Intrinsic brainstem circuits comprised of Chx10-expressing neurons contribute to reticulospinal output in mice

Glutamatergic reticulospinal neurons in the gigantocellular reticular nucleus (GRN) of the medullary reticular formation can function as command neurons, transmitting motor commands to spinal cord circuits to instruct movement. Recent advances in our understanding of this neuron-dense region have been facilitated by the discovery of expression of the transcriptional regulator, Chx10, in excitatory reticulospinal neurons. Here, we address the capacity of local circuitry in the GRN to contribute to reticulospinal output. We define two sub-populations of Chx10-expressing neurons in this region, based on distinct electrophysiological properties and somata size (small and large), and show that these populations correspond to local interneurons and reticulospinal neurons, respectively. Using focal release of caged glutamate combined with patch clamp recordings, we demonstrated that Chx10 neurons form microcircuits in which the Chx10 local interneurons project to and facilitate the firing of Chx10 reticulospinal neurons. We discuss the implications of these microcircuits in terms of movement selection.

Brainstem Neurons that Command Left/Right Locomotor Asymmetries

Descending command neurons instruct spinal networks to execute basic locomotor functions, such as which gait and what speed. The command functions for gait and speed are symmetric, implying that a separate unknown system directs asymmetric movements—the ability to move left or right. Here we report the discovery that Chx10-lineage reticulospinal neurons act to control the direction of locomotor movements in mammals. Chx10 neurons exhibit ipsilateral projection, and can decrease spinal limb-based locomotor activity ipsilaterally. This circuit mechanism acts as the basis for left or right locomotor movements in freely moving animals: selective unilateral activation of Chx10 neurons causes ipsilateral movements whereas inhibition causes contralateral movements. Spontaneous forward locomotion is thus transformed into an ipsilateral movement by braking locomotion on the ipsilateral side. We identify sensorimotor brain regions that project onto Chx10 reticulospinal neurons, and demonstrate that their unilateral activation can impart left/right directional commands. Together these data identify the descending motor system which commands left/right locomotor asymmetries in mammals.

Unsupervised identification of the internal states that shape natural behavior

SummaryInternal states can shape stimulus responses and decision-making, but we lack methods to identify internal states and how they evolve over time. To address this gap, we have developed an unsupervised method to identify internal states from behavioral data, and have applied it to the study of a dynamic social interaction. During courtship, Drosophila melanogaster males pattern their songs using feedback cues from their partner. Our model uncovers three latent states underlying this behavior, and is able to predict the moment-to-moment variation in natural song patterning decisions. These distinct behavioral states correspond to different sensorimotor strategies, each of which is characterized by different mappings from feedback cues to song modes. Using the model, we show that a pair of neurons previously thought to be command neurons for song production are sufficient to drive switching between states. Our results reveal how animals compose behavior from previously unidentified internal states, a necessary step for quantitative descriptions of animal behavior that link environmental cues, internal needs, neuronal activity, and motor outputs.

Starting and stopping movement by the primate brain

We review the current knowledge about the part that motor cortex plays in the preparation and generation of movement, and we discuss the idea that corticospinal neurons, and particularly those with cortico-motoneuronal connections, act as ‘command’ neurons for skilled reach-to-grasp movements in the primate. We also review the increasing evidence that it is active during processes such as action observation and motor imagery. This leads to a discussion about how movement is inhibited and stopped, and the role in these for disfacilitation of the corticospinal output. We highlight the importance of the non-human primate as a model for the human motor system. Finally, we discuss the insights that recent research into the monkey motor system has provided for translational approaches to neurological diseases such as stroke, spinal injury and motor neuron disease.

Regulation of histone H4 acetylation in the CNS and defensive behavior command neurons of the mollusk Helix mediated by serotonin and neuropeptide FMRFamide

Epigenetic mechanisms are commonly known to underlie memory formation. Presently, scientists’ attention is focused on changes in the levels of histone modifications (mainly acetylation and methylation) in the chromatin of CNS cells tested in various experimental models. Owing to their relatively simple CNSs, mollusks are among the most popular models. Our experiments were con-ducted with the molluskHelix lucorumbecause its CNS had been investigated in detail and most of its neurons had been proven to participate in the formation of different behavior patterns, including the prolonged response to various stimuli. This work concerns the influence of various effectors (serotonin and FMRFamide, associated with CNS activator and inhibitory pathways, respectively) on the acetylation of H4 histone in the subesopha­geal ganglion complex and in defensive behavior command neurons of the right and left parietal ganglia (RPa3/2 and LPa3/2) in the snail. Western blot analysis showed that FMRFamide inhibited histone H4 acetylation induced by serotonin in the subesophageal complex of CNS ganglia. How­ever, serotonin and FMRFamide cooperatively enhanced the induction of histone H4 acetylation in RPa3/2 defensive behavior command neurons. No changes were found in the counterpart LPa3/2. It is a new piece of evidence for functional asym­metry inHelix. The inhibitory pathways mediated by FMRFamide not only inhibit the activatory in­tracellular processes in the entire CNS but can also enhance them, as in RPa3/2 defensive behavior command neurons.

Induced expression of a vestigial sexual signal

Vestigial morphological traits are common and well known in a variety of taxa. Identification of vestigial genes has illustrated the potential for evolutionary reversals and the re-expression of atavistic traits. Here we induce expression of a behavioural sexual signal, male calling song, in a cricket species, Gryllus ovisopis, which lacks a functional calling song. We successfully used acetylcholine injections in the frontal space of the head of male crickets to activate cerebral command neurons for cricket calling, and we recorded calling songs with a temporal chirp pattern similar to that of G. ovisopis ' close evolutionary relatives, G. firmus and G. pennsylvanicus , implying that the neural pattern generators that underlie cricket calling behaviour persist in a vestigial state in G. ovisopis . To our knowledge, this is the first demonstration of the induced expression of a vestigial behaviour in any organism. The retention of latent neural capacity to express sexual behaviours could have important implications for rapid evolution, trait re-emergence and reproductive isolation.