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C H A P T E R 11 Brainstem: Pons Introduction External Features Internal Structure • Structure of the basilar part • Structure of the tegmental part Transverse Section Through the Lower Pons INTRODUCTION Pons, in a literal sense, means ‘the bridge’. It acts as a conduit for the passage of impulses from one side of the cerebellum to the other and also between the medulla, below, and midbrain, above. Physiologically, the centre of respiration is present in the pons. Pons also has nuclei of origin for the following cranial nerves: Trigeminal (V), abducent (VI), facial (VII) and vestibulocochlear (VIII). EXTERNAL FEATURES Pons is a part of the brainstem, situated between the medulla, below, and midbrain, above (see Figs 10.1–10.4). It lies in front of the cerebellum. It is about 2.5 cm long and presents a ventral and a dorsal surface. The ventral surface is convex and is bounded by upper and lower borders. This surface presents transversely running ridges (fibres). Laterally, these ridges come closer to form a bundle, the middle cerebellar peduncle. The point of junction between the anterior surface of the pons and the middle cerebellar peduncle is marked by the emergence of the trigeminal nerve. The trigeminal nerve emerges in the form of a large sensory root and a small motor root. The ventral surface also presents a shallow groove in the midline, known as the basilar groove. This groove lodges the ‘basilar artery’. The ventral surface of the pons is in relation to ‘dorsum sellae’ and ‘basiocciput’. These are the bony landmarks present on the basilar surface (internal aspect) of the skull. On the ventral aspect, three cranial nerves are seen to emerge from the pontomedullary junction (i.e. lower border of pons). The abducent nerve emerges just above the Ch_11_NA.indd 127 • Arrangement of grey matter • Arrangement of white matter Transverse Section Through the Upper Pons • Arrangement of grey matter in tegmentum • Arrangement of white matter in tegmentum • Tumours of the pons pyramid and facial and vestibulocochlear nerves emerge above the olive (see Figs 10.1–10.4). The posterior surface of the pons is formed by the upper part of the floor of the fourth ventricle. This surface is limited laterally by the superior cerebellar peduncles. The posterior surface of the pons is related to the cerebellum. INTERNAL STRUCTURE A transverse section passing through the pons is divided into ventral (basilar) and posterior (tegmentum) parts (Fig. 11.1). The basilar (ventral) part contains transversely and vertically running white fibres and scattered masses of grey matter (pontine nuclei). The transverse fibres run laterally to form the middle cerebellar peduncle while the vertical fibres run downwards to form the pyramid of medulla. The structure of the basilar part is the same throughout the pons (at all levels). The tegmentum of the pons is a direct upward continuation of the medulla except for the pyramidal tracts (which run through the basilar part of pons). This part consists of many ascending and descending tracts and nuclei for the cranial nerves. Structure of the Basilar Part The basilar part of the pons consists of descending longitudinal fibres, transverse pontine fibres and pontine nuclei. The descending fibres enter the pons as a compact bundle but soon they break up into many bundles due to the presence of pontine nuclei and transversely running pontine fibres (Fig. 11.2). 8/28/2012 1:17:46 PM 128 Illustrated Textbook of Neuroanatomy Tegmental part Middle cerebellar peduncle Basilar part Nucleus of trapezoid body Trapezoid body Pontine nuclei Descending longitudinal fibres Figure 11.1 Schematic diagram showing the transverse section through the lower part of the pons. The basilar part of the pons lies ventral to the trapezoid body while the tegmental part lies dorsal to it. Descending Longitudinal Fibres The descending longitudinal fibres consist of corticospinal, corticonuclear and corticopontine fibres. • Corticospinal fibres: These fibres traverse the pons and converge again to form pyramids in medulla. • Corticonuclear fibres: These fibres descend along with the corticospinal to form pyramids in medulla. However, many corticonuclear fibres terminate in the contralateral (and some ipsilateral) motor nuclei of cranial nerves and reticular formation of pons. • Corticopontine fibres: These fibres arise from the frontal, parietal, temporal and occipital cortices and terminate on pontine nuclei of the same side. Pontine