Midbrain Overview and Anatomy

MIDBRAIN ANATOMY AND FUNCTIONS

Objectives

• Identify clinically relevant internal and external midbrain anatomy

• Describe the components and functions of the midbrain tectum

• List the anatomical structures for voluntary eye movements

• Identify the CNS implications of a pupil that is dilated and unreactive to light

• Discuss pathoanatomy of decorticate and decerebrate posturing

MIDBRAIN OVERVIEW

• Mesencephalon:

  • Smallest division of the brainstem

  • Location: Caudal to diencephalon, rostral to pons

  • Functions:

  • Control of vertical saccadic eye movements

  • Gross motor control of upper extremities

  • Orienting eyes and head toward stimuli

  • Movements of eyes and pupillary constriction

  • Functionally related to basal nuclei

MIDBRAIN STRUCTURES

• Divided between tectum and tegmentum

The Tectum

• Portion of the midbrain dorsal to cerebral aqueduct:

  • Superior Colliculi: Integral for visual processing and reflexive orientation

  • Inferior Colliculi: Involved in auditory processing

  • Pretectal Area: Involved in visual reflexes and control of pupil size

Tegmentum

• Location: Ventral to cerebral aqueduct

• Contains:

  • Ascending/descending tracts

  • Red nuclei: Involved in motor control

  • Periaqueductal gray matter: Associated with pain modulation

  • Substantia nigra: A functional component of the basal nuclei

  • Cranial Nerves III and IV nuclei: Involved in eye movements

  • Trigeminal mesencephalic nucleus: Involved in sensory processing

  • Midbrain reticular formation: Involved in arousal

  • Superior cerebellar peduncles

  • Cerebral Peduncles: Descending fiber tracts from cerebral cortex

MIDBRAIN EXTERNAL STRUCTURES

Dorsal:

• Superior Colliculi

• Inferior Colliculi

• CN IV: Trochlear nerve

• Superior Cerebellar Peduncle

Ventral:

• CN III: Oculomotor nerve

• Cerebral Peduncles: A major structure formed by descending axons

• Mamillary Bodies: Part of the hypothalamus

PEDUNCLES

• Cerebral Peduncle:

  • Bilateral structure

  • Continuation of the internal capsule

  • Contains large bundles of axons from the cerebral cortex

  • Middle 1/3: Composed of corticospinal and corticobulbar tracts

• Superior Cerebellar Peduncle:

  • Bilateral structure

  • Major efferent pathway from the cerebellum

  • White matter structure consisting of only axons

  • Axons from cerebellar nuclei project into midbrain to:

  • Influence rubrospinal tract via the red nucleus

  • Project onto the thalamus (output feedback loop)

  • Thalamus projects to supplementary motor cortex, aiding in motor precision

  • Influence corticospinal tract

  • Ventral Spinocerebellar Tract: Enters at this region

SUPERIOR COLLICULUS

• Function: Orienting head and eyes toward objects or noises

• Arrangement:

  • Each neuron receives input from a specific point in the visual field

  • Creates a topographical map of the visual field

• Key Functions:

  • Pinpoints location of visual stimuli

  • Sends coordinates to the Paramedian Pontine Reticular Formation (PPRF) for saccadic eye movement

  • Activates lower motor neurons in spinal cord to orient head toward stimulus

Superior Colliculus - Eye Movement Control

• Voluntary Control of Eye Movements:

  1. Frontal Eye Field (FEF) initiates contralateral saccadic eye movements

  2. Superior Colliculus (SC) receives location information from FEF

  3. SC pinpoints exact target location in visual field

  4. SC sends coordinates to PPRF

  5. PPRF generates motor plan for saccade and communicates with abducens nuclei

• Loss of FEF: Results in the inability to voluntarily move eyes contralaterally

• Reflex Control of Eyes:

  1. SC receives direct input from optic tracts (retinal input)

  2. SC pinpoints location of unexpected peripheral stimuli

  3. SC sends coordinates for saccade to PPRF

  4. PPRF executes motor plan and communicates with abducens nuclei

Efferent Pathways from SC:

• Tectospinal Tract: Efferent fibers from SC to spinal cord lower motor neurons (LMNs)

• Tectobulbar Tract: Efferent fibers from SC to PPRF

Abducens Nuclei:

• Two populations of neurons:

  • Population 1: Forms CN VI (abducens nerve)

  • Population 2: Joins contralateral medial longitudinal fasciculus (MLF) to convey saccadic plans to CN III (oculomotor)

TECTUM

• Inferior Colliculi: Part of the auditory pathway

  • Receives auditory inputs via lateral lemniscus from cochlear nuclei

  • Utilizes echolocation to determine sound direction

  • Sends outputs to SC to orient visual attention towards auditory stimuli

PRETECTAL AREA

• Location: Rostral to superior colliculus

• Functions:

