NEU 101: Neurons & Glia
Learning Outcomes
Recognize and match the function of organelles.
Define and recognize key terms.
Label the parts of a neuron and identify them based on function.
Explain the process of action potentials in broad terms (excitatory vs. inhibitory inputs, the axon hillock, electric/chemical components of the signal, synapses, neurotransmitters).
Match types of glia with functions & shapes of glia.
Neurons: Core Definition & Count
Neurons are nervous system cells that send signals to each other.
The human nervous system has about N \,\approx\ 8.5\times 10^{10} neurons.
Basic Neuron Anatomy (Parts Highlighted)
Dendrites: incoming signals are received here.
Soma (cell body): contains nucleus and metabolic machinery.
Nucleus: stores genetic material.
Axon: conducts electrical impulses away from the soma.
Axon hillock: the site where the action potential is initiated (integration point for inputs).
Axon terminals: release neurotransmitters into the synapse.
Myelin sheath: insulates the axon to speed conduction.
White matter vs Grey matter: axons with myelin constitute white matter; cell bodies and dendrites constitute grey matter.
Neuron–Glia Related Structures (From Slides)
Dendritic spines: small protrusions on dendrites where synapses form; have a spine neck and spine head (dimensions shown as examples: roughly 5\ \mu\text{m} and 3\ \mu\text{m} in diagrams).
Node of Ranvier: gaps in the myelin sheath along the axon that facilitate rapid signal conduction.
Dendrite, Soma, Axon, Axon terminals as core compartments in labeled diagrams.
Neurons: Similarities to Other Cells; Key Organelles
Neurons share many organelles with typical animal cells:
Cytoplasm, Nucleus, Golgi apparatus, Endoplasmic Reticulum (rough and smooth), Mitochondria, Ribosomes, Cytoskeleton, Lysosome, Microtubules, Dendritic spines, Myelin sheath, Dendrite.
Golgi apparatus: involved in protein packaging and trafficking.
Rough ER: protein synthesis (ribosome-studded).
Smooth ER: lipid synthesis and other metabolic processes.
Lysosome: waste processing and recycling.
Mitochondria: energy production.
Eukaryotic Cell Organization (Animal Cells)
Core components common to many eukaryotic cells:
Membrane (phospholipid bilayer), Cytosol, Nucleus, Mitochondria, Ribosomes, Endoplasmic Reticulum (RER & SER), Golgi apparatus, Cytoskeleton.
The Cell Membrane (Phospholipid Bilayer)
Hydrophilic (water-loving) head: polar phosphate group; interacts with water.
Hydrophobic (water-hating) tail: nonpolar hydrocarbon chains.
Cytoskeleton: Structure & Roles
Microtubules (thickest): maintain shape; highways for transport of proteins & organelles.
Actin filaments (thinnest): cell movement, shape changes, protein anchoring.
Dendritic Spines (Structure)
Dendritic spine components: spine neck and spine head connected to the dendrite.
Visual examples often labeled as A, B, C, D with approximate dimensions of 5\ \mu\text{m} (spine length) and 3\ \mu\text{m} (spine width) in some slides.
Specialized vs General Neuron Parts
Although neurons share core parts with other cells, they have distinct and specialized regions:
Dendrites, Cell body (soma), Nucleus, Axon hillock, Axon, Synapse, Presynaptic terminal, Postsynaptic cell, Myelin sheath, Node of Ranvier.
Neurons: Shape Diversity
Neurons vary in the numbers and shapes of processes (the parts that stick out from the cell body).
For neurons, the processes are dendrites (receive signals) and axons (transmit signals).
Interconnection: Neuronal Networks
Neurons are interconnected via synapses; signals travel from presynaptic to postsynaptic cells.
Synapses: The Gaps Between Neurons
Between each neuron is a space called the synapse.
Pre-synaptic (sending) cell and Post-synaptic (receiving) cell.
Electrochemical Signaling in the Nervous System
Neurons communicate using two modes: electrical signals within a neuron (action potentials) and chemical signals between neurons (neurotransmitter release).
Action Potential (AP): the primary electrical signal in neurons.
AT: Signals within a single neuron are electrical; Signals between neurons are chemical.
Action Potentials: Basic Concepts
Incoming signals are integrated until threshold at the axon hillock is met or not.
If threshold is met, an action potential travels down the axon (all-or-none event).
If threshold is not met, nothing happens.
When the AP reaches the terminal buttons, neurotransmitters are released into the synapse, producing excitation or inhibition in the postsynaptic soma.
Excitatory vs Inhibitory Inputs
Excitatory action potentials promote depolarization toward the threshold.
Inhibitory signals reduce the likelihood of reaching threshold, promoting hyperpolarization.
Threshold & Action Potential Propagation
Threshold is defined at the axon hillock; once met, voltage-gated channels open and the AP propagates along the axon to transmit signal.
The AP is an all-or-none event: above threshold triggers a spike; below threshold yields no spike.
Synaptic Transmission: Neurotransmitters & Effects
Action potentials arriving at axon terminals trigger the release of neurotransmitters into the synapse.
Neurotransmitter binding to receptors on the postsynaptic cell determines whether the postsynaptic neuron experiences excitation or inhibition.
Discussion Question (Glia): Non-Electrical Roles
The nervous system contains glia that do not send electrical signals between neurons.
They contribute via support, maintenance, and modulation roles (speculative prompts to think beyond notes).
Glial Cells: Overview & Functions
Neuroglia (glia) perform various essential tasks:
Myelinate axons (increase conduction velocity).
Remove toxic materials.
Encase and protect cell bodies.
Produce cerebrospinal fluid (CSF).
