CA

CH 3 A: Biology and Behavior – Nervous System & Brain

Organization of the Nervous System

  • Two macroscopic divisions
    • Central Nervous System (CNS)
    • Brain, spinal cord, retina
    • Peripheral Nervous System (PNS)
    • All neural tissue outside CNS
    • Sub-divisions
      • Somatic Nervous System → voluntary skeletal‐muscle control, sensory input from skin/muscles (somatosensation)
      • Autonomic Nervous System (ANS) → involuntary visceral regulation
      • Sympathetic Division → “fight / flight” responses
      • Parasympathetic Division → “rest / digest” responses

Neurons – Basic Units of Information Processing

  • Everything we think, feel, & do is mediated by neural activity
  • Neuron = cell that receives, integrates, transmits information
    • Operates via electrical impulses, communicates via chemical signals, forms neural networks
  • Two main branch types
    • Dendrites → receive signals
    • Axons → send signals

Detailed Anatomy

  • Dendrite – branch-like receivers of input from other neurons
  • Cell body (soma) – integrative hub; sums thousands of inputs
  • Axon – long fiber carrying action potential (AP) away from soma
  • Myelin sheath – fatty insulation; speeds AP propagation
  • Nodes of Ranvier – gaps between myelin segments where ion exchange occurs; enable saltatory conduction
  • Terminal buttons – swellings at axon end; release neurotransmitter (NT)
  • Synapse
    • Microscopic gap between presynaptic terminal & postsynaptic membrane
    • Site of chemical communication

Functional Types

  • Sensory neurons – transduce physical energy → neural code; e.g., somatosensory nerves from skin & muscles
  • Motor neurons – command muscle contraction / relaxation → movement

Electrical Signaling

  • Resting Membrane Potential (RMP)
    • At rest inside of neuron ≈ -70\,\text{mV} relative to outside
    • Polarized – more negative ions inside than outside; stores potential energy
  • Action Potential (AP)
    • All-or-none electrical spike traveling along axon
    • Triggered when depolarization reaches threshold ≈ -55\,\text{mV}
    • Sequence
    1. Stimulus opens voltage-gated \text{Na}^+ channels → rapid depolarization to ≈ +40\,\text{mV}
    2. \text{Na}^+ channels inactivate; \text{K}^+ channels open → repolarization
    3. Hyperpolarization / Refractory period ensures unidirectional travel; cell cannot fire again immediately
    • After AP, \text{Na}^+/\text{K}^+-ATPase pump restores ionic gradients (3 \text{Na}^+ out / 2 \text{K}^+ in)

Why Myelin Matters

  • Myelin resists ion leakage → AP jumps node-to-node (saltatory conduction)
  • Greatly increases conduction velocity & energy efficiency; myelin loss (e.g., multiple sclerosis) slows signaling

Chemical Signaling – Synaptic Transmission

  1. AP arrives at terminal → \text{Ca}^{2+} influx
  2. Vesicles fuse → neurotransmitter released into synaptic gap
  3. NT binds postsynaptic receptors → ion channels open / close, changing postsynaptic potential
  4. Termination mechanisms
    • Reuptake into presynaptic cell
    • Enzymatic degradation
    • Diffusion away from synapse

Key Neurotransmitters

  • Glutamate – primary excitatory NT; increases likelihood of firing
  • GABA (gamma-aminobutyric acid) – primary inhibitory NT; decreases likelihood of firing

Drug Effects on Transmission

  • Psychoactive drugs modify any synaptic step
    • Stimulate / inhibit release (e.g., amphetamine)
    • Mimic NT (agonists; e.g., morphine mimics endorphins)
    • Block receptors (antagonists; e.g., naloxone vs opioids)
    • Inhibit reuptake (e.g., SSRIs prolong serotonin signaling)

Protective Structures

  • Meninges – three connective-tissue layers surrounding brain & spinal cord
  • Cerebrospinal Fluid (CSF) – cushions, provides buoyancy, nutrient waste exchange

Brainstem

  • Extension of spinal cord; automatic survival functions
    • Medulla oblongata – heart rate & blood pressure
    • Pons – sleep, arousal
    • Midbrain (contains thalamus & hypothalamus in this lecture’s classification)
    • Reticular formation – sleep–wake regulation

Thalamus & Basal Ganglia

  • Thalamus – “gateway to cortex”; almost all sensory info (vision, hearing, touch) synapses here before cortex
  • Basal Ganglia (BG) – large subcortical nuclei controlling voluntary movement & reward learning
    • Degeneration → Parkinson’s disease (tremor, rigidity) due to dopamine loss in BG circuitry

