Chapter 2 Notes: Neurons and Glia

THE NEURON DOCTRINE

  • The brain is the most complex organ, but unraveling its function starts with studying brain cells individually and then in circuits.

  • Core idea: mind-brain are not separable; understanding single-cell and network actions yields understanding of mental abilities.

  • The book’s structure follows a “neurophilosophy”: start with cells (neuron and glia), then circuits mediating sensation, perception, movement, speech, and emotion.

  • Neurons vs. glia distinction is crucial:

    • Neurons and glial cells are present in roughly equal numbers in the adult human brain (~85 billion each).

    • Neurons are primarily responsible for sensing changes, communicating them, and commanding body responses.

    • Glia mainly insulate, support, and nourish neurons.

  • Analogy: if the brain were a chocolate chip cookie, neurons are the chips and glia the cookie dough that fills spaces and holds chips in place; glia derive from Greek for “glue.”

  • Neuroscience has its own language; review keywords after reading to build vocabulary.

THE PROTOTYPICAL NEURON

  • Neuronal structure basics: soma (cell body), dendrites, axon; inside/outside separated by the neuronal membrane.

  • Size context: neurons are very small (typical diameter ~0.01−0.05 mm0.01-0.05\,\text{mm}); visible with compound microscope only after fixation and sectioning.

  • Historical breakthroughs enabling brain cell study:

    • Fixation with formaldehyde to harden tissue.

    • Microtome to slice very thin sections.

    • Staining methods to reveal cellular components (Nissl stain, Golgi stain).

  • The Golgi stain (Camillo Golgi, 1873) revealed that:

    • A small percentage of neurons are darkly colored in their entirety.

    • The neuron is composed of a soma and radiating neurites (axons and dendrites).

    • The soma is only a portion of the total neuronal structure; neurites extend from it.

  • The Nissl stain (Franz Nissl) stains nuclei and surrounding material (Nissl bodies) and is useful to:

    • Distinguish neurons from glia.

    • Study cytoarchitecture (regional neuronal arrangement).

  • Early histology showed two competing views:

    • Golgi supported a reticular theory (neurites fused into a continuous network).

    • Cajal argued for the neuron doctrine: neurites are not continuous; neurons communicate by contact.

  • Electron microscopy later provided definitive proof (1950s): neurites from different neurons are not continuous.

  • Key consequences:

    • The starting point for brain study is the individual neuron, not a continuous reticulum.

BOX 2.1 Advances in Microscopy (special interest)

  • Human eye resolution limit: about 100 μm100\,\mu\text{m} between distinguishable points.

  • Neuron diameter ~20 μm20\,\mu\text{m}; neurites can be much thinner.

  • Light microscope resolution limit ~∼0.1 μm\sim 0.1\,\mu\text{m}; space between neurons is ~0.02 μm0.02\,\mu\text{m}, which made early conclusions about continuity challenging.

  • Electron microscope resolution limit ~0.1 nm0.1\,\text{nm} (a million times better than the naked eye; ~1000× better than light microscopy).

  • Modern microscopy combines lasers and computer-based image reconstruction; fluorescence labeling enables live imaging and super-resolution (~down to ~20 nm20\,\text{nm}).

  • Figure concept: electron vs. light microscopy; super-resolution imaging.

THE PROTOTYPICAL NEURON (internal organization)

  • The neuron comprises: soma, dendrites, axon, and a membrane barrier enclosing cytoplasm.

  • Internal components (soma): nucleus, rough ER, smooth ER, Golgi apparatus, mitochondria; cytoplasm excludes nucleus.

  • The nucleus:

    • Spherical, centrally located; about 5−10 μm5-10\,\mu\text{m} in diameter.

    • Enclosed by a double membrane (nuclear envelope) with ~0.1 μm0.1\,\mu\text{m} pores.

    • Contains chromosomes with DNA; DNA in neurons is the same as in other cells, but gene expression patterns differ.

  • DNA, genes, and gene expression:

    • DNA laid out as a double helix; genes are segments that are used to assemble proteins.

    • Gene expression ( transcription ) produces mRNA; translation then assembles proteins.

