Midbrain (mesencephalon)

External structure

• Two large cerebral peduncles (crura cerebri) lie on either side.
• They are separated by the interpeduncular fossa.
• The oculomotor nerve (CN III) emerges from the medial side of each cerebral peduncle.
• Cerebral peduncles carry major descending cortical fibers, including corticospinal, corticonuclear, and corticopontine tracts.

• Called the tectum or quadrigeminal plate.
• It has four elevations, the corpora quadrigemina:
  • Two superior colliculi, for visual reflexes and orienting the eyes and head toward visual stimuli.
  • Two inferior colliculi, for auditory reflexes and auditory pathway processing.
• The trochlear nerve (CN IV) emerges dorsally just below the inferior colliculus. It is the only cranial nerve to emerge from the dorsal aspect of the brainstem and its fibers decussate before emerging.
• The superior and inferior colliculi connect to the lateral and medial geniculate bodies through the superior and inferior brachia, respectively.

Internal structure

Important nuclei and tracts

• Oculomotor nucleus: somatic motor fibers of CN III to most extraocular muscles and levator palpebrae superioris.
• Edinger-Westphal nucleus: parasympathetic fibers in CN III to sphincter pupillae and ciliary muscle, producing pupillary constriction and accommodation.
• Red nucleus: motor coordination; gives rise to the rubrospinal tract.
• Substantia nigra: dopamine-producing pars compacta modulates basal ganglia circuits. Degeneration is a central feature of Parkinson disease.
• Mesencephalic nucleus of CN V: proprioceptive sensory neurons associated with jaw muscles.
• Ascending pathways: medial lemniscus, spinothalamic and trigeminothalamic tracts.
• Descending pathways: corticospinal and corticonuclear tracts.

• Inferior colliculus: auditory relay and reflex center.
• Trochlear nucleus: motor fibers of CN IV to the contralateral superior oblique muscle.
• Decussation of superior cerebellar peduncles: important crossing of cerebellar output fibers.
• Substantia nigra, mesencephalic nucleus of CN V, reticular formation, and major long tracts are also present.

Functions of the midbrain

1. Eye movement
   • CN III controls most extraocular muscles, eyelid elevation, pupillary constriction, and accommodation.
   • CN IV controls superior oblique muscle action.
2. Visual reflexes
   • Superior colliculi coordinate reflex turning of the eyes, head, and neck toward a visual stimulus.
3. Auditory reflexes
   • Inferior colliculi process auditory input and mediate reflex orientation toward sound.
4. Motor control
   • Substantia nigra influences initiation and smooth control of movement through basal ganglia circuits.
   • Red nucleus and rubrospinal connections contribute to motor coordination.
   • Cerebral peduncles transmit voluntary motor commands from cortex to brainstem and spinal cord.
5. Sensory conduction
   • Major ascending pathways transmitting fine touch, proprioception, pain, temperature, and facial sensory information traverse the tegmentum.
6. Consciousness and arousal
   • Midbrain reticular formation contributes to the ascending reticular activating system, hence wakefulness and alertness.
7. Pain and autonomic modulation
   • Periaqueductal gray participates in endogenous pain control and autonomic/behavioral responses.

Intermediate brain (diencephalon)

1. Thalamus
2. Hypothalamus
3. Epithalamus
4. Subthalamus

1. Thalamus

• Principal relay and processing station for nearly all sensory information headed to cerebral cortex, except olfaction.
• Relays motor information between the cerebral cortex, basal ganglia, cerebellum, brainstem and spinal cord.
• Participates in attention, consciousness, sleep, emotion and pain perception.

• Lateral geniculate body: visual relay to visual cortex.
• Medial geniculate body: auditory relay to auditory cortex.
• Ventral posterolateral nucleus: relay of body somatosensation.
• Ventral posteromedial nucleus: relay of facial sensation and taste.
• Ventral anterior and ventral lateral nuclei: motor relays from basal ganglia and cerebellum to motor cortex.

2. Hypothalamus

• Maintains homeostasis.
• Regulates body temperature, hunger, satiety, thirst and water balance.
• Controls autonomic nervous system functions.
• Regulates sleep-wake and circadian rhythms.
• Integrates emotional and behavioral responses.
• Controls pituitary endocrine activity:
  • Produces releasing and inhibiting hormones for the anterior pituitary.
  • Synthesizes ADH and oxytocin, released through the posterior pituitary.

3. Epithalamus

• Pineal gland
• Habenular nuclei
• Posterior commissure

• Pineal gland secretes melatonin and helps regulate circadian rhythms.
• Habenular nuclei link limbic structures with midbrain centers and contribute to emotional, reward and aversive responses.

4. Subthalamus

• It is part of the basal ganglia motor circuit.
• Helps suppress unwanted movements and regulate movement amplitude.
• A lesion may cause contralateral hemiballismus, characterized by violent, flinging limb movements.

Short comparison