EM 398 Pyramidal Neuron

Pyramidal Neuron

??? Preliminary outline of description

Transmission electron microscopy (TEM) of the cell body and dendrites of a pyramidal neuron in the cerebral cortex.

Neurons vary in shape and size, but most share a common structure.

• Cell Body (or Soma) - contains the nucleus and is rich in organelles
• Dendrites - branching extensions that receive signals from other neurons
• Axon - single, long fiber that sends signals to other neurons or cells
• Axon Terminals - ends of the axon that transmits signals to other cells
• Synapses - junctions between the axon terminal and another cell

Surrounding neurons is the neuropil composed of neuronal processes, including synapses, as well as glial cells and their processes.

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.

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Cell Body

While many neurons have spherical or oval cell bodies, often referred to as the soma, pyramidal neurons have a distinctive pyramid-shaped cell body that contains the nucleus and a cytoplasm rich in organelles.

• Nucleus (blue) / Nuclear Envelope (purple) - rounded nucleus of dispersed chromatin (euchromatin) with one or more nucleoli (none visible)
• Golgi Apparatus (yellow) - usually more than one well-developed Golgi apparatus distributed around the nucleus
  • Critical role in protein sorting and trafficking to specific locations, including the cell body, dendrites, axons, and synapses
• Mitochondria (red) - dispersed throughout the cytoplasm
  • Necessary to meet the high energy for maintaining the neuron’s resting potential, transmit signals, and perform other functions
• Endoplasmic Reticulum (cyan) -especially abundant in the cell body
  • Rough Endoplasmic Reticulum (RER) - cisternae with bound ribosomes responsible for protein synthesis and folding
    • Nissl Substance - clusters of RER and polyribosomes that are a distinguishing feature of neurons
    • RER in the cell body produces all proteins needed in the axon and its terminal
  • Smooth Endoplasmic Reticulum (SER) - cisternae that lack ribosomes and are involved in lipid synthesis and calcium storage
• Lysosomes (black) - break down waste materials and cellular debris
• Lipofuscin Granules (black) -remnants from the incomplete breakdown of cellular debris
  • Size and number increases with aging
• Neurofibrils - neurons contain an extensive array of protein filaments to provide structural support
  • Microtubules (dark blue) - also involved in the transport of materials
  • Neurofilaments - intermediate filaments specific to neurons

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Dendrites

Like most neurons, pyramidal neurons have multiple dendrites that extend from the cell body. Only the apical dendrite and one branch, which extend from the top of the cell body, are visible here. Dendrites from other neurons (dark green) are visible in longitudinal and cross-sections in the neuropil surrounding the neuron.

• Dendritic Spines (blue) - small protrusions that increase the surface area to provide sites for synapses
  • Longitudinal Section - one spine is seen originating from a dendrite
  • Cross-Sections - most profiles are cross-sections of spines
• Golgi Outposts (yellow)- additional Golgi apparatus usually found in the larger dendrites.
  • Allows the packaging of locally synthesized proteins into vesicles destined for synapses or plasma membrane.
• Mitochondria (red) - needed to meet local energy demands
• Endoplasmic Reticulum (cyan) - contains both smooth endoplasmic reticulum (SER) and rough endoplasmic reticulum (RER)
• Neurofibrils - dendrites contain many longitudinal protein filaments
  • Microtubules (dark blue) - provide structural support and are involved in the transport of materials in both directions through the dendrites
  • Neurofilaments - intermediate filaments that provide structural support

Dendrites and spines are highly dynamic structures that can alter their shape and size based on experience. This plasticity plays a role in learning and memory.

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Axons

The axon of this pyramidal neuron is not visible in this section. However, a multitude of axons (orange) from other neurons are visible in both longitudinal and cross-sections surrounding this neuron.

• Unmyelinated Axons (orange) - most of the axons are visible in cross-section
• Myelinated Axons (orange) - some axons are wrapped by a myelin sheath (dark brown)
  • Oligodendrocytes - often appear darker than surrounding tissue
  • Allows faster signal transmission

Axons contain mitochondria and a smooth endoplasmic reticulum (SER), but unlike dendrites, they do not possess rough endoplasmic reticulum (RER).

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Axon Terminals / Synapses

Many axon terminals (yellow) form synapses with the spines of the dendrites and, less often, with the dendrites themselves.

The smooth endoplasmic reticulum (SER) is found in axons and axon terminals, where it plays a role in calcium signaling and synaptic vesicle release.

These terminals release neurotransmitters from synaptic vesicles (purple) into the synapse, enabling communication between neurons.

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Astrocytes

Astrocytes are the largest and most populous of the glial cells in the brain.

Protoplasmic astrocytes possess stellate-shaped cell bodies with intricate branched processes extending into the neuropil.

The cell body of an astrocyte (tan) is visible next to the apical dendrite of the pyramidal neuron.

• Nucleus (blue) / Nuclear Envelope (purple) - rounded nucleus of dispersed chromatin with a thin rim of heterochromatin
• Intermediate Filaments - larger processes contain bundles of protein filaments composed of glial fibrillary acidic protein (GFAP)
• Cytoplasm - relative sparseness of electron-dense material

Astrocyte processes have irregular and angular shapes that frequently ensheath synapses.

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