Neurons & Glia

Course logistics and learning goals

• Quiz #1 scheduled for Tuesday, Sept 16 (week after next). Preview tools: "Sham Quiz" to understand format; Study Guide with carefully chosen verbs.
• Office hours: Wednesdays & Thursdays (by appointment). Honors contracts spots filled; TAs provide study tips.
• Honors contracts will be submitted to Canvas by the deadline listed in Canvas.
• Questions, comments, thoughts, or anything else? Reach out during announcements and reminders.

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 their functions & shapes.

Neurons: overview

• Neurons are nervous system cells that send signals to each other.
• The human nervous system has approximately $N \approx 8.5 \times 10^{10}$ neurons.
• Basic regions mentioned: Grey matter, White matter.
• Key components labeled: Dendrites, Soma (cell body), Axon, Axon terminals.

Neuron structure and direction of signaling

• Neurons send incoming signals via dendrites and outgoing signals via axon terminals.
• Dendrites receive signals; axon terminals emit signals.
• Direction of electrical impulse travels along the axon.
• Axon hillock is a critical site where threshold for signaling is evaluated (see action potentials).

The anatomy of a neuron (detailed views)

• Soma (cell body) contains nucleus.
• Dendrites extend from the soma to receive signals.
• Axon conducts electrical impulses away from the soma toward the axon terminals.
• Axon terminals release neurotransmitters into synapses.
• Myelin sheath surrounds axons and influences conduction.

Neurons vs other cells; common organelles

• Neurons are similar to other eukaryotic cells in having: cytoplasm, nucleus, ribosomes, endoplasmic reticulum (rough and smooth), Golgi apparatus, mitochondria, lysosomes, cytoskeleton, etc.
• Dendritic spines are specialized to receive synaptic inputs.
• Distinct yet specialized structures include dendrites, cell body, nucleus, axon hillock, axon, synapse, myelin sheath, and axon terminals.

The cell membrane and cytoskeleton

• Cell membrane is a phospholipid bilayer with:
  • Hydrophilic (water-loving) heads containing polar phosphate groups that interact with water.
  • Hydrophobic (water-hating) tails that are nonpolar hydrocarbon chains.
• Cytoskeleton components:
  • Microtubules: thickest, maintain shape, act as highways for transport of proteins and organelles.
  • Actin filaments: thinnest, involved in cell movement, shape changes, and protein anchoring.

Dendritic spines and their dimensions

• Dendritic spines have distinct shapes (A–D in the figure).
• Example dimensions shown: spine neck around $5\,\mu\text{m}$; spine head around $3\,\mu\text{m}$.
• Spines contribute to synaptic strength and plasticity.

Regional specialization in neurons

• Neurons vary in shape and have different numbers of processes (dendrites and axons).
• Processes are the parts that stick out from the cell body; in neurons, these are primarily dendrites and axons.

Interconnectivity of neurons

• Neurons are interconnected via synapses, gaps between neurons where signaling occurs.
• A signal travels from a presynaptic cell to a postsynaptic cell across the synapse.
• The synapse is the critical site for communication between neurons.

Electrochemical signaling in neurons

• Neurons communicate using two modalities:
  • Electrical signals (within a single neuron): action potentials.
  • Chemical signals (between neurons): neurotransmitter release at the synapse.
• Action potentials (AP) are the fundamental electrical events in neurons.

Signaling types: within vs between neurons

• Signals within a neuron are electrical (action potentials).
• Signals between neurons are chemical (neurotransmitter release across the synapse).

Action potentials: broad overview

• Incoming signals are integrated to determine whether the threshold is reached at the axon hillock.
• If the threshold is not met, nothing happens (no AP).
• If the threshold is met, an action potential travels down the axon – the neuron fires.
• Action potentials rely on the all-or-none principle: a threshold event triggers a full spike.
• Neurons do not physically touch each other across synapses; there is a synaptic gap.
• The presynaptic cell is the signaling (sending) neuron; the postsynaptic cell is the receiving neuron.

Synapses and neurotransmission

• When an action potential reaches the terminal buttons, it triggers the release of neurotransmitters into the synapse.
• Neurotransmitters then cause either excitation or inhibition in the soma (or target region) of the postsynaptic cell.
• The excitation/inhibition determines whether the postsynaptic neuron will fire an action potential.

Glial cells: roles and types

• Glia (neuroglia) do not fire electrical signals between neurons, but support neural function.
• Roles include:
  • Myelinating axons (insulation to increase conduction speed).
  • Removing toxic materials from the neural environment.
  • Enclosing and protecting cell bodies.
  • Producing cerebrospinal fluid (CSF) and contributing to the blood-brain barrier and nutrient delivery.
  • Other supportive functions (various glial types).

