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Pontine tegmentum
- The dorsal part of the pons that contains:
- Motor cranial nerve nuclei (VI, VII, and motor nuclei of V)
- Sensory cranial nerve nuclei (VIII, main sensory nucleus of V, spinal nucleus of V)
- Salivatory nuclei (GVE nuclei)
- Solitary nucleus and tract
- Spinal trigeminal nucleus and tract
- Medial lemniscus & spinothalamic tracts
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Basis Pontis
- Ventral part of the pons that contains:Pontine nuclei
- Corticofugal tracts (include corticobulbar, corticoreticulobulbar, corticospinal, and corticopontine fibers)
- Pontocerebellar fibers
- Reticular formation
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Abducens and facial nuclei location and function
- Location: Pontine tegmentum
- Facial function: Innervates muscles of facial expression, stylohyoid, posterior digastric, and stapedius; this is a branchiomotor structure (SVE) and has migrated away from the floor of the IVth ventricle (see picture)
- Abducens function: Innervates the lateral rectus muscle; remains close to the midline beneath the floor of the IVth ventricle
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Motor & spinal nuclei of CN V location and function
- Location: Caudal pontine tegmentum
- Motor function: Innervate muscles of mastication MATT (mylohyoid, anterior digastric, tensor tympani, and tensor veli palatini); these are branchiomotor neurons
- Spinal function: Crude touch from face and cornea
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Main sensory nucleus of CN V location and function
- Location: Mid pontine tegmentum; cross to form ventral trigeminothalamic tract (VTT) or do not cross to form dorsal trigeminothalamic tract (DTT); ascends in trigeminal lemniscus
- Function: 2-point discriminatory touch and vibration from the face (NOT proprioception)
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Trigeminal lemniscus location and function
- Location: Pontine tegmentum
- Function: Carries ventral and dorsal trigeminothalamic tracts to the ventral posterior medial nucleus of the thalamus; relays 2-point discriminative touch and vibration from the face
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Medial lemniscus location and function
- Location: Pontine tegmentum
- Function: Carries conscious proprioception, vibration, and discriminative touch from the contralateral body
- Becomes associated with spinothalamic tract by mid pons because the inferior portion of the medial lemniscus is pushed laterally
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Spinothalamic tract location and function
- Location: Pontine tegmentum
- Function: Carries pain, temp, and crude touch from contralateral body
- Becomes associated with medial lemniscus by mid pons because the inferior portion of the medial lemniscus is pushed laterally
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CN VIII related structures
- Auditory structures: Superior olive, lateral lemniscus, trapezoid body
- Major connections with vestibular nerve, spinal cord, cerebellum, and nuclei of extraocular muscles: Lateral and superior vestibular nuclei
- Variable function: Medial longitudinal fasciculus
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Medial longitudinal fasciculus function based upon location
- Medulla: Contains the medial vestibulospinal tract
- Pons: Tracts that connect the vestibular nuclei with the abduscens, trochlear, and oculomotor nuclei to coordinate eye movements with the head; and connects the abducens and oculomotor nuclei to enable lateral conjugate gaze
- Remember, this is associated with CN VIII and is found in the pontine tegmentum!