Nuclei The pontine nuclei are small masses of grey matter scattered between longitudinal (vertical) and transversely arranged fibres. The pontine nuclei give rise to transverse fibres of the pons. These fibres are known as pontocerebellar because they terminate in the cerebellum (cerebellar cortex). The pontine nuclei are a relay station in the corticopontocerebellar pathway, that is, between the cerebral cortex and contralateral cerebellar hemisphere. Corticopontine fibres from various lobes of cerebral cortex end in pontine nuclei (Fig. 11.2). Transverse Pontine Fibres Transverse pontine fibres are pontocerebellar fibres that run transversely across the midline of the pons. While running transversely in the pons, the bundles of pontocerebellar fibres cross the vertically running bundles of the descending fibres (corticospinal and corticonuclear). The pontocerebellar fibres together form a bundle (on the opposite side) which is referred to as ‘middle cerebellar peduncle’. The pontocerebellar fibres are a part of the corticopontocerebellar pathway. Corticopontine Midbrain Corticonuclear fibres Pons Motor cranial nerve nuclei and reticular formation Pontocerebellar fibre Middle cerebellar peduncle Pontine nuclei Olive Pyramid Medulla Corticospinal fibres Figure 11.2 Components of descending longitudinal fibres (i.e. corticospinal, corticonuclear and corticopontine) in the basilar part of pons. Corticospinal fibres converge to form pyramid, corticonuclear fibres end in motor nuclei of cranial nerves or in reticular formation and corticopontine fibres end in pontine nuclei. Pontocerebellar fibres form the middle cerebellar peduncle. They originate in pontine nuclei and run transversely to cerebellum. The basilar part of pons consists of both longitudinal and transverse and running fibres. Ch_11_NA.indd 128 8/28/2012 1:17:50 PM Brainstem: Pons 129 (Pons) (a) (b) Figure 11.3 Transverse sections of pons (cranial and caudal levels). (a) Illustration depicting levels of transverse section. (b) Photograph depicting the transverse section of pons. Photograph courtesy: Professor RK Zargar and Dr Sunita Athavale, People’s Medical College, Bhopal. Structure of the Tegmental Part The tegmental (dorsal) part of the pons is the upward continuation of the medullary reticular formation. The white matter in the tegmentum consists of various ascending and descending tracts. The grey matter consists of the cranial nerve nuclei of V, VI, VII and VIII nerves and reticular nuclei. The posterior surface of pons forms the floor of the fourth ventricle. The structure of the tegmentum is different in the upper and lower parts of pons. Hence, it is customary to study the internal structure of the tegmental part of pons at two different levels—caudal (lower) part, transverse section passing through the facial colliculus, and cranial (upper) part, transverse section passing through trigeminal nuclei (Fig. 11.3). • The vestibular nuclei complex receives afferent fibres from • • TRANSVERSE SECTION THROUGH THE LOWER PONS The transverse section through the lower part of pons corresponds to the level of facial colliculus (Fig. 11.4). • Arrangement of Grey Matter • The floor of the fourth ventricle, at this level, is lined by grey matter and shows the presence of two cranial nerve nuclei—abducent and vestibular. Also, seen are the cranial nerve nuclei of trigeminal nerve (spinal nucleus of the trigeminal nerve) and facial nerve, at deeper levels. • The abducent nerve nucleus lies beneath the facial colliculus and lateral to the medial longitudinal bundle. The facial colliculus is formed due to a complicated loop formed by the fibres of the facial nerve winding around the abducent nucleus. This nucleus gives origin to the abducent nerve, which is the motor nerve for the lateral rectus muscle of eye. Ch_11_NA.indd 129 • the vestibular division of the vestibulocochlear nerve. The efferents from this nuclei form the vestibulocerebellar tract, vestibulospinal tract, medial longitudinal bundle and lateral lemniscus. The vestibular nuclei convey impulses concerned with equilibrium and orientation in three-dimensional space. There are two cochlear nuclei which are designated as ‘ventral’ and ‘dorsal’. The ventral and dorsal nuclei lie on the ventral and dorsal aspects of the inferior cerebellar peduncle, respectively. These two nuclei are relay stations in the pathway of hearing or in the auditory pathway. These nuclei receive the afferents from the cochlear nerve. The motor nucleus of the facial nerve is present