EM 287 Dendritic Spine

Dendritic Spine

Transmission electron microscopy (TEM) of a dendritic spine of a neuron in the central nervous system (CNS).

Dendritic spines are small, mushroom-shaped protrusions that extend from dendrites to increase the surface area available for synapses.

• Dendritic Spine (green) - base of the spine, where it attaches to a dendrite, is cut off at the bottom
• Myelinated Axon - forms a synapse after exiting its myelin sheath (brown)
• Synapses (#1, #2,#3) - axon terminals (orange) that form synapses with this dendritic spin (green)
  • Synaptic Vesicles (purple or blue)
• Unmyelinated Axons and Axon Terminals (yellow) - axons in the surrounding neuropil with no apparent synapses
  • Synaptic Vesicles (dark yellow)
• Astrocytes (tan) - irregularly-shaped processes that fill spaces between axons

Subcellular Structures:

• Mitochondria (red)
• Endoplasmic Reticulum (cyan)
• Synaptic Vesicles (purple / blue)

Courtesy of Kohi Uchizono, Department of Physiology, University of Tokyo, Tokyo, Japan.

Synapses

This dendritic spine is covered with multiple synapses (#1, #2,#3; yellow), known as axo-dendritic synapses.

Synapses are specialized junctions between a neuron and a target cell (like another neuron, muscle, or gland cells) where information is transmitted.

A synapse has three main parts:

• Presynaptic Terminal - swellings (orange) at the end of the axon of the transmitting neuron
  • Synaptic Vesicles (purple and blue) - small, secretory vesicles filled with neurotransmitters
• Synaptic Cleft - narrow gap between the two cell membranes
  • Contains enzymes that break down neurotransmitters
• Postsynaptic Membrane - electron-dense region of the cell membrane of the receiving neuron
  • Contains receptors that respond to neurotransmitters

When an action potential travels down the axon, synaptic vesicles in the presynaptic terminal fuse with the cell membrane and release neurotransmitters into the synaptic cleft. They bind to receptors on the postsynaptic membrane, either exciting or inhibiting the next cell, depending on the type of neurotransmitter and receptor involved.

Asymmetrical and Symmetrical Synapses

In the central nervous system, chemical synapses are classified into two structural types based on their appearance in electron microscopy.

• Asymmetrical Synapses (#1,#2) - usually excitatory
  • Clear asymmetry between pre- and postsynaptic sides
    • Postsynaptic density is thicker than the presynaptic side
  • Synaptic Vesicles (purple) - round or ovoid shapes
    • Store excitatory neurotransmitters (often glutamate)
  • Increase the likelihood that the postsynaptic neuron will fire an action potential
• Symmetrical Synapses - usually inhibitory
  • Symmetry between pre- and postsynaptic sides
    • Thinner and more evenly matched densities on both sides of the synapse
  • Synaptic Vesicles (blue) - flattened or pleomorphic (irregular) shapes
    • Store inhibitory neurotransmitters (often GABA)
  • Decrease the likelihood that the postsynaptic neuron will fire an action potential