Brain: Embryology and the Cerebrum

Anencephaly

  • Definition: A neural tube defect occurring between days 23 and 26 of fetal development.
  • Cause: Failure of the anterior end of the neural tube to close.
  • Consequences: Conditions include absence of calvaria (skull bones) and telencephalon (part of the brain).

Brain Development Process

  • Neural Tube Formation:
  • Week 3: Development starts from ectoderm forming the neural plate.
  • Week 4: Invagination creates the neural groove and folds, leading to the fusion and formation of the neural tube.
  • Neural Crest Formation: Cells from the neural folds migrate to form the neural crest, which contributes to the PNS.
  • Differentiation:
  • Anterior (Rostral) End: Becomes the brain.
  • Posterior (Caudal) End: Becomes the spinal cord.

Brain Vesicles

  • Primary Brain Vesicles (Week 5):
  • Prosencephalon: (Forebrain)
  • Mesencephalon: (Midbrain)
  • Rhombencephalon: (Hindbrain)
  • Secondary Brain Vesicles:
  • Telencephalon: (Cerebrum)
  • Diencephalon: (Thalamus, Hypothalamus)
  • Mesencephalon: (Midbrain)
  • Metencephalon: (Pons, Cerebellum)
  • Myelencephalon: (Medulla Oblongata)

Ventricles of the Brain

  • General Features:
  • Hollow chambers filled with cerebrospinal fluid (CSF), lined by ependymal cells.
  • Continuous with the central canal of the spinal cord.
  • Specific Ventricles:
  • Lateral Ventricles: Paired, C-shaped, located in the cerebrum.
  • Third Ventricle: Located in the diencephalon.
  • Fourth Ventricle: In the hindbrain, communicates with the third ventricle and spinal canal.

Structure of the Cerebrum

  • Overview:
  • Most superior region of the brain, accounting for 83% of total brain mass.
  • Grey Matter: Surface layer containing neuron cell bodies, dendrites, unmyelinated axons, and glial cells.
  • White Matter: Deep layer consisting of myelinated and unmyelinated axons.
  • Terminology:
  • Gyrus: Elevated ridges of tissue.
  • Sulcus: Shallow grooves between gyri.
  • Fissure: Deep grooves separating large regions of the brain.

Anatomical Landmarks

  • Lobes:
  • Frontal, Parietal, Occipital, Temporal, and Insula (deep within the lateral sulcus).
  • Fissures and Sulci:
  • Longitudinal Fissure: Separates the left and right hemispheres.
  • Transverse Fissure: Separates the cerebellum from the cerebrum.
  • Central Sulcus: Between frontal and parietal lobes.
  • Lateral Sulcus: Between frontal, parietal, and temporal lobes.

Brain Function Areas

  • Motor Areas:

  • Primary Motor Cortex: Located in the precentral gyrus of the frontal lobe for voluntary movements.

  • Premotor Area: Anterior to primary motor cortex, involved in learned motor skills.

  • Broca’s Area: Involved in speech production, located in the left frontal lobe.

  • Sensory Areas:

  • Primary Somatosensory Cortex: In the postcentral gyrus, processes sensory information from the body.

  • Visual and Auditory Areas: Located in the occipital and temporal lobes respectively.

  • Association Areas:

  • Integrate and interpret sensory information, link memories, and tie them to prior experiences.

  • Prefrontal Cortex: Responsible for personality, decision making, and complex cognitive behavior.

Cerebral White Matter

  • Composition: Bundles of myelinated and unmyelinated axons, critical for communication between brain regions.
  • Types:
  • Commissural fibers: Connect hemispheres (e.g., corpus callosum).
  • Association fibers: Connect different parts in the same hemisphere.
  • Projection fibers: Link higher brain functions with lower brain regions, passing through the internal capsule.

Basal Nuclei (Ganglia)

  • Location: Masses of grey matter deep within cerebral white matter.
  • Functions:
  • Regulation of attention, cognition, and motor control (starting, stopping, and monitoring movements).
  • Inhibition of unnecessary movements.

Key Learning Points

  • Anencephaly is a severe neural tube defect affecting brain formation.
  • The brain develops from the neural tube and differentiates into primary and secondary brain vesicles, each giving rise to specific structures.
  • Understanding the structure and function of the cerebrum and its various components is critical for grasping how the brain operates as a whole.