Synaptic sequences in mouse SmI cortex involving pyramidal cells labeled by retrograde filling with horseradish peroxidase.

A method combining anterograde degeneration and the retrograde transport of horseradish peroxidase (HRP) has been used to study synapses involving pyramidal cells in mouse SmI cortex. Neurons labeled with HRP are so well filled that even their finer processes, such as dendritic spines and axon collaterals, are clearly visible with both the light and electron microscopes. Results indicate that pyramidal cells projecting from SmI to ipsilateral MsI cortex receive thalamocortical input and have local axon collaterals which form asymmetrical synapses with spines and with varicose, non-spiny dendrites.