Modulatory and Command Interneurons for Behaviour

Abstract

Modulatory neurons change the properties of synapses and neurons, and thereby change the excitability or performance of neural circuits. Command neurons activate specific neural circuits that mediate discrete behaviour patterns. By operating on command neurons, on the inputs that excite them and on the circuits that they excite, modulatory neurons can transform individual patterns of behaviour and ensembles of behaviour in an adaptive manner.

Keywords: neuromodulation; stomatogastric nervous system; crustacea; spinal pattern generator; rhythmic motor pattern; reconfiguration; command neuron; mauthner neuron; lamprey; sensorimotor integration

Figure 1.

Command‐activated circuits. In a circuit (red) representative of both the lateral giant tailflip escape circuit in crayfish and the M‐cell escape circuit in fish, a command neuron (CN) is excited by converging sensory afferents (primary A) and initiates a pattern of motor neuron (MN1) excitation that leads to a stereotyped movement (M1). Excitation of the CN also leads to recurrent inhibition (I) of the primary afferents in order to protect against reafferent excitation produced by command‐induced movements. Inhibition is also directed at other, mutually exclusive, sensorimotor circuits, like that excited by a command system (CS) (blue). A CS is composed of an ensemble of interneurons that can activate a set of motor neurons (MN2) in distinct but related patterns to give rise to a variety of movements (M2). Both circuits can be modulated. A neuromodulator (MOD1) is directed at the CN in a fashion like the serotonergic modulation of the lateral giant (LG) nucleus and the dopaminergic modulation of the M cell, and a second neuromodulator (MOD2) is directed at multiple targets in the CS circuit. This last pattern of modulation is similar to the serotonergic modulation of the abdominal postural circuit in lobster by the A1/5‐hydroxytryptamine cells.

Figure 2.

Reconfiguration by modulatory neurons. Two diagrams of the same network illustrate how different modulatory neurons, red in (a) and green in (b), have a suite of effects on the network that has the effect of constructing and activating a new circuit. Cells are indicated by circles, connections by lines. The different colours of the cells contacted by the modulatory neuron represent the different effects that the modulator has on neuronal properties. The different thicknesses of the connecting lines represent the different effects of the modulator on synaptic connections and properties.

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Further Reading

Edwards DH, Heitler WJ and Krasne FB (1999) 50 years of a command neuron: the neurobiology of escape behavior in the crayfish. Trends in Neurosciences 22 (4): 153–161.

Katz PS (1999) Neuromodulation: Its Role in Information Flow and Neuronal Circuit Flexibility. Oxford: Oxford University Press.

Stein PSG, Grillner S, Selverston AI and Stuart DG (1997) Neurons, Networks and Motor Behavior. Cambridge, Massachusetts: MIT Press.

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Edwards, Donald H(Apr 2001) Modulatory and Command Interneurons for Behaviour. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0000183]