Crosses and uncrossed synaptic actions on motoneurones of back muscles in the cat

Motoneuronal Spinal Circuits in Degenerative Motoneuron Disease

The most evident phenotype of degenerative motoneuron disease is the loss of motor function which accompanies motoneuron death. In both amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA), it is now clear that dysfunction is not restricted to motoneurons but is manifest in the spinal circuits in which motoneurons are embedded. As mounting evidence shows that motoneurons possess more elaborate and extensive connections within the spinal cord than previously realized, it is necessary to consider the role of this circuitry and its dysfunction in the disease process. In this review article, we ask if the selective vulnerability of the different motoneuron types and the relative disease resistance of distinct motoneuron groups can be understood in terms of their intraspinal connections.

Electrophysiological properties of Ia excitation and recurrent inhibition in cat abdominal motoneurons

Ia excitation and recurrent inhibition are basic neuronal circuits in motor control in hind limb. Renshaw cells receive synaptic inputs from axon collaterals of motoneurons and inhibit motoneurons and Ia inhibitory interneurons. It is important to know properties of Ia excitation and recurrent inhibition of trunk muscle such as abdominal muscles. The abdominal muscles have many roles and change those roles for different kind of functions. Intracellular recordings were obtained from the abdominal motoneurons of the upper lumbar segments in cats anesthetized. First, dorsal roots were left intact, and sensory and motor axons were electrically stimulated. Ia excitatory post-synaptic potentials were elicited in five of eight motoneurons at same segment stimulated. Second, dorsal roots were sectioned, and motor axons were electrically stimulated. Recurrent inhibitory post-synaptic potentials were elicited in one of 11 abdominal motoneurons. Renshaw cells extracellularly fired high-frequency bursts at short latency and at same segment stimulated.

Escape from homeostasis: spinal microcircuits and progression of amyotrophic lateral sclerosis

In amyotrophic lateral sclerosis (ALS), loss of motoneuron function leads to weakness and, ultimately, respiratory failure and death. Regardless of the initial pathogenic factors, motoneuron loss follows a specific pattern: the largest α-motoneurons die before smaller α-motoneurons, and γ-motoneurons are spared. In this article, we examine how homeostatic responses to this orderly progression could lead to local microcircuit dysfunction that in turn propagates motoneuron dysfunction and death. We first review motoneuron diversity and the principle of α-γ coactivation and then discuss two specific spinal motoneuron microcircuits: those involving proprioceptive afferents and those involving Renshaw cells. Next, we propose that the overall homeostatic response of the nervous system is aimed at maintaining force output. Thus motoneuron degeneration would lead to an increase in inputs to motoneurons, and, because of the pattern of neuronal degeneration, would result in an imbalance in local microcircuit activity that would overwhelm initial homeostatic responses. We suggest that this activity would ultimately lead to excitotoxicity of motoneurons, which would hasten the progression of disease. Finally, we propose that should this be the case, new therapies targeted toward microcircuit dysfunction could slow the course of ALS.

Origin and Segmental Diversity of Spinal Inhibitory Interneurons

Motor output varies along the rostro-caudal axis of the tetrapod spinal cord. At limb levels, ∼60 motor pools control the alternation of flexor and extensor muscles about each joint, whereas at thoracic levels as few as 10 motor pools supply muscle groups that support posture, inspiration, and expiration. Whether such differences in motor neuron identity and muscle number are associated with segmental distinctions in interneuron diversity has not been resolved. We show that select combinations of nineteen transcription factors that specify lumbar V1 inhibitory interneurons generate subpopulations enriched at limb and thoracic levels. Specification of limb and thoracic V1 interneurons involves the Hox gene Hoxc9 independently of motor neurons. Thus, early Hox patterning of the spinal cord determines the identity of V1 interneurons and motor neurons. These studies reveal a developmental program of V1 interneuron diversity, providing insight into the organization of inhibitory interneurons associated with differential motor output.

