PONS MIDBRAIN Trigeminal (V) chewing Abducens (VI) movement of eyeballs

Transcription

1 Anatomy of brain stem and associated lesions: PONS and MIDBRAIN Brain stem Underneath the limbic system is the brain stem. This structure is responsible for basic vital life functions such as breathing, heartbeat, and blood pressure. The brain stem is made of Midbrain Pons Medulla. Pons Forepart of the hind-brain Situated in front of the cerebellum Behind and below, the pons is continuous with the medulla oblongata, but is separated from it in front by a furrow in which the abducent, facial, and acoustic nerves appear. Associated with cranial nerves: Trigeminal (V) chewing Abducens (VI) movement of eyeballs Facial (VII)- facial expression; secretion of saliva and tears Vestibulcochlear (VIII) balance, equilibrium, hearing

2 Ventral or Anterior surface of pons Basilar sulcus Midline sulcus occupied by basilar artery Transverse running fibres on the surface: pontocerebellar fibres which continues as Middle cerebellar peduncle. Trigerminal nerve emerge at lateral part of pons Dorsal or Posterior surface of Pons Hidden by cerebellum Bounded laterally by superior cerebellar peduncle Median sulcus in the midline Facial colliculus Paramedian elevation raised by underlying abducent nerve covered by winding fibres of facial nerve. Trochlear nerve Internal structure of pons Basilar or ventral portion Contains both longitudinal and transverse fibers intermixed with pontine nuclei Dorsal tegmental portion Continuation of the reticular formation of the medulla oblongata Structures present at the level of facial nucleus. CN VI nucleus Abducens nerve CN VII nucleus Facial nerve CN VIII Vestibular Nuclei Pure sensory location lateral Pontine nuclei Project to cerebellum Continuation of arcuate nuclei (over pyramids)

3 Structures present at the level of facial nucleus. CN VI nucleus Abducens nerve CN VII nucleus Facial nerve CN VIII Vestibular Nuclei Pure sensory location lateral Level of facial nucleus (CN VII) CN VIII Vestibular Nuclei Balance 4 th ventricle Connection of pons to cerebellum Restiform body (inf. cerebellar peduncle) Middle cerebellar peduncle Medial lemniscus Ascending 2 nd order sensory neurons Descending upper motor neurons CN VI nucleus = Abducens nerve somatic efferent nerve that controls the movement of a single muscle, the lateral rectus muscle of the eye. Abduction of eye CN VII nucleus Facial nerve controls the muscles of facial expression, and functions in the conveyance of taste sensations from the anterior two-thirds of the tongue and oral cavity

4 Internal structure of pons Structures present at the level of middle cerebellar peduncle. Medial lemniscus Lateral lemniscus Trapezoid body Trigerminal nucleus (sensory and motor) Pontine nuclei Level of middle cerebellar peduncle Middle cerebellar peduncle At the level of mid pons Lateral lemniscus 4 th ventricle Medial lemniscus fibers from dorsal column Trapezoid body transverse fibers in pontine tegmentum Trigerminal tract The pontine nuclei are a part of the pons which store the memory of intention during motor activity Corticospinal tract, corticobulbar tract, corticopontine fibers

5 Cranial Nerve of the Mid Pons Principal trigerminal Sensory nucleus CN V Motor trigeminal nucleus 4 th ventricle Trigerminal fascicles Trigerminal nerve Internal structure of pons Structures present at the level of upper pons MLF Superior cerebellar peduncle Locus ceruleus Parabrachial nucleus Pediculopontine nucleus Level of locus ceruleus

6 Periaqueductal gray matter Superior cerebellar peduncle Upper Pons Locus ceruleus Transverse pontocerebellar fibers 4 th ventricle MLF Decussation Descending upper motor neurons Midbrain Landmarks Cerebral peduncles Optic chiasm Interpeduncular fossa (Superior colliculi) (Inferior colliculi) (Superior cerebellar peduncle) Cranial Nerves III, IV Midbrain is divided at the level of cerebral aqueduct into Dorsal portion called tectum which largely consists of inferior and superior colliculi Ventral portion, known as tegmentum

