Internal structure of pons

1. Internal structure of pons • There are 3 levels of transversesections :1-lower level : level of facial colliculus. 2-mid-level : level of trigeminal nerve. 3-upper level : level of 4 lemnisci. • The pons is divided into 2 main divisions by the transversely running fibres of trapezoid body :1-Basilar part (Basis pontis) : it is the ventral part of pons, -its contents are constant in all levels. 2-Tegmentum : it is the dorsal part ofpons. Superiorly,tegmentum of pons is continuous with tegmentum of midbrain. -its contents vary in the 3 levels of pons.

2. Ventral (basilar) part of pons in all levels: • It is marked by numerous transverse pontocerebellar fibres which arise from pontine nuclei. • These Pontocerebellar transverse fibres cross the midline to pass to contralateral cerebellum, forming middle cerebellarpeduncle (brachium pontis),where trigeminal nerve (V) pierce it. . Caudal pons at level of facial colliculus Caudal pons Mid-pons at level of trigminal nerve.

3. Ventral (Basilar)part of pons in all levels: • Corticospinal & corticobulbar fibres (pyramidal tract) appear as small,separate bundles running longitudinally between fascicles of transverse pontine fibres. T.S of caudal pons at level of facial colliculus T.S of rostral pons at level of 4 lemnisci T.S of mid-pons at level of trigeminal nerve

4. Dorsal( tegmental) part of caudal pons at level of facial colliculus : • Trapezoid body consists of acoustic fibresresponsible for hearing, arising from cochlear nuclei (dorsal & ventral cochlear nuclei lying dorsal & ventral to inferior crebellar peduncle in the lower part of pons). -These acoustic fibres crossing opposite side of pons forming decussation oftrapezoid body, then the axons ascend into rostral pons & midbrain as lateral lemniscus and terminate in inferior colliculus. -it lies in the anterior part of tegmentum. T.S of caudal pons T.S of caudal pons T.S of rostral pons

5. Dorsal (tegmental)part of caudal pons at level of facial colliculus : • Abducent nerve nucleus :site : in posterior aspect of caudal pons near floor of 4th ventricle. -It is encircled byfibres of facial N., forming elevation in the the floor of 4th ventricle called facial colliculus.-Its efferent fibres : pass downwards traversing medial lemniscus & pyramidal Tract bundles to emerge anteriorly at junction between pons & pyramid of medulla, supplying lateral rectus muscle T.S through caudal part of pons at the level of facial colliculus.

6. Dorsal (tegmental)part of caudal pons at level of facial colliculus : • Facial motor nucleus :site : in posterior part of caudal pons.Its efferent fibres encircle abducent nucleus,then pass anterolaterally to emerge at the junction between pons & olive of medulla, (supplying muscles of facial expression). • Medial longitudinal fasciculus or bundle : -it is anassociation tract, lies close to midline. -found throughout the brain stem,and descend into spinal cord. -It links vestibular nuclei with motor ocularnuclei in the brain stem, supplying extraocular muscles (oculomotor, trochlear & abducent nuclei) to serve corrdination of head and eye movements.

7. Dorsal (tegmental) part of caudal pons at level of facial colliculus : • Vestibular nuclei : they are 4 nuclei lie near to vestibular area of floor of 4th ventricle. -they receive afferent fibres for unconscious deep sensation (equilibrium), from the vestibular nerve. -they send efferent fibres as :vestibulo-ocular fibres through medial longitudinal fasciculus. • Spinal tract & nucleus of trigeminal nerve: lie on the anteromedial aspect of inferior cerebellar peduncle. -carrying pain &temperature sensationsfrom the face & scalp. -The axons of the cells of this nucleus cross to opposite side, ascending as trigeminallemniscus to end in thalamus. T.S through caudal part of pons at the level of facial colliculus.

8. Dorsal (tegmental)part of mid-pons at level of trigeminal nerve : • Trigeminal motor nucleus :medial in position. its axons form the motor root of trigeminal N. which passes along mandibular N. (supplying muscles of mustication). • Trigeminal sensory nucleus :lateral in position. It receives afferent touchsensation from face & scalp. -It sends efferent fibres, cross to opposite side to join axons of spinal nucleus of trigeminal, forming trigeminal lemniscus in rostral pons – midbrain - thalamus. • Superior cerebellar peduncle : lies posterolateral to motor nucleus of trigeminal • Medial longitudinal fasciculus, spinal lemniscus , medial lemniscus & trapezoid body. T.S of mid-pons a level of trigeminal N. . T.S of mid-pon at level of trigeminal nerve

9. Dorsal (tegmental) part of Rostral Pons at level of 4 lemnisci : • Superior cerebellar peduncle :-lies in the rostral part of pons, forming lateral walls of 4th ventricle, right & left.-They are connected together by sup.medullary velum which forms roof of 4th ventricle.-At the level of trigeminal nerve, it lies posterolateral to motor nucleus of trigeminal nerve. T.S of rostral pons T.S of mid-pons at level of trigeminal nerve. T.S of rostral pons

