Lecture 03 Neurons Glia

Neurons and Glia

Goals

  • Introduce neurons and glial cells as the two principal cell types of the brain.
  • Understand the structure, function, and diversity of neurons.
  • Explore the historical foundations of the neuron doctrine.
  • Examine the roles of glial cells in supporting neural activity and brain function.

Neurons

Overview

  • Neurons are excitable cells responsible for processing and transmitting information in the nervous system.
  • Key Features:
    • ~86 billion neurons in the human brain:
      • ~16 billion in the cerebral cortex.
      • ~69 billion in the cerebellum.
    • Neurons are characterized by functional polarity:
      • Information flows from dendrites → soma (cell body) → axon.

Anatomy of a Neuron

  • Dendrites:
    • Receive inputs from other neurons via synapses.
    • Highly branched with spines, sites for synaptic plasticity.
  • Soma (Cell Body):
    • Contains the nucleus and other organelles.
    • Integrates synaptic inputs.
  • Axon:
    • Transmits output signals.
    • Can be myelinated to speed up signal conduction.
  • Synapse:
    • Junction for communication between neurons.
    • Types:
      • Chemical Synapses: Use neurotransmitters.
      • Electrical Synapses: Direct ion flow via gap junctions.

Neuron Classification

  1. Projection Neurons (Principal Neurons):
    • Long axons that connect distant brain regions.
    • Examples:
      • Pyramidal Neurons: ~70% of cortical neurons; excitatory.
      • Purkinje Cells: Cerebellum; inhibitory.
      • Medium Spiny Neurons: Basal ganglia; inhibitory.
  2. Interneurons:
    • Short axons for local circuit modulation.
    • Mostly inhibitory (release GABA).

Neural Circuits

  • Integration of excitatory (glutamate) and inhibitory (GABA) signals enables complex computations.
  • Circuit Motifs:
    • Feedforward inhibition: Excitatory neurons activate inhibitory interneurons to suppress downstream activity.
    • Disinhibition: Inhibition of an inhibitory neuron enhances activity of the target.

Historical Foundations: The Neuron Doctrine

Santiago Ramón y Cajal and Camillo Golgi

  • Golgi:
    • Developed the silver nitrate stain to visualize entire neurons.
    • Proposed a reticular theory, where neurons form a continuous network.
  • Cajal:
    • Used Golgi’s staining technique to demonstrate that neurons are discrete entities.
    • Advocated for the neuron doctrine.

Tenets of the Neuron Doctrine

  1. Neurons are the fundamental structural and functional units of the nervous system.
  2. Neurons are discrete cells, not continuous networks.
  3. A neuron consists of three parts: dendrites, axon, and soma.
  4. Information flows unidirectionally from dendrites to axons (functional polarity).

Modern Challenges to the Neuron Doctrine

  • Electrical Synapses:
    • Direct ion flow between neurons violates unidirectional information flow.
  • Retrograde Transmission:
    • Postsynaptic neurons send signals back to presynaptic neurons.
  • Dendritic Computations:
    • Some dendrites can process information independently of the soma.

Glia

Overview

  • Glial cells (“glue”) support neurons and modulate brain function.
  • Outnumber neurons in some brain regions.

Types of Glial Cells

  1. Astrocytes:
    • Support synapses and regulate the extracellular environment.
    • Influence local blood flow.
    • Connected by gap junctions, forming a syncytium.
  2. Oligodendrocytes (CNS) and Schwann Cells (PNS):
    • Form myelin sheaths to insulate axons and speed conduction.
    • Myelin degeneration occurs in diseases like multiple sclerosis.
  3. Microglia:
    • Immune cells of the brain.
    • Remove debris and synaptic connections; implicated in forgetting.
  4. Ependymal Cells:
    • Line brain ventricles and help produce cerebrospinal fluid.

Advances in Neuron and Glia Research

  • Quantitative Morphometry:
    • New techniques (e.g., electron microscopy) allow detailed mapping of neuronal and glial networks.
  • Brainbow Technique:
    • Visualizes individual neurons with unique fluorescent colors.

Applications and Implications

  • Plasticity and Learning:
    • Dendritic spines grow and shrink during learning.
    • Astrocytes influence synaptic strength and connectivity.
  • Neurodegenerative Diseases:
    • Role of glial cells in Alzheimer’s, Parkinson’s, and multiple sclerosis.
  • Computational Neuroscience:
    • Neurons modeled as logic gates (e.g., McCulloch-Pitts neuron) inspire artificial intelligence and neural networks.

This lecture provided a foundational understanding of neurons and glia, highlighting their structure, function, and role in the nervous system. It also traced the historical and modern advancements in neuroscience, setting the stage for deeper exploration of neural circuits and systems.

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