--- module: 081-02 language: en chapter: 81 title: "Structural Neuroanatomy and Lesion Localisation" module_title: "Brainstem nuclei, cranial pathways, reticular formation, and cerebellar circuits" source_sha256: 163da7d2577d82fecd21c8cf923a97b9d76fa6ec88ea95d629661fd80c008c33 --- # Brainstem nuclei and cerebellar circuits ## Orienting the brainstem ### Small lesion gives crossed findings #### Ipsilateral cranial nerve, contralateral body ### Medulla: pyramids, olives, fourth ventricle opens ### Pons: ventral basis, dorsal tegmentum #### Middle cerebellar peduncles form lateral bulk ### Midbrain surrounds cerebral aqueduct #### Peduncles, substantia nigra, red nucleus #### Tectum: superior and inferior colliculi ## Cranial nerve columns ### Ventricle opening pushes sensory nuclei lateral ### Somatic motor nearest midline #### Hypoglossal, abducens, trochlear, oculomotor ### Branchial motor #### Nucleus ambiguus, facial, trigeminal motor ### Parasympathetic nuclei #### Dorsal motor vagus, salivatory nuclei #### Edinger-Westphal with oculomotor ### Sensory: solitary, vestibular, cochlear, trigeminal ## Trigeminal systems ### Principal nucleus: discriminative facial touch ### Spinal nucleus: pain and temperature ### Mesencephalic nucleus: primary neurons inside CNS ### Second-order fibres cross to ventral posteromedial ### Motor nucleus supplies mastication ### Lateral lesion: crossed pain-temperature loss ## Ocular motor integration ### Trochlear fibres cross before exiting dorsally #### Nuclear lesion contralateral, nerve ipsilateral ### Abducens nucleus holds interneurons #### Nuclear lesion: ipsilateral horizontal gaze palsy #### Fascicular lesion: isolated abduction weakness ### Paramedian pontine reticular formation: saccades ### Midbrain coordinates vertical gaze ### Medial longitudinal fasciculus lesion #### Internuclear ophthalmoplegia ### Pupil fibres superficial in third nerve #### Aneurysm compresses, microvascular may spare ## Medullary visceral networks ### Solitary nucleus integrates visceral reflexes ### Dorsal vagal and ambiguus: parasympathetic output ### Respiratory rhythm from distributed networks #### Pre-Bötzinger complex, respiratory groups #### Bilateral injury or opioids: respiratory failure ### Area postrema detects blood-borne emetics #### Hiccup and vomiting localise to medulla ## Auditory and vestibular pathways ### Cochlear nuclei at pontomedullary junction ### Bilateral ascending auditory projection #### Unilateral lesion rarely deafens one ear ### Vestibular nuclei stabilise gaze and posture ### Signs separate peripheral from central ### Nodulus and flocculus shape vestibulo-ocular function ## Cerebellar architecture ### Vestibulo-, spino-, cerebrocerebellum ### Cortex: molecular, Purkinje, granular layers #### Mossy fibres via granule and parallel fibres #### Climbing fibres convey error signals #### Purkinje cells inhibit deep nuclei ### Deep nuclei: fastigial, interposed, dentate ### Peduncles #### Inferior: spinal, vestibular, olivary input #### Middle: contralateral pontine input #### Superior: deep-nuclear output ### Double or no crossing, ipsilateral effect ## Cerebellar dysfunction ### Vermis: truncal and gait ataxia ### Hemisphere: ipsilateral limb dysmetria, tremor ### Flocculonodular: disequilibrium, ocular signs ### Gait inability may be the only stroke sign ### Cognitive-affective syndrome via cortical loops ## Reticular formation and consciousness ### Arousal, autonomic, pain, tone, sleep ### Ascending arousal via thalamus and hypothalamus ### Consciousness needs both hemispheres and arousal #### Small upper-brainstem lesion can impair it #### Unilateral cortical lesion usually does not ### Posturing reflects severe lesions, not precise level ## Vascular brainstem syndromes ### Paramedian and circumferential territories ### Posterior inferior cerebellar: lateral medulla ### Lateral medullary syndrome #### Vertigo, ipsilateral ataxia, facial loss #### Dysphagia, hoarseness, Horner syndrome ### Medial medullary syndrome #### Contralateral weakness, ipsilateral tongue ### Basilar thrombosis: locked-in syndrome #### Consciousness and vertical gaze preserved ### Examination builds anatomical hypothesis