Cerebellum

  • Maintains balance, posture, motor coordination, motor learning, & motor memory
  • Also contributes to language, cognition, perception

Limbic System – Emotion & Motivation

  • Hypothalamus – homeostasis & motivated behaviors: fight, flight, feeding, sex; regulates endocrine & ANS
  • Hippocampus – formation & retrieval of long-term memories
  • Amygdala – detection / generation of fear & aggression; emotional tagging of memories
  • Cingulate Cortex
    • ACC – decision making, reward anticipation, empathy, autonomic control (via hypothalamus)
    • PCC – memory & visual–spatial processing

Reward Circuit & Addiction

  • Ventral Tegmental Area (VTA), Nucleus Accumbens (NAc), & prefrontal cortex release dopamine during pleasurable experiences (food, sex, drugs) → reinforcement & addiction vulnerability

Cerebral Hemispheres & Corpus Callosum

  • Corpus callosum – bundle of millions of axons linking hemispheres
  • Split-brain surgery (callosotomy)
    • Treats intractable epilepsy by preventing seizure spread
    • Revealed lateralized cortical functions (e.g., left = language, right = spatial)
    • Research pioneered by Michael Gazzaniga

Neocortex – Site of Complex Cognition

  • Highly folded outer layer of cerebrum; four lobes per hemisphere

Occipital Lobe

  • Primary visual cortex (V1); further processing in V2, V3, V3A, V4, MT/V5, etc.
  • Interfaces with temporal & parietal lobes to derive “what” & “where” information

Parietal Lobe

  • Primary somatosensory cortex (S1) → touch, pain, temperature, proprioception
  • Posterior areas integrate visual‐spatial info

Temporal Lobe

  • Primary auditory cortex (tonotopically organized \approx 500\text{–}16{,}000\,\text{Hz})
  • Specialized regions for object & face recognition (fusiform gyrus)
  • Houses hippocampus & amygdala (medial temporal)

Frontal Lobe

  • Primary motor cortex (M1) → voluntary movement (motor homunculus)
  • Prefrontal cortex (PFc)
    • Planning, attention, judgment, working memory
    • Orbitofrontal cortex – olfaction & emotion
  • Famous case: Phineas Gage (railroad spike) → profound personality change → PFc role in social behavior
    • Lobotomy (Egas Moniz, Nobel 1949) destroyed PFc to treat mental illness; produced lethargy, emotional blunting

Sensory & Motor Maps

  • Adjacent body parts map onto adjacent cortical tissue (somatotopy)
    • Over-representation of hands, lips, tongue (fine control / acuity)
  • Motor map mirrors sensory map anterior to central sulcus

Olfactory & Gustatory Cortex

  • Olfactory cortex (ventral frontal / temporal) processes smell
  • Gustatory cortex (insula & frontal operculum) processes taste

Auditory Processing Details

  • Primary → frequency (pitch) map; secondary areas integrate complex sounds (language, music)

Visual Processing Details

  • Left & right visual fields project to contralateral hemispheres
  • Right hemisphere excels at spatial relations; left at language‐based interpretation of visuals

Human Brain Imaging & Stimulation Techniques

  • Electroencephalography (EEG)
    • Measures scalp electrical potentials; excellent temporal (ms), poor spatial (cm)
  • Positron Emission Tomography (PET)
    • Inject \gamma-emitting tracer; shows metabolic activity; good spatial (mm–cm), poor temporal (min)
  • Magnetic Resonance Imaging (MRI)
    • Static structural images via magnetic fields & radio waves; high spatial resolution
  • Functional MRI (fMRI)
    • Measures Blood-Oxygen-Level Dependent (BOLD) signal; good spatial, seconds-level temporal
  • Transcranial Magnetic Stimulation (TMS)
    • Brief magnetic pulses create localized, reversible “virtual lesions”; causal inference about cortical function

Ethical, Clinical & Real-World Notes

  • Myelin diseases (Multiple Sclerosis) illustrate importance of conduction speed
  • Parkinson’s Disease highlights basal ganglia role & dopamine dependency
  • Psychoactive drug action (e.g., SSRIs) demonstrates modulation of synaptic chemistry for therapy
  • Lobotomy history underscores ethical evolution of neuroscience practices
  • Imaging & stimulation enable diagnosis (tumors, stroke) & treatment (TMS for depression)