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Neurobiology Chapter 2 Notes

Discovering the Cells of the Brain

  • The origin of understanding brain cells traces back to the 1600s with discovery of brain cells; neurons were not identified for centuries after that due to technical limits (they are very small, brain tissue is mushy, preservatives were needed).
  • Early neuroscience relied on histology to link structure and function: the idea that the brain does something unique implies neurons may have distinctive structure.

Neuroscience Methods: Histology

  • Histology = microscopic study of anatomy (shape and organization) to infer structure–function relationships.
  • Core steps in histology (1800s methods) to study fixed tissue:
    • Step 1: Fix the tissue to prevent disintegration
    • Step 2: Stain the tissue
    • Step 3: Slice the tissue
    • Step 4: Visualize the tissue
  • H&E staining (basic histology) allowed visualization but raised the question: can we tell which cell is a neuron just by this stain?
  • Major limitation of general stains: often cannot uniquely identify neuron vs other cells without more specific staining methods.

The Neuron-Specific Stains: Nissl vs Golgi

  • Nissl stain:
    • Stains the cell body primarily (soma) because neurons have a high density of protein synthesis organelles (rough endoplasmic reticulum).
    • Useful for seeing soma size/shape but does not reveal full neuronal morphology.
  • Golgi stain (silver stain):
    • Stains the entire structure of the neuron (cell body plus all processes).
    • Invented by Camillo Golgi; uses a different set of liquids; fixes, stains, slices, and visuals the full neuron.
    • Key point: Golgi staining reveals the complete morphology of a neuron, enabling study of dendrites and axons.

The Golgi Stain Method (details)

  • The Golgi stain reveals the full structure of a neuron; however, only about 1\%-5\% of neurons get stained in a given sample.
  • Typical sample characteristics observed with Golgi staining:
    • Slice thickness around 50\,\mu\mathrm{m}
    • Dendritic spines can be as small as 2\,\mu\mathrm{m} on the surface.
    • The technique produced a sparse, stained population of neurons that allowed tracing of entire cell morphology.
  • Why only 1–5%? The exact impregnation process randomly labels a small subset of neurons, permitting detailed single-neuron morphologies without overplotting.

The Big Question in Early Neuroscience: Reticular Theory vs Neuron Doctrine

  • The pettiest debate: Is the nervous system a single continuous network (reticular theory) or a collection of discrete units (neuron doctrine)?
  • Golgi and Cajal contributed opposing views:
    • Golgi suggested a network where neurons were connected in a continuous reticulum.
    • Cajal argued for discrete neurons that are functionally connected but anatomically separate.
  • The neuron doctrine argues that the nervous system is composed of separate cells (Ramón y Cajal) with the neuron as the basic unit; there are gaps between neurons (synapses) rather than a continuous network.

Nobel Prize Context (1906)

  • The debate culminated in Nobel Prizes awarded to Camillo Golgi and Santiago Ramón y Cajal in 1906.
  • Cajal championed the neuron doctrine, emphasizing that neurons are distinct units with specialized functions and that the nervous system is not a single continuous reticulum.
  • Golgi, while acknowledging his own staining method's insights, remained skeptical of replacing terminology; he warned against equating the word neuron with nerve cell itself.
  • Key statements from Golgi's Nobel Lecture touched on: neurons as morphological entities; terminal arrangement of nerve fibers; reciprocal, not continuous, connections; evidence from Golgi staining across brain regions.

What Can a Stain Tell Us About Neurons? (Key Concepts)

  • Neurons are distinct morphological units rather than a single intertwined network, as evidenced by selective Golgi staining and Cajal’s observations.
  • The presence of synaptic contacts (gaps) supports the neuron doctrine: neurons contact each other at specialized sites (synapses) rather than sharing a continuous cytoplasm.
  • Although the neuron doctrine is central, later discoveries revealed that some cells and glial processes do connect via gap junctions and tunneling nanotubes, indicating that there are exceptions to strict separation.

Neurons: What Makes Them Unique? (Major Features)

  • Neuron features can be summarized as several unique and extreme characteristics compared to other cells:
    • Cytoskeleton and polarization:
    • Neurons are highly polarized; structure dictates function.
    • They have a crazy, highly extended morphology with neurites (axons and dendrites).
    • Cellular diversity vs genetic diversity:
    • Neurons exhibit vast cellular diversity without a correspondingly large increase in genetic diversity; many neuronal types arise from regulated expression and modifications.
    • Central dogma challenges:
    • Neurons tend to exploit mechanisms beyond simple one-gene-one-protein flow; they “cheat” the central dogma by using alternative transcriptional and post-translational strategies to increase protein diversity.
    • High energy demand:
    • Neurons are energy hogs and rely heavily on mitochondria.
    • The neurons you are born with are the neurons you die with (non-replacing). They don’t regenerate or replace themselves readily.
  • Implications: These features underpin why neuronal dysfunction can be so impactful and why understanding neuron-specific processes is crucial for neuroscience.

