Anatomy of the Central Nervous System: Midbrain (Mesencephalon)

Overview

• Central Nervous System (CNS) consists of:
  • Encephalon
  • Spinal cord
• Encephalon includes:
  • Brainstem: medulla, pons, midbrain (mesencephalon)
  • Cerebellum
  • Diencephalon
  • Telencephalon

Midbrain (Mesencephalon)

• Located above the pons, in front of the cerebellum, below the diencephalon
• Focus: anatomy of the midbrain

External Surfaces of the Midbrain

Anterior Surface

• Cerebral Peduncles: Large structures containing tracts from the cortex; responsible for voluntary movement.
• Interpeduncular Fossa: Contains structures of the diencephalon.
• Posterior Perforated Substance: Contains grey matter with holes for blood vessels.
• Oculomotor Sulcus: Groove for cranial nerve III (oculomotor nerve) which innervates eye muscles.

Posterior Surface

• Tectal Plate (Lamina Tecti): Contains the superior and inferior colliculi.
  • Superior Colliculi: Associated with rapid eye movements; involved in visual pathway.
  • Inferior Colliculi: Part of the auditory pathway.
• Lateral Sulcus: Boundary between the cerebral peduncles and posterior midbrain.
• Trigon of the Lateral Lemniscus
• Trochlear Nerve (CN IV): Innervates the superior oblique muscle of the eye.

Internal Anatomy

Division

• Tectum: Contains colliculi.
• Tegmentum: Middle portion.
• Cerebral Peduncles: Contain descending tracts.

Grey Matter Structures

• Red Nucleus (Nucleus Ruberi): Coordinate voluntary muscle control.
• Substantia Nigra: Includes pars compacta and pars reticulata; involved in movement modulation.
  • Relation to Parkinson's Disease due to dopamine neuron loss.
• Nuclei of Cranial Nerves:
  • Oculomotor Nerve (CN III)
  • Trochlear Nerve (CN IV)
  • Trigeminal Nerve at inferior colliculi level
• Reticular Formation: Vital functions and balance.
• Periaqueductal Gray: Associated with pain modulation.

White Matter Structures

• Ascending Tracts:
  • Medial Lemniscus: Conscious proprioception and mechanoreceptors.
  • Spinal Lemniscus: Sensory input for pain, temperature, pressure, touch.
  • Trigeminal Lemniscus: Sensory input from the face.
  • Lateral Lemniscus: Hearing pathway.
• Descending Tracts:
  • Tectospinal Tract: From superior colliculus for neck muscle engagement.
  • Rubrospinal Tract: Supports voluntary muscle movements.
  • Reticulospinal Tract: Balance and posture.
  • Medial Longitudinal Fasciculus: Coordinates head, neck, and eye movement.
  • Corticospinal Tract: Innervates skeletal muscles.
  • Corticonuclear Tract: Controls head and neck muscles.
  • Corticopontine Tracts: Include frontopontine, occipitopontine, parietopontine, and temporopontine tracts supporting voluntary movement.

Study Aid

• Quiz with identification of structures based on images and descriptions.

Next Steps

• Next video will focus on the diencephalon.

Note: For effective study, identify structures and understand their functions and pathways, particularly focusing on the role of substantia nigra in movement disorders like Parkinson's disease.