Medullary reticular neurons in the Japanese toad: morphologies and excitatory inputs from the optic tectum

Possible neural network mediating jaw opening during prey-catching behavior of the frog

The prey-catching behavior of the frog is a complex, well-timed sequence of stimulus response chain of movements. After visual analysis of the prey, a size dependent program is selected in the motor pattern generator of the brainstem. Besides this predetermined feeding program, various direct and indirect sensory inputs provide flexible adjustment for the optimal contraction of the executive muscles. The aim of the present study was to investigate whether trigeminal primary afferents establish direct contacts with the jaw opening motoneurons innervated by the facial nerve. The experiments were carried out on Rana esculenta (Pelophylax esculentus), where the trigeminal and facial nerves were labeled simultaneously with different fluorescent dyes. Using a confocal laser scanning microscope, close appositions were detected between trigeminal afferent fibers and somatodendritic components of the facial motoneurons. Quantitative analysis revealed that the majority of close contacts were encountered on the dendrites of facial motoneurons and approximately 10% of them were located on the perikarya. We suggest that the identified contacts between the trigeminal afferents and facial motoneurons presented here may be one of the morphological substrate in the feedback and feedforward modulation of the rapidly changing activity of the jaw opening muscle during the prey-catching behavior.

A single pair of interneurons commands the Drosophila feeding motor program

Many feeding behaviors represent stereotyped, organized sequences of motor patterns that have been the subject of neuroethological studies^1,2 such as electrophysiological characterization of neurons governing prey capture in toads^1,3. Technical limitations, however, have prevented detailed study of the functional role of these neurons as in other studies on vertebrate organisms. Complexities involved in studies of whole animal behavior can be resolved in Drosophila, where remote activation of brain cells by genetic means⁴ allows one to interrogate the nervous system in freely moving animals to identify neurons that govern a specific behavior, and then to repeatedly target and manipulate these neurons to characterize their function. Here we show finding of neurons that generate the feeding motor program in Drosophila. We performed an unbiased screen using remote neuronal activation and identified a critical pair of brain cells that induces the entire feeding sequence when activated. These Fdg (feeding)-neurons are also essential for normal feeding as their suppression or ablation eliminates the sugar-induced feeding behavior. Activation of a single Fdg-neuron induced asymmetric feeding behavior and ablation of a single Fdg-neuron distorted the sugar-induced feeding behavior to be asymmetric, indicating the direct role of these neurons in shaping motor program execution. Simultaneously recording neuronal activity with calcium imaging during feeding behavior⁵ further revealed that the Fdg-neurons respond to food presentation, but only in starved flies. Our results demonstrate that Fdg-neurons operate firmly within the sensori-motor watershed, downstream of sensory and metabolic cues and at the top of the feeding motor hierarchy to execute the decision to feed.

Schema-based learning of adaptable and flexible prey-catching in anurans I. The basic architecture

A motor action often involves the coordination of several motor synergies and requires flexible adjustment of the ongoing execution based on feedback signals. To elucidate the neural mechanisms underlying the construction and selection of motor synergies, we study prey-capture in anurans. Experimental data demonstrate the intricate interaction between different motor synergies, including the interplay of their afferent feedback signals (Weerasuriya 1991; Anderson and Nishikawa 1996). Such data provide insights for the general issues concerning two-way information flow between sensory centers, motor circuits and periphery in motor coordination. We show how different afferent feedback signals about the status of the different components of the motor apparatus play a critical role in motor control as well as in learning. This paper, along with its companion paper, extend the model by Liaw et al. (1994) by integrating a number of different motor pattern generators, different types of afferent feedback, as well as the corresponding control structure within an adaptive framework we call Schema-Based Learning. We develop a model of the different MPGs involved in prey-catching as a vehicle to investigate the following questions: What are the characteristic features of the activity of a single muscle? How can these features be controlled by the premotor circuit? What are the strategies employed to generate and synchronize motor synergies? What is the role of afferent feedback in shaping the activity of a MPG? How can several MPGs share the same underlying circuitry and yet give rise to different motor patterns under different input conditions? In the companion paper we also extend the model by incorporating learning components that give rise to more flexible, adaptable and robust behaviors. To show these aspects we incorporate studies on experiments on lesions and the learning processes that allow the animal to recover its proper functioning.

How the mesencephalic locomotor region recruits hindbrain neurons

This chapter summarizes experiments which were designed to reveal how repetitive electrical stimulation of the mesencephalic locomotor region (MLR) recruits nearby hindbrain neurons into activity, such that locomotion can ensue in the tiger salamander, A. tigrinum. The MLR stimulus strength was subthreshold or near-threshold for locomotor movements to ensue. Such relatively weak stimulation of the MLR produced locomotor movements after a relatively long delay, which featured neuronal interactions in the hindbrain. MLR-evoked spike responses of single hindbrain neurons were recorded before locomotor movements began. This allowed consideration of the build-up of the hindbrain neuronal activity, which was subsequently impressed upon the spinal cord such as to evoke locomotor movements. Each train of MLR stimulus pulses evoked monosynaptic responses in but a small proportion of the hindbrain's neurons. Rather, oligosynaptic responses were routinely evoked, even in the "input" neurons that were activated monosynaptically. Consecutive stimulus volleys recruited a given neuron after a variable number of synaptic translations. It is argued that the hindbrain's input neurons excited a much larger number of other hindbrain neurons. By this means, an MLR-evoked, short-lived propagating wave of excitation (i.e., approximately 2-4 successive synaptic activations) can be spread throughout the hindbrain.

