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Biological Basis of Behavior #3 – Comprehensive Lecture Notes

Central vs. Peripheral Nervous System

  • The nervous system is divided into two main "major units":
    • Central Nervous System (CNS)
    • Composed of the brain and spinal cord
    • Primary processing center for integration and coordination
    • Peripheral Nervous System (PNS)
    • All neural tissue outside the CNS
    • Carries information to/from the CNS
    • Branches further into the Somatic and Autonomic divisions
  • Autonomic Division (of PNS) splits into:
    • Sympathetic Nervous System
    • Activates “fight‐or‐flight” responses
    • Increases heart rate, dilates pupils, mobilizes energy
    • Parasympathetic Nervous System
    • Promotes “rest‐and‐digest” activities
    • Slows heart rate, fosters digestion, conserves energy

Macro-Organization of the Brain

  • Brain regions never act in isolation; complex functions emerge from interconnections.
  • Cerebral Cortex Lobes
    • Frontal Lobe
    • Complex thought & planning
    • Movement control; contains a map of the body’s muscles
    • Parietal Lobe
    • Processes touch & spatial awareness
    • Houses a map of the body’s skin surface
    • Temporal Lobe
    • Hearing and object memory
    • Occipital Lobe
    • Vision processing hub
    • Insular (Insula) Lobe
    • Taste perception
    • Interoceptive awareness of internal organs
  • Subcortical & Brainstem Structures (overview only, detailed coverage implied)
    • Limbic system (emotion & memory)
    • Midbrain (dopamine nuclei, e.g., Ventral Tegmental Area)
    • Brainstem (basic life functions)

Mesoscopic Organization – Cortical Layers & Networks

  • Neurons are arranged in layers (especially in cortex)
    • Each layer gets a distinct mix of inputs and sends outputs to specific targets.
    • Layered arrangement enables rapid complexity: different input combinations → sophisticated processing.
  • The brain behaves like a social network
    • Question 1: “Does a given neuron follow many similar accounts or a diverse mix?”
    • Question 2: “Does a neuron broadcast publicly or send private messages?”
    • Analogy emphasizes variability in connectivity patterns and information flow.

Microscopic Anatomy – Neuron Shapes & Polarity

  • Shape dictates function: dendritic architecture determines what inputs a neuron “listens” to.
  • Despite shape diversity, all neurons share a fundamental structure:
    • Dendrites
    • Receive chemical & electrical input
    • Integration across many synapses
    • Cell Body (Soma)
    • Integrates dendritic signals
    • Contains DNA, receptors, protein‐making machinery
    • Axon
    • Conducts electrical signal (action potential) away from soma
    • Speed enhanced by myelin (from glia) → “white matter”
    • Axon Terminals
    • Release neurotransmitters when action potential arrives

Long-Range Projections Example – Dopamine Cells

  • Ventral Tegmental Area (VTA) in midbrain:
    • Cell bodies & dendrites reside locally.
    • Axons project widely: terminals found across multiple brain regions simultaneously.
    • Myelination by glia essential for rapid long-distance transmission.

Electro-Chemical Signaling: Action Potentials

  • Neurons “fire” when combined inputs exceed a voltage threshold.
    • Sequence:
    1. Resting state
    2. Depolarizing inputs (< threshold) → still no spike
    3. Summed input crosses threshold → Action Potential (AP)
    4. Repolarization & Refractory Period
    5. Return to resting potential
  • Joke slide: the struggle from first‐time to 1000th‐time readers trying to grasp AP mechanics.
  • Key ionic basis (implied): \text{Na}^+ influx, \text{K}^+ efflux, membrane potential swings from \approx -70\ \text{mV} to +30\ \text{mV}.

Synaptic Transmission – Turning Electricity into Chemistry & Back

  • AP arriving at terminals → vesicles release neurotransmitter into synaptic cleft.
  • Neurotransmitter crosses the gap → binds receptors on the postsynaptic neuron.
  • Binding opens ion channels → positive ions enter → membrane becomes less negative (depolarization).
  • Depolarization can contribute to the next neuron’s threshold crossing.

Receptors, Agonists & Antagonists

  • Endogenous neurotransmitters (made inside body) have high affinity & activate specific receptors.
  • Exogenous molecules (drugs/pharmacological agents) can also bind these receptors:
    • Agonist: activates receptor (mimics or amplifies natural ligand)
    • Antagonist: binds without activation, prevents receptor from being activated by agonist.
  • Key Insight: Receptors are less picky than assumed; structural similarity allows many drugs to hijack signaling.

Life-Saving Example – Naloxone

  • Endogenous opioids: small, precise doses → pleasure & analgesia.
  • Opioid drugs (morphine, fentanyl, oxycontin, heroin): powerful agonists.
    • Benefits: pain relief.
    • Risks: respiratory & cardiac depression at high doses → overdose.
  • Naloxone (Narcan): competitive opioid receptor antagonist.
    • Displaces opioid agonists, blocks receptor, reverses overdose, saves lives.

Glia – The Other Half of the Brain

  • At least half of brain cells are glia (support cells), not neurons.
  • Major functions:
    • Myelination: Oligodendrocytes (CNS) & Schwann cells (PNS) wrap axons with fatty myelin → faster conduction.
    • Waste Patrol & Cleanup: Microglia remove debris, excess neurotransmitter, prune synapses.
    • Provide metabolic & structural support to neurons.
  • Take-home: neurons get the spotlight, but glia make neural signaling possible.

Quick Recap – “Tell All Your Friends How Cool the Brain Is”

  • Nervous system units: CNS vs. PNS; Sympathetic vs. Parasympathetic.
  • Brain organization: cortical lobes, limbic system, midbrain, brainstem, layered circuitry.
  • Brain composition: neurons and glia.
  • Communication: electrical (action potentials) × chemical (neurotransmitters).
  • Pharmacology: agonists & antagonists (e.g., naloxone) illustrate real-world impact.