External and internal features of midbrain Flashcards
(31 cards)
What are the three main parts of the midbrain?
Tectum
Tegmentum
Cerebral Peduncles
Which cranial nerves emerge from the midbrain, and where?
CN III (Oculomotor): Emerges from the interpeduncular fossa (anterior).
CN IV (Trochlear): Emerges dorsally, below the inferior colliculi (posterior).
What are the functions of the midbrain?
Motor control, vision, hearing, arousal, and temperature regulation.
What are the roles of the superior and inferior colliculi?
Superior colliculus: Visual reflexes center; connects to the lateral geniculate body.
Inferior colliculus: Lower auditory center; connects to the medial geniculate body.
What structures are found at the inferior colliculus level of the midbrain?
Tectum: Inferior colliculus.
Tegmentum: Trochlear nucleus, decussation of superior cerebellar peduncles, medial lemniscus, spinothalamic tract, PAG.
Basis pedunculi: Corticospinal and corticobulbar tracts.
What structures are found at the superior colliculus level of the midbrain?
Tectum: Superior colliculus.
Tegmentum: Oculomotor & Edinger-Westphal nuclei, red nucleus, medial lemniscus, PAG.
Basis pedunculi: Corticospinal, corticobulbar, corticopontine tracts, substantia nigra.
What is the function of the red nucleus, and what are its afferent and efferent connections?
Function: Motor control.
Afferents: Cortex, cerebellum, substantia nigra, spinal cord.
Efferents: Spinal cord (rubrospinal tract), reticular formation, thalamus, substantia nigra.
What is the role of the substantia nigra, and how is it related to Parkinson’s disease?
Role: Produces dopamine and projects to the basal ganglia (caudate nucleus and putamen).
Parkinson’s: Degeneration of the pars compacta (dopamine-producing neurons) leads to Parkinson’s disease.
What are the “Big Three” tracts in the midbrain?
Corticospinal tract
Medial lemniscus
Spinothalamic tract
What are the key features of Weber’s, Benedikt’s, and Parinaud’s syndromes?
Weber’s: CN III palsy + contralateral hemiparesis.
Benedikt’s: CN III palsy + tremor/ataxia (red nucleus involvement).
Parinaud’s: Vertical gaze palsy + eyelid retraction (superior colliculus lesion).
Which arteries supply the midbrain?
Branches of the Superior Cerebellar Artery (SCA) and Posterior Cerebral Artery (PCA).
Lateral midbrain also supplied by the anterior choroidal artery (branch of ICA).
What is the pretectal nucleus, and what reflex is it involved in?
Location: Rostral to the superior colliculus.
Function: Mediates the pupillary light reflex via connections with the Edinger-Westphal nucleus (CN III).
What is the function of the superior colliculus in the visual system?
Coordinates visual reflexes (e.g., eye and head movements toward stimuli).
Receives input from the visual cortex and frontal eye fields.
Sends efferents via the tectospinal and tectobulbar tracts for reflex responses.
What is the role of the medial longitudinal fasciculus (MLF) in the midbrain?
Links the oculomotor and trochlear nuclei to the abducens nucleus in the pons.
Vital for coordinating conjugate eye movements (e.g., gaze control
What is the inferior colliculus part of, and what is its pathway?
Part of the ascending auditory pathway.
Receives input from the lateral lemniscus.
Projects to the medial geniculate nucleus (thalamus), which relays signals to the auditory cortex.
What is unique about the trochlear nerve (CN IV) compared to other cranial nerves?
Only cranial nerve that emerges dorsally from the brainstem (below the inferior colliculus).
Its fibers decussate (cross midline) before exiting.
Innervates the superior oblique muscle (controls downward eye movement).
What are the parasympathetic functions of the Edinger-Westphal nucleus?
Controls pupillary constriction (via pupillary sphincter muscle).
Adjusts lens shape (accommodation reflex) via the ciliary ganglion.
Part of the light reflex circuit (connects to pretectal nucleus).
What is the decussation of the superior cerebellar peduncles, and where does it occur?
Fibers from the cerebellum cross midline in the caudal midbrain (inferior colliculus level).
Some terminate in the red nucleus; others ascend to the thalamus.
What is the somatotopic organization of the medial lemniscus in the midbrain?
Arranged laterally to medially:
Leg (most lateral) → Arm → Trunk (most medial).
Carries fine touch, vibration, and proprioception signals to the thalamus.
How does the substantia nigra pars compacta differ from the pars reticulata?
Pars compacta:
Pigmented (melanin-containing).
Produces dopamine (projects to basal ganglia).
Pars reticulata:
Non-pigmented.
Acts as an output nucleus (GABAergic, projects to thalamus).
What is the role of the periaqueductal gray (PAG) in the midbrain?
Involved in pain modulation, defense behaviors, and autonomic control.
Coordinates responses to stress (e.g., freezing, fight-or-flight).
What are saccadic eye movements, and how are they controlled?
Rapid, voluntary shifts of gaze (e.g., reading).
Controlled by corticotectal fibers from the frontal eye fields to the superior colliculus.
Which muscles are innervated by CN III, CN IV, and CN VI?
CN III (Oculomotor):
Superior, inferior, and medial rectus; inferior oblique; levator palpebrae.
CN IV (Trochlear):
Superior oblique.
CN VI (Abducens):
Lateral rectus.
What is the afferent and efferent pathway of the pupillary light reflex?
Afferent: Optic nerve → pretectal nucleus.
Efferent: Pretectal nucleus → Edinger-Westphal nucleus → CN III → ciliary ganglion → pupillary constrictor.
