Peduncle lies dorso-lateral to motor nucleus of 5th nerve.

    • T/S Through The Caudal Part

  • T/S Through The Caudal Part
    white matter

  1. The median raphe contains inferior central nuclei & subspendymal area.

  2. Trapeziod body fibres derived from cochlear nuclei & trapezoid nuclei run transversely on the ventral side of PONS.

  3. The basilar part of PONS contain pontine nuclei which contain corticospinal fibres—transverse fibres--- middle cerebellar peduncle----- cerebellum

  • T/S Through The Caudal Part

  • The lateral SPINO-THALAMIC tract (spinal leminiscus) lies lateral to medial leminiscus.

  • Inferior Cerebellar peduncle lies lateral to floor of fourth ventricle.


  • Similar as seen in caudal part.

  • The spinal nucleus of trigerminal nerve & its tract lie on the anteromedial aspect of ICP.

  • TRAPEZOID BODY: Is made up of fibers derived from cochlear nuclei & nuclei of trapezoid body .

  • They run transversely in the anterior part of tegmenteum.

  • The basilar part of PONS contains small masses of nerve cells called “Pontine Nuclei”.

  • The sorticospinal fibers of mid brain terminated in the pontine nuclei.

  • The axom of these cells give origin to transverse fibers of PONS, which cross the midline & intersects the C/S & contricinuclear tract breaking them up into small bundles.

  • The transverse fibers of PONS enter the MCP and are distributed to cerebellar hemisphere.

  • Forms main pathway linking cerebrall cortex to the cerebellum.


  1. Contains principal & motor sensory nucleus of trigerminal nerve.

  2. The motor nucleus of trigerminal nerve is situated beneath the lateral part of 4th ventricle within reticular formation.

  3. The emerging motor fibers travels anteriorly through the substance of PONS & exit on its anterior surface.

  4. Principal sensor of nucleus of trigerminal nerve is situated on lateral side of motor nucleus is contineous inferiorly with nucleus of spinal tract.

  5. The entering sensory fibers travels through the substance of PONS & lie lateral to motor fibers.

  • The superior cerebellar peduncle is situated postero-lateral to motor nucleus of trigerminal nerve & is joined by Anterior-Spino Cerebellar Tract.

  • The trapezoid body & medial leminiscus are situated lies in some portion.

  • The lateral & spinal terminisci lie at lateral extremity of medial laminiscus.

  • Function

  • Pons contains neural pathways, connection cerebellum, spinal chord, and cerebrum. It serves as a relay station of motor (cerebrum) and sensory (spinal cord) messages. Contained within pons are important centers for regulation of breathing.

  • Important role in the coordination of movement of the right and left sides of the body.

  • The horseshoe-shaped structure is also involved in information integration and transfer, respiration, control of eye movement and head muscles, taste, and states of arousal.

  • Nuclei located in the area where the pons and midbrain meet have been of particular interest in studies of the rapid eye movement (REM) stage of the sleep cycle.

  • Most individuals are prevented from acting out their dreams due to another function of the pons, which sends signals to inhibit neurons in the spinal cord so that the muscles of the limbs are temporarily paralyzed during REM sleep.

  • Motor activity may continue, however, in patients with a rare condition called REM sleep behavior disorder.

  • Blood Supply of Pons

  • The pontine arteries that branch from the basilar artery, as well as the anterior inferior and superior cerebellar arteries, maintain a constant supply of blood to the pons

  • SCA (superior cerebellar artery syndrome)

  • Main symptoms are ipsilateral cerebellar ataxias (middle and/or superior cerebellar peduncles), nausea and vomiting, slurred (pseudobulbar) speech, loss of pain and temperature over the opposite side of the body. Partial deafness, tremor of the upper extremity, an ipsilateral Horner syndrome and palatal myoclonus have been reported. Clinically, this stroke may be impossible to distinguish from a partial AICA or PICA territory stroke. It is much rarer than either one. Ocular pulsion away from the side of lesion has been reported in SCA syndrome. Diagnosis of the stroke is via MRI.