    • The length of the human genome spans roughly 2 m if stretched end-to-end for the 46 chromosomes; genes are 0.1 to several µm in length.

  • The central dogma of molecular biology (summarized):
    Transcription: DNA→mRNA;Translation: mRNA→Protein\text{Transcription: } \text{DNA} \rightarrow \text{mRNA}; \quad \text{Translation: } \text{mRNA} \rightarrow \text{Protein}

  • Key molecular biology terms:

    • DNA, chromosome, gene, promoter, transcription factor, introns, exons, RNA splicing, mRNA, translation, ribosome.

  • Transcription process:

    • RNA polymerase binds promoter with transcription factors to initiate transcription.

    • Transcripts may contain introns and exons; introns are removed during RNA splicing to create a mature mRNA.

    • Alternative splicing can produce multiple mRNAs from a single gene.

  • mRNA transport and protein synthesis:

    • mRNA exits nucleus through nuclear pores into cytoplasm where translation occurs.

    • Protein assembly uses amino acids; 20 standard amino acids exist; ribosomes translate the mRNA blueprint into protein sequences.

  • Neuronal genes, genetic variation, and genetic engineering:

    • The human genome comprises ~25,00025{,}000 genes; neurons express particular genes as proteins guiding their unique functions.

    • Gene copy number variations (CNVs) can cause disease susceptibility (e.g., autism, schizophrenia).

    • Mutations (e.g., single nucleotide polymorphisms, SNPs) can affect protein function.

    • Gene expression studies in post-genomic era rely on comparing mRNA levels across tissues or conditions (e.g., microarrays).

  • Genetic engineering in neuroscience:

    • Knockout mice: deletion of a gene to study its function.

    • Transgenic mice: introduction and overexpression of a gene (transgene).

    • Knock-in mice: native gene replaced with modified transgene.

    • Nobel Prize 2007 (Physiology or Medicine): Martin Evans, Oliver Smithies, Mario Capecchi, for gene targeting in mice (Box 2.3).

  • Box 2.2 Expressing One’s Mind in the Post-Genomic Era:

    • The Human Genome Project identified ~25,00025{,}000 genes; the post-genomic era uses gene expression data to diagnose and treat diseases.

    • DNA microarrays enable large-scale measurement of mRNA expression differences between samples by labeling transcripts with fluorescent tags (green/red) on a DNA microarray; highly expressed genes show bright fluorescence.

  • Box 2.3 Path of Discovery: Gene Targeting in Mice (Mario Capecchi):

    • Early DNA uptake experiments showed efficient uptake and expression of exogenous DNA in mammalian cells when DNA was directly injected into the nucleus.

    • Discovery of concatemer formations and evidence for homologous recombination as the mechanism of integration.

    • Development of gene targeting in mice required embryonic stem (ES) cells capable of germline transmission; collaboration with Martin Evans enabled EK-like cells.

    • Over 10 years to establish gene targeting, enabling mouse models of disease and mutation.

    • Impact on modern neuroscience: knockouts, transgenics, knock-ins, microglia studies, and links to psychiatric disorders (e.g., depression, autism, schizophrenia, Alzheimer’s).

    • Figures illustrate microinjection of DNA into fertilized eggs and the concept of targeted mutations.

BOX 2.2 Expressing One’s Mind in the Post-Genomic Era (expanded)

  • Sequencing the human genome (completed 2003) identified ~25,00025{,}000 genes.

  • Post-genomic era: gene expression differences across brains or tissues help diagnose and understand neurological and psychiatric conditions.

  • How gene expression is assessed:

    • Collect mRNA from two samples, label one with green fluorescence and the other with red.

    • Hybridize to a microarray; differential expression appears as color differences corresponding to transcript abundance.

  • The concept of tissue- and cell-type-specific gene expression underpins neural classification and functional studies.

BOX 2.3 PATH OF DISCOVERY: Gene Targeting in Mice by Mario Capecchi

  • In 1980 Capecchi demonstrated high-efficiency transformation by direct microinjection of DNA into cultured mammalian cells.

  • Observation: many copies of a gene injected become ordered in a head-to-tail concatemer, suggesting nonrandom integration.

