The road from thought to action: a brainwide atlas of spinal projecting neurons

Abstract

The brain executes control of nearly all bodily functions via spinal projecting neurons (SPNs) that carry command signals from numerous supraspinal regions to the spinal cord. Despite significant progress in identifying the major anatomical tracts and their functions, the molecular and cellular mechanisms underlying SPN connectivity and function remain unknown. In this dissertation, using retrograde labeling, whole-brain imaging, and high-throughput transcriptional profiling, I generated a unified brain-wide anatomic and transcriptomic atlas of adult mouse SPNs at single-cell resolution. This atlas revealed important insights for understanding the anatomic (Chapter 1), transcriptomic (Chapter 2), and electrophysiologic (Chapter 3) features of these neurons:

Chapter 1 provides insights into the anatomical distribution of cervical- and lumbar-projecting SPNs via serial two-photon tomography and registration to the Allen Mouse Common Coordinate Framework. This work comprehensively mapped the distribution of SPNs, confirming concentrations in the cortex, hypothalamus, midbrain, cerebellum, pons, and medulla.

Chapter 2 showcases a multi-level transcriptomic taxonomy developed using single-nucleus transcriptomic profiling of 65,002 SPNs. This taxonomy revealed a three-component organization of SPNs: (1) molecularly homogeneous excitatory SPNs in the cortex, red nucleus, and cerebellum with somatotopic spinal terminations suitable for point-to-point communication; (2) highly heterogeneous excitatory and inhibitory populations in the reticular formation with broad spinal termination patterns, suitable for relaying commands to the entire spinal cord; and (3) modulatory neurons expressing slow-acting neurotransmitters and/or neuropeptides in the hypothalamus, midbrain, and reticular formation for gain control of brain-spinal signals.

Chapter 3 focuses on the electrophysiological properties of SPNs. Cell-attached and whole-cell recordings of retrogradely labeled SPNs revealed electrophysiological features of a subset of SPNs that highly express transcriptional signatures correlating with fast-firing properties (namely, Pvalb/Kcng4/Spp1). These results show that Spp1+ SPNs are defined by fast-conducting properties, suggesting the presence of different cable lines (i.e., “fast and slow”) transmitting brain signals to the spinal cord.

This body of work is the first to systematically describe brain-wide spinal projecting neurons by integrating their anatomic, transcriptomic, and electrophysiologic features, promising to deepen our understanding of how the brain controls the body, and may contribute to the development of new therapeutic approaches for disorders affecting descending pathways.