Crayfish escape behavior: Neurobehavioral analysis of phasic extension reveals dual systems for motor control

The giant escape neurons of crayfish: Past discoveries and present opportunities

Crayfish are equipped with two prominent neural circuits that control rapid, stereotyped escape behaviors. Central to these circuits are bilateral pairs of giant neurons that transverse the nervous system and generate escape tail-flips in opposite directions away from threatening stimuli.

Heinrich Reichert (1949-2019)

Heinrich Reichert, Professor Emeritus at the University of Basel, Switzerland, passed away on the 13th of June 2019 after a prolonged illness. Heinrich described himself as ‘a hedonist when it came to science’ because he said it gave him great pleasure. It was this quality that made working with Heinrich thrilling and deeply fulfilling. Heinrich's long and versatile career spanned the breadth of neuroscience – from development, to evolution and behaviour. In his passing we have lost not just an astute scientist, but also an impassioned educator and an adventurer of science.

Not so fast: giant interneurons control precise movements of antennal scales during escape behavior of crayfish

High-speed video recordings of escape responses in freely behaving crayfish revealed precisely coordinated movements of conspicuous head appendages, the antennal scales, during tail-flips that are produced by giant interneurons. For tail-flips that are generated by the medial giants (MG) in response to frontal attacks, the scales started to extend immediately after stimulation and extension was completed before the animal began to propel backwards. For tail-flips that are elicited by caudal stimuli and controlled by the lateral giants (LG), scale extensions began with significant delay after the tail-flip movement was initiated, and full extension of the scales coincided with full flexion of the tail. When we used implanted electrodes and stimulated the giant neurons directly, we observed the same patterns of scale extensions and corresponding timing. In addition, single action potentials of MG and LG neurons evoked with intracellular current injections in minimally restrained preparations were sufficient to activate scale extensions with similar delays as seen in freely behaving animals. Our results suggest that the giant interneurons, which have been assumed to be part of hardwired reflex circuits that lead to caudal motor outputs and stereotyped behavior, are also responsible for activating a pair of antennal scales with high temporal precision.

Motor control of Drosophila feeding behavior

The precise coordination of body parts is essential for survival and behavior of higher organisms. While progress has been made towards the identification of central mechanisms coordinating limb movement, only limited knowledge exists regarding the generation and execution of sequential motor action patterns at the level of individual motoneurons. Here we use Drosophila proboscis extension as a model system for a reaching-like behavior. We first provide a neuroanatomical description of the motoneurons and muscles contributing to proboscis motion. Using genetic targeting in combination with artificial activation and silencing assays we identify the individual motoneurons controlling the five major sequential steps of proboscis extension and retraction. Activity-manipulations during naturally evoked proboscis extension show that orchestration of serial motoneuron activation does not rely on feed-forward mechanisms. Our data support a model in which central command circuits recruit individual motoneurons to generate task-specific proboscis extension sequences.

Motor neurons in the escape response circuit of white shrimp (Litopenaeus setiferus)

Many decapod crustaceans perform escape tailflips with a neural circuit involving giant interneurons, a specialized fast flexor motor giant (MoG) neuron, populations of larger, less specialized fast flexor motor neurons, and fast extensor motor neurons. These escape-related neurons are well described in crayfish (Reptantia), but not in more basal decapod groups. To clarify the evolution of the escape circuit, I examined the fast flexor and fast extensor motor neurons of white shrimp (Litopenaeus setiferus; Dendrobranchiata) using backfilling. In crayfish, the MoGs in each abdominal ganglion are a bilateral pair of separate neurons. In L. setiferus, the MoGs have massive, possibly syncytial, cell bodies and fused axons. The non-MoG fast flexor motor neurons and fast extensor motor neurons are generally found in similar locations to where they are found in crayfish, but the number of motor neurons in both the flexor and extensor pools is smaller than in crayfish. The loss of fusion in the MoGs and increased number of fast motor neurons in reptantian decapods may be correlated with an increased reliance on non-giant mediated tailflipping.