in the reticular formation, medial to the nucleus of the spinal tract of the trigeminal nerve and dorsal to the superior olivary nucleus. This nucleus gives origin to the motor fibres of facial nerves for the innervations of muscles of facial expression. The salivatory nuclei are usually divided into superior salivatory, inferior salivatory and lacrimatory nuclei. These nuclei are located between the upper end of dorsal vagal nucleus and inferior end of facial nucleus. The upper end of the dorsal vagal nucleus is situated at the level of pontomedullary junction and the inferior end of facial nucleus is located in the lateral part of reticular formation. The salivatory nuclei are a group of parasympathetic motor nuclei. These nuclei send secretomotor fibres via facial nerve (to pterygopalatine and submandibular ganglia) and glossopharyngeal nerve (to otic ganglion). The postganglionic fibres from the ganglia supply to the salivary and lacrimal glands. The nucleus of the spinal tract of the trigeminal nerve and the tract are present ventromedial to the inferior 8/28/2012 1:17:51 PM 130 Illustrated Textbook of Neuroanatomy Facial nerve Medial longitudinal bundle nucleus Tectospinal Facial colliculus Abducent nucleus Vestibular nuclei complex Inferior cerebellar peduncle Cochlear nuclei Superior salivatory nucleus Reticular formation Spinal nucleus and tract of trigeminal nerve Middle cerebellar peduncle Lateral lemniscus VIII nerve Spinal lemniscus Pontocerebellar fibres Trigeminal lemniscus Nucleus of tractus solitarius Medial lemniscus VII nerve Trapezoid nucleus Corticospinal and corticonuclear (Or corticopontine) fibres Trapezoid body Rubrospinal tract VI nerve Pontine nuclei Figure 11.4 Transverse section of pons at the level of facial colliculus. The facial nerve, after taking origin from the nucleus, follows an unusual course. At first, the fibres of the facial nerve pass dorsomedially below the abducent nerve nucleus. Then they pass upwards on the medial side of the abducent nucleus, close to midline. Fibres then curve forwards and laterally above the abducent nucleus. At this place, fibres lie deep to the floor of the fourth ventricle forming the facial colliculus. Now these fibres pass forwards, laterally and downwards through the reticular formation lying between its own nucleus on the medial side and the nucleus of the spinal tract of the trigeminal nerve on the lateral side. • • • • cerebellar peduncle and lateral to the nucleus of the facial nerve. Nucleus of tractus solitarius: At the level of the lower pons, this nucleus lies lateral to the superior salivatory nucleus. The reticular nuclei are a small collection of grey matter scattered in the network of white fibres. Superior olivary nuclear complex: The complex of superior olivary nuclei mainly consists of the lateral superior olivary nucleus, medial superior olivary nucleus and retro-olivary group of nuclei. These nuclei are situated lateral to the reticular formation, in the lower pons, at the pontomedullary junction (not shown in Fig. 11.4 as the section is at a higher level). The nucleus of the trapezoid body is situated in the ventral part of tegmentum. This nucleus is placed in the auditory pathway and its fibres constitute the lateral lemniscus. Arrangement of White Matter At the level of lower pons, many new features appear in the white matter of the tegmentum. The trapezoid body is seen as a mass of white matter in the ventral part of tegmentum, just dorsal to the basilar part. The medial and spinal lemnisci have changed their positions. The trigeminal and spinal lemnisci are new ascending tracts seen at this level. Trapezoid Body The trapezoid body is formed by the fibres of both ventral and dorsal cochlear nuclei. These fibres cross the midline in the ventral part of the tegmentum. Fibres of the trapezoid body relay in the trapezoid nucleus and in the superior olivary complex. The efferents of these nuclei form the lateral lemniscus (ascending auditory pathways). Medial, Trigeminal, Spinal and Lateral Lemnisci • These lemnisci are situated posterior to the trapezoid body (Fig. 11.4). • The medial lemniscus is the most medial and now orient- ed transversely. The body is somatotopically represented in such a way that fibres from the neck and upper limb lie most medially and those of the lower limb most laterally. The medial lemniscus is joined by the fibres of the ventral spinothalamic tract.