Ch 1. Brain Basics

Brain Overview

  • The brain is the literal “nerve-center” of the body, containing billions of neurons that simultaneously manage movement, thought, emotion, memory, and autonomic functions.
  • Multitasking examples: throwing a ball while talking, planning dinner while shopping, day-dreaming while driving—all possible because the brain is partitioned into specialized regions working in parallel.
  • Key principle: functional specialization + massive inter-connection = high-level cognition.

Major Brain Landmarks and Lobes

  • Cerebrum
    • Largest brain portion; split into left & right hemispheres.
    • Hemispheres connected by the corpus callosum (largest commissural tract).
    • Surface layer = cerebral cortex; deep folds (gyri & sulci) expand surface area, allowing more neurons → greater processing power.
  • Lobes & Core Functions
    • Frontal (above eyes): voluntary movement, speech, memory, emotion, planning, problem-solving, personality.
    • Parietal (top, behind frontal): integrates skin sensations, taste, portions of visual processing (spatial aspects).
    • Occipital (back): primary & secondary visual processing, color/shape recognition, complex visual comprehension.
    • Temporal (sides, level of eyes): auditory interpretation, some visual processing.
      • Hippocampus (curved, under cortex): encodes new memories.
      • Amygdala (deep, anterior to hippocampus): integrates memory with emotion.

Forebrain Structures: Limbic System & Basal Ganglia

  • Limbic System (emotion & motivation regulation)
    • Hippocampus & amygdala (temporal lobe).
    • Thalamus: sensory integration & relay hub.
    • Hypothalamus: hormonal control via pituitary; bodily homeostasis.
  • Basal Ganglia
    • Formed by portions of forebrain + midbrain.
    • Regulate complex voluntary movements; provide excitatory/inhibitory feedback loops to motor cortex (fine motor skills, writing, instrument playing).

Midbrain & Brainstem Components

  • Midbrain (under thalamus)
    • Coordinates eye movements (blink, focus).
    • Generates auditory startle reflex.
    • Contains nuclei that inhibit unwanted body movements & synchronize sensory ↔ motor signals.
  • Brainstem = Midbrain + Pons + Medulla
    • Core conduit between spinal cord & higher brain; houses life-support nuclei.

Hindbrain: Cerebellum, Pons, Medulla

  • Cerebellum
    • Second-largest volume; holds >50 % of neurons.
    • Two hemispheres, folded like cortex.
    • Functions: coordinate voluntary movement, motor learning, spatial & temporal perception.
      • Clinical example: cerebellar damage ⇒ jerky, arrhythmic gait; cannot touch finger to nose accurately.
  • Pons (below cerebellum): influences breathing & posture.
  • Medulla (lowest brainstem)
    • Connects brain ↔ spinal cord; controls swallowing, heart rate, breathing.

Brain Evolution

  • Originated from a simple neural tube (seen in lancelet Amphioxus).
  • Early vertebrate brain developed three bulges → forebrain, midbrain, hindbrain.
  • Evolutionary expansions:
    • Olfactory bulbs (chemical detection).
    • Visual processing areas as image-forming eyes evolved.
    • Cerebellum added for escape & orientation in active swimmers.
    • Cerebral hemispheres ballooned; cortex folded in mammals, greatly amplifying neuron count & processing power.

Neural Networks & Nerve Tracts

  • Nerve tract: bundle of long-range axons linking regions (e.g., corpus callosum, anterior commissure).
  • Neural network: ordered series of tracts + regions routing information in milliseconds.
  • Movie-watching pathway example:
    1. Retina photoreceptors → optic nerve.
    2. Optic tract → thalamus (basic shape/color/motion coding).
    3. Thalamus → primary visual cortex (edge detection, binocular 3-D integration).
    4. Divergence into two streams:
      • Temporal “what”: object identity.
      • Parietal “where”: spatial location.
    5. Visual cortex sends feedback to thalamus ⇒ continual refinement (thalamo-cortical loop).