Innate visual object recognition in vertebrates: some proposed pathways and mechanisms

Almost all vertebrates are capable of recognizing biologically relevant stimuli at or shortly after birth, and in some phylogenetically ancient species visual object recognition is exclusively innate. Extensive and detailed studies of the anuran visual system have resulted in the determination of the neural structures and pathways involved in innate prey and predator recognition in these species [Behav. Brain Sci. 10 (1987) 337; Comp. Biochem. Physiol. A 128 (2001) 417]. The structures involved include the optic tectum, pretectal nuclei and an area within the mesencephalic tegmentum. Here we investigate the structures and pathways involved in innate stimulus recognition in avian, rodent and primate species. We discuss innate stimulus preferences in maternal imprinting in chicks and argue that these preferences are due to innate visual recognition of conspecifics, entirely mediated by subtelencephalic structures. In rodent species, brainstem structures largely homologous to the components of the anuran subcortical visual system mediate innate visual object recognition. The primary components of the mammalian subcortical visual system are the superior colliculus, nucleus of the optic tract, anterior and posterior pretectal nuclei, nucleus of the posterior commissure, and an area within the mesopontine reticular formation that includes parts of the cuneiform, subcuneiform and pedunculopontine nuclei. We argue that in rodent species the innate sensory recognition systems function throughout ontogeny, acting in parallel with cortical sensory and recognition systems. In primates the structures involved in innate stimulus recognition are essentially the same as those in rodents, but overt innate recognition is only present in very early ontogeny, and after a transition period gives way to learned object recognition mediated by cortical structures. After the transition period, primate subcortical sensory systems still function to provide implicit innate stimulus recognition, and this recognition can still generate orienting, neuroendocrine and emotional responses to biologically relevant stimuli.

Behavior of hindbrain neurons during the transition from rest to evoked locomotion in a newt

Trains of electrical stimuli were delivered to the mesencephalic 'locomotor region' in the rough skin newt. The current (3-12 mcA) and the interstimulus interval (100 to 200 ms) were adjusted so that locomotion arose in approximately 10 s, or so that the train remained subthreshold for initiation of locomotion. Impulses of single neurons in the hindbrain were recorded during the transition period from rest to locomotion. Time-locked synaptic responses were bi- or unimodal with typical latencies close to 18, 23 or 28 ms, and weak irregular mode near 13 ms. Impulses that were not locked to the stimuli arose in some silent neurons, and the rate of firing of neurons with background discharge was sometimes enhanced. Composite responses consisted of both time-locked component and impulses distributed throughout the interstimulus interval. The data suggest that short-lived, wave-like propagation of the input volley ceases or is transformed into asynchronous activity after three or four translations. The latter variant could occur if the train reached the threshold for initiation of locomotion. The asynchronous activity persisted throughout interstimulus interval and could coexist with time-locked impulses. Some neurons generated only a few impulses, while others remained active from beginning to end of the train. These active neurons could either spike at a steady rate, or decrement or augment their rate of firing during the train. The time course of their activity was related to the initial rate of firing. The augmenting type of firing in a subset of neurons may arise due to the interaction of neurons with unstable, steady state and decrementing activity.

Neuromuscular control of prey capture in frogs

While retaining a feeding apparatus that is surprisingly conservative morphologically, frogs as a group exhibit great variability in the biomechanics of tongue protraction during prey capture, which in turn is related to differences in neuromuscular control. In this paper, I address the following three questions. (1) How do frog tongues differ biomechanically? (2) What anatomical and physiological differences are responsible? (3) How is biomechanics related to mechanisms of neuromuscular control? Frog species use three non-exclusive mechanisms to protract their tongues during feeding: (i) mechanical pulling, in which the tongue shortens as its muscles contract during protraction; (ii) inertial elongation, in which the tongue lengthens under inertial and muscular loading; and (iii) hydrostatic elongation, in which the tongue lengthens under constraints imposed by the constant volume of a muscular hydrostat. Major differences among these functional types include (i) the amount and orientation of collagen fibres associated with the tongue muscles and the mechanical properties that this connective tissue confers to the tongue as a whole; and (ii) the transfer of intertia from the opening jaws to the tongue, which probably involves a catch mechanism that increases the acceleration achieved during mouth opening. The mechanisms of tongue protraction differ in the types of neural mechanisms that are used to control tongue movements, particularly in the relative importance of feed-forward versus feedback control, in requirements for precise interjoint coordination, in the size and number of motor units, and in the afferent pathways that are involved in coordinating tongue and jaw movements. Evolution of biomechanics and neuromuscular control of frog tongues provides an example in which neuromuscular control is finely tuned to the biomechanical constraints and opportunities provided by differences in morphological design among species.

