The thalamocortical projection in Pseudemys turtles: a quantitative electron microscopic study

Area of Science:

• Neuroscience
• Electron Microscopy
• Comparative Anatomy

Background:

• Thalamocortical projections are fundamental for sensory processing and cognitive functions.
• Understanding the precise synaptic targets of thalamic fibers is crucial for deciphering cortical circuitry.
• The Pseudemys turtle provides a valuable model for studying vertebrate brain organization.

Purpose of the Study:

• To characterize the synaptic connections of thalamic fibers in the Pseudemys turtle cortex.
• To determine the types of vesicles, membrane differentiations, and neuronal processes contacted by thalamic axons.
• To quantify the distribution and density of thalamic synapses on different neuron types.

Main Methods:

• Unilateral thalamus removal in Pseudemys turtles to induce Wallerian degeneration of thalamic axons.
• Electron microscopy was used to examine degenerating axon terminals in the general cortex.
• Synaptic profiles were analyzed for vesicle type, membrane specialization, and contacted neuronal structures.

Main Results:

• Degenerating thalamic axon terminals were localized to the outer molecular layer, constituting up to 25% of profiles.
• These terminals contained round, agranular vesicles and formed asymmetrical synapses, primarily on dendritic spines (86%) and shafts (14%).
• Stellate cells in the molecular layer received significantly more thalamic synapses (approx. 1,800 per cell) compared to pyramidal cells (approx. 300 per cell).

Conclusions:

• Thalamic fibers in turtle cortex predominantly synapse on pyramidal cell dendrites, with a substantial proportion targeting stellate cells.
• Stellate cells appear to be the primary recipients of thalamic input, receiving at least six times more synapses than pyramidal cells.
• The findings suggest that stellate cells in the thalamic input zone function as inhibitory interneurons, mediating thalamic volleys to efferent pyramidal cells.