Specific glial cell types

• Ependymal cells: line the ventricles and produce CSF.
• Astrocytes: regulate chemical environment by interacting with blood to regulate nutrient and ion supply to neurons.
• Oligodendrocytes: myelinate neurons in the CNS.
• Microglia: act as the brain's cleanup crew (phagocytic maintenance).

Neuron and glia interactions

• Neurons and glia are interwoven in neural circuits.
• Example cell types involved in support and signaling: astrocytes, oligodendrocytes, microglia, and neurons.
• Diagrammatic reference: dendrite – astrocyte – oligodendrocyte – axon – terminology for connectivity (nucleus, axon terminals).
• Node of Ranvier: gaps in the myelin sheath along the axon that facilitate rapid conduction.

Methods in neuroscience tools and approaches

• Observation and the scientific method: observation, hypothesis formation, experimentation, naturalistic vs. artificial conditions, controlled experiments.
• Experimental methods include:
  • Patch clamp to measure patterns of action potentials.
  • DNA extraction from nucleus to measure gene expression.
  • 3D virtual reality (VR) technologies to map cell morphology.

The nervous system and experimental design

• The world, cells, and behavior relate through causal and correlational relationships in experiments.
• Variables in experiments:
  • Independent Variable (IV): what is manipulated by scientists.
  • Dependent Variable (DV): what is measured by scientists.
  • The mapping of variables can be: Cells ⇄ Behavior, Cells ⇄ World, Behavior ⇄ World.

Experimental scenarios: practice questions

• 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.

Techniques to study the nervous system

• Ex Vivo: studies without life (e.g., isolated tissue).
• In Vivo: studies within a living organism.
• In Vitro: studies within glass (e.g., cell cultures).

Model species

• Model organisms and species used in neuroscience research.

Ethics in animal research

• The three R rule:
  • Replace: use alternatives when available.
  • Reduce: use the minimum number of animals necessary.
  • Refine: optimize procedures to minimize discomfort or distress.
• IACUC: Institutional Animal Care & Use Committee; oversees animal research ethics and compliance.
• There are IACUC branches at major universities; a reference page is provided for ASU in the course materials.

Spatial and temporal resolution in neuroscience imaging

• Spatial resolution (how finely a structure is spatially resolved) and temporal resolution (how finely signaling events are timed).
• A scale from low to high resolution is depicted, illustrating trade-offs.

Imaging quality and resolution examples

• High resolution vs. low resolution examples:
  • High resolution image: $300\,\text{dpi}$.
  • Low resolution image: $72\,\text{dpi}$.

Microscopy and histology basics

• Microscopy: using microscopes to study tiny things.
• Histology: study of the microscopic structure of tissues.
• Staining techniques highlight specific cellular components.

Classic neuronal stains

• Golgi stain: stains soma, dendrites, and axons (entire cell) dark brown/black; famously used by Santiago Ramón y Cajal; reference to historical figure.
• Nissl stain: stains various parts of the cell body blue/purple; used to study cytoarchitecture.
• Weigert stain: stains axons (white matter).

Electron microscopy

• Uses electrons instead of light to achieve very high resolution imaging of cellular ultrastructure.

Connections to foundational principles and real-world relevance

• Understanding neuron structure and signaling underpins many fields: physiology, cognition, neurology, and medical sciences.
• Glial roles are essential for metabolic support, myelination, and immune defense in the CNS.
• Imaging and staining techniques allow visualization of structure and connectivity, informing models of brain function and disease.
• Ethical considerations (3 R rule, IACUC) guide responsible animal research and translational potential.
• The distinction between electrical (intraneuronal) and chemical (interneuronal) signaling is central to understanding neurocommunication and synaptic plasticity.

Key terms for quick recall

• Neuron, soma, dendrite, axon, axon hillock, axon terminals, dendritic spines, synapse, presynaptic cell, postsynaptic cell, neurotransmitters, action potential, excitatory vs inhibitory signals, membrane, phospholipid bilayer, hydrophilic head, hydrophobic tail, cytoskeleton, microtubules, actin, myelin sheath, nodes of Ranvier, glia, astrocyte, oligodendrocyte, microglia, ependymal cells, CSF, blood-brain barrier, patch clamp, cytoarchitecture, Golgi stain, Nissl stain, Weigert stain, electron microscopy

Foundational diagrams and terms to review

• The anatomy of a neuron diagram (dendrites, soma, axon, axon hillock, axon terminals).
• The basic layout of a neuron-glia network (astrocytes, oligodendrocytes, microglia, ependymal cells).
• Synapse diagram: presynaptic terminal, synaptic cleft, postsynaptic membrane.

Topical reminder: key terms to define (from lecture prompts)

• Topological magnification
• Cortical magnification
• Gating at the axon hillock (threshold concept)
• All-or-none principle of action potentials
• Excitatory vs inhibitory synaptic inputs