Distinct limb and trunk premotor circuits establish laterality in the spinal cord

Movement coordination between opposite body sides relies on neuronal circuits capable of controlling muscle contractions according to motor commands. Trunk and limb muscles engage in distinctly lateralized behaviors, yet how regulatory spinal circuitry differs is less clear. Here, we intersect virus technology and mouse genetics to unravel striking distribution differences of interneurons connected to functionally distinct motor neurons. We find that premotor interneurons conveying information to axial motor neurons reside in symmetrically balanced locations while mostly ipsilateral premotor interneurons synapse with limb-innervating motor neurons, especially those innervating more distal muscles. We show that observed distribution differences reflect specific premotor interneuron subpopulations defined by genetic and neurotransmitter identity. Synaptic input across the midline reaches axial motor neurons preferentially through commissural axon arborization, and to a lesser extent, through midline-crossing dendrites capturing contralateral synaptic input. Together, our findings provide insight into principles of circuit organization underlying weighted lateralization of movement.

Absence of synergy for monosynaptic Group I inputs between abdominal and internal intercostal motoneurons

Internal intercostal and abdominal motoneurons are strongly coactivated during expiration. We investigated whether that synergy was paralleled by synergistic Group I reflex excitation. Intracellular recordings were made from motoneurons of the internal intercostal nerve of T8 in anesthetized cats, and the specificity of the monosynaptic connections from afferents in each of the two main branches of this nerve was investigated. Motoneurons were shown by antidromic excitation to innervate three muscle groups: external abdominal oblique [EO; innervated by the lateral branch (Lat)], the region of the internal intercostal muscle proximal to the branch point (IIm), and muscles innervated from the distal remainder (Dist). Strong specificity was observed, only 2 of 54 motoneurons showing excitatory postsynaptic potentials (EPSPs) from both Lat and Dist. No EO motoneurons showed an EPSP from Dist, and no IIm motoneurons showed one from Lat. Expiratory Dist motoneurons fell into two groups. Those with Dist EPSPs and none from Lat (group A) were assumed to innervate distal internal intercostal muscle. Those with Lat EPSPs (group B) were assumed to innervate abdominal muscle (transversus abdominis or rectus abdominis). Inspiratory Dist motoneurons (assumed to innervate interchondral muscle) showed Dist EPSPs. Stimulation of dorsal ramus nerves gave EPSPs in 12 instances, 9 being in group B Dist motoneurons. The complete absence of heteronymous monosynaptic Group I reflex excitation between muscles that are synergistically activated in expiration leads us to conclude that such connections from muscle spindle afferents of the thoracic nerves have little role in controlling expiratory movements but, where present, support other motor acts.

Etv1 inactivation reveals proprioceptor subclasses that reflect the level of NT3 expression in muscle targets

The organization of spinal reflex circuits relies on the specification of distinct classes of proprioceptive sensory neurons (pSN), but the factors that drive such diversity remain unclear. We report here that pSNs supplying distinct skeletal muscles differ in their dependence on the ETS transcription factor Etv1 for their survival and differentiation. The status of Etv1-dependence is linked to the location of proprioceptor muscle targets: pSNs innervating hypaxial and axial muscles depend critically on Etv1 for survival, whereas those innervating certain limb muscles are resistant to Etv1 inactivation. The level of NT3 expression in individual muscles correlates with Etv1-dependence and the loss of pSNs triggered by Etv1 inactivation can be prevented by elevating the level of muscle-derived NT3-revealing a TrkC-activated Etv1-bypass pathway. Our findings support a model in which the specification of aspects of pSN subtype character is controlled by variation in the level of muscle NT3 expression and signaling.

