KK

Biological Basis of Behavior #2 – Brain Structures, Functions, and Interconnections

Overview of This Week’s Lectures

  • Scope of the unit
    • Major anatomical divisions of the nervous system
    • Central vs. Peripheral Nervous Systems
    • Sympathetic vs. Parasympathetic Nervous Systems
    • How the brain is organized
    • Cerebral cortex (lobes + their functional maps)
    • Sub-cortical structures (limbic system, midbrain, brainstem/hindbrain)
    • Cellular makeup of the brain to be covered next class
    • Neurons & glia
    • How neurons communicate
    • Intracellular machinery of the neuron

Key Caution: Brain Regions as an Oversimplification

  • Color-coded brain diagrams are pedagogical tools, not reality
  • Functional units are:
    • Much smaller (micro-circuits, individual neurons, synapses)
    • Highly interconnected (no lobe works in isolation)
  • Take-home message: think networks, not isolated “chunks”

Cerebral Cortex: The Five Lobes

  • Frontal Lobe
    • "Complex thought & planning" headquarters
    • Primary motor cortex (M1) = map of body muscles (somatotopic)
    • Language production → Broca’s area (left inferior frontal gyrus)
    • Damage ⇒ Broca’s aphasia (non-fluent speech, intact comprehension)
    • Pre-frontal cortex (PFC)
    • Executive functions: evaluation, self-control, working memory, attention, effort allocation
  • Parietal Lobe
    • Primary somatosensory cortex (S1) = map of skin surface (homunculus)
    • Spatial awareness & proprioception (sense of body position/movement)
    • Lesions can cause hemispatial neglect (ignoring half of visual/body space)
  • Temporal Lobe
    • Auditory cortex → decoding sound features & meaning
    • Object recognition (with occipital/parietal inputs)
    • Memory interface (direct projections to hippocampus & amygdala)
    • Wernicke’s area (language comprehension)
    • Damage ⇒ Wernicke’s aphasia (fluent but meaningless speech; impaired understanding)
    • Clinical tie-in: Patient H.M. lost portions of temporal lobe → profound anterograde amnesia
  • Occipital Lobe
    • Primary visual cortex (V1) converts light into edges, borders, surfaces
    • Higher-order visual areas integrate contours → example: illusory square created by black circles
    • Even cats show susceptibility to illusory contours (citizen-science study; "If I fits I sits")
  • Insular Lobe (Insula)
    • Interoception hub: taste, visceral sensations, bodily states (hunger, nausea, pain, “gassy”)
    • Integrates internal body signals with emotional/cognitive context

Aphasia: Two Classic Cortical Syndromes

  • \text{Broca’s area} \rightarrow \text{Production}
    • Stroke → non-fluent, effortful speech; good comprehension
  • \text{Wernicke’s area} \rightarrow \text{Comprehension}
    • Stroke → fluent “word salad”; poor comprehension
  • Demonstrates division of labor & necessity of inter-lobe communication

Sub-Cortical Forebrain (Limbic System)

  • General note: evolutionarily older paleocortex sitting beneath neocortex; critical for emotion, memory, motivation
  • Hippocampus
    • Encodes spatial & episodic memories ("what happened, where & when")
    • Place cells: single neurons fire for specific environmental locations
  • Amygdala
    • Tags experiences with emotional salience (both positive & negative)
    • Loss → blunted fear & dampened impact of good/bad events on choices & memory
  • Basal Ganglia
    • Nucleus accumbens: motivation & reward valuation (dopaminergic input)
    • Caudate & Putamen: action selection, initiation & termination
    • Parkinson’s (hypokinetic): impaired action initiation (dopamine loss in substantia nigra → striatum)
    • Huntington’s (hyperkinetic): impaired action stopping
    • Habit formation: cue → automatic action (illustrated by dog chasing habit loop)
  • Thalamus
    • “Relay station” linking sensory inputs, cortex, and limbic/motor circuits (highly selective & modulatory)
  • Hypothalamus
    • Interface with endocrine system: stress (HPA axis), feeding, thermoregulation, reproduction
    • Sits under thalamus; employs hormonal release & autonomic control

Midbrain (Mesencephalon)

  • Houses small nuclei that flood the brain with neuromodulators
    • Ventral tegmental area (VTA) & substantia nigra pars compacta → dopamine
    • Others produce serotonin & acetylcholine
  • Dopamine misconceptions
    • Dopamine ≠ pleasure; it signals prediction error & motivates actions to obtain rewards
    • Pleasure (“liking”) thought to involve other transmitters (e.g., opioids, endocannabinoids)

Hindbrain (Rhombencephalon)

  • Pons & Medulla Oblongata
    • Vital autonomic functions: \text{breathing}, \text{heart rate}, swallowing, coughing
    • Barrington’s nucleus (in pons) controls micturition; stress inputs can inhibit urination
  • Cerebellum
    • Contains roughly half of all brain neurons (!!)
    • Coordinates fine motor control, balance, timing, precision learning (“making us better at things”)
    • Despite complexity, still poorly understood relative to cortex
  • No lobe/structure operates solo; e.g., temporal ↔ occipital ↔ parietal exchange is required for object identification
  • Executive function (PFC) uses:
    • Sensory maps (parietal, occipital)
    • Emotional/motivational signals (limbic, midbrain)
    • Motor pathways (frontal)
  • Clinical correlations sprinkled throughout the lecture highlight function–structure links (aphasia, neglect, Parkinson’s, H.M.)
  • Evolutionary layering: newer cortex refines & contextualizes signals from ancient limbic & brainstem circuits

What’s Next (Preview of Next Class)

  • Cellular level
    • Neuron anatomy (dendrites, soma, axon, synaptic terminals)
    • Neuroglia types & roles (astrocytes, oligodendrocytes, microglia, etc.)
  • Neurophysiology
    • Resting membrane potential & ion gradients
    • Action potentials (all-or-none spikes) & conduction
    • Synaptic transmission (chemical & electrical)
  • Intracellular components & molecular machinery