Neuron Anatomy: Key Components

  • Basic parts:
    • Soma (cell body) with nucleus, smooth and rough ER (Nissl), Golgi apparatus.
    • Dendrites: input sites that receive signals from other neurons.
    • Axon: output conduit that transmits signals to other neurons or target cells.
    • Axon hillock: initiation site of axon near the soma.
    • Axon terminals: synaptic endpoints that release neurotransmitters.
    • Myelin and Schwann cells (PNS) or oligodendrocytes (CNS): insulate axons to speed transmission.
    • Nodes of Ranvier: gaps in myelin that facilitate saltatory conduction.
    • Neurites: generic term for dendrites and axons; branching illustrated as dendrites and axon with terminals.

Dendrites: receive signals

  • Dendrites are often postsynaptic structures that receive signals from adjacent neurons.
  • Dendritic tree: all dendritic branches.
  • Dendritic spines: small postsynaptic protrusions that host receptors at synapses; axons form synapses onto spines but axons themselves are not visible in a spine image.
  • Function: Convert synaptic input into electrical signals integrated by the neuron.

The Axon: output and transmission

  • Axon transmits signals away from the soma to other neurons or target cells.
  • Axon hillock: start of the axon, near the soma, where action potentials often initiate.
  • Axon collaterals: branches off the axon.
  • Axon terminals: presynaptic ends that release neurotransmitters in vesicles.
  • Axon terminals contain a high density of membrane proteins and mitochondria; rough ER is not present in the axon (no ribosomes for protein synthesis).
  • Synapses can form on muscle or other targets via axon terminals.

Synapse: The Communication Junction

  • Presynaptic terminal: contains synaptic vesicles and abundant membrane proteins; releases neurotransmitters.
  • Postsynaptic site: typically on a dendritic spine; contains receptors.
  • Synaptic cleft: the gap between pre- and postsynaptic elements where neurotransmitters diffuse.

The Cytoskeleton: Structure and Transport

  • Three key components:
    • Microtubules (largest): built from tubulin, polymerized; regulated by microtubule-associated proteins (MAPs).
    • Neurofilaments (medium): provide structural support.
    • Microfilaments (smallest): important in dendritic spines and neurite structure; actin-based.
  • Purpose: provide internal scaffolding, enable transport, and support morphology.
  • Tau and microtubules:
    • Tau is a MAP that stabilizes microtubules.
    • In Alzheimer’s disease, kinases can phosphorylate tau excessively, causing it to detach from microtubules and form neurofibrillary tangles; loss of microtubule function leads to neuronal death.

Axonal Transport: Moving Materials Along Neurons

  • Proteins are produced mainly in the soma but must reach axon terminals and dendrites.
  • Transport along the axon uses motor proteins powered by ATP:
    • Anterograde transport: away from the soma to the axon/dendrites; driven by kinesins.
    • Retrograde transport: toward the soma; driven by dyneins.
  • Proper targeting of proteins is crucial; mislocalization can cause dysfunction and disease.

If Proteins Don’t Get to the Right Place: Illustrative Patient Case

  • Patient file (pseudonym): Ava Reynolds, 7 months old, seizures with eye movements, muscle stiffness and flaccidity episodes, inconsolable crying.
  • History: Normal development until 5 months; seizures began at 6 months and escalated.
  • Highlights the clinical relevance of proper protein localization and neuronal transport for healthy brain function.

Tool #1: Immunocytochemistry (ICC)

  • Purpose: locate where proteins are in cells.
  • Principle: uses antibodies that bind specifically to target proteins.
  • Visualization: uses light to detect where the antibody-bound proteins are located.
  • Significance: allows mapping of protein distribution within neurons and glia.

Unique Features of Neurons (Recap)

  • Cytoskeleton and polarity: highly polarized cells with distinct compartments.
  • Structure–function relationship: the structure of neurons enables their specialized functions.
  • Diversity with limited genetic diversity: neurons use regulatory mechanisms to expand protein repertoires beyond what the gene count would suggest.
  • Central dogma challenges: neurons do not strictly follow one gene→one protein; they employ alternative splicing and post-translational modifications to diversify proteins.
  • Energy demands: neurons require substantial energy; discussed more below.
  • Non-replication: neurons are largely non-replicating; there are limited pools of progenitor cells that can form new neurons in adulthood.

How Neurons Maximize Protein Diversity (Two Main Strategies)

  • Alternative splicing: introns are removed and exons rearranged to produce multiple protein variants from a single gene.
    • In the brain, this dramatically expands protein diversity beyond the ~2\times 10^{4} protein-coding genes.
    • Example: DSCAM gene can generate enormous exon combinations; one estimate shows up to 3.8\,1\,6 exon combinations (commonly reported as 38,016) allowing thousands of possible protein products.
  • Post-translational modification (PTM): after translation, proteins are modified (e.g., phosphorylation) to alter function.
    • Kinases add phosphate groups; phosphatases remove them.
    • CAMKII is a brain kinase linked to learning; blocking CAMKII impairs learning because proteins aren’t in the required phosphorylation state.
  • Together, alternative splicing and PTMs greatly expand neuronal protein diversity from a finite gene set.