* • The trigeminal lemniscus begins to form at this level and is situated lateral to the medial lemniscus. It is formed by the axons arising from the contralateral spinal nucleus of the trigeminal nerve. This tract conveys the exteroceptive impulses (pain, touch, temperature) from the area of supply of the trigeminal nerve (face, nose, mouth, tongue, conjunctiva, etc.) *According to some authors, the ventral spinothalamic tract joins the lateral spinothalamic tract to form the spinal lemniscus. Ch_11_NA.indd 130 8/28/2012 1:17:53 PM Brainstem: Pons • The spinal lemniscus is formed by the fibres of lateral • • • • spinothalamic and spinotectal tracts. It lies lateral to the trigeminal lemniscus. The lateral lemniscus begins in the lower half of pons. It is a part of the auditory pathway. It lies most laterally in the lemniscal band, that is, lateral to the spinal lemniscus. The ventral spinocerebellar tract is situated dorsolateral to the lateral lemniscus. The spinal nucleus and tract of trigeminal nerve are situated lateral to the facial nerve nucleus. The lateralmost area of the tegmentum is occupied by the inferior cerebellar peduncle. TRANSVERSE SECTION THROUGH THE UPPER PONS The transverse section through the upper part of pons corresponds to the level of trigeminal nuclei (Fig. 11.5). This level passes through the motor and principal sensory nuclei of the trigeminal nerve. The dorsal part of the tegmentum now contains the cavity of the fourth ventricle, which is bounded dorsolaterally on either side by a superior cerebellar peduncle and roofed by the superior medullary velum. Arrangement of Grey Matter in Tegmentum 131 Below this level, the nucleus is continuous with the spinal nucleus of the trigeminal nerve. This nucleus is the relay station for the sensation of touch (pain and temperature being carried by the spinal nucleus). Fibres arising from this nucleus ascend in the trigeminal tract and after crossing to the opposite side terminate in thalamus. • Motor nucleus of the trigeminal nerve: This nucleus lies medial to the sensory nucleus in the floor of the fourth ventricle, within the reticular formation. It gives origin to the motor root of the trigeminal nerve. • Nucleus of lateral lemniscus: This small nucleus is present medial to lateral lemniscus. This nucleus is considered as a relay station in the auditory pathway and is associated with the trapezoid nucleus. Arrangement of White Matter in Tegmentum At the upper level of pons, the trapezoid body is usually not seen. However, some fibres of the trapezoid body may be visible just ventral to the medial lemniscus. • The medial, trigeminal and spinal lemnisci are in the same position. • The lateral lemniscus is more prominent at this level. • The ventral spinocerebellar tract now joins the superior cerebellar peduncle. • The superior cerebellar peduncle is situated at the dorso- The following nuclei are seen in tegmentum: lateral position of the section. • Principal (superior) sensory nucleus of the trigeminal nerve: It is situated lateral to the motor nucleus (Fig. 11.5). • The medial longitudinal bundle, tectospinal tract and ru- brospinal tracts are seen in the paramedian position. Superior medullary velum Superior cerebellar peduncle Ventral spinocerebellar tract Principal sensory nucleus of V nerve Cavity of the fourth ventricle Motor nucleus of V nerve Central tegmental tract LL Nucleus of lateral lemniscus Tegmental part of pons MLB TST Middle cerebellar peduncle RST SL TL ML Sensory root of V nerve Motor root of V nerve V Corticospinal fibres V Basilar part of pons Trapezoid body Pontine nuclei Figure 11.5 Transverse section of pons at the level of trigeminal nucleus (upper pons). LL—lateral leminsicus, ML—medial lemniscus, MLB—medial longitudinal bundle, RST—rubrospinal tract, SL—spinal lemniscus, TL—trigeminal lemniscus, TST—tectospinal tract. Ch_11_NA.indd 131 8/28/2012 1:17:53 PM 132 Illustrated Textbook of Neuroanatomy CLINICAL ASPECT Vascular Lesions of the Pons Vascular lesions of the pons may result due to occlusion or haemorrhage of pontine arteries which are branches from the basilar artery. Pontine Haemorrhage Pontine haemorrhage is usually fatal. This condition results in hemiplegia on the opposite side of the lesion due to damage to corticospinal fibres and results in facial paralysis on the same side due to involvement of the facial nerve and nucleus. (But if the haemorrhage is extensive and bilateral, then paralysis of face and limbs is also bilateral.) Similarly, due to the involvement of sympathetic fibres in the reticular formation the patient suffers from hyperpyrexia and shows