Biological Pattern Generation: The Cellular and Computational Logic of Networks in Motion

In 1900, Ramón y Cajal advanced the neuron doctrine, defining the neuron as the fundamental signaling unit of the nervous system. Over a century later, neurobiologists address the circuit doctrine: the logic of the core units of neuronal circuitry that control animal behavior. These are circuits that can be called into action for perceptual, conceptual, and motor tasks, and we now need to understand whether there are coherent and overriding principles that govern the design and function of these modules. The discovery of central motor programs has provided crucial insight into the logic of one prototypic set of neural circuits: those that generate motor patterns. In this review, I discuss the mode of operation of these pattern generator networks and consider the neural mechanisms through which they are selected and activated. In addition, I will outline the utility of computational models in analysis of the dynamic actions of these motor networks

Neuronal Control of Swimming Behavior: Comparison of Vertebrate and Invertebrate Model Systems

Swimming movements in the leech and lamprey are highly analogous, and lack homology. Thus, similarities in mechanisms must arise from convergent evolution rather than from common ancestry. Despite over 40 years of parallel investigations into this annelid and primitive vertebrate, a close comparison of the approaches and results of this research is lacking. The present review evaluates the neural mechanisms underlying swimming in these two animals and describes the many similarities that provide intriguing examples of convergent evolution. Specifically, we discuss swim initiation, maintenance and termination, isolated nervous system preparations, neural-circuitry, central oscillators, intersegmental coupling, phase lags, cycle periods and sensory feedback. Comparative studies between species highlight mechanisms that optimize behavior and allow us a broader understanding of nervous system function

From lamprey to salamander: an exploratory modeling study on the architecture of the spinal locomotor networks in the salamander

The evolutionary transition from water to land required new locomotor modes and corresponding adjustments of the spinal "central pattern generators" for locomotion. Salamanders resemble the first terrestrial tetrapods and represent a key animal for the study of these changes. Based on recent physiological data from salamanders, and previous work on the swimming, limbless lamprey, we present a model of the basic oscillatory network in the salamander spinal cord, the spinal segment. Model neurons are of the Hodgkin-Huxley type. Spinal hemisegments contain sparsely connected excitatory and inhibitory neuron populations, and are coupled to a contralateral hemisegment. The model yields a large range of experimental findings, especially the NMDA-induced oscillations observed in isolated axial hemisegments and segments of the salamander Pleurodeles waltlii. The model reproduces most of the effects of the blockade of AMPA synapses, glycinergic synapses, calcium-activated potassium current, persistent sodium current, and -current. Driving segments with a population of brainstem neurons yields fast oscillations in the in vivo swimming frequency range. A minimal modification to the conductances involved in burst-termination yields the slower stepping frequency range. Slow oscillators can impose their frequency on fast oscillators, as is likely the case during gait transitions from swimming to stepping. Our study shows that a lamprey-like network can potentially serve as a building block of axial and limb oscillators for swimming and stepping in salamanders

From lamprey to salamander: an exploratory modeling study on the architecture of the spinal locomotor networks in the salamander

The evolutionary transition from water to land required new locomotor modes and corresponding adjustments of the spinal "central pattern generators” for locomotion. Salamanders resemble the first terrestrial tetrapods and represent a key animal for the study of these changes. Based on recent physiological data from salamanders, and previous work on the swimming, limbless lamprey, we present a model of the basic oscillatory network in the salamander spinal cord, the spinal segment. Model neurons are of the Hodgkin-Huxley type. Spinal hemisegments contain sparsely connected excitatory and inhibitory neuron populations, and are coupled to a contralateral hemisegment. The model yields a large range of experimental findings, especially the NMDA-induced oscillations observed in isolated axial hemisegments and segments of the salamander Pleurodeles waltlii. The model reproduces most of the effects of the blockade of AMPA synapses, glycinergic synapses, calcium-activated potassium current, persistent sodium current, and $$h$$ -current. Driving segments with a population of brainstem neurons yields fast oscillations in the in vivo swimming frequency range. A minimal modification to the conductances involved in burst-termination yields the slower stepping frequency range. Slow oscillators can impose their frequency on fast oscillators, as is likely the case during gait transitions from swimming to stepping. Our study shows that a lamprey-like network can potentially serve as a building block of axial and limb oscillators for swimming and stepping in salamander

