Alan Peters and Claire Folger Sethares
The Fine Structure of the Aging Brain
(www.bu.edu/agingbrain)
Department of Anatomy & Cell Biology
Boston University School of Medicine
72 East Newton Street
Boston, Massachusetts
(Supported by the Institute on Aging of the National Institute of Health, Grant Number P 01-AG 000001)
Copying or redistribution in any manner for commercial use, including commercial publication, or for personal gain is strictly prohibited.
Faculty/Retired
University of Minnesota
Department of Genetics, Cell Biology and Development
6-160 Jackson Hall
321 Church St SE
Minneapolis, MN 55455
Robert L. Sorenson, Ph.D.
Professor Emeritus
University of Minnesota
Department of Genetics, Cell Biology and Development
6-160 Jackson Hall
321 Church St SE
Minneapolis, MN 55455
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EM 398 Pyramidal Neuron
Pyramidal Neuron
Transmission electron microscopy (TEM) of a pyramidal neuron in the prefrontal cortex of a 5-year-old monkey.
are the most abundant and functionally important cells in the cerebral cortex. They are the primary excitatory neurons responsible for cognitive processing, memory formation, and integrating information across cortical regions. Their distinctive pyramid-shaped cell bodies and elaborate dendritic trees allow them to receive and process input from large numbers of neurons,
Fundamental Neuronal Architecture
All neurons share a basic structural organization optimized for intercellular signaling:
: Contains the nucleus and most organelles; center of metabolic activity
: Branching input structures that receive signals from other neurons
Axon: Single output fiber that transmits signals to target cells
: Specialized endings that form synaptic connections
: Precise junctions enabling cell-to-cell communication
Neuropil
The space surrounding neurons, known as the neuropil, consists of densely packed neuronal elements (dendrites, axons, and synapses), glial cells and their processes, and blood vessels.
Structure and Function
The following pages discuss these elements in more detail.
Courtesy of Alan Peters and Claire Folger Sethares, The Fine Structure of the Aging Brain ((www.bu.edu/agingbrain), Boston University School of Medicine, Boston, MA.
Cell Body (Soma)
The , often called the soma, of pyramidal neurons, exhibits a distinctive pyramidal shape containing the nucleus surrounded by the cellular machinery necessary for its demanding metabolic requirements.
Nucleus:
(blue): Large, centrally located with dispersed chromatin (euchromatin), indicating active transcription
Nuclear Envelope (purple): Double membrane system regulating molecular transport between the nucleus and cytoplasm
Actin Filaments: Small dynamic filaments that maintain cell shape, dendritic spine formation and plasticity, and axonal growth during development
Dendrites
Pyramidal neurons possess elaborate dendritic trees that serve as the primary sites for synaptic input integration. These structures exhibit remarkable plasticity throughout life.
Dendritic Architecture:
: Extends from the top of the soma towards the cortical surface
Dendritic Spines: Small protrusions (1-2 μm) that dramatically increase synaptic surface area
Morphology: Mushroom-shaped spines
Occasional longitudinal sections ( and ) showing connection to a dendrite
Most profiles are cross-sections ( and )
Dynamic Nature: Continuously remodels based on synaptic activity
Plasticity: Key substrates for learning and memory function
The surrounding neuropil contains the profiles of many dendrites from other neurons.
Dendritic Organelles:
: Satellite Golgi apparatus are often found in larger dendrites
Enable local protein processing and trafficking
Mitochondria: meet local energy demands and sustain electrical activity
Endoplasmic Reticulum (ER):
contain rough and smooth ER
Smooth ER within spines ( and ) is important in calcium regulation at synapses
Dendritic Cytoskeleton:
Microtubules ( and ): Long, hollow tubules also involved in the transport of cellular components
Motor Proteins: Kinesin and dynein attach and crawl along microtubules
Actin Filaments: Important in dendritic spine formation and plasticity
Axons
While the pyramidal neuron's own axon may not be visible in every section, the neuropil contains numerous axons from other neurons, illustrating the complexity of cortical connectivity.
Axon Classification:
Unmyelinated Axons ( and ): Smaller diameter fibers (0.1-1.0 µm)
Slower conduction velocities
Energy-efficient for short-distance communication
Myelinated Axons ( and ): Larger-diameter fibers wrapped in myelin sheaths
Myelin Sheath: Insulating layers produced by oligodendrocytes
Oligodendrocytes often appear darker than other cells or processes
Faster Conduction: Saltatory propagation between nodes of Ranvier
Microtubules: Provide structural support and transport pathways
Neurofilaments: Maintain axon diameter and mechanical strength
Mitochondria: Meet energy demands along the axon length
Smooth ER: Absence of rough ER distinguishes axons from dendrites
Synaptic Terminals and Neurotransmission
Pyramidal neurons integrate thousands of synaptic inputs across their dendritic trees. The axons of other neurons end in axon terminals ( and ) that form synapses ( and ), highly specialized structures optimized for rapid chemical signaling between neurons, with the dendritic spines.
Axon Terminal:
Synaptic Vesicles: Small membrane-bound vesicles containing neurotransmitters
Mitochondria: Provide ATP for vesicle recycling and calcium pumping
Smooth ER: Regulates local calcium concentrations
Neurotransmission:
Action Potential: Propagates along the axon, arriving at the axon terminal
