Biological signaling in the mammalian nervous system spans a dizzying range of spatial and temporal scales. To understand how cellular and molecular signals contribute to physiology and behavior and to treat the neurological and psychiatric conditions our group designs tools that mimic biological complexity yet match the materials properties of tissues. By combining polymer engineering, fiber drawing, and solid-state microelectronics we create scalable fiber-based tools that record and modulate cell signaling in the central and the autonomic nervous systems in behaving rodents. Using these fiber-based tools we reveal the contributions of gut-brain circuits not only to ingestive and metabolic functions but also to high-level behaviors previously attributed exclusively to brain signaling. To probe receptor contributions to neural circuit dynamics, we synthesize magnetic nanotransducers that convert externally applied magnetic fields into thermal, chemical, mechanical, and electrical signals. Since biological tissues exhibit negligible magnetic permeability and low conductivity, magnetic fields can penetrate deep into the body with no attenuation allowing us to apply the nanomagnetic transducers to remotely modulate ion channel function in arbitrarily deep tissues. We employ magnetic neuromodulation to control reward and motivation circuits and extend their applications to relieve motor dysfunctions in mouse models of Parkinson’s disease.