Transition of pattern generation: the phenomenon of post-scratching locomotion

A fundamental problem in neurophysiology is the understanding of neuronal mechanisms by which the central nervous system produces a sequence of voluntary or involuntary motor acts from a diverse repertory of movements. These kinds of transitions between motor acts are extremely complex; however, they could be analyzed in a more simple form in decerebrate animals in the context of spinal central pattern generation. Here, we present for the first time a physiological phenomenon of post-scratching locomotion in which decerebrate cats exhibit a compulsory locomotor activity after an episode of scratching. We found flexor, extensor and intermediate single interneurons rhythmically firing in the same phase during both scratching and the subsequent post-scratching locomotion. Because no changes in phase of these neurons from scratching to post-scratching locomotion were found, we suggest that in the lumbar spinal cord there are neurons associated with both motor tasks. Moreover, because of its high reproducibility we suggest that the study of post-scratching fictive locomotion, together with the unitary recording of neurons, could become a useful tool to study neuronal mechanisms underlying transitions from one rhythmic motor task to another, and to study in more detail the central pattern generator circuitry in the spinal cord.

A mobility gradient in the organization of vertebrate movement: The perception of movement through symbolic language

Ordinary language can prevent us from seeing the organization of whole-animal movement. This may be why the search for behavioral homologies has not been as fruitful as the founders of ethology had hoped. The Eshkol-Wachman (EW) movement notational system can reveal shared movement patterns that are undetectable in the kinds of informal verbal descriptions of the same behaviors that are in current use. Rules of organization that are common to locomotor development, agonistic and exploratory behavior, scent marking, play, and dopaminergic drug-induced stereotypies in a variety of vertebrates suggest that behavior progresses along a “mobility gradient” from immobility to increasing complexity and unpredictability. A progression in the opposite direction, with decreasing spatial complexity and increased stereotypy, occurs under the influence of the nonselective dopaminergic drugs apomorphine and amphetamine and partly also the selective dopamine agonist quinpirole. The behaviors associated with the mobility gradient appear to be mediated by a family of basal ganglia-thalamocortical circuits and their descending output stations. Because the small number of rules underlying the mobility gradient account for a large variety of behaviors, they may be related to the specific functional demands on these neurological systems. The EW system and the mobility gradient model should prove useful to ethologists and neurobiologists.

Loss of escape responses and giant neurons in the tailflipping circuits of slipper lobsters, Ibacus spp. (Decapoda, Palinura, Scyllaridae)

In many decapod crustaceans, escape tailflips are triggered by lateral giant (LG) and medial giant (MG) interneurons, which connect to motor giant (MoG) abdominal flexor neurons. Several decapods have lost some or all of these giant neurons, however. Because escape-related giant neurons have not been documented in palinurans, I examined tailflipping and abdominal nerve cords for giant neurons in two scyllarid lobster species, Ibacus peronii and Ibacus alticrenatus. Unlike decapods with giant neurons, Ibacus do not tailflip in response to sudden taps. Ibacus can perform non-giant tailflipping: the frequency of tailflips during swimming is adjusted by altering the gap between each individual tailflip. Abdominal nerve cord sections show no LG or MG interneurons. Backfilling nerve 3 of abdominal ganglia revealed no MoG neurons, and the fast flexor motor neuron population is otherwise identical to that described for crayfish. The loss of giant neurons in Ibacus represents an independent deletion of these cells compared to other reptantian decapods known to have lost these giant neurons. This loss is correlated with the normal posture in scyllarids, in which the last two abdominal segments are flexed, and an alternative defensive strategy, concealment by digging into sand.

Functional organization of crayfish abdominal ganglia: II. Sensory afferents and extensor motor neurons

Abdominal ganglia of crayfish contain identifiable neuropils, commissures, longitudinal tracts, and vertical tracts. To determine the functional significance of this ganglionic framework, we backfilled the following types of neurons with cobalt chloride: sensory hair afferents, slow and fast extensor motor neurons, the segmental stretch receptor neurons, and their inhibitory accessory cells. After the cobalt ions were precipitated and intensified, we studied the central projections of the filled neurons within the ganglionic structures. All of the axons of these neurons exit or enter each of the first five abdominal ganglia through the second pair of nerves. Our description of the central projections of the hair afferents is the first in the literature. These afferents innervate the large ventral horseshoe neuropil (HN) in the core of each ganglion. This neuropil is homologous to the insect ventral association centers, which also process sensory information. Furthermore, we discovered that some of the crayfish afferents innervate glomeruli within the HN. The slow and fast extensor motor neurons, the stretch receptor neurons, and the accessory cells branch mostly in the dorsal part of the ganglion. We reinterpret previous identifications of the extensor neurons that were based largely on soma position. Together with our previous descriptions of the flexor motor neurons, these results allow us to relate both rapid tail-flips and slower postural movements to the structure of the segmental ganglia.