Brain Structure and Function
Goals
- Explore brain structure and function through evolutionary, developmental, and anatomical lenses.
- Discuss the vertebrate brain plan (Bauplan) and its implications for understanding human brain organization.
- Highlight the roles of major brain divisions and their functions.
- Examine historical perspectives on brain function and modern comparative approaches.
Overview of Brain Structure and Function
- Why Brains?
- Brains improve biological fitness by enabling adaptive movement, which supports:
- Acquiring nutrients.
- Escaping predation.
- Reproducing.
- Brains are metabolically expensive and require:
- High-calorie diets (e.g., meat, cooked food).
- Energy-saving adaptations (e.g., bipedalism, high body fat, shorter alimentary canal).
- Brains improve biological fitness by enabling adaptive movement, which supports:
- Human Brain Specialization:
- Humans have the highest encephalization quotient (EQ).
- Unique features include high neuron density in the cerebral cortex and exceptional energy demands.
Historical Perspectives on Brain Function
Galen (129–199 AD)
- Proposed the vital spirits theory, associating brain ventricles with imagination, memory, and reasoning.
- Emphasized structure-function relationships but based observations on ox brains (e.g., rete mirabile, absent in humans).
- Longevity of theory (~1000 years) underscores its historical importance.
Renaissance and Beyond
- Vesalius and Da Vinci:
- Accurate anatomical depictions challenged ventricular-centric theories.
- Rembrandt:
- Public dissections advanced understanding of brain structure.
- Descartes (1662):
- Pineal gland as the “seat of the soul”; hydraulic model of nervous function.
Carving the Brain at Its Joints
- Structure-Function Relationships:
- Different structures often imply different functions (an intuition, not a law).
- Functional dissociations observed with localized brain damage provide insights.
- Single vs. Double Dissociation:
- Single: Damage to Region A affects Function X but not Function Y.
- Double: Regions A and B show complementary deficits in Functions X and Y.
Vertebrate Brain Bauplan
- Shared features across vertebrates:
- Forebrain (Telencephalon): Includes cerebral cortex and basal ganglia.
- Midbrain (Mesencephalon): Tectum and tegmentum.
- Hindbrain (Metencephalon/Myelencephalon): Includes cerebellum, pons, and medulla.
- Heterochrony:
- Timing of developmental processes leads to variation in brain size and structure.
Comparative Insights
- Evolutionary elaboration of vertebrate brains:
- Example: Six-layered neocortex in mammals.
- Shared neurotransmitter systems (e.g., glutamate and GABA) traceable to simple organisms like sponges.
Developmental Considerations
Gastrulation and Neurulation
- Early processes establish the neural tube, which differentiates into:
- Dorsal-Ventral Axis:
- Dorsal: Sensory regions (alar plate).
- Ventral: Motor regions (basal plate).
- Rostral-Caudal Axis:
- Forebrain, midbrain, and hindbrain segmentation (e.g., rhombomeres).
- Dorsal-Ventral Axis:
Regulatory Genes
- Genes pattern neural tube regions into distinct brain areas.
- Example: Hox genes control segmentation along the anterior-posterior axis.
Key Anatomical Divisions of the Brain
Central Nervous System (CNS)
- Cerebral Cortex:
- Higher-order functions like reasoning, language, and memory.
- Brainstem:
- Vital autonomic functions (e.g., breathing, heart rate).
- Cerebellum:
- Coordinates movement and contributes to cognitive processes.
Peripheral Nervous System (PNS)
- Somatic Nervous System:
- Voluntary muscle control.
- Autonomic Nervous System (ANS):
- Sympathetic (“fight or flight”).
- Parasympathetic (“rest and digest”).
- Enteric: Gut function; produces 90% of serotonin.
Challenges to Simplistic Models
Recapitulation Theory (Haeckel)
- Discredited theory claiming embryonic development mirrors evolutionary history.
- Embryonic similarities reflect shared evolutionary ancestry, not direct recapitulation.
Triune Brain Hypothesis (MacLean)
- Layers:
- Reptilian brain: Basic survival functions.
- Limbic brain: Emotions and memory.
- Neocortex: Higher-order cognition.
- Criticism:
- Oversimplifies brain evolution; all layers have evolved in complexity.
Comparative Evolutionary Approaches
- Tool Use and Niche Construction:
- Example: Homo erectus used advanced tools, fostering social and dietary changes.
- Social Brain Hypothesis:
- Correlation between group size and neocortical expansion in primates.
Summary
This lecture emphasized the evolutionary, developmental, and structural organization of the brain. It highlighted the historical progression of ideas about brain function, the shared vertebrate brain plan, and modern insights into brain evolution and development. Understanding these principles sets the stage for detailed exploration of specific brain systems.
