Anatomy of the Central Nervous System: The Midbrain (Mesencephalon)

Overview

• The central nervous system consists of:
  • Encephalon
  • Spinal Cord
• Encephalon subdivided into:
  • Brainstem (Medulla, Pons, Midbrain)
  • Cerebellum
  • Diencephalon
  • Telencephalon

Midbrain (Mesencephalon)

• Located above Pons, in front of the Cerebellum, below the Diencephalon.
• Focus on external and internal anatomy.

External Surfaces

Anterior View

• Cerebral Peduncles:
  • Large peduncles containing tracts from the Cortex.
  • Responsible for voluntary body movement.
• Interpeduncular Fossa:
  • Pit between peduncles housing structures like Hypothalamus, Pituitary gland (not part of the midbrain).
  • Posterior Perforated Substance: Contains grey matter and blood vessels.
• Ocuculomotor Sulcus:
  • Groove where the oculomotor nerve (3rd cranial nerve) exits to innervate eye muscles.

Posterior View

• Tectal Plate (Lamina Tecti):
  • Includes Superior and Inferior Colliculi.
  • Superior Colliculus: Involved in rapid eye movement control via visual pathway.
  • Inferior Colliculus: Part of the auditory pathway.
• Lateral Sulcus of the Mesencephalon: Separates cerebral peduncles from posterior surface.
• Trochlear Nerve (4th cranial nerve): Innervates superior oblique muscle of the eye.

Internal Anatomy

Grey Matter Structures

• Red Nucleus (Nucleus Ruber):
  • Coordinates voluntary muscle control.
• Substantia Nigra:
  • Produces dopamine, part of basal ganglia.
  • Involved in movement modulation (nigrostriatal pathway).
  • Loss of function linked to Parkinson’s Disease.
• Nuclei at Colliculi Levels:
  • Superior Colliculi: Oculomotor nerve nucleus.
  • Inferior Colliculi: Trochlear nerve nucleus, Mesencephalic trigeminal nucleus.
• Reticular Formation: Supports vital functions and balance.
• Periaqueductal Grey: Pain modulation.

White Matter Structures

Ascending Tracts

• Medial Lemniscus: Conscious proprioception, mechanoreceptor pathway.
• Spinal Lemniscus: Sensory input of pain, temperature, pressure, touch.
• Trigemnical Lemniscus: Sensory pathway for facial area.
• Lateral Lemniscus: Part of hearing pathway.

Descending Tracts

Tegmentum

• Tectospinal Tract: Engages neck muscles based on visual stimuli.
• Rubrospinal Tract: Coordinates voluntary muscle movements.
• Reticulospinal Tract: Balance and posture control.
• Medial Longitudinal Fasciculus: Coordinates head, neck, eye movements.

Cerebral Peduncles

• Corticospinal Tract: Innervates skeletal muscles.
• Corticonuclear Tracts: Voluntary control of head and neck muscles.
• Corticopontine Tract: Supports voluntary movement.

Key Takeaways

• The midbrain plays a critical role in coordinating voluntary and involuntary movements.
• Involvement in sensory pathways (vision, hearing) and motor pathways.
• Understanding structures of the midbrain is essential for grasping neurophysiology and potential clinical implications like Parkinson’s Disease.

Quiz

• Review table of grey and white matter structures to test understanding.

The next video will cover the Diencephalon.