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Spinally projecting serotonergic (5-HTsp) neurons represent a heterogeneous population of neurons in the brainstem whose relevance in the control of movement has largely been inferred. Numerous studies across a variety of species have suggested that 5-HTsp neurons exert a widespread influence on spinal sensorimotor networks, operating at multiple levels (primary afferents, interneurons, and motoneurons) through various serotonin receptor subtypes. However, despite the anatomical and neurochemical complexity of the 5-HTsp system, most supporting evidence has largely been derived from indirect approaches (e.g., exogenous application of 5-HT and agonists/antagonists of 5-HT receptors). Direct demonstrations of specific anatomical and functional connectivity have been limited, occasionally yielding apparent discrepant results. Consequently, as the primary provider of serotonin to the spinal cord, the exact contributions of the 5-HTsp neurons remain to be fully elucidated. For this mini-review, we sifted through the literature of the last six decades, starting after the characterization of the brainstem raphe nuclei and monoaminergic systems [1–3], to provide a clearer picture of what is currently known of the anatomy and influences of the different populations of 5-HTsp neurons on sensorimotor circuits and motor behaviors. We focused on studies reporting direct manipulation of brainstem 5-HTsp neurons, excluding those targeting 5-HT neurotransmission by exogenous application of 5-HT. This emphasis aims to highlight the urgency of resolving how 5-HTsp neuron subpopulations differentiate anatomically and functionally, so that they can be integrated as dedicated components in current models of supraspinal control of movement and motor diseases such as Parkinson‘s and amyotrophic lateral sclerosis. Along the way, we point out gaps in knowledge that may be filled using newly available research tools. 

The limited information about how descending inputs from the brain and sensory inputs from the periphery use spinal cord interneurons (INs) is a major barrier to understanding how these inputs may contribute to motor functions under normal and pathologic conditions. Commissural interneurons (CINs) are a heterogeneous population of spinal INs that has been implicated in crossed motor responses and bilateral motor coordination (ability to use the right and left side of the body in a coordinated manner) and, therefore, are likely involved in many types of movement (e.g., dynamic posture stabilization, jumping, kicking, walking). In this study, we incorporate mouse genetics, anatomy, electrophysiology, and single-cell calcium imaging to investigate how a subset of CINs, those with descending axons called dCINs, are recruited by descending reticulospinal and segmental sensory signals independently and in combination. We focus on two groups of dCINs set apart by their principal neurotransmitter (glutamate and GABA) and identified as VGluT2+ dCINs and GAD2+ dCINs. We show that VGluT2+ and GAD2+ dCINs are both extensively recruited by reticulospinal and sensory input alone but that VGluT2+ and GAD2+ dCINs integrate these inputs differently. Critically, we find that when recruitment depends on the combined action of reticulospinal and sensory inputs (subthreshold inputs), VGluT2+ dCINs, but not GAD2+ dCINs, are recruited. This difference in the integrative capacity of VGluT2+ and GAD2+ dCINs represents a circuit mechanism that the reticulospinal and segmental sensory systems may avail themselves of to regulate motor behaviors both normally and after injury. SIGNIFICANCE STATEMENTThe way supraspinal and peripheral sensory inputs use spinal cord interneurons is fundamental to defining how motor functions are supported both in health and disease. This study, which focuses on dCINs, a heterogeneous population of spinal interneurons critical for crossed motor responses and bilateral motor coordination, shows that both glutamatergic (excitatory) and GABAergic (inhibitory) dCINs can be recruited by supraspinal (reticulospinal) or peripheral sensory inputs. Additionally, the study demonstrates that in conditions where the recruitment of dCINs depends on the combined action of reticulospinal and sensory inputs, only excitatory dCINs are recruited. The study uncovers a circuit mechanism that the reticulospinal and segmental sensory systems may avail themselves of to regulate motor behaviors both normally and after injury. 

Abstract A first synthesis of available data for the period of Rome’s expansion in Italy (about 400–29 b.c.e.) shows the role of climate and environment in early Roman imperialism. Although global indices suggest a warmer phase with relatively few short-term climate events occuring around the same time as the expansion, local data emphasize the highly variable timing and expression of these trends. This variability casts doubt on ideas of a unitary, historically consequential “Roman Warm Period.” The historical importance of climate and environment to socioeconomic development merits emphasis, but should be understood in terms of evolving, contingent forms of resilience and risk-mitigating behavior by Italian communities during Roman expansion.