Abstract:
Understanding neuronal morphologies is essential for unraveling the complexity of neural
networks and the intricate mechanisms underlying brain function. That’s because the
unique morphological signature of a neuron and its architecture dictate its function
through orchestrating the neuron’s intrinsic properties and synaptic connections that
paves the way to learning and behavior. The songbird’s brain circuitry serves as an
excellent model for investigating the neural basis of vocal learning and production, a
complex behavior that requires precise control over the vocal production system. The
High Vocal Center (HVC), a premotor nucleus in the avian forebrain analogous to layer
V in the motor cortex, is a crucial brain region involved in this process, as it is responsible
for the production of learned song. Among the several neuronal populations in HVC, the
premotor HVCRA neurons, which project to the robust nucleus of the arcopallium, play a
critical role in controlling the precise timing and sequence of the vocal output. These
neurons had been under long-sought investigations due to their significance in our
understanding of rhythmic pattern generators in the brain, as they are known to be part of
one of the most temporally precise neural sequences known in nature to date. Recent
electrophysiological studies have identified four subclasses of HVCRA neurons (Daou and
Margoliash, under review), yet their anatomical and morphological properties are still
unclear. To address this gap, we analyzed a large dataset of 3D confocal images of
HVCRA neurons collected by Daou et al. using intracellular neurobiotin injections that
were processed histologically post-hoc and imaged on a confocal microscope. Our results
show four unique subclasses of HVCRA neurons that show differences in soma sizes, spine
densities, number of primary dendrites, branching, Sholl analysis, and other dendritic
patterns and complexities. This study shed light on the morphological properties of
HVCRA neurons in zebra finches, and their possible implications for the neural
mechanisms underlying vocal learning and production. By identifying these differences
between the subclasses of HVCRA neurons, we can gain a better understanding of how
these neurons contribute to the precise control of vocal output, and ultimately, how the
various morphological variations orchestrate the underlying behavior.