Brain Waves (EEG Rhythms)

  • Rhythmic activity from thalamo-cortical loops detectable as brain waves.
  • Frequency bands (with typical cortical origins & states):
    • Alpha 8!\text{–}!13\,\text{Hz} (parietal/occipital, relaxed eyes-closed).
    • Beta 14!\text{–}!30\,\text{Hz} (frontal/parietal during sensory processing, concentration).
    • Theta 4!\text{–}!7\,\text{Hz} (light sleep).
    • Delta <3.5\,\text{Hz} (deep sleep).
  • Typical scalp amplitudes:
    • Alpha & Delta 20\text{–}200\,\mu\text{V}
    • Beta & Theta 5\text{–}10\,\mu\text{V}

Neural Circuits: Columns, Excitation & Inhibition

  • Cortical microcircuit: neurons arranged in stacked layers forming vertical columns dedicated to specific features (e.g., a single pixel, pitch, or tactile point).
  • Signal flow: feed-forward down the column; each relay transforms information (edges → shapes → faces).
  • Lateral connectivity: neurons talk to neighboring columns, allowing dynamic modulation (context-dependent perception).
  • Neuron types
    • Excitatory \approx80\% (e.g., pyramidal cells): push postsynaptic neurons toward firing.
    • Inhibitory \approx20\% (local interneurons): suppress activity; essential for tuning & preventing runaway excitation (epilepsy if imbalanced).
  • Recurrent network motifs
    • Feed-forward inhibition: column A excites its own path while inhibiting neighbors → sharper contrast.
    • Feedback inhibition: downstream excitation loops back via interneurons to dampen earlier layers → stability & timing.

Neuron Anatomy

  • Soma (cell body): nucleus + protein machinery.
  • Dendrites: branched receivers; thousands of synapses per neuron.
  • Axon: single long output cable; can span >1\,\text{m}; ends in axon terminals.
  • Myelin (oligodendrocyte/Schwann cell sheath): increases conduction speed via saltatory transmission between nodes of Ranvier.

Glial Cells (≈ equal numbers to neurons in primate cortex)

  • Astrocytes: ion buffering, nutrient supply, synapse formation/cleanup.
  • Microglia: immune surveillance, phagocytosis, synaptic pruning.
  • Ependymal cells: secrete cerebrospinal fluid.
  • Oligodendrocytes: form myelin in CNS (Schwann cells in PNS).

Ion Channels & Action Potentials

  • Resting potential \approx -70\,\text{mV} (inside negative).
  • Synaptic inputs depolarize or hyperpolarize dendritic membrane.
  • If summed depolarization reaches threshold (voltage-gated Na⁺ channels open), an action potential fires—an all-or-nothing electrical pulse propagating down the axon.

Synapses & Neurotransmission

  • Chemical synapse components: presynaptic axon terminal, synaptic cleft, postsynaptic density.
  • Sequence:
    1. Action potential arrives → voltage-gated Ca²⁺ channels open.
    2. Ca²⁺ triggers fusion of synaptic vesicles → neurotransmitter release.
    3. NTs diffuse <1\,\mu\text{m} across cleft → bind postsynaptic receptors.
    4. Receptors open/close ion channels → postsynaptic potential.
    5. Termination: enzymatic breakdown or reuptake into presynaptic terminal; astrocytes assist by “mopping up.”
  • Receptor families
    • Ionotropic: receptor = part of ion channel (fast, ms scale).
    • Metabotropic: receptor triggers G-protein cascade → indirect channel modulation or gene effects (slower, modulatory).

Neurotransmitters: Glutamate & GABA

  • Glutamate (excitatory; \approx50\% of synapses)
    • AMPA receptors: fast, transient depolarization.
    • NMDA receptors: slower, require multiple spikes & Mg²⁺ unblock; crucial for synaptic plasticity, learning & memory.
  • GABA (inhibitory)
    • GABA_A (ionotropic): Cl⁻ influx → hyperpolarization.
    • GABA_B (metabotropic): K⁺ efflux → hyperpolarization.

Receptors & Molecular Signaling (Hormones, Neuromodulators)

  • Hormones (e.g., cortisol, estradiol) carry systemic information; can act on membrane receptors or, if lipid-soluble, intracellular receptors → act as transcription factors.
  • Neuromodulators (e.g., endocannabinoids) fine-tune neurotransmission—often suppress release probability.
  • Prostaglandins: small lipids raising pain sensitivity during inflammation.
  • Signal transduction: ligand binding → conformational change → intracellular cascade (second messengers, kinases, ion balance shifts, gene regulation).

Genes, Gene Expression & Neurological Disorders

  • All neurons share identical DNA; functional diversity arises from differential gene expression.
  • Chromatin remodeling (tight vs. open) controls which genes are accessible; reversible → neurons adapt to environment & hormones.
  • Alleles: sequence variations ⇒ protein variants with differing efficacy.
    • Example: Tay-Sachs disease—mutations in HEX A gene → defective β-hexosaminidase A; toxic lipid accumulation, fatal neurodegeneration.
  • Advances in whole-genome sequencing will clarify genetic contributions to brain disorders within the next decade.