EM 336 Thalamus

Neuron

Transmission electron microscopy (TEM) of a neuron and the surrounding neuropil in the thalamus of the central nervous system.

• Neuron - cell body (green) and dendrites (dark green)
• Nerve Fibers (yellow) - cross-sections of unmyelinated axons
• Synapses (#1,#2; orange) - occur on the cell body and dendrites
• Astrocytes (tan) - processes fill spaces the in between axons

We will take a closer look at these in the following pages.

Subcellular Structures

• Nuclear Envelope (purple)
• Golgi Apparatus (yellow)
• Mitochondria (red)
• Endoplasmic Reticulum (cyan) - Nissl Substance
• Synaptic Vesicles (purple)
• Microtubules (blue)

Courtesy of Michael J. Cullen, National Institute of Neurological and Communicative Disorders and Stroke, National Institutes of Health, Bethesda, Maryland.

Cell Body

The cell body of neurons, often referred to as the soma or perikaryon, contains a single, rounded nucleus surrounded by cytoplasm rich in organelles.

These structures are visible in this neuron:

• Nuclear Envelope (purple) - small patch from the edge of the nucleus with nuclear pores
• Golgi Apparatus (yellow) - highly-developed with multiple stacks of flattened sacs and nearby vesicles
• Mitochondria (red) - abundant to support the high levels of cellular and electrical activity
• Endoplasmic Reticulum (cyan) - abundant throughout the cell body
  • Rough Endoplasmic Reticulum (RER) - cisternae with bound ribosomes responsible for the synthesis of proteins destined for insertion into membranes, delivery to other organelles or secretory vesicles
    • Nissl Substance - clusters of RER and polyribosomes that are a distinguishing feature of neurons
  • Smooth Endoplasmic Reticulum (SER) - cisternae that lack ribosomes involved in lipid synthesis and calcium storage
• Ribosomes - numerous and scattered throughout the cytoplasm
• Lysosomes (black) - break down waste materials and cellular debris

Dendrites

Dendrites are branching processes that increase the surface area for synapses with other neurons. Electrical signals travel along the dendrites toward the cell body, where the neuron integrates them to determine whether to fire an action potential down its axon.

Cross-sections of several dendrites are visible.

• Mitochondria (red) - meet the local energy demands and sustain electrical activity
• Endoplasmic Reticulum (cyan) - contains both smooth endoplasmic reticulum (SER) and rough endoplasmic reticulum (RER)
• Cytoskeleton - longitudinal arrays of filaments that maintain their structural integrity
  • Microtubules (blue) - motor proteins (kinesin and dynein) attach and crawl along microtubules to transport proteins, vesicles, and organelles
  • Neurofilaments (red) - smaller intermediate filaments that provide structural support

Synapses

Axons terminate in swellings called axon terminals, which create synapses with other cells.

• Synapses (orange) - specialized sites where communication occurs between neurons and other neurons or target cells.
• Mitochondria (red) - needed to meet local energy demands
• Smooth Endoplasmic Reticulum (SER; cyan) - helps control the local concentrations of Ca²⁺ ions
• Cytoskeleton - microtubules (blue) and smaller neurofilaments

Synapses between neurons are also defined by their location:

• Axo-Somatic -axon terminates on the cell body of another neuron
  • Stronger influence on whether the neuron will generate an action potential
• Axo-Dendritic - axon terminates on a dendrite of another neuron

Neuropil

The neuropil is the space between the cell bodies of neurons and is the location of most synaptic activity.

Primarily composed of:

• Dendrites (dark green)
• Synapses (yellow)
• Axons (orange) - only unmyelinated axons (mostly cross-sections) are visible
  • Except for myelinated axons, axons in the central nervous system (CNS) lack coverings (unlike axons outside the CNS)
• Glial Cells - only the irregularly shaped processes of astrocytes (tan) are visible
• Blood Vessels (not visible)