Neuronal mechanisms of feedback postural control

Different species maintain a basic body posture due to the activity of the postural control system. An efficient control of the body orientation, as well as the body configuration, is important for standing and during locomotion. A general goal of the present study was to analyze neuronal feedback mechanisms contributing to stabilization of the trunk orientation in space, as well as those controlling the body configuration. Two animal models of different complexity, the lamprey (a lower vertebrate) and the rabbit (a mammal), were used. Neuronal mechanisms underlying lateral stability were analyzed in rabbits. The dorsalside- up trunk orientation in standing quadrupeds is maintained by the postural system driven mainly by somatosensory inputs from the limbs. Postural limb reflexes (PLRs) represent a substantial component of this system. To characterize spinal neurons of the postural networks, in decerebrate rabbit, activity of individual spinal neurons in L4-L6 was recorded during PLRs caused by lateral tilts of the supporting platform. Spinal neurons mediating PLRs have been revealed, and different parameters of their activity were characterized. All neurons were classified into four types according to the combination of tilt-related sensory inputs to a neuron from the ipsi- and contralateral limb (determining the modulation of a neuron). A hypothesis about the role of different types of PLR-related neurons for trunk stabilization in different planes has been proposed. To reveal contribution of supraspinal influences to modulation of PLR-related neurons, the activity of individual spinal neurons was recorded during stimulation causing PLRs under two conditions: (i) when spinal neurons received supraspinal influences, and (ii) when these influences were temporarily abolished by a cold block of spike propagation in spinal pathways at T12 (“reversible spinalization”). The effects of reversible spinalization on individual neurons were diverse. Neurons, which did not receive supraspinal influences, were located mainly in the dorsal horn, whereas most neurons, receiving excitatory supraspinal influences were located in the intermediate zone and ventral horn. The population of PLRrelated neurons presumably responsible for disappearance of muscle tone and PLRs after spinalization was revealed. The effects of manipulation with the tonic supraspinal drive (by means of binaural galvanic vestibular stimulation, GVS) on the postural system were studied. GVS creates asymmetry in tonic supraspinal drive, resulting in a lateral body sway towards the anode. This new body orientation is actively stabilized. To reveal the underlying mechanisms, spinal neurons were recorded during PLRs with and without GVS. It was found that GVS enhanced PLRs on the cathode side and reduced them on the anode side. It was suggested that GVS changes the set-point of the postural system through the change of the gain in antagonistic PLRs. Two sub-groups of PLR-related neurons presumably mediating the effect of GVS on PLRs were found. An artificial feedback system was formed in which GVS-caused body sway was used to counteract the lateral body sway resulting from a mechanical perturbation of posture. It was demonstrated that the GVS-based artificial feedback was able to restore the postural function in rabbits with postural deficit. We suggested that such a control system could compensate for the loss of lateral stability of different etiology. Neuronal mechanisms underlying control of body configuration were analyzed in lampreys. The lamprey is capable of different forms of motor behavior: fast forward swimming (FFS), slow forward swimming (SFS), backward swimming (BS), forward and backward crawling, and lateral turns (LT). The amplitude of the body flexion (characterizing the body configuration) differs in different forms of motor behavior. In the lamprey, signals about the body configuration are provided by intraspinal stretch receptor neurons (SRNs). To clarify whether the networks generating different forms of motor behavior are located in the spinal cord, in chronic spinal lampreys, electrical stimulation of the spinal cord was performed. It was demonstrated that all forms of motor behavior are generated by the spinal networks. To study SRN-mediated reflexes and their contribution to the control of body configuration in different motor behaviors, in the in vitro preparation we recorded responses of reticulospinal (RS) neurons and motoneurons (MNs) to bending of the spinal cord in different planes and at different rostro-caudal levels during different forms of fictive motor behavior Bending in the pitch plane during FFS caused SRN-mediated reflexes. MNs on the convex side were activated by pitch bending in the mid-body region. These reflexes will reduce the bend, thus contributing to maintenance of rectilinear body axis in the pitch plane during FFS. It was found that bending in the yaw plane activated MNs on the convex side during FFS, but on the concave side during different forms of escape behavior (SFS, BS, LT). It was demonstrated that a reversal of reflex responses was due to ipsilateral supraspinal commands causing modifications of the spinal network located in the ipsi-hemicord. A population of RS neurons (residing in the middle rhombencephalic reticular nuclei) presumably transmitting these commands has been revealed. We suggest that modifications of SRN-mediated reflex responses will result in the decrease and increase of the lateral bending amplitude during FFS and escape behaviors, respectively, thus reinforcing movements generated in each specific behavior. Thus in the present study, for the first time, some neuronal mechanisms underlying reflex reversal in vertebrate animals have been revealed

