Axonal trajectories between mouse somatosensory thalamus and cortex

Area of Science:

• Neuroscience
• Cellular Biology
• Anatomy

Background:

• Understanding the precise anatomical organization of neural pathways is crucial for deciphering brain function.
• The somatosensory thalamocortical and corticofugal pathways are fundamental for sensory processing and motor control.
• Previous knowledge of these pathways' organization in vivo has limitations in resolving fine details.

Purpose of the Study:

• To visualize and characterize the organization of thalamocortical and corticofugal fiber bundles in an in vitro mouse brain slice preparation.
• To investigate the spatial relationships of axons as they project from the thalamus to the cortex and vice versa.
• To elucidate the mechanisms underlying the observed mediolateral rotation between thalamic and cortical sensory maps.

Main Methods:

• Utilized an in vitro brain slice preparation (400-800 microns) of the mouse brain.
• Labeled afferent and efferent fibers using horseradish peroxidase (HRP) with various enhancers (NP40, DMSO).
• Visualized and traced fiber pathways using direct microscopy under controlled conditions.

Main Results:

• Thalamocortical and corticofugal fibers form a consistent plexiform system of bundles.
• Axon bundles rotate approximately 180 degrees within the internal capsule, explaining the mediolateral map reversal.
• Fibers diverge significantly below the white matter, and afferent/efferent fibers exhibit distinct looping patterns and branching in cortical layers IV, VI, and I.

Conclusions:

• The in vitro slice preparation effectively visualizes complex fiber organization in the somatosensory thalamocortical system.
• The observed fiber rotation and divergence explain the topographical organization of sensory maps.
• Axonal branching patterns in cortical layers IV and VI are similar to in vivo findings, validating the model.