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Pontine nuclei location and function
- Location: Make the bulk of the grey matter of basis pontis
- Function: Receive information from virtually every area of the ipsilateral cerebral cortex via corticopontine fibers and connect the cerebrum with the cerebellum
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Corticopontine tract location and function
- The largest descending system in the brain (14 million axons) and forms much of white matter of rostral pons
- Location: Basis pontis
- Function: Carry information from the ipsilateral cerebral cortex to the pontine nuclei
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Pontocerebellar fibers location and function
- Location: Basis pontis
- Function: Carry information from the pontine nuclei (via axons of those cells) to the opposite cerebellum via the middle cerebellar peduncle
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Corticofugal fibers location and tracts included
- Location: Basis pontis
- Tracts: Corticobulbar, corticoreticulobulbar, corticospinal, and corticopontine fibers
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Corticonuclear fibers location and function
- Location: Basis pontis
- Function: Carry information from the cerebral cortex to nuclei of CN V, VI, and VII in the tegmentum of the upper pons
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Corneal blink reflex
- Mediated by fibers in the pons, this reflex uses Aδ (mechanoreceptor) and C (nociceptor) fibers to carry information to the facial nucleus (motor to orbicularis oculi) and rostral spinal trigeminal nuclei via long ciliary branches of V (opthalmic nerve)
- Results in a consensual reflex to unilateral stimuli
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Vestibulo-ocular reflex (VOR)
Involves vestibular nuclei, medial longitudinal fasciculus, and CN VI nuclei in the pons
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Pontine micturition centers
- Barrington's Nucleus
- Found in the dorsolateral upper pons, it is connected with pre-ganglionic parasympathetics (excitatory) and Onuf's nucleus (inhibitory) in the sacral cord
- Activated to cause urination when the bladder is full
- Pontine lesions result in urine retention
- Pontine Storage Center
- Inhibits detrusor but disinhibits Onuf's allowing for urinary retention
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Pneumotaxic center location and function
- Found in the upper pons near the cerebellar peduncle in the pontine tegmentum
- Ends inspiration by inhibiting the dorsal respiratory group; also important in establishing respiration rate
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Apneustic center
- Located in lower pontine tegmentum
- Drives inspiration; inhibited by the pneumotaxic center and by input from lung stretch receptors; possibly a second center of breathing
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Blood supply of pons
- All supply is from the basilar artery
- Paramedian vessels: Supply medial parts of basis pontis and pontine tegmentum
- Short circumferentials: Supply ventrolateral region of basis pontis
- Long circumferentials: Supply lateral and dorsal pontine tegmentum; includes AICA and superior cerebellar arteries
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Differences between lesions to abducens nerve versus abducens nucleus versus MLF
- Nerve: Loss of abduction of the eye on the lesioned side (lateral rectus palsy)
- Nucleus: In addition to above, loss of conjugate gaze to the lesioned side (conjugate gaze palsy)
- This loss of conjugate gaze occurs because the neurons of the abducens nucleus send their axons to the contralateral III nucleus, specifically to neurons exciting the contralateral medial rectus; this no longer can happen, resulting in conjugate gaze palsy
- MLF: Internuclear opthalmoplegia (INO) because the axons running to the III nucleus are damaged; failure of adduction of eye on lesioned side on gaze to the opposite side and nystagmus of the eye on unaffected side on abduction
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Differences between lesions to facial nerve versus facial nucleus
- Nerve: Facial hemiplegia
- Nucleus: In addition to facial hemiplegia, bilateral hyperacusis will be present because of loss of innervation of stapedius muscle (which receives bilateral input)
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Differences between lesions to UMN versus LMN of face
- Central VII lesion: UMNs (above facial nucleus) go to both nuclei for upper face and same side for lower face. If we lesioned these, we would only see a deficit on the opposite lower face since the upper face would still receive input from the opposite cortex
- Peripheral VII lesion: Lesions of VII nucleus or LMNs will affect the same side of the face--both upper and lower
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Psedo bulbar palsy
- Damage to corticonuclear fibers, resulting in UMN defects of CN V, VII, IX, XII
- Speech problems (dysarthria) and dysphagia
- Emotional facial reactions intact, resulting in inappropriate outbursts of laughter and crying
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Eight-and-a-half syndrome
One and a half syndromes (conjugate gaze palsy and part of internuclear opthalmoplegia) plus CN VII hemiplegia
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Locked-in syndrome
- Large lesions of the basal pons, damaging the corticospinal and corticobulbar pathways bilaterally, thus interfering with facial expression, speech, and movement
- Some eye movements spared (inputs to CN III); medulla intact so breathing/cardiac is okay; somatosensory pathways and reticular formation (no coma) are usually spared
- Results in patient being awake and aware of his surroundings
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Generalizations of pontine lesions
- Tegmental: Same-side CN signs and contralateral sensory loss over body; rostral lesions affect MLF, medial lemniscus, superior cerebellar peduncle, and spinal lemniscus; caudal lesions cause VII palsy and conjugate gaze palsy
- Basal: Corticospinal, corticobulbar, and VI and VII nerve fibers; caudal have same side facial LMN hemiplesia and contralateral UMN hemiplegia
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Regions of the midbrain
- Tectum: Superior and inferior colliculi
- Tegmentum: CN nuclei, major ascending and descending tracts, periaqueductal grey, and reticular formation
- Substantia nigra
- Basis pedunculi: Corticofugal projections (corticopontine, corticoreticular/corticoreticulobulbar, corticobulbar, and corticospinal)
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General contents of midbrain
- CN III and IV nuclei (GSE) and Edinger-Westphal nucleus (GVE)
- Mesencephalic nucleus of CN V (GSA; carries proprioception of face)
- Red nucleus and substantia nigra (large motor-related areas) and superior colliculus (small motor-related area)
- Superior cerebellar peduncle tract (???)