  • Smooth pursuit tracking of moving objects

  • Pupillary light reflex

• Receives afferent fibers from retinal cells that specialize in light detection via optic tract

• Efferent fibers project to the Edinger-Westphal nucleus of CN III (oculomotor nerve)

EDINGER-WESTPHAL NUCLEUS

• Origin of preganglionic parasympathetic fibers of the oculomotor nerve

• Responsible for reflexive pupil constriction in light presence

• Damage: Resulting in loss of parasympathetic control leads to "blown pupil"—dilated and unreactive to light rather than constricting

TEGMENTUM

• Contains key structures:

  • Spinothalamic Tract: Pathway for pain and temperature sensation

  • Medial Lemniscus: Pathway for touch and proprioception sensation

  • Periaqueductal Gray Matter: Pain modulation pathway that surrounds the cerebral aqueduct

  • Red Nucleus: Participates in motor control and learning

  • Substantia Nigra: Key neurotransmitter roles in motor control and behavioral regulation

• Also home to nuclei for cranial nerves III and IV, and critical for vertical gaze motor planning

INTERNAL MIDBRAIN STRUCTURES

RED NUCLEUS

• Functions:

  • Involved in motor learning, particularly gross movements of neck and upper extremities

• Input from:

  • Cerebellum via superior cerebellar peduncle

  • Cerebral cortex

• Outputs:

  • Projects to inferior olivary nuclei, facilitating motor learning

  • Rubrospinal Tract: Important for LMN modulation in gross voluntary movements, favoring flexion in the upper extremities

SUBSTANTIA NIGRA

• Divided into two parts:

  • Pars Compacta: Produces dopamine and has projections to the basal nuclei

  • Pars Reticulata: Major output structure of the basal nuclei, using GABA (inhibitory)

OCULOMOTOR NERVE AND NUCLEI

• Oculomotor Nucleus: Located near MLF, ventral to the cerebral aqueduct

• Oculomotor Nerve Innervates:

  • Superior rectus (elevation)

  • Medial rectus (adduction)

  • Inferior rectus (depression)

  • Inferior oblique (extorsion)

  • Levator palpebrae superioris (elevation of eyelid)

MEDIAL LONGITUDINAL FASCICULUS (MLF)

• Connects abducens nucleus with contralateral oculomotor nucleus

• Essential for horizontal conjugate gaze movements

• Coordinates medial and lateral rectus muscles for saccades and smooth pursuit in horizontal motion

• Often the first structure affected by demyelinating diseases, such as multiple sclerosis

TRIGEMINAL NERVE

• The mesencephalic nucleus resides in the midbrain

• Involved in proprioception of mastication muscles, TMJ, teeth, and gums

• Plays a role in regulating the force of the bite

RETICULAR FORMATION

• Critical structure for:

  • Ascending arousal system primarily located in the pons and midbrain

  • Vital for arousal signaling to thalamus and cerebral cortex

  • Damage can lead to coma due to disrupted pathways

• Uncal Herniation: May compress the midbrain, leading to coma and dilated pupils unresponsive to light (due to damage to Edinger-Westphal nucleus)

• Also provides vertical gaze center for vertical saccadic motor planning

PERIAQUEDUCTAL GRAY MATTER

• Gray matter surrounding the cerebral aqueduct

• Extension of the hypothalamic function

• Plays a significant role in pain inhibition via projections to the raphe nucleus

BLOOD SUPPLY

• Dual supply mainly by:

  • Posterior Cerebral Artery

  • Interpeduncular branches

  • Quadrigeminal arteries

  • Posterior Choroidal artery

  • Basilar artery

  • Interpeduncular arteries

  • Superior Cerebellar Artery

IMPORTANT CLINICAL APPLICATIONS

DECORTICATE RIGIDITY

• Caused by lesions in rostral midbrain (above red nucleus)

• Mechanism: Interrupted descending influences from cortex result in an excessive excitatory effect of the red nucleus on upper extremity flexion.

  • Red nucleus influences flexion: UEs remain flexed

  • Lower extremities are extended (due to intact rubrospinal and vestibulospinal tracts)

  • Often associated with coma

DECEREBRATE RIGIDITY

• More severe damage occurring with lesions in the caudal midbrain.

• Mechanism: Damage to red nucleus halts UE flexion, while vestibular nuclei excitatory signals lead to extension bias without cortical inhibition.

  • Often indicative of more severe brain injury and also associated with coma

DIFFERENCE BETWEEN DECORTICATE AND DECEREBRATE

• Decerebrate posture is generally considered worse due to greater extent of brain damage, leading to more severe deficits in body posturing and autonomic control.