Contribute to the Blood-Brain Barrier and nutrient support.
Additional supportive roles and protection.
Types of Glia and Their Roles
Ependymal cells: line the ventricles; produce CSF.
Astrocytes: support neurons; help transfer materials from blood to neurons.
Oligodendrocytes: myelinate neurons in the CNS.
Microglia: immune-like “clean up crew” cells in the CNS.
Neuron & Supporting Glia: Interaction Diagram
Diagrammatic view (neurons with dendrites and axons) often shows glial cells such as astrocytes supporting neurons and oligodendrocytes myelinating axons.
Model of the Cell & Connectivity (Membrane, Spines, Glia)
Cell membrane remains the phospholipid bilayer.
Dendrite, cell body (soma), axon, oligodendrocyte, node of Ranvier, myelin sheath, and synapse are key components interacting in networks.
Methods in Neuroscience: Experimental Approaches
Observation; The Scientific Method; Experimentation; Naturalistic vs. Chaotic vs. Controlled approaches.
Experimental Contexts & Data Collection
Cells? Networks of cells? Learning? Behavior? Cognition? Disorders? Memory? Injuries?
Possible experimental angles include exogenous substances and their impact on nervous system activity and behavior.
Observation & Techniques for Studying Cells
Observational approaches include:
Extract living cells from human brains.
Use patch-clamp methods to measure action potential patterns.
Extract DNA from nucleus to measure gene expression.
Use 3D virtual reality to map cell morphology.
Variables in Experiments
Independent Variable (IV): what the experimenter manipulates.
Dependent Variable (DV): what is measured.
Conceptual mapping examples: Cells → Behavior; Behavior → Cells; World → Cells; World → Behavior.
Discussion Question: Experimental Scenarios
Think of examples where:
The world is the IV and cells are the DV.
Cells are the IV and behavior is the DV.
Behavior is the IV and cells are the DV.
Ways to Study the Nervous System (Contexts)
Ex Vivo: without life (dead tissue studied outside the organism).
In Vivo: within life (studies in the living organism).
In Vitro: within glass (cultured cells or tissues).
Model Species
Various model species are used in neuroscience research (slides reference this concept).
Ethics in Animal Research
The Three Rs:
Replace: use alternatives when available.
Reduce: use the minimum number of animals necessary.
Refine: optimize procedures to minimize discomfort.
The Institutional Animal Care & Use Committee (IACUC) strictly monitors use of animals in research.
There are IACUC branches at major universities.
For example, ASU has an IACUC page.
Imaging & Resolution: Spatial vs Temporal
Spatial & Temporal Resolution:
High vs Low resolution affects image clarity and timing.
A simple visualization shows that higher temporal resolution captures rapid changes; low temporal resolution may miss fast dynamics.
A common example: high-resolution image at 300 dpi vs low-resolution image at 72 dpi.
Microscopy & Histology: Staining & Visualization
Microscopy: using microscopes to study tiny objects.
Histology: study of the microscopic structure of tissues.
Staining techniques highlight parts of cells or tissues to reveal structure.
Classic Stains in Neuroanatomy
Golgi stain: stains soma, dendrites, and axons (entire cell) dark brown/black; famously used by Santiago RamĂłn y Cajal to reveal neuron structure.
Nissl stain: stains various parts of the cell body blue/purple; highlights Nissl bodies.
Weigert stain: stains axons (white matter).
Nissl Stain & Cytoarchitecture
Nissl stains help illustrate cytoarchitecture: different brain regions show different cell types and organization.
Electron Microscopy
Electron microscopy uses a beam of electrons (not light) and provides very high resolution.
Key Formulas & Notation (Summary)
Neuron count estimate: N \approx 8.5\times 10^{10}
Dendritic spine dimensions (examples): d{neck} \approx 5\ \mu\text{m}, \ d{head} \approx 3\ \mu\text{m}
Action potential threshold concept (conceptual): If the membrane potential Vm \ge V{th} at the axon hillock, an AP is triggered; otherwise, the neuron remains at rest.
Resolution examples: ext{High resolution} \approx 300\,\text{dpi} \quad\text{vs}\quad \text{Low resolution} \approx 72\,\text{dpi}
Connections to Foundational Principles & Real-World Relevance
Neurons use both electrical (APs) and chemical (neurotransmitter) signaling to rapidly communicate across networks, enabling perception, learning, and behavior.
Glial cells provide critical support that maintains neuronal function, protects CNS tissue, and shapes neural signaling environments.
Ethical considerations and rigorous methodological controls (IV/DV design, ex vivo/in vivo/in vitro options) guide responsible neuroscience research.
Practical Implications & Ethical/Philosophical Notes
The “three Rs” framework (Replace, Reduce, Refine) seeks to minimize animal suffering while enabling scientific advancement.
High-resolution imaging and advanced staining techniques (Golgi, Nissl, Weigert, EM) have transformed our understanding of brain structure and connectivity.
Understanding glial roles challenges neuron-centric views and supports more integrated models of brain function.
Quick Reference: Key Terms
Neuron; soma; nucleus; axon; dendrite; dendritic spine; axon hillock; axon terminals; synapse; presynaptic cell; postsynaptic cell; neurotransmitters; receptors; excitatory/inhibitory inputs; action potential; myelin sheath; nodes of Ranvier; glia; astrocyte; oligodendrocyte; microglia; ependymal cell; CSF; blood-brain barrier; Golgi stain; Nissl stain; Weigert stain; electron microscopy; patch clamp; ex vivo; in vivo; in vitro; ICACU/C; IACUC; independent variable; dependent variable; high vs low spatial/temporal resolution.