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Rostral versus caudal midbrain contents
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Inferior colliculus location and function
- Location: Dorsal caudal midbrain
- Function: Transmits auditory information from lower relay nuclei via the lateral lemniscus to the medial geniculate body of the thalamus
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Trochlear nucleus location, function, and lesions
- Location: Caudal dorsal midbrain
- Function: Innervate superior oblique (GSE)
- Superior oblique depresses the eye when adducted and intorts it when abducted
- Lesion (of nucleus or nerve): Leads to torsional diplopia; eye is extorted, but patient can correct by tilting head; gives this lesion the name pathetic
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Superior cerebellar peduncle
- Output bundle of the cerebellum
- Fibers arise from deep cerebellar nuclei, decussate, follow the dentato-rubro-thalamic tract, and end in the VL thalamic nucleus or in the red nucleus of midbrain
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Mesencephalic nucleus and tract of CN V location and function
- Location: Rostral midbrain
- Function: Conscious & unconscious proprioception of jaw and relay of periodontal reflex; sends axons near the principal nucleus of V (proprioception) and to the motor nucleus of V (reflexes)
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Location of trigeminal ganglion, motor nuclei of CN V, pontine trigeminal nucleus (principal?), spinal trigeminal nucleus, and mesencephalic nucleus of CN V
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Periaqueductal grey location and function
- Location: Surrounding the cerebral aqueduct in the midbrain
- Function: Mediates stereotypic adaptive behaviors and reproduction while integrating analgesic inputs from the spinothalamic tract
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Mesopontine cholinergic neurons location and function
- Location: Caudal midbrain tegmentum
- Function: The ascending reticular activating system (ARAS) (as part of the mesencephalic reticular formation) innervates the thalamus by these neurons; these neurons are important for forced attention and consciousness
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Midbrain dorsal raphe location and function
- Location: Midline of upper medulla, pons, and midbrain in the raphe
- Function: Contains a large population of serotonin-containing neurons; part of mesencephalic reticular formation, which is important in sleep/wake, attention, etc.
