Midbrain

What are the functions of the midbrain?

• Control of vertical saccadic eye movements

• Gross motor control of UEs

• Orienting eyes & head toward stimuli

• Movements of eyes & pupillary constriction

• Functionally related to basal nuclei

What is the tectum, and what structures does it include?

• Portion of midbrain dorsal (posterior) to cerebral aqueduct

• Superior colliculi, inferior colliculi, pretectal areal

What is the tegmentum and what structures does it include?

• Portion of midbrain ventral (anterior) to cerebral aqueduct

• Ascending/descending tracts, red nuclei, periaqueductal gray, SN, CN III, CN IV nuclei, Trigeminal mesencephalic nucleus, reticular formation, SCP, STT, medial lemniscus, vertical gaze center

What are the dorsal external structures?

• Superior & Inferior colliculi

• CN IV

• SCP

What are the ventral external structures?

• CN III

• Cerebral peduncles

• Mammillary bodies

Describe the SCP.

• Major efferent pathway from cerebellum

• White matter structure

• Axons/fibers project to red nucleus & thalamus

• Ventral spinocerebellar tract enters here

Describe the superior colliculus.

• Orients movements of head & eyes toward an object or noise

• Creates a map of our visual field

• Pinpoints location of visual stimuli

• Sends coordinates to PPRF for saccade toward object

• Activates LMNs in SC to orient head toward stimulus

What are the steps of the superior colliculus’s voluntary control of eye movements?

1. FEF initiates contralateral saccadic eye movements

2. Superior colliculus receives info from FEF on location, pinpoints exact location in visual field

3. Sends precise coordinates to PPRF

4. PPRF generates motor plan for saccade & passes it off to CN VI nuclei

What are the steps of the superior colliculus’s reflex control of eye movements?

1. Superior colliculus receives direct retinal input from optic tracts

2. Pinpoints exact location of unexpected object in periphery

3. Sends coordinates to PPRF for saccade to unexpected object

4. PPRF generates motor plan for saccade & passes it off to CN VI nuclei

What is the tectospinal tract?

Efferent fibers from superior colliculus to SC LMNs

What is the tectobulbar tract?

• Efferent fibers from superior colliculus to PPRF

• PPRF makes motor plan & passes it off to CN VI nuclei

What are the 2 populations of neurons from the Abducens nuclei?

1. Gives axons that form CN VI, abducens nerve

2. Gives off axons that joint contralateral MLF, carrying plan to CN III nucleus

What is the function of the inferior colliculi?

• Part of auditory system

• Receives auditory input from cochlear nuclei via lateral lemniscus

• Uses echolocation to ID direct sound

• Sends output to superior colliculus (pinpoints location)

What is the pretectal area?

• Just rostral to superior colliculus

• Nuclei participating in smooth pursuit & pupillary light response

How does the pretectal area participate in pupillary light response?

• Receives afferent fibers from retinal cells (light detection)

• Project efferent fibers to Edinger-Westphal nucleus (CN III)

What is the Edinger-Westphal nucleus?

• Origin of preganglionic PNS fibers of CN III

• Reflexive constriction of pupil in light

What occurs with damage to the Edinger-Westphal nucleus?

• Loss of parasympathetic tone

• Blown pupil with no reaction to light

• Sympathetic takes over

What is the periaqueductal gray matter?

• Gray matter surrounding cerebral aqueduct

• Extension of hypothalamus

• Does pain modulation through projections to raphe nucleus

What is the vertical gaze center?

• Reticular structure for vertical saccade motor plan generation

• Part of reticular formation

What is the red nucleus?

• Large gray matter that participates in motor learning, gross movements

• ONLY UEs

• Receives info via SCP & cortex

• Sends output to inf olivary nuclei → ICP → cerebellum & ruprospinal tract to SC LMNs

What is an UE flexor bias?

When left alone it will flex UEs, fingers, wrist, & elbow

What are the 2 divisions of substantia nigra (SN)?

1. Pars compacta (SNc) = dopamine projections to basal nuclei

2. Pars reticulata (SNr) = inhibitory (GABA) output of basal nuclei

What muscles does CN III innervate?

• Superior rectus

• Medial rectus

• Inferior rectus

• Inferior oblique

• Levator palpebrae superioris

What is the medial longitudinal fasciculus (MLF)?

• Axons connecting abducens nucleus with contralateral oculomotor nucleus

• Essential for horizontal conjugate gaze

• Coordinates medial & lateral rectus during saccades & smooth pursuit

Which structure is most often first affected by MS?

• MLF

• Will get double vision

What is the function of the mesencephalic nucleus?

• Proprioception for muscles of mastication, TMJ, teeth, & gums

• Controls force of bite

What is the reticular formation?

• Mostly found in pons & midbrain

• All arousal system projections to thalamus & cortex

What does damage to the midbrain cause?

Coma

What is an uncal herniation and what does it cause?

• Uncus compressing midbrain in tentorial notch

• Causes coma & blown pupils + unreactive to light (EW nucleus)

Where does the midbrain receive its blood supply from?

• Posterior cerebral artery (PCA)

• Basilar artery

• Superior cerebellar artery

What is decorticate rigidity?

• Lesion in rostral midbrain (above red nucleus)

• Flexion in UEs & extension in LEs

• Coma

What is decerebrate rigidity?

• Lesion in caudal midbrain (below red nucleus)

• Extension in UEs & LEs

• Coma

• WORSE