Network dynamics and motor pattern generation

Our lab uses the central pattern generating (CPG) networks of neurons found in the stomatogastric ganglion (STG), a small part of the central nervous system in lobsters and crabs that controls movements of the stomach, to study the regulation of neuron and network properties which give rise to proper motor activity. CPGs are found in both vertebrates and invertebrates that generate rhythmic activity for crucial behaviors like walking and breathing. The STG contains a small number of individually identified neurons with known synaptic connectivity, which makes it an ideal test bed for regulation of network output.

The stomatogastric nervous system produces spontaneous rhythmic activity in vitro. The connectivity of the central pattern generating networks in the STG is known. Shown here are a simplified circuit diagram of the pyloric network and the tri-phasic rhythm it produces.

Network homeostasis in a motor system
A neuron's identity and function is determined by its morphology, the densities and spatial distribution of its specific types of receptors and ion channels, and its synaptic connections within the network. These features are all subject to dynamic regulation and must be matched to the functional requirements in the face of changing environmental and behavioral demands, both during growth and development, and in adult life. Homeostatic mechanisms are needed to ensure that dynamic changes occur only within certain boundaries that keep neuron and network activity in a functional range. We are only beginning to understand how nervous systems strike a balance between altering individual neurons and synapses in the name of plasticity, while maintaining long-term stability in neuronal system function. Homeostatic mechanisms have been investigated in a variety of systems at the level of intrinsic membrane properties and synaptic strengths in single neurons. However, ultimately, behavior depends on the performance of entire networks. Our research focuses on the question of how stability of network function is achieved through regulation of neuronal properties. We combine anatomical and imaging techniques, electrophysiology and biophysical measurements to investigate how tightly neuronal and synaptic properties need to be regulated to achieve functional network performance, and how such regulation can be achieved at the cellular and synaptic level.

Despite a dramatic increase in membrane surface from juvenile to adult, underlying intrinsic neuronal and synaptic properties must be regulated to keep network performance stable.

Dopamine modulation of a motor axon

It has long been assumed that the sole function of axons is the faithful conduction of action potentials from one location to another. Signal integration, voltage-dependent dynamics, and sensitivity to neuromodulators are usually thought to be confined to dendrites and terminals. Axons are thought to have only a simple complement of voltage-activated channels, those minimally required for action potential propagation. Recently, this view has been challenged by a number of studies showing diverse functional capabilities of axons. These findings argue that axonal computation may play an important role in the short-term dynamics of neural communication. However, in most systems this problem is hard to address, because physiological recordings from axons are difficult. We use an easily accessible motor axon in the peripheral nerves of the stomatogastric nervous system to better establish the emerging idea that axons indeed have complex dynamics and contribute to the generation of output activity. We have shown that this axon has complex intrinsic membrane properties and can initiate action potentials in response to dopamine application. We are now characterizing the contribution of different voltage-gated ion channels and their modulation by dopamine to the complex behavior of the axon membrane, and test how these properties affect the conduction of temporal patterns of action potentials generated in the central nervous system.
The axon of the Pyloric Dilator motor neuron (PD) can be recorded intracellularly in the peripheral nerves. It has complex intrinsic membrane properties and depolarizes in response to dopamine application.
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