the pinpointed pupil. Hyperpyrexia is due to damage to sympathetic fibres through which the hypothalamic heat regulating centre exerts its effect. The pinpointed pupil results due to over-activity of parasympathetic fibres in the absence of sympathetic fibres. Damage to reticular formation leads to deep coma as it controls consciousness. If the vital centres of reticular formation controlling respiration and circulation are also damaged, then sudden death may occur. The occlusion of the pontine branches of the basilar artery may result in various syndromes. These are described in Table 11.1. Tumours of the Pons Tumours of the pons arise from neuroglial cells (mostly astrocytes). Astrocytoma occurs in childhood. In this lesion, cranial nerves of the same side are affected (cranial nerves V, VI, VII and VIII). However, involvement of the corticospinal tract leads to hemiparesis on the opposite side. Table 11.1 Syndromes Associated with Occlusion of Pontine Branches of Basilar Artery Syndrome Site of Damage Clinical Features Raymond’s syndrome (Due to occlusion of pontine branches of the basilar artery) Corticospinal and other descending motor fibres Damage to abducent nerve fibres (Fig. 11.6) Hemiplegia/hemiparesis on the opposite side of the body (UMN type of lesion) Paralysis of the lateral rectus muscle of eye ball. This results in medial squint Millard–Gubler syndrome (It is a more dorsolaterally located lesion than Raymond’s. In this syndrome, axons and nucleus of the VI nerve are spared.) Damage to corticospinal fibres Damage to fibres of the VII cranial nerve and its nucleus (Fig. 11.7) Hemiparesis on the opposite side of the body (UMN type of lesion) Paralysis of the same side of face (LMN type of paralysis) Foville’s syndrome Lesion of abducent nerve nucleus (It is a more dorsally and medially located Facial nucleus and its axons (Fig. 11.8) lesion as compared to the Millard– Transient ischaemia of the corticospinal Gubler syndrome; Fig. 11.8.) tract LMN type of paralysis of lateral rectus muscle resulting in medial squint on the same side of the lesion LMN type of paralysis of facial muscles on the same side Hemiparesis of a brief duration on the opposite side of the body Facial nerve nucleus Facial nerve Abducent nerve Abducent nerve Descending fibres Figure 11.6 Raymond’s syndrome. This syndrome results in hemiplegia on the opposite side of the body due to involvement of corticospinal fibres. This also results in medial squint on the same side due to damage to the abducent nerve. Ch_11_NA.indd 132 Figure 11.7 Millard–Gubler syndrome. This results in hemiparesis on the opposite side of the body and paralysis of the face on the same side. Hemiparesis results due to damage to only few corticospinal fibres. The abducent nerve is spared. 8/28/2012 1:17:55 PM Brainstem: Pons 133 Facial nerve Abducent nerve Figure 11.8 Foville’s syndrome. This results in medial squint and facial paralysis on the same side of the body. SUMMARY External Features of Pons Pons lies between the medulla and midbrain and is situated in front of the cerebellum. It presents ventral and dorsal surfaces. The ventral surface of pons is convex and presents a shallow groove in midline—the basilar groove. On each side, the ventral surface is continuous with the middle cerebellar peduncle and shows the attachment of V cranial nerve. The posterior surface of pons is formed by the upper part of the floor of the fourth ventricle. Three cranial nerves (VI, VII and VIII) emerge from the pontomedullary junction. Tegmentum, at the level of facial colliculus (lower pons), shows the following features: Arrangement of Grey Matter in Tegmentum • • • Internal Structure of Pons A transverse section passing through pons is divided into basilar and tegmental parts (Fig. 11.1). • • The structure of the basilar part of pons consists of descending fibres (corticospinal, corticonuclear and corticopontine) and transverse pontine fibres (pontocerebellar). The pontine nuclei are small masses of grey matter scattered between longitudinally and transversely arranged fibres. The structure of the tegmental part of pons consists of ascending and descending tracts, cranial nerve nuclei of V to VIII nerves and reticular formation. The structure of basilar part of pons remains constant throughout the pons. However, the structure of tegmentum differs in upper and lower parts of pons. • • The floor of the fourth ventricle, at this level, shows the presence of abducent and vestibular nerve nuclei (Fig. 11.3). The ventral and dorsal