Energy Demands of Neurons

  • Neurons are energy-hogs, consuming a large portion of the body's energy (
    • They account for about 0.20\text{ of total energy} consumption, i.e., 20\% of energy while comprising only \approx 2\% of body weight: the energy demand is disproportionately high.
  • Mitochondria in neurons are abundant: up to 2\times 10^{6} mitochondria per neuron, compared to roughly a few hundred to a few thousand in typical somatic cells.
  • Why so much energy?
    • To maintain electrically excitable membranes, support synaptic transmission, and sustain mitochondrial ATP production.
    • The resting ATP consumption by an average neuron is enormous: about 4.7\times 10^{9} ATPs per second (4.7\text{ billion ATP/s}).
  • Consequences: mitochondrial dysfunction can severely affect neural function and contribute to neurodegenerative diseases.

Mitochondrial Health and Neurological Risk (Hyperlink to common crises)

  • Acute energy deficits manifest as physical and cognitive symptoms: e.g., hypoglycemia can trigger shivering, hunger, confusion, dizziness, and other systemic effects due to insufficient ATP production in neurons and other tissues.
  • A memorable, light-hearted example used in lectures: “You’re not you when you’re hungry” to illustrate energy depletion effects on brain function.

Neuronal Regeneration and Progenitors

  • Neurons largely do not reproduce or replace themselves; however, there are small pools of neuron progenitors that can form neurons throughout adulthood.
  • Implications for brain injury and neurodegenerative disease: limited regeneration capacity contributes to lasting deficits after injury.

The Brain’s Other Cells: Glia

  • The brain contains non-neuronal glial cells that provide support and regulation:
    • Macroglia:
    • Astrocytes – approximately 20% of glia; support synapses, regulate extracellular chemicals (e.g., glutamate), help convert glutamate to glutamine for recycling in neurons, and regulate the blood-brain barrier (BBB).
    • Oligodendrocytes – ~25% of glia in the brain; form myelin in the CNS; contribute to rapid signal conduction along axons.
    • Precursors – about ~5%; can give rise to new glial cells.
    • Microglia – ~10%; resident immune cells of the brain; clear dead cells, prune synapses, respond to injury and disease; overactivation can contribute to neurodegenerative diseases (e.g., Alzheimer’s).
  • Myelination and nodes of Ranvier:
    • Myelin sheaths (oligodendrocytes in CNS; Schwann cells in PNS) insulate axons to speed up conduction.
    • Node of Ranvier is a bare region of axonal membrane rich in voltage-gated channels, enabling saltatory conduction.

Oligodendrocytes vs Schwann Cells: Myelination Across the Nervous System

  • CNS: oligodendrocytes wrap multiple axons with myelin in the central nervous system.
  • PNS: Schwann cells wrap individual axons with myelin in the peripheral nervous system.
  • The myelin sheath contains layers of membrane and cytoskeletal components to insulate and speed neural signals.

Disease Spotlight: Multiple Sclerosis (MS)

  • MS is an autoimmune attack on myelinating cells.
  • Demyelination damages signal conduction, leading to slowed or blocked nerve signaling and various neurological symptoms.

Astrocytes and Microglia: Roles in Health and Disease

  • Astrocytes:
    • Maintain extracellular chemical balance and regulate neurotransmitters (e.g., glutamate -> glutamine recycling).
    • Regulate the BBB and interact with blood vessels to maintain brain homeostasis and nutrient delivery.
    • Promote synapse formation and support neuronal health.
  • Microglia:
    • Immune cells of the brain; remove dead cells, prune synapses, and clear protein aggregates.
    • Dysregulated microglia activity can contribute to neurodegenerative diseases (e.g., Alzheimer's).

Re-visiting Golgi’s Theory: Connections Between Cells

  • Reticular theory vs neuron doctrine revisited in modern context:
    • Gap junctions provide direct cytoplasmic connections between some neurons and glia, implying some level of direct continuity.
    • Tunneling nanotubes are channels that connect cells and allow direct exchange of materials.
  • These discoveries do not negate the neuron doctrine but indicate that brain connectivity includes both discrete units and certain direct intercellular channels.

Chapter 2 Learning Objectives (Summary of Goals)

  • Describe the basic steps of staining in histology (fixation, staining, slicing, visualization).
  • Differentiate between the Nissl stain and the Golgi stain, including what each reveals about neuron structure.
  • Understand the importance of the structure–function relationship in neurons.
  • Define the Neuron Doctrine and contrast it with Reticular Theory; explain why the Neuron Doctrine progressed neuroscience.
  • Identify all parts of a neuron (membrane, cytoskeleton, soma, neurites, axons, dendrites, axon hillock, nodes of Ranvier, synapse, axon terminal, dendritic spines) and describe their functions and how dysfunction could affect neural signaling.
  • Differentiate cytoskeletal components (microtubules, neurofilaments, microfilaments) and explain anterograde vs retrograde transport.
  • Describe the purpose and method of immunocytochemistry.
  • Describe the three main types of glia, their functions, and roles in disease.
  • Explain how Golgi staining contributed to the neuron doctrine, and discuss two ways in which Golgi was not entirely wrong.
  • Connect these concepts to real-world relevance, including disease mechanisms and brain energy demands.