Electrophysiological and morphological characterization of propriospinal interneurons in the thoracic spinal cord

Propriospinal interneurons in the thoracic spinal cord have vital roles not only in controlling respiratory and trunk muscles, but also in providing possible substrates for recovery from spinal cord injury. Intracellular recordings were made from such interneurons in anesthetized cats under neuromuscular blockade and with the respiratory drive stimulated by inhaled CO(2). The majority of the interneurons were shown by antidromic activation to have axons descending for at least two to four segments, mostly contralateral to the soma. In all, 81% of the neurons showed postsynaptic potentials (PSPs) to stimulation of intercostal or dorsal ramus nerves of the same segment for low-threshold (≤ 5T) afferents. A monosynaptic component was present for the majority of the peripherally evoked excitatory PSPs. A central respiratory drive potential was present in most of the recordings, usually of small amplitude. Neurons depolarized in either inspiration or expiration, sometimes variably. The morphology of 17 of the interneurons and/or of their axons was studied following intracellular injection of Neurobiotin; 14 axons were descending, 6 with an additional ascending branch, and 3 were ascending (perhaps actually representing ascending tract cells); 15 axons were crossed, 2 ipsilateral, none bilateral. Collaterals were identified for 13 axons, showing exclusively unilateral projections. The collaterals were widely spaced and their terminations showed a variety of restricted locations in the ventral horn or intermediate area. Despite heterogeneity in detail, both physiological and morphological, which suggests heterogeneity of function, the projections mostly fitted a consistent general pattern: crossed axons, with locally weak, but widely distributed terminations.

Selfishness examined: Cooperation in the absence of egoistic incentives

Social dilemmas occur when the pursuit of self-interest by individuals in a group leads to less than optimal collective outcomes for everyone in the group. A critical assumption in the human sciences is that people's choices in such dilemmas are individualistic, selfish, and rational. Hence, cooperation in the support of group welfare will only occur if there are selfish incentives that convert the social dilemma into a nondilemma. In recent years, inclusive fitness theories have lent weight to such traditional views of rational selfishness on Darwinian grounds. To show that cooperation is based on selfish incentives, however, one must provide evidence that people do not cooperate without such incentives. In a series of experimental social dilemmas, subjects were instructed to make single, anonymous choices about whether or not to contribute money for a shared “bonus” that would be provided only if enough other people in the group also contributed their money. Noncontributors cited selfish reasons for their choices; contributors did not. If people are allowed to engage in discussion, they will contribute resources at high rates, frequently on irrational grounds, to promote group welfare. These findings are consistent with previous research on ingroup biasing effects that cannot be explained by “economic man” or “selfish gene” theories. An alternative explanation is that sociality was a primary factor shaping the evolution of Homo sapiens. The cognitive and affective mechanisms underlying such choices evolved under selection pressures on small groups for developing and maintaining group membership and for predicting and controlling the behavior of other group members. This sociality hypothesis organizes previously inexplicable and disparate phenomena in a Darwinian framework and makes novel predictions about human choice.

On the function of muscle and reflex partitioning

Studies have shown that in the mammalian neuromuscular system stretch reflexes are localized within individual muscles. Neuromuscular compartmentalization, the partitioning of sensory output from muscles, and the partitioning of segmental pathways to motor nuclei have also been demonstrated. This evidence indicates that individual motor nuclei and the muscles they innervate are not homogeneous functional units. An analysis of the functional significance of reflex localization and partitioning suggests that segmental control mechanisms are based on subdivisions of motor nuclei–muscle complexes. A partitioned organization of segmental control mechanisms could utilize (1) the potential functional diversity of muscle fiber types, (2) the variety of mechanical actions of individual muscles arising from their distributed origins and insertions, and (3) diverse architectural features such as intramuscular variations in pinnation and complex in-series and in-parallel arrangements of muscle fibers. The differentiated activity observed in some muscles during natural movements also calls for localized segmental control mechanisms. Partitioning may also play a role in mechanical interactions between contracting motor units and in increasing the stability of neuromuscular systems. The functional advantages of reflex localization and partitioning suggest they are probably common features of segmental systems, whose organization reflects the structure and function of their associated neuromuscular systems.