10. Dorsal (tegmental) part of Rostral Pons at level of 4 lemnisci : • Types of fibres in the S.C.P. :(A) Afferent fibres :1-ventral spino-cerebellar tract : it carries proprioceptive impulses from the limbs to cerebellum. 2-tecto-cerebellar tract : it carries auditory & visual impulses from tectum of midbrain to cerebellum. (B)Efferent fibres :1-Dendato-rubral tract : it is concerned with coordination of movement. it ends in red nucleus of midbrain – thalamus- motor & premotor area of cortex.2-Dentato-thalamic tract : to carry proprioceptive deep sensation from dentate nucleus of cerebellum to end in V.L.N. of thalamus. T.S of rostral pons

11. Dorsal (tegmental) part of Rostral Pons at level of 4 lemnisci : • Lateral lemniscus : themost lateral lemniscus. it is a band of ascending fibres carryinghearing sensation from both ears ( mainly from opposite side). Ends inauditory area in temporal lobe. • Spinal lemniscus :justmedial to lateral lemniscus. it is a band of ascending Fs. Carryingpain, tempreture & light touch fromopposite side of body.Ends inP.L.V.N.of thalamus.

12. Dorsal (tegmental) part of Rostral Pons at level of 4 lemnisci : • Trigeminal lemniscus: just medial to spinal lemniscus.it is a band of ascending Fs.carrying superficial sensation, from opposite side of face & scalp.endsinP.M.V.N.of thalamus. • Medial lemniscus : -(the most medial lemniscus) ismarking the boundary between ventral & tegmental portions of pons. - it is a band of ascending fibres carrying proprioceptive sensation from opposite side of body.ends in P.L.V.N. of thalamus.

14. Internal structure of Midbrain • The midbrain is divided into dorsal & ventralportions at the level of cerebral aqueduct.(A)Tectum : the smaller dorsal part behind aqueduct. It is composed of 4 rounded swellings (colliculi) : 1-2 superior colliculi : reflex centers of vision.2-2 inferior colliculi : reflex centers of hearing.(B)2 Cerebral peduncles : the larger ventralpart in front of aqueduct. It consists of 3 parts, arranged from before backwards as follows : 1-Crus cerebri (Basis pedunculi) : the most anterior part which consists entirely of pyramidal &cortico-pontine fibres2-Substantia nigra :a thick lamina of grey matter formed of deeply pigmented nerve cells lying behind crus cerebri. It is anExtrapyramidal motor centre.3-Tegmentum : the post. part of cerebral peduncle. It contains ascending tract,certain nuclei, decussations & reticular formation of midbrain.

15. Caudal midbrain at level of Inferior colliculus • The inferior colliculus is a mass of grey matter which receives ascending auditory pathway ,which run in lateral lemniscus to end in inferior colliculus. -Its Efferent Fs. end in medial geniculate nucleus of thalamus, which projects to auditory cortex of temporal lobe. So,it is a centre of hearing reflex. • The cerebral aqueduct runs ventral to colliculi, and surrounding by area of grey matter, the peri-aqueductal (or central grey ). • Trochlear nucleus : lies ventral to peri-aqueductal grey, its efferent Fs. cross to opposite side to emerge from back of midbrain, to supply extraocular ms.(sup.oblique). T.S.of caudal midbrain at level of inferior colliculus.

16. Auditory pathway & inferior colliculus of midbrain (reflex center of hearing) :

17. Caudal midbrain at level of Inferior colliculus • Medial longitudinal fasciculus : is a well defined bundle of fibres lies on each side of median plane in midbrain tegmentum. -It extends throughout the brain stem, and descends into spinal cord. -It lies ventral and close to oculomotor, trochlear & abducent nuclei. -it receives fibres from vestibular nuclei.-it sends efferents to ocular motor nuclei-Its function is to control of gaze by coordination of eye,and head & neckmovements.

18. Medial longitudinal fasiculus in midbrain :

19. Caudal midbrain at level of Inferior colliculus • Decussation of superior cerebellar peduncles(brachium conjunctivum) : the fibres of each peduncle cross to opposite side, forming decussation in thecentral part of tegmentum. • Medial lemniscus : it is a band of ascending Fs. carrying proprioceptive &fine touch sensation from opposite side of body. -It is the upward continuation of gracile &cuneate tracts of opposite side. -It lies in tegmentum, posterior to substantia nigra. .