The roles of dopamine and the sodium pump in the spinal control of locomotion

Rhythmically active, locomotor networks of the spinal cord are subject to both neuromodulation and activity-dependent homeostatic regulation. I first show that the neuromodulator dopamine exerts potent inhibitory effects on the central pattern generator (CPG) circuit controlling locomotory swimming in post-embryonic Xenopus tadpoles. Dopamine, acting endogenously on spinal D2-like receptors, reduces spontaneous fictive swimming occurrence and shortens, slows and weakens swimming. The mechanism involves a TTX-resistant hyperpolarisation of rhythmically active CPG neurons, mediated by the direct opening of a K+ channel with GIRK-like pharmacology. This increases rheobase and reduces spike probability. I next explore how sodium pumps contribute to the activity-dependent regulation of the Xenopus swim circuit, and possible interactions of the pumps with modulators, temperature and ionic conductances. I characterise the pump-mediated ultra-slow afterhyperpolarisation (usAHP), and show that monensin, a sodium ionophore, enhances pump activity, converting the usAHP into a tonic hyperpolarisation; this decreases swim episode duration and cycle frequency. I also characterise a ZD7288-sensitive Ih current, which is active in excitatory dIN interneurons and contributes to spiking. Blocking Ih with ZD7288 decreases swim episode duration and destabilises swim bursts. Both Ih and the usAHP increase with temperature, which depolarises CPG neurons, decreases input resistance, and increases spike probability; this increases cycle frequency, but the enhanced usAHP shortens swimming. I also show that the usAHP is diminished by nitric oxide, but enhanced by dopaminergic signalling. Finally, I explore sodium pumps in the neonatal mouse. The sodium pump blocker ouabain increases the duration and frequency of drug- and sensory-induced locomotion, whilst monensin has opposite effects. Decreasing inter-episode interval also shortens and slows activity, a relationship abolished by ouabain, implicating sodium pumps in a feedforward motor memory mechanism. Finally, I show that the effects of ouabain on locomotion are dependent on dopamine, which enhances a TTX- and ouabain-sensitive usAHP in spinal neurons

Noradrenergic control of spinal motor circuitry in two related amphibian species 'Xenopus laevis' and 'Rama temporaria'

1. The role of the catecholamine noradrenaline (NA) was examined during fictive swimming in Xenopus laevis tadpoles. 2. The primary effects of the amine in both embryonic and larval Xenopus was to markedly decrease motor frequency whilst simultaneously reducing rostrocaudal delays during swimming. 3. The NA-mediated modulation of swimming activity in Xenopus larvae can be reversed with phentolamine, a non-selective an adrenergic receptor antagonist, suggesting that NA may be acting through either α₁ or α₂ receptors, or a combination of both. 4. Intracellular recordings made from embryo spinal motorneurones revealed that reciprocal inhibitory glycinergic potentials are enhanced by NA. This effect is most prominent in caudal regions of the spinal cord where inhibitory synaptic drive is generally weaker. 5. NA was also found to enhance glycinergic reciprocal inhibition during swimming in larval spinal cord motomeurones. 6. Intracellular recordings, under tetrodotoxin, reveal that NA enhances the occurrence of spontaneous glycinergic inhibitory post synaptic potentials arising from the terminals of inhibitory intemeurones, suggesting that the amine is acting presynaptically to enhance evoked release of glycine during swimming. 7. The effects of NA on swimming frequency and rostrocaudal delay appear to be largely mediated through an enhancement of glycinergic reciprocal inhibition as blockade of glycine receptors with strychnine weakens the ability of the amine affect these parameters of motor output. 8. The effects of NA on motor output were also examined in embryos of the amphibian Rana temporaria. Whilst NA did not obviously affect swimming activity, the amine induced a non-rhythmic pattern of motor activity. 9. The free radical gas, nitric oxide also induced a non-rhythmic pattern of motor discharge that was remarkably similar to that elicited by NA, indicating that this neural messenger may be important for motor control

The development and neuromodulation of motor control systems in pro-metamorphic Xenopus laevis frog tadpoles

