In the preceding companion article (Berkowitz and Stein, 1994b), we showed that many descending propriospinal neurons in the turtle were rhythmically activated during two different motor patterns, fictive rostral scratching and fictive pocket scratching. In this article, we present phase analyses of the activity of each such neuron during fictive scratching. Each neuron's activity was concentrated in a particular phase of the ipsilateral hip flexor muscle nerve (VP-HP) activity cycle; each had a distinct "preferred phase." Each neuron's preferred phase during fictive rostral scratching was similar to its preferred phase during fictive pocket scratching. This result is consistent with the idea that some descending propriospinal neurons may contribute to the generation of both rostral scratching and pocket scratching. Many descending propriospinal neurons were rhythmically activated during fictive scratching evoked on either side of the body. This activity may contribute to production of bilateral hindlimb movements during scratching. It is also possible that synaptic interactions between the two sides of the spinal cord may be important in generating the motor patterns for movement of a single hindlimb. In addition, we present a model which illustrates that a population of propriospinal neurons, each of which is broadly tuned to a region of the body surface and is rhythmically activated in a constant phase of the hip control cycle, could mediate the selection and generation of rostral scratching and pocket scratching. Thus, the selection of an appropriate motor pattern and the production of the required knee-hip synergy may each be distributed over a diverse population of spinal cord neurons. This model requires that each such neuron project to both knee muscle and hip muscle motoneurons. According to this model, the process of selecting a motor pattern would not be completed until knee muscle motoneurons integrate overlapping excitatory and inhibitory inputs.

In principle, nervous systems could generate a behavior either via neurons that are relatively specialized for producing one behavior or via multifunctional neurons that are shared among multiple, diverse behaviors. I recorded extracellularly from individual turtle spinal cord neurons while evoking hindlimb scratching, swimming, and withdrawal motor patterns. The majority of spinal neurons recorded were activated during both scratching and swimming motor patterns, consistent with the existence of shared circuitry for these types of limb movements. These neurons tended to have a similar degree of rhythmic modulation of their firing rate and a similar phase preference within the hip flexor activity cycle during scratching and swimming motor patterns. In addition, a substantial minority of neurons were activated during scratching motor patterns but silenced during swimming motor patterns. This raises the possibility that inhibitory interactions between some scratching and swimming neural circuitry play a role in motor pattern selection. These scratch-specialized neurons were also less likely than the putative shared neurons to be activated during withdrawal motor patterns. Thus, these neurons may represent two separate classes, one of which is used generally for hindlimb motor control and the other of which is relatively specialized for a subset of hindlimb movement types.

Berkowitz A. Physiology and morphology of shared and specialized spinal interneurons for locomotion and scratching. J Neurophysiol 99: 2887-2901, 2008. First published April 2, 2008 doi:10.1152/jn.90235.2008. Distinct types of rhythmic movements that use the same muscles are typically generated largely by shared multifunctional neurons in invertebrates, but less is known for vertebrates. Evidence suggests that locomotion and scratching are produced partly by shared spinal cord interneuronal circuity, although direct evidence with intracellular recording has been lacking. Here, spinal interneurons were recorded intracellularly during fictive swimming and fictive scratching in vivo and filled with Neurobiotin. Some interneurons that were rhythmically activated during both swimming and scratching had axon terminal arborizations in the ventral horn of the hindlimb enlargement, indicating their likely contribution to hindlimb motor outputs during both behaviors. We previously described a morphological group of spinal interneurons ("transverse interneurons" or T neurons) that were rhythmically activated during all forms of fictive scratching at higher peak firing rates and with larger membrane potential oscillations than scratch-activated spinal interneurons with different dendritic orientations. The current study demonstrates that T neurons are activated during both swimming and scratching and thus are components of the shared circuitry. Many spinal interneurons activated during fictive scratching are also activated during fictive swimming (scratch/swim neurons), but others are suppressed during swimming (scratch-specialized neurons). The current study demonstrates that some scratch-specialized neurons receive strong and long-lasting hyperpolarizing inhibition during fictive swimming and are also morphologically distinct from T neurons. Thus this study indicates that locomotion and scratching are produced by a combination of shared and dedicated interneurons whose physiological and morphological properties are beginning to be revealed.

We recorded the activity of descending propriospinal axons at the caudal end of a seven-segment (D3-D9) turtle spinal cord preparation. These seven spinal segments contain sufficient neural circuitry to select and generate fictive rostral scratching or fictive pocket scratching in response to tactile stimulation in the appropriate region of the body surface. Each turtle received two spinal transections, one just caudal to the forelimb enlargement and one in the middle of the hindlimb enlargement. Descending propriospinal axons were recorded extracellularly from the hindlimb enlargement on one side of the body, while the ipsilateral or contralateral body surface was stimulated. Concurrent recordings were made from ipsilateral and contralateral hindlimb muscle nerves to monitor fictive scratch motor patterns. We found that most tactilely responsive descending propriospinal axons were excited by stimulation anywhere within the rostral scratch or pocket scratch receptive fields on at least one side of the body, and often on both sides. The activity of these neurons was usually rhythmically modulated during fictive rostral scratching and fictive pocket scratching. Many neurons with large excitatory receptive fields generated action potentials at their highest rate during stimulation of a particular region of the body surface on one side, and generated action potentials at progressively lower rates during stimulation of sites progressively farther away. Thus, these units were broadly tuned to a region of the body surface. Some were tuned to a region of the rostral scratch receptive field and others were tuned to a region of the pocket scratch receptive field. These data suggest that selection of the appropriate form of scratching, rostral or pocket, may be mediated by populations of broadly tuned neurons rather than by highly specialized neurons.

The hindbrain and spinal cord can produce multiple forms of locomotion, escape, and withdrawal behaviors and (in limbed vertebrates) site-specific scratching. Until recently, the prevailing view was that the same classes of central nervous system neurons generate multiple kinds of movements, either through reconfiguration of a single, shared network or through an increase in the number of neurons recruited within each class. The mechanisms involved in selecting and generating different motor patterns have recently been explored in detail in some non-mammalian, vertebrate model systems. Work on the hatchling Xenopus tadpole, the larval zebrafish, and the adult turtle has now revealed that distinct kinds of motor patterns are actually selected and generated by combinations of multifunctional and specialized spinal interneurons. Multifunctional interneurons may form a core, multipurpose circuit that generates elements of coordinated motor output utilized in multiple behaviors, such as left-right alternation. But, in addition, specialized spinal interneurons including separate glutamatergic and glycinergic classes are selectively activated during specific patterns: escape-withdrawal, swimming and struggling in tadpoles and zebrafish, and limb withdrawal and scratching in turtles. These specialized neurons can contribute by changing the way central pattern generator (CPG) activity is initiated and by altering CPG composition and operation. The combined use of multifunctional and specialized neurons is now established as a principle of organization across a range of vertebrates. Future research may reveal common patterns of multifunctionality and specialization among interneurons controlling diverse movements and whether similar mechanisms exist in higher-order brain circuits that select among a wider array of complex movements.