1.1 - building a nervous system
Study Notes: Neurons, Glia, and Eukaryotic Cell Fundamentals
1. Similarities and Differences Between Neurons, Glia, and Eukaryotic Cells (LO 1.1.1)
Common Features of Eukaryotic Cells
All eukaryotic cells (domain Eukarya) have:
Cell membrane enclosing the cell.
Genetic material (DNA) contained within a nucleus.
Membrane-bound organelles such as mitochondria, endoplasmic reticulum (ER), Golgi apparatus, and lysosomes for compartmentalization and specialized functions.
Central dogma: DNA is transcribed into messenger RNA (mRNA) inside the nucleus; mRNA exits to cytoplasm where proteins are synthesized by ribosomes.
Unique Features of Neurons and Glia
Neurons and glia are fundamentally eukaryotic cells.
Neurons are specialized for rapid electrical and chemical communication:
Have excitable membranes.
Produce and transmit electrical signals and release neurotransmitters.
Glia generally do not generate electrical signals; they provide structural, metabolic, and regulatory support to neurons.
Cell identity arises from gene regulation: all cells have the same DNA, but selective gene expression produces different proteins, defining cell function (e.g., neuron-specific ion channels vs insulin production in pancreatic cells).
2. Functional Specializations of Neurons (LO 1.1.2)
Basic Neuronal Structure
Soma (cell body): Contains the nucleus and typical organelles; site of protein synthesis.
Dendrites: Tree-like branched projections specialized for receiving information at synapses; can be covered in dendritic spines that compartmentalize synapses, important in learning and memory.
Axon: Usually single, long process specialized for sending information via action potentials.
Originates from soma at the axon hillock, the site of action potential initiation.
Can branch into axon collaterals connecting with multiple partner cells.
Ends at axon terminals that release neurotransmitters.
Diversity of Neuronal Types
Multipolar neurons: One axon, many dendrites (e.g., motor neurons).
Bipolar neurons: One axon, one dendrite (e.g., olfactory neurons).
Unipolar neurons: Single continuous process, often sensory (e.g., some peripheral sensory neurons).
Functional Classification
Afferent (sensory) neurons: Transmit signals towards the central nervous system (CNS).
Efferent (motor) neurons: Transmit signals away from the CNS to muscles and glands.
Interneurons: Connect and relay information between afferent and efferent neurons within CNS.
3. Types and Functions of Glia (LO 1.1.3)
Glial Cell Type | Location | Main Functions |
|---|---|---|
Astrocytes | Central nervous system (CNS) | Maintain the blood-brain barrier (BBB), provide metabolic and structural support to neurons, recycle neurotransmitters by reuptake. |
Microglia | CNS | Immune cells of the brain; phagocytize pathogens, damaged cells, and prune synapses during development. |
Oligodendrocytes | CNS | Myelinate axons by wrapping multiple segments, enabling rapid electrical signal transmission. |
Schwann cells | Peripheral nervous system (PNS) | Myelinate single axon segments in PNS, support axon regeneration and function. |
Additional and Historical Notes
Blood-brain barrier (BBB): Protects the brain by filtering chemicals and pathogens; astrocytes help maintain it.
Myelin sheath: Fatty covering around axons by oligodendrocytes (CNS) and Schwann cells (PNS) to speed conduction.
Loss of myelin causes diseases like Multiple Sclerosis (MS), leading to impaired neural communication.
Microglial dysfunction has implications for neuropsychiatric disorders such as autism and schizophrenia.
4. Visualization Techniques for Brain Cells
Golgi stain: Randomly stains a small subset of whole neurons; revealed neurons as separate units (Neuron Doctrine, Ramón y Cajal).
Other chemical stains:
Nissl & Hematoxylin and Eosin (H&E): Stain cell bodies and nuclei.
Luxol fast blue: Stains myelin to identify white matter.
Immunofluorescence: Uses fluorescent-tagged antibodies targeting specific proteins, allowing cell-type identification and protein localization.
GFP and Brainbow: Genetic fluorescent protein labeling techniques; Brainbow provides multicolor labeling for individual neurons, facilitating circuit tracing.
5. Key Facts
The human brain contains approximately 86 billion neurons, mostly in the cerebellum.
Neurons and glia share fundamental eukaryotic structures but are functionally specialized.
The nervous system’s complexity requires sophisticated visualization techniques developed over centuries.
Neurons are highly diverse in morphology and function across brain regions.
Glia play critical roles beyond “nerve glue,” including metabolic support, defense, and myelination.