A mesencephalic relay for visual inputs to reticulospinal neurones in lampreys

Visual stimuli elicit motor responses in lampreys. These responses rely, in part, on the activation of reticulospinal (RS) neurones which constitute the main descending pathway in these early vertebrates. This study sought to identify and characterize possible mesencephalic relays for visual inputs to RS neurones of the rhombencephalon. The anatomical substrate subserving this function was investigated by iontophoretically ejecting cobalt-lysine, a retrograde tracer, in the middle rhombencephalic reticular nucleus in the in vitro isolated brainstem preparation of young adult Petromyzon marinus. Several populations of cells were retrogradely labeled in the brainstem. Of particular interest were the cell populations found on each side of rostral mesencephalon, located in the tectum and pretectum. There were, on average, 113 cells labeled contralateral to the injection site and 80 cells labeled ipsilateral to the injection site. The cells were morphologically similar on both sides, except that the contralateral group had larger cell bodies as compared to the group on the ipsilateral side. To determine whether the axons of the cells contacted reticulospinal neurones, electrophysiological experiments were carried out in which the region containing these cells was microstimulated. Large post-synaptic potentials were recorded intracellularly in RS neurones. Furthermore, microstimulation of the optic nerve on the same side as the recorded cell (i ON) evoked responses with a pattern similar to those resulting from stimulation of the optic tectum contralateral to the cell recorded (co OT), except for the longer response latencies. Local ejection of xylocaine (1% lidocaine hydrochloride) or CNQX (1 mM) onto the co OT reversibly abolished the responses evoked from stimulation of the i ON. There were no significant effects observed when the drug was ejected onto optic tectum ipsilateral to the cell. Taken together, the results from this study indicate that the crossed tectoreticular pathway is involved in relaying optic nerve inputs to RS neurones of the middle rhombencephalic reticular nucleus. Moreover, cells of origin of this pathway appear, in all respect, homologous to cells giving rise to the crossed tectobulbar pathway in other vertebrates.

The role of hypoglossal sensory feedback during feeding in the marine toad, Bufo marinus

Behavioral observations demonstrate that bilateral deafferentation of the hypoglossal nerves in the marine toad (Bufo marinus) prevents mouth opening during feeding. In the present study, we used high-speed videography, electromyography (EMG), deafferentation, muscle stimulation, and extracellular recordings from the trigeminal nerve to investigate the mechanism by which sensory feedback from the tongue controls the jaw muscles of toads. Our results show that sensory feedback from the tongue enters the brain through the hypoglossal nerve during normal feeding. This feedback appears to inhibit both tonic and phasic activity of the jaw levators. Hypoglossal feedback apparently functions to coordinate tongue protraction and mouth opening during feeding. Among anurans, the primitive condition is the absence of a highly protrusible tongue and the absence of a hypoglossal sensory feedback system. The hypoglossal feedback system evolved in parallel with the acquisition of a highly protrusible tongue in toads and their relatives.

Responses of medullary neurons to moving visual stimuli in the common toad. I. Characterization of medial reticular neurons by extracellular recording

The concept of coded 'command releasing systems' proposes that visually specialized descending tectal (and pretectal) neurons converge on motor pattern generating medullary circuits and release--in goal-specific combination--specific action patterns. Extracellular recordings from medullary neurons of the medial reticular formation of the awake immobilized toad in response to moving visual stimuli revealed the following main results. (i) Properties of medullary neurons were distinguished by location, shape, and size of visual receptive fields (ranging from relatively small to wide), by trigger features of various moving configural stimulus objects (including prey- and predator-selective properties), by tactile sensitivity, and by firing pattern characteristics (sluggish, tonic, warming-up, and cyclic). (ii) Visual receptive fields of medullary neurons and their responses to moving configural objects suggest converging inputs of tectal (and pretectal) descending neurons. (iii) In contrast to tectal monocular 'small-field' neurons, the excitatory visual receptive fields of comparable medullary neurons were larger, ellipsoidally shaped, mostly oriented horizontally, and not topographically mapped in an obvious fashion. Furthermore, configural feature discrimination was sharper. (iv) The observation of multiple properties in most medullary neurons (partly showing combined visual and cutaneous sensitivities) suggests integration of various inputs by these cells, and this is in principle consistent with the concept of command releasing systems. (v) There is evidence for reciprocal tectal/medullary excitatory pathways suitable for premotor warming-up. (vi) Cyclic bursting of many neurons, spontaneously or as a post-stimulus sustaining event, points to a medullary premotor/motor property.