  • Demonstrated homologous recombination as the mechanism for chromosomal integration of injected DNA.

  • 1985–1987: developmental work to extend gene targeting to mice; required germline-competent ES cells.

  • Christmas 1985: collaboration with Martin Evans, learning EK-like cells; success followed after persistent effort.

  • Result: creation of mutant mice by targeted gene disruption; opened new directions including studying microglia and immune-brain interactions related to neuropsychiatric disorders and neurodegenerative diseases.

  • Real-world impact: Nobel Prize in 2007 for Capecchi and colleagues; key references include Capecchi 1980; Folger et al. 1982; Thomas & Capecchi 1987.

BOX 2.4 Alzheimer’s Disease and the Neuronal Cytoskeleton

  • Neurites and dendrites possess elaborate cytoskeletal architectures crucial for information processing.

  • Alzheimer’s disease (AD) involves disruption of neuronal cytoskeleton, particularly neurofilaments and tau pathology, correlating with dementia.

  • Tau pathology: paired helical filaments (tau protein) aggregations form neurofibrillary tangles that disrupt microtubule stability and axonal transport.

  • Amyloid pathology: abnormal amyloid secretion is implicated as an initiating event in AD that leads to tangles and neuronal death.

  • AD hallmarks include tau tangles and neurofibrillary tangles; tangles are associated with progressive cognitive decline.

BOX 2.5 Hitching a Ride with Retrograde Transport

  • Fast anterograde transport moves materials from soma to axon terminal via kinesin along microtubules; rate can reach up to 1000 mm/day1000\ \text{mm/day}.

  • Retrograde transport moves materials from terminal to soma via dynein; signals changes in metabolic needs at the terminal.

  • Both directions rely on microtubule tracks and ATP.

  • Retrograde labeling methods (HRP) and viral transport exploit retrograde pathways to map neural connections.

  • Box 2.5 also mentions how certain pathogens (e.g., rabies) and herpesviruses exploit retrograde transport.

BOX 2.6 Intellectual Disability and Dendritic Spines

  • Dendritic structure intricately relates to synaptic connectivity and cognitive function.

  • Intellectual disability (ID) is subaverage cognitive functioning that impairs adaptive behavior; prevalence ~2−3%2-3\% of humans.

  • Causes include genetic disorders (e.g., phenylketonuria, Down syndrome) and adverse prenatal environments (e.g., maternal infection like rubella, malnutrition, fetal alcohol exposure).

  • Golgi staining of brains from intellectually disabled children revealed dendritic abnormalities: fewer dendritic spines, and spines that are longer and thinner; spine morphology correlates with degree of disability.

  • Purpura proposed that immature spine morphology resembles that of a normal fetus, suggesting developmental circuit disruption.

  • Intervention can reverse deprivation-induced changes if applied early enough; later chapters discuss experience’s role in brain development.

BOX 2.7 Understanding Neuronal Structure and Function with Incredible Cre

  • Gene expression differences define neuronal identity; genetic tools enable selective manipulation of specific neuron types.

  • Example: cholinergic neurons express ChAT (choline acetyltransferase) and use acetylcholine as a neurotransmitter.

  • Cre recombinase system (Cre/loxP) enables cell-type specific genetic manipulation:

    • Cre recombinase recognizes loxP sites to excise DNA between them, allowing cell-type specific knockout of gene X when Cre is driven by a specific promoter (e.g., ChAT promoter in cholinergic neurons).

    • Lox-Stop-Lox strategy allows transgene expression only in cells where Cre is expressed by removing a stop cassette.

  • Applications include cell-type specific knockout, transgene expression, and activity monitoring (e.g., fluorescent reporters, calcium indicators) in targeted neuronal populations.

  • Figures illustrate Cre-lox strategies for cholinergic neuron-specific gene knockout (A) and transgene expression (B).

THE GLIA and NON-NEURONAL CELLS

  • Glia are not merely support cells; they contribute to information processing, but still primarily support neuronal function.

  • Astrocytes (most numerous glia):

    • Fill much of the extracellular space between neurons.

