Introduction

Some neuroscientists interested in the neural control of movement study rhythmical behaviors, such as locomotion, respiration, or mastication. Others examine more episodic behaviors, such as reaching movements of the arm or orienting movements of the eyes and head. Because the neural circuits involved in rhythmical behavior often continue to generate cyclical patterns of activity in isolated preparations (dish, slice, or slab), detailed descriptions of the biophysical and pharmacological properties of neurons and circuits are possible. These descriptions extend to studies of the effects of manipulating different subtypes of ion channels and transmitters or second-messenger systems on the behavior of the circuit (Grillner 2003). Some model systems have identified cells (the same cell can be found in different members of the species), and the anatomical connections of local networks of cells are completely described. The mechanisms by which neuromodulators alter the properties of a "hard-wired" circuit have been the focus of much work on the stomatogastric ganglion (Katz and Harris-Warrick 1999; Marder and Thirumalai 2002). Analytical models of circuit and cell behavior can be combined with experiments by using the dynamic clamp technique (Marder and Abbott 1995).

The level of analysis possible in model systems employed to study rhythmical behavior is extremely difficult to obtain in model systems used to study episodic movements. It is hard to study episodic behavior in slice because, unlike the cyclical activity underlying rhythmical behavior, an electrophysiological signature of motor command signals is missing. In fact, command signals may not occur when premotor neurons are separated from sensory inputs or, if they do occur, they may not be recognized because there is no movement to observe. Consequently, the goals of studies of episodic movements have been quite different, and the focus has been on molar, rather than molecular, issues. These include, for example, quantifying the relationship between neural and muscular activity as well as the direction, amplitude, and speed of movements, and understanding the neural basis of the variability in simple reaction time tasks. The effects of local perturbations of the spatial and temporal pattern of network activity on the latency, accuracy, speed, and trajectory of a movement inform us about how neurons with coarsely tuned movement fields can produce precise movements. Other studies focus on the transformations of sensory signals required to interface with motor command signals generated by neurons organized in motor maps, and understanding the manner in which the format of motor commands constrains the types of sensory processing that must occur. Considerable progress has been made in understanding the neural mechanisms underlying various types of motor plasticity. Studies of motor preparation examine neural activity that occurs well in advance of the executed movement. Increasingly, episodic behaviors are used to investigate cognitive factors (e.g., decision making, spatial attention, motor memory, motor intention, and movement cancellation) that influence motor control.

For those studying episodic movements, the route to detailed descriptions of the microcircuits involved and the biophysical and biochemical properties of the neurons in the circuits is usually indirect. For example, the network involved in

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