My thesis has accomplished 3 significant contributions to neuroscience. Firstly, I have discovered a novel example of vertebrate deep-brain photoreception. Spontaneously generated fictive locomotion from the isolated nervous system of pro-metamorphic Xenopus tadpoles is sensitive to the ambient light conditions, despite input from the classical photoreceptive tissues of the retina and pineal complex being absent. The photosensitivity is found to be tuned to short wavelength UV light and is localised to a small region of the caudal diencephalon. Within this region, I have discovered a population of neurons immuno-positive for a UV-specific opsin protein, suggesting they are the means of phototransduction. This may be a hitherto overlooked mechanism linking environmental luminance to motor behaviour. Secondly, I have advanced the collective knowledge of how both nitric oxide and dopamine contribute to neuromodulation within motor control systems. Nitric oxide is shown to have an excitatory effect on the occurrence of spontaneous locomotor activity, representing a switch in its role from earlier in Xenopus development. Moreover, this excitatory effect is found to be mediated in the brainstem despite nitric oxide being shown to depolarise spinal neurons. Thirdly, I have developed a new preparation for patch-clamp recording in pro-metamorphic Xenopus tadpoles. My data suggest there are several changes to the cellular properties of neurons in the older animals compared with the embryonic tadpole; there appears to be an addition of Ih and K[sub](Ca) channels and the presence of tonically active and intrinsically rhythmogenic neurons. In addition, I have shown that at low doses dopamine acts via D2-like to hyperpolarise the membrane potential of spinal neurons, while at higher doses dopamine depolarises spinal neurons. These initial data corroborate previously reported evidence that dopamine has opposing effects on motor output via differential activation of dopamine receptor subtypes in Xenopus tadpoles

Ventral spinocerebellar tract neurons are essential for mammalian locomotion

Locomotion, including running, walking, and swimming, is a complex behavior enabling animals to interact with the environment. Vertebrate locomotion depends upon sets of interneurons in the spinal cord, known as the central pattern generator (CPG). The CPG performs multiple roles: pattern formation (left-right alternation and flexor-extensor alternation) and rhythm generation (the onset and frequency of locomotion). Many studies have begun to unravel the organization of the neuronal circuits underlying left-right and flexor-extensor alternation. However, despite pharmacologic, lesion, and optogenetic studies suggesting that the rhythm generating neurons are ispilaterally-projecting glutamatergic neurons, the precise cellular identification of rhythm generating neurons remains largely unknown. Traditionally, CPG networks (both pattern formation and rhythm generation) are thought to reside upstream of motor neurons, which serve as the output of the spinal cord. Recently however, it has been discovered that direct stimulation of lumbar motor neurons using the intact ex vivo neonate mouse spinal cord preparation can activate CPG networks to produce locomotor-like behavior. Furthermore, depressing motor neuron discharge decreases locomotor frequency, whereas increasing motor neuron discharge accelerates locomotor frequency, suggesting that motor neurons provide ongoing feedback to the CPG. However, the circuit mechanisms through which motor neurons can influence activity in the CPG in mammals remain unknown. Here, I used motor neurons as a means of accessing CPG interneurons by asking how motor neuron activation might induce locomotor-like activity. Through intracellular recording and morphological assays, I discovered that ventral spinocerebellar tract (VSCT) neurons are activated monosynaptically following motor neuron axon stimulation through chemical and electrical synapses. A subset of VSCT neurons were located close to or within the motor neuron nucleus. VSCT neurons were found to be excitatory, have descending spinal axon collaterals, and influence motor neuron output, suggesting that VSCT neurons are positioned advantageously to initiate and maintain locomotor-like rhythmogenesis. Intracellular recording from VSCT neurons revealed that they exhibit rhythmic activity during locomotor-like activity. VSCT neurons were found to contain the rhythmogenic pacemaker Ih current and to be connected to other VSCT neurons, at least through gap junctions. Optogenetic and chemogenetic manipulation of VSCT neuron activity provided evidence that VSCT neurons are both necessary and sufficient for the production of locomotor-like activity. Silencing VSCT neurons prevented the induction of such activity, whereas activation of VSCT neurons was capable of inducing locomotor-like activity. The production of locomotor-like activity by VSCT neuron photoactivation was dependent upon both electrical communication through gap junctions as well as the pacemaker Ih current. The evidence presented in this thesis suggests that VSCT neurons are critical components for rhythm generation in the mammalian CPG and are key mediators of locomotor activity