  • This is a catastrophic event, typically a hypertensive bleed. It presents with of coma, quadriplegia, small reactive pupils and absent horizontal eye movements. In most quadriplegic patients a hematoma in the middle of the pons is centered at the junction of the tegmentum and basis pontis. Ocular bobbing is a less constant feature. Lateral tegmental hemorrhages present with 1 1/2 syndrome, small reactive pupils, limb ataxia of the cerebellar type, and contralateral hemisensory loss (Caplan and Goodwin, 1982). Those that survive may develop oculopalatal myoclonus. Diagnosis may be made via MRI (best) or CT scan, or a combination of both.

  • Medial medullary infarction (syndrome of Dejerine)

  • 0.5% of all brain infarcts. Contralateral hemiparesis sparing the face, hemisensory loss of the posterior column type (contralateral). Weakness of the tongue is ipsilateral to the infarct. Pathology may be in vertebral artery or mesial limb of vertebral artery after PICA. Upbeat nystagmus may occur. Small vessel disease (diabetes, hypertension, hypercholesterolemia) is the usual cause.

  • MRI scan of person with central pontine myelinolysis. Saggital view The dark area inside the circle is the region of damage.

  • MRI scan of person with central pontine myelinolysis, axial view. Note the "I" shaped area in the center of the pons.


  • It is common to encounter areas of increased signal on T2 MRI in the pons in older persons with unsteadiness. These patients often display symptoms of disequilibrium, difficult with speech and swallowing. (Kwa et al, 1998). In the author's experience, these patients often exhibit rebound nystagmus, which is a variant of gaze-evoked nystagmus.

  • A rare source of pontine hyperintense lesions is central pontine myelinolysis (see above). This is caused by rapid fluctuations in electrolyte status, usually in the context of a hospitalization. The individual shown above had a liver transplant done. After the liver transplant, he was fine for a couple of days but then gradually became comatose. His MRI at that time showed the picture above. Examination nine months later revealed an ambulatory individual with some mild cerebellar signs. About 2% of persons with liver transplant develop central pontine myelinolysis.

  • Individuals with midline pontine infarcts usually have normal ABR testing

  • Larger Vessel Disease
    Vertebrobasilar Insufficiency

  • Decreased blood flow in the vertebrobasilar system is invoked as a potential explanation for a myriad of symptoms possibly attributable to the brainstem.

  • At this writing (12-2007), CT-angiography is the best way to establish this diagnosis. While MRI technology has advanced greatly, MRA is presently not of high enough resolution to reliably visualize the tiny arteries involved in this area. Conventional angiography is the most reliable way to infer the diagnosis, but is usually unreasonably risky compared to CT-angiography.

  • CT angiogram showing a hypoplastic right vertebral, in a person with symptoms of vertebrobasilar insufficiency. Left vertebral (left lower) is large and dominant. Right vertebral (right lower) is small and hypoplastic. This is the same case as shown in the selective vertebral angiogram below.

  • Basilar Artery Thrombosis

  • As the basilar artery supplies most of the brainstem, occlusion is commonly catastrophic resulting in quadriplegia. Death from respiratory failure is common. The "locked in syndrome", denoting a state where the unfortunate patient can think and see but may be unable to respond may occur.   Occlusion of the "top" of the basilar artery can result in a large number of complex syndromes that may include visual hallucinations, somnolence, various ocular findings mainly involving vertical gaze and/or convergence/retraction nystagmus. The classic paper is by Kubik and Adams (1946) . Diagnosis is via MRI/MRA.


  • Usually manifests as PICA territory infarct (see below). Bilateral occlusions are much rarer than unilateral, and have a slowly progressive course and poor prognosis (Caplan, 1983). Ct-angiography combined with MRI is usually the best way to make this diagnosis.


  • Carotid disease rarely causes vertigo. This is because the parts of the brain that control motion perception are in the back, and are supplied by different arteries (the vertebral and basilar arteries). When carotid disease is severe, and accompanied by disease in the back arteries, carotid disease can be associated with vertigo, but this is unusual. Diagnosis is via MRA, doppler, or angiography.


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