Week 4: Structural and Functional Organization of the Brain*

Introduction to the Brain

• Adult brain weighs about three pounds, primarily composed of water and fat.
• Essential for basic functions (walking, talking, etc.) and complex tasks (writing, designing AI).
• Brain acts as the system guiding experiences and cultural practices across generations.

Brain Development

• Brainstem: Connects brain to spinal cord; regulates basic functions.
• Cerebellum: Located posteriorly to reconnect with brain; possesses tracts exclusive to the brain.
• Embryological Development: Brain begins as a cluster of cells leading to:
  • Neural Tube: Forms the foundation for brain and spinal cord.
  • Vesicles: At two months of development, three primary vesicles develop:
  1. Forebrain (Prosencephalon)
  2. Midbrain (Mesencephalon)
  3. Hindbrain (Rhombencephalon)

Structures That Protect the Brain

• Brain originates from primary vesicles, developing into five secondary vesicles:
  1. Telencephalon → Cerebrum
  2. Diencephalon → Thalamus, Hypothalamus, Epithalamus
  3. Mesencephalon → Midbrain
  4. Metencephalon → Pons, Cerebellum
  5. Myelencephalon → Medulla Oblongata
• Cranium: Bony structure provides primary protection.
• Meninges: Layers of connective tissue (Dura mater, Arachnoid mater, Pia mater) protect the brain:
  • Dura Mater: Two fused layers; separates to form venous sinuses.
  • Dural Folds: Additional support structures:
  1. Falx Cerebri: Separates cerebral hemispheres.
  2. Falx Cerebelli: Separates cerebellar hemispheres.
  3. Tentorium Cerebelli: Separates cerebrum from cerebellum; clinically significant for evaluating brain injuries.

Brain Ventricles and Cerebrospinal Fluid (CSF)

• Ventricles: Four connected cavities filled with CSF, continuous with spinal cord's central canal.
  • Right & left lateral ventricles: Resemble ram’s horns; horseshoe shape.
  • Third Ventricle: Between diencephalon lobes; connects to lateral ventricles via interventricular foramen.
  • Fourth Ventricle: Between pons and cerebellum; connects to third ventricle through cerebral aqueduct.
• CSF: Nourishes and protects the brain; produced by choroid plexuses covered with ependymal cells.
  • Circulates through ventricles and exits to subarachnoid space, maintaining brain buoyancy.

Blood-Brain Barrier (BBB)

• Protects brain from harmful substances, maintains CSF-brain fluid separation.
• Selective permeability: Allows water, oxygen, carbon dioxide, and nonpolar compounds; restricts large polar molecules.
• Formed by endothelial cells with tight junctions and astrocytes supporting capillaries.

Structural Organization of the Brain

• The brain is divided into four principal regions:
  1. Cerebrum: Largest part, divided into left and right hemispheres; associated with higher functions such as cognition, memory, and voluntary movement.
  2. Diencephalon: Composed of:
  • Thalamus: Relay station for sensory impulses (excluding smell).
  • Hypothalamus: Regulates autonomic functions, hormones, emotional behavior, and homeostasis.
  • Epithalamus: Includes pineal gland, releases melatonin affecting circadian rhythm.
  1. Brainstem: Controls essential life functions, includes:
  • Midbrain, Pons, Medulla Oblongata: Involved in reflexes, respiration, and sensory relay.
  1. Cerebellum: Coordinates muscle movements and balance.

Functional Regions of the Cerebral Cortex

Lobes of the Brain

• Frontal Lobe: Responsible for higher intellectual functions, motor control; contains primary motor cortex and Broca’s area for speech production.
• Parietal Lobe: Sensory interpretation; contains primary somatosensory cortex and Wernicke's area for comprehension.
• Temporal Lobe: Involved in auditory processing; contains primary auditory cortex.
• Occipital Lobe: Processes visual information; responsible for visual perception.
• Insula: Processes taste and visceral sensations.

Association Areas

• Link sensory inputs and execute higher processing:
  • Premotor area: Coordinates action sequences.
  • Wernicke’s area: Responsible for language comprehension; damage causes expressive aphasia.
  • Prefrontal Cortex: Involved in complex decision-making and personality.

Brainstem and Cerebellum Functions

• Medulla Oblongata: Controls autonomic functions such as heartbeat and breathing; contains nuclei for cranial nerves.
• Pons: Acts as a bridge between medulla and cerebellum; involved in breathing control.
• Midbrain: Controls visual and auditory reflexes; contains substantia nigra (movement regulation).
• Cerebellum: Manages subconscious muscle actions and body balance.

Limbic System

• Role: Center for emotional processing and memory.
• Components:
  • Hippocampus: Key for memory formation.
  • Amygdala: Processes emotions, especially fear.
  • Sense of Smell: Strong emotional connections through olfactory structures.

Summary of Key Functions

• Thalamus: Sensory signal relay.
• Hypothalamus: Autonomic system control, hunger and thirst regulation.
• Amygdala: Emotional states.
• Cerebellum: Balance and coordination.
• Sensory and Motor Cortex: Processes various sensory modalities and voluntary movements respectively.