Internal structure of pons There are 3 levels of transverse sections : 1-lower level : level of facial colliculus. 2-midlevel : 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 of pons. Superiorly,tegmentum of pons is continuous with tegmentum of midbrain. -its
. Ventral (basilar) part of pons in all levels: It is marked by numerous transverse pontocerebellar fibres which arise from pontine nuclei. Caudal pons at level of facial colliculus Caudal pons Mid-pons at level of trigminal nerve. These Pontocerebellar transverse
fibres cross the midline to pass to contralateral cerebellum, forming middle cerebellar peduncle (brachium pontis),where trigeminal nerve (V) pierce it. 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 mid-pons at level of
trigeminal nerve T.S of rostral pons at level of 4 lemnisci Dorsal( tegmental) part of caudal pons at level of facial colliculus : Trapezoid body consists of acoustic fibres responsible for hearing, T.S of caudal pons 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 of trapezoid 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 rostral pons Dorsal (tegmental)part of caudal pons at level of facial colliculus : Abducent nerve nucleus : T.S through caudal part of pons at the level of facial colliculus.
site : in posterior aspect of caudal pons near floor of 4th ventricle. -It is encircled by fibres 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 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 an association tract, lies close to midline. -found throughout the brain stem,and descend into spinal cord. -It links
vestibular nuclei with motor ocular nuclei in the brain stem, supplying extraocular muscles (oculomotor, trochlear & abducent nuclei) to serve corrdination of Dorsal (tegmental) part of caudal pons at level of facial colliculus : T.S through caudal part of pons at the 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 sensations from the face & scalp. -The axons of the cells of this nucleus cross to opposite side, ascending as trigeminal lemniscus to end in thalamus. 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). T.S of mid-pons a level of trigeminal N. . Trigeminal sensory nucleus : lateral in position. It receives afferent touch sensation 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 T.S of mid-pon at level of trigeminal nerve
Medial longitudinal fasciculus, spinal lemniscus , medial lemniscus & trapezoid body. Dorsal (tegmental) part of Rostral Pons at level of 4 lemnisci : Superior cerebellar peduncle : T.S of rostral pons T.S of mid-pons at level of trigeminal nerve. T.S of rostral pons -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. Dorsal (tegmental) part of Rostral Pons at level of 4 lemnisci : Types of fibres in the S.C.P. : T.S of rostral pons (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. Dorsal (tegmental) part of Rostral Pons at level of 4 lemnisci : Lateral lemniscus : the most lateral lemniscus. it is a band of ascending fibres carrying hearing sensation from both ears ( mainly from opposite side).
Ends in auditory area in temporal lobe. Spinal lemniscus : just medial to lateral lemniscus. it is a band of ascending Fs. Carrying pain, tempreture & light touch from opposite side of body. Ends in P.L.V.N.of thalamus. 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. ends in P.M.V.N.of thalamus.
Medial lemniscus : -(the most medial lemniscus) is marking 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. Internal structure of Midbrain The midbrain is divided into dorsal & ventral portions 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 ventral part 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 fibres 2-Substantia nigra :a thick lamina of grey matter formed of deeply pigmented nerve cells lying behind crus cerebri. It is an Extrapyramidal motor centre. 3-Tegmentum : the post. Caudal midbrain at level of inferior colliculus is a mass Inferior colliculus ofThe 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. T.S.of caudal midbrain at level of inferior colliculus. 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). Auditory pathway & inferior colliculus of midbrain (reflex center of hearing) : 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 & neck movements. Medial longitudinal fasiculus in midbrain : 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 the central 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. Caudal midbrain at level of Inferior colliculus Substantia Nigra : It is a large T.S.of caudal midbrain at level of inferior colliculus. motor nucleus, lies at the most ventral part of midbrain tegmentum. -It contains subdivision part, the pars compacta, which consists
of pigmented, melanin-containing neurones that synthesise dopamine as their transmitter. -It projects to caudate nucleus + putamen of basal ganglia in the forebrain. -It has extra-pyramidal motor function, concerned with movements. -lesion of pars compacta leads to parkinsons disease. It is due to absence of dopamine into basal ganglia, this is manifested by a mask face, resting tremors, rigidity of muscles and a shuffling Caudal midbrain at level of Inferior colliculus Crus Cerebri : lies ventral to substantia 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 fibres which traverse pons, down to medullary pyramid and spinal cord. T.S.of caudal midbrain at level of inferior colliculus. -on either side of corticobulbar & 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-pontocerebellar pathway ) to involve in coordination of movement.
Rostral midbrain at the level of Superior colliculus : Superior Colliculus: T.S. of rostral midbrain at level of superior colliculus. -lies in upper part of tectum of midbrain. -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 Rostral midbrain at the level of Peri-aqueductal (central) grey. Superior Colliculus : Oculomotor nucleus : lies ventral to peri-aqueductal grey at level of superior colliculus of midbrain. -efferent Fs. emerge from the medial surface of crus cerebri. as
oculomotor nerve to supply extraocular ms. of eye (except S.O + L.R.). T.S. of rostral midbrain at level of superior colliculus. 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 rubrospinal tract, which cross in ventral tegmental decussation. and to inferior olivary nucleus of medulla, via central tegmental tract as rubroolivary Fs. Reticular formation
It is a complex matrix of neurones that extends throughout the length of brain stem, 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 activating system, for activation of thalamus, hypothalamus & cerebral cortex
and heightened arousal. Reticular formation Raphe nuclei 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 Locus coeruleus
neurones that lies in brain stem tegmentum of caudal midbrain & 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, 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). 3contalateral hemisensory loss.
4-ipsilateral incoordination. 5-it can affect eye movements through demyelination of medial longitudinal fasciculus, producing internuclear ophthalmoplegia which interferes with conjugate ocular deviation (abducting eye moves normally, but adducting eye fails to follow), adduction is preserved on convergence. A bilateral lesion : destroys the vital centers for respiration & circulation, leading to
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