7 External Structure of Midbrain Optic chiasm 1 Interpeduncular fossa 2 Oculomotor nerve (CN III) 3 Trochlear nerve (CN IV) 4 Pons 5 Cerebral peduncles (crus cerebri) 6 Cranial Nerves of the Midbrain Anterior exit CN III (1) CN VI (5) Posterior exit CN IV (2) MLF - Medial longitudinal fasciculus (7) Vestibular nuclei (6) Pons (3) Internal divisions of midbrain Tegmentum is bounded ventrally by the massive fibre system of the crus cerebri The term cerebral peduncle is sometimes used as a synonym for crus cerebri Cerebral peduncle refers to the whole midbrain on either side excluding the tectum Ventral to the colliculi the cerebral aqueduct runs the length of the midbrain Surrounding the aqueduct is a pear shaped arrangement of grey matter called periaqueductal grey

8 Substantia nigra These neurones project to the caudate nucleus and putamen of the basal ganglia in the forebrain Degeneration of the pars compacta of the substantia nigra is associated with Parkinson s disease Other non pigmented subdivision of the substantia nigra is called the pars reticulata Pars reticulata is considered to be a functional homologue of the medial segment of the globus pallidus which is also part of the basal ganglia Internal Structure of Midbrain Cross section at the level of inferior colliculus CN IV Trochlear nerve MLF Dorsal raphe nucleus Substantia nigra Mesencephalic nucleus of V Level of inferior colliculus Lower Midbrain Mesencephalic nucleus of V analogous to dorsal root ganglion but within CNS Substantia nigra Crus cerebri (cerebral peduncle) Cerebral aqueduct Inferior colliculus CN IV Trochlear nerve MLF Basis peduncularis

9 Caudal part of midbrain In the caudal part of the midbrain the inferior colliculus constitutes part of the ascending acoustic projection Ascending auditory fibres run in the lateral lemniscus which terminates in the inferior colliculus Efferent fibres from the colliculus terminate in the medial geniculate nucleus of the thalamus This nucleus projects to the auditory cortex of the temporal lobe Internal Structure of Midbrain Cross section at the level of superior colliculus: Medial geniculate body Superior colliculus Crus cerebri (cerebral peduncle) Red nucleus Controls arm movement Substantia nigra Level of superior colliculus

10 Upper midbrain Superior colliculus Medial geniculate body hearing Substantia nigra Red nucleus relay from cortex and cerebellum to spinal cord, inferior olive, reticular formation, cerebellum Controls arm movement Crus cerebri (cerebral peduncle) Cranial Nerves of Upper Midbrain CN III Oculomotor nucleus Edinger Westfal nucleus Parasympathetic to ciliary ganglion Pupillary sphincter ciliary muscles MLF

11 Rostral part of midbrain The superior colliculus of the rostral area of the midbrain is part of the visual system Its main afferents are corticotectal fibres originating from the visual cortex of the occipital lobe and from the frontal eye field of the frontal lobe These inputs are concerned with controlling movements of the eyes Pontine hemorrhage Pons supplied by basilar artery, anterior, inferior and superior cerebellar arteries. If hemorrhage is unilateral Facial paralysis on the side of lesion (facial nerve nucleus) Paralysis of limbs on opposite side (corticospinal fibres) Paralysis of conjugate ocular deviation (abducent nerve nucleus) If bilateral Pinpoint pupils (ocular sympathetic fibres) Bilateral paralysis of face and limbs poikilothermia Tumors of pons Astrocytoma of pons, most common tumor of brainstem Blockage of cerebral aqueduct Cerebral aqueduct is one of the narrowest part of ventricular system. In congenital hydrocephalus, or tumors of midbrain, aqueduct may be blocked which blocks the flow of CSF.