    • Enwrap synapses, regulate neurotransmitter levels, and help remove excess neurotransmitters from the synaptic cleft.

    • Contain receptors and signaling machinery, suggesting active roles in signaling.

    • Regulate extracellular potassium ion concentration (K+ homeostasis).

  • Myelinating glia:

    • Oligodendroglia (CNS) and Schwann cells (PNS) provide myelin sheaths around axons to speed signal transmission.

    • Myelin is interrupted by nodes of Ranvier; myelin increases conduction speed along axons.

    • Oligodendroglia wrap multiple axons; Schwann cells wrap a single axon.

  • Other non-neuronal cells:

    • Ependymal cells line ventricular system and contribute to neural development.

    • Microglia act as phagocytes to remove debris and may remodel synapses; they can migrate into the brain from blood during development or disease.

  • Vasculature supplies nutrients and oxygen to neurons via blood vessels.

CONCLUDING REMARKS

  • Structure informs function: absence of ribosomes in the axon predicts local protein synthesis occurs in the soma and transported to the axon terminal via axoplasmic transport.

  • The abundance of mitochondria at the axon terminal reflects high energy demands for neurotransmission and ion pumping.

  • The elaborate dendritic tree is specialized for receiving inputs; most synapses form on dendrites.

  • Rough endoplasmic reticulum (Nissl bodies) indicates protein synthesis for membrane proteins and secreted proteins.

  • The next chapters will examine how membrane proteins confer neurons’ electrical signaling abilities and how synaptic signaling underlies memory and learning.

CLASSIFYING NEURONS

  • Goals: categorize neurons so that understanding group-specific function helps explain brain computation.

  • Based on neuronal structure: number of neurites from the soma; dendritic architecture; presence or absence of spines; axon length.

  • Number of neurites:

    • Unipolar: one neurite; Bipolar: two neurites; Multipolar: three or more neurites (most brain neurons are multipolar).

  • Dendrites: shapes vary (e.g., stellate cells, pyramidal cells); pyramidal cells are typically spiny; stellate cells may be spiny or aspinous.

  • Dendritic spines: small protrusions on dendrites that host synapses; spine morphology reflects synaptic activity and plasticity; changes associated with cognitive impairments (intellectual disability box).

  • Golgi type I vs II neurons (axon length):

    • Golgi type I (projection) neurons have long axons that extend to distant targets.

    • Golgi type II (local circuit) neurons have short axons.

  • Classification based on gene expression: different neuronal types express distinct genes and neurotransmitters; promoter-driven genetic reporters (e.g., GFP) enable visualization and manipulation.

  • Common neurotransmitter-based classification: neurons releasing a specific transmitter are termed by that system (e.g., cholinergic neurons use acetylcholine).

GLIA (summary of major glial cell types)

  • Astrocytes: regulate extracellular milieu, neurotransmitter clearance, and synapse function; maintain ion balance.

  • Oligodendroglia and Schwann cells: form myelin sheaths; speed up action potential conduction; CNS vs PNS differences.

  • Other non-neuronal cells: ependymal cells, microglia, vasculature; glia contribute to brain health and disease.

REVIEW QUESTIONS

  1. State the neuron doctrine in a single sentence. To whom is this insight credited?

  2. Which parts of a neuron are shown by a Golgi stain that are not shown by a Nissl stain?

  3. What are three physical characteristics that distinguish axons from dendrites?

  4. Of the following structures, state which ones are unique to neurons and which are not: nucleus, mitochondria, rough ER, synaptic vesicle, Golgi apparatus.

  5. What are the steps by which the information in the DNA of the nucleus directs the synthesis of a membrane-associated protein molecule?

  6. Colchicine is a drug that causes microtubules to break apart (depolymerize). What effect would this drug have on anterograde transport? What would happen in the axon terminal?

  7. Classify the cortical pyramidal cell based on (1) the number of neurites, (2) the presence or absence of dendritic spines, (3) connections, and (4) axon length.

  8. Knowledge of genes uniquely expressed in a particular category of neurons can be used to understand how those neurons function. Give one example of how you could use genetic information to study a category of neuron.

  9. What is myelin? What does it do? Which cells provide it in the central nervous system?