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Mesencephalic locomotor center location and function
- Location: Caudal midbrain
- Function: Serves as output of basal ganglia;
- part of mesencephalic reticular formation; important for hard-wired locomotion
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Summary of mesencephalic reticular formation
- Ascending reticular activating system: Contains mesopontine cholinergic cells that innervate the thalamus; important for forced attention and consciousness
- Midline midbrain: Serotonin-containing neurons of the midbrain dorsal raphe
- Mesencephalic locomotor center: Output of basal ganglia; important for hard-wired locomotion
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Hydraencephaly
- Most of the telencephalon and diencephalon is replaced by CSF, leaving only the hypothalamus, midbrain, pons, and medulla functional
- Patients survive but lack cognitive abilities; they can survive, grow, and exhibit emotional responses
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Summary of cell groups in midbrain specified by specific neurotransmitters
- Mesopontine cholinergic cells: Acetylcholine; forced attention and consciousness
- Midbrain dorsal raphe cells: Serotonin; sleep/wake, attention, etc
- Substantia nigra cells: Dopamine in the pars compacta; GABA in the pars reticulata
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Substantia nigra location and function
- Location: Rostral midbrain (black stuff); pars compacta located next to the tegmentum (uses neuromelanin/polymer of DOPA and dopamine); pars reticulata located next to crus cerebri
- Function: Closely associated with basal ganglia; degeneration of pars compacta causes Parkinson's disease
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Basis pedunculi somatotopy
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Oculomotor nucleus location and functional divisons
- Location: Rostral midbrain
- GSE component: Innervates levator palpebrae superioris; superior, inferior, and medial recti (receives directions for conjugate gaze from VI nucleus); and inferior oblique
- GVE component: Aka Edinger-Westphal nucleus; provides preganglionic parasympathetic innervation to ciliary ganglion, which sends postganglionic fibers to constrictor pupillae and ciliary muscles (lens accomodators)
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Lesion of III nucleus or nerve
- Both cause "down and out" syndrome aka third nerve palsy (but most lesions occur to nerve usually in the midbrain tegmentum)
- Ipsilateral strabismus (squint) from loss of medial rectus and depression from loss of superior rectus and inferior oblique
- Ipsilateral ptosis from loss of levator palpebrae superioris
- Ipsilateral mydriasis (dilation) from loss of constrictor pupillae
- Loss of direct and consensual pupillary light reflexes in ipsilateral eye (since CN III carries constriction info from Edinger-Westphal nucleus)
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Vertical gaze center location, function, and lesion
- Location: Rostral dorsal midbrain
- Function: Coordinates SR, IR, SO, and IO for vertical gaze
- Lesion: Parinaud's syndrome result in paralysis of upward gaze
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Pupillary light reflex
- Afferent arc goes through optic nerve
- Neurons come off and go into midbrain tectum to synapse in pretectal area
- Pretectal neurons send axons to both Edinger-Westphal nuclei (they go to contralateral via posterior commissure)
- Efferent arc goes through CN III with a synapse in the ciliary ganglion
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Superior colliculus function, somatotopy, and output tract
- Function: Mediates movements of the eyes and head in response to visual, auditory, and somatic stimuli
- Somatotopy: Outer layers receive inputs from retina and cerebral cortex; deep layers receive input from inferior colliculus (auditory info), spinal nucleus of V, and the spinal cord
- Output: Extend ventromedially around periaqueductal grey to cross dorsal tegmental decussation and descend to the spinal cord via tectospinal tract and oculomotor nuclei via tectobulbar fibers
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Red nucleus location and function
- Location: Rostral midbrain
- Function: Rubrospinal tract is questioned in humans, but in other mammals, it is responsible for generating gait; the major output of the red nucleus in humans is to the inferior olive via the rubro-spinal tract
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Common causes of midbrain lesions
- Vascular (blood supply to midbrain below)
- Necrotic (such as uncal herniation)
- Multiple sclerosis
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Weber's syndrome
- Lesions involving the basal midbrain (basis pedunculi)
- Cranial nerve III: Although found in the tegmentum, damage to these fibers causes 'alternating oculomotor hemiplesia' on the ipsilateral side
- Corticobulbar/corticospinal fibers: Contralateral UMN paralysis
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Benedikt's syndrome
- Lesions involving midbrain tegmentum results in alternating oculomotor hemianesthesia with hemitremor
- Cranial nerve III: Ipsilateral III nerve palsy
- Cerebellothalamic fibers/red nucleus: Coarse tremor upon movement
- Substantia nigra: Resting tremor
- Medial and spinal lemnisci and ventral trigeminothalamic tract: Contralateral hemianesthesia (all modalities)
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