20. Caudal midbrain at level of Inferior colliculus • Substantia Nigra : It is a large motor nucleus, lies at the most ventral part of midbrain tegmentum.-It contains subdivision part, the parscompacta, which consists of pigmented, melanin-containingneurones that synthesise dopamine astheir transmitter.-It projects to caudate nucleus + putamen of basal ganglia in the forebrain. -It has extra-pyramidal motor function,concerned with movements. -lesionof pars compacta leads to parkinson’s disease.It is due toabsence of dopamine into basal ganglia, this is manifested by a mask face, resting tremors, rigidity of muscles and a shuffling gait. T.S.of caudal midbrain at level of inferior colliculus.

21. Caudal midbrain at level of Inferior colliculus • Crus Cerebri :lies ventral tosubstantia nigra. It consists entirely of descending cortical efferent Fs.-50% of crus consists of pyramidal tract consists of cortico-bulbar Fibres (end in motor cranial nerve nuclei of brain stem) +cortico-spinal fibreswhich traverse pons, down to medullary pyramid and spinal cord. -on either side ofcorticobulbar & corticospinal fibres, crus cerebri contains cortico-pontine,temporo-pontine + fronto-pontine fibres-These Fs. arise from cerebral cortex and ends in pontine nuclei of ventral pons to pass via M.C.P into cerebellum,(cortico-ponto-cerebellar pathway ) to involve in coordination of movement. T.S.of caudal midbrain at level of inferior colliculus.

22. Rostral midbrain at the level of SuperiorColliculus: • Superior colliculus : -lies in upper part of tectum ofmidbrain. -it is a centre of visual reflexes. -Its main afferent Fs. are : Cortico-tectal Fs. arise from -visual cortex of occipital lobe. -frontal eye field of frontal lobe.-function : control movements of eyes + accomodation reflex. • Pretectal nucleus : It lies close to superio- lateral part of superior colliculus. -The visual Fs. running in optic tract just rostral to superior colliculus end in pretectal N. -it has connections with parasymp. nucleus of oculomotor N.(Edinger-Westphal nucleus) to control smooth ms. of eye (sphincter pupillae) and to mediate pupillary light reflex. T.S. of rostral midbrain at level of superior colliculus.

23. Rostral midbrain at the level of SuperiorColliculus: • Peri-aqueductal (central) grey. • Oculomotor nucleus : lies ventral to peri-aqueductal grey at level of superiorcolliculus of midbrain. -efferent Fs. emerge from the medialsurface of crus cerebri. as oculomotor nerve to supply extraocular ms. of eye (except S.O + L.R.). • Red nucleus : -it is a large mass of grey matter lies in tegmentum of rostral midbrain. -it is involved in motor control. -It receives afferents from motor cortex of frontal lobe (cortico-rubral Fs.). -it sends efferents to spinal cord as rubro-spinal tract, which cross in ventral tegmental decussation. and to inferior olivary nucleusof medulla, via central tegmental tract as rubroolivary Fs. T.S. of rostral midbrain at level of superior colliculus.

24. Reticular formation • It is a complex matrix of neurones that extends throughout the length of brainstem, made up of deeply placed nerve cells & fibres. • It has widespread afferent & efferent connections with other parts of CNS. • It has important functions for vital centers as respiratory & cardiovascular centres that are located in medullary + caudal pontine reticular formation. • It has descending reticulospinal tracts arise from medullary + pontine reticular formation ,that influence muscle tone &posture. • It has ascending fibres, the reticular activatingsystem, for activation of thalamus, hypothalamus & cerebral cortex and heightened arousal.

25. Reticular formation • Raphe nuclei : are a group of midline nuclei that extend throughout the length of brain stem. -they are serotonergic nuclei (their transmitter is serotonine). -Their ascending fibres to forebrain are involved in neural mechanisms regulating sleep. -Descending fibres to the spinal cord are involved in modulation of nociceptive mechanisms. • Locus coeruleus : is a group of pigmented neurones that lies in brain stem tegmentum of caudalmidbrain & rostral pons. -it is noradrenergic cell group. -it has ascending fibres to cerebellum, thalamus,hypothalamus, limbic system and cerebral cortex. -its descending fibres project to brain stem & spinal cord. -involved in neural mechanisms regulating sleep,particularly REM (rapid eye movement) sleep. Raphe nuclei Locus coeruleus

26. Brain stem lesions : • A unilateral brain stem lesion : caused by stroke,tumour or multiple sclerosis causes :1-epsilateral cranial nerve dysfunction + contralateral spastic hemiparesis. 2-hyperreflexia & an extensor plantar response (upper motor neurone lesion). 3-contalateral hemisensory loss. 4-ipsilateral incoordination. 5-it can affect eye movements through demyelination of medial longitudinalfasciculus, producing internuclear ophthalmoplegia which interferes withconjugate ocular deviation (abducting eye moves normally, but adducting eye fails to follow), adduction is preserved on convergence. • A bilateral lesion : destroys the ‘vitalcenters’ for respiration & circulation, leading to coma & death.