and cochlear nuclei are located on the dorsal and ventral aspects of the inferior cerebellar peduncle, respectively. The motor nucleus of the facial nerve is present in the area of reticular formation. The facial nerve follows an unusual course before it comes out at the pontomedullary junction. Nucleus of the spinal tract of trigeminal nerve is located ventromedial to the inferior cerebellar peduncle. Nucleus of tractus solitarius and superior salivatory nucleus are situated ventromedial to facial nucleus. Arrangement of White Matter • • • The trapezoid body is seen at the junction of basilar and tegmental parts. Posterior to the trapezoid body, four lemnisci (medial, trigeminal, spinal and lateral) are arranged from medial to lateral side. The medical longitudinal bundle, tectospinal tract and rubrospinal tract are situated at the paramedian position (Fig. 11.3). The inferior cerebellar peduncle is present in the lateralmost area of tegmentum. Multiple Choice Questions 1. Which of the following statements about pons is false? a. It lies in front of cerebellum b. The posterior surface of pons is formed by the upper surface of the roof of the fourth ventricle c. The posterior surface of pons is limited laterally by superior cerebellar peduncles d. The posterior surface of pons is related to cerebellum (Refer to pages 127 and 128) Ch_11_NA.indd 133 2. Which of the following statements about pons is false? a. The ventral surface of pons is bounded by upper and lower borders b. The ventral surface of pons is convex c. There is a shallow basilar groove in midline, which lodges the basilar plexuses of veins d. Laterally, the ventral surface is continuous with the middle cerebellar peduncle 8/28/2012 1:17:56 PM 134 Illustrated Textbook of Neuroanatomy e. The ventral surface of pons is in relation to basisphenoid (clivus) bone (Refer to page ) 3. All the following statements about the transverse section of pons are true except a. The section is divided into basilar and tegmental parts b. The basilar part contains transversely and vertically running white fibres c. The basilar part does not contain grey matter (nuclei) d. Transverse fibres run laterally to form the middle cerebellar peduncle e. Vertical fibres run downwards to form the pyramid (Refer to pages 129 and 130) 4. Which of the following descending fibres are present in the basilar part of the pons? a. Corticospinal b. Corticonuclear c. Corticopontine d. All of the above (Refer to page 129) 5. Which of the following statements about the corticopontocerebellar pathway is false? a. It arises from frontal, temporal, parietal and occipital lobes b. It terminates on pontine nuclei of the same side c. Fibres arising from pontine nuclei constitute transverse fibres (pontocerebellar) of the pons d. Pontocerebellar fibres enter the cerebellum on the same side through the middle cerebellar peduncle e. Pontine nuclei are a relay station in the corticopontocerebellar pathway (Refer to page 129) 6. The efferents of the vestibular nuclear complex form the following except a. Vestibulocerebellar tract b. Vestibulospinal tract c. Medial longitudinal bundle 7. 8. 9. 10. d. Lateral lemniscus e. Medial lemniscus (Refer to page 130) The trapezoid body is formed by a. Fibres of both dorsal and ventral cochlear nuclei b. Efferents arising from superior olivary nuclei c. Efferents arising from the nucleus of the trapezoid body d. All of the above (Refer to page 131) Which of the following lemnisci are present in the white matter of tegmentum at the upper level of pons? a. Medial lemniscus b. Spinal lemniscus c. Trigeminal lemniscus d. Lateral lemniscus e. All of the above (Refer to page 132) The following statements about pontine haemorrhage are true except a. It occurs due to occlusion or haemorrhage of pontine arteries b. Corticospinal fibres are damaged leading to hemiplegia on the opposite side of the body c. Facial paralysis on the opposite side of the body due to damage to facial nucleus d. There is hyperpyrexia due to damage to sympathetic fibres through which the heat regulating centre exerts its effect e. Pinpoint pupil due to damage to sympathetic fibres (Refer to page 132) Which of the following facts about Raymond’s syndrome are true? a. It results due to occlusion of pontine branches of the basilar artery b. Damage occurs to corticospinal fibres c. Damage to abducent nerve fibres d. All of the above (Refer to page 132) Answers 1. b; Ch_11_NA.indd 134 2. c; 3. c; 4. d; 5. d; 6. e; 7. d; 8. e; 9. c; 10. d. 8/28/2012 1:17:56 PM