Crayfish escape behavior

Motor behavior: A feedforward circuit for zebrafish escape

A recent study of motor control in zebrafish demonstrates the critical role of an excitatory neural relay network in the transformation of a unilateral turn command into a subsequent bilateral swim signal. A rapid and smooth transition between these motor phases is critical for successfully escaping danger.

Identification of mechanoafferent neurons in terrestrial snail: response properties and synaptic connections

In this study, we describe the putative mechanosensory neurons, which are involved in the control of avoidance behavior of the terrestrial snail Helix lucorum. These neurons, which were termed pleural ventrolateral (PlVL) neurons, mediated part of the withdrawal response of the animal via activation of the withdrawal interneurons. Between 15 and 30 pleural mechanosensory neurons were located on the ventrolateral side of each pleural ganglion. Intracellular injection of neurobiotin revealed that all PlVL neurons sent their axons into the skin nerves. The PlVL neurons had no spontaneous spike activity or fast synaptic potentials. In the reduced "CNS-foot" preparations, mechanical stimulation of the skin covering the dorsal surface of the foot elicited spikes in the PlVL neurons without any noticeable prepotential activity. Mechanical stimulus-induced action potentials in these cells persisted in the presence of high-Mg(2+)/zero-Ca(2+) saline. Each neuron had oval-shaped receptive field 5-20 mm in length located on the dorsal surface of the foot. Partial overlapping of the receptive fields of different neurons was observed. Intracellular stimulation of the PlVL neurons produced excitatory inputs to the parietal and pleural withdrawal interneurons, which are known to control avoidance behavior. The excitatory postsynaptic potentials (EPSPs) in the withdrawal interneurons were induced in 1:1 ratio to the PlVL neuron spikes, and spike-EPSP latency was short and highly stable. These EPSPs also persisted in the high-Mg(2+)/high-Ca(2+) saline, suggesting monosynaptic connections. All these data suggest that PlVL cells were the primary mechanosensory neurons.

Premotor interneurons in generation of adaptive leg reflexes and voluntary movements in stick insects

We investigated the role of local nonspiking interneurons involved in motor control of legs in the stick insects, Carausius morosus. In a preparation that allowed the animals to perform active leg movements such as adaptive tactile reflexes, proprioceptive reflexes, and walking, we gathered the following results. Almost all tested nonspiking interneurons that provide synaptic drive onto moto-neurons of the proximal leg muscles contribute to all of the motor programs underlying tactile reflexes and voluntary leg movements such as walking, searching, and rocking. Most of them are also involved in the generation of proprioceptive reflexes. All motor programs for coactivation, avoidance reflexes, resistance reflexes, and voluntary leg movements result from parallel pathways including nonspiking interneurons that support and others that oppose the motoneuronal activity. The contribution of a single interneuron to the different motor program is specific: it can be supporting for one motor program but opposing for the other. Even for the same motor program, for example, coactivation, the contribution of an individual interneuron can depend on the stimulus site from where the response is elicited. Our results support the idea that the different motor patterns for adaptive tactile reflexes, resistance reflexes, and voluntary leg movements emerge from a multifunctional neuronal circuit that is reorganized corresponding to the motor behavior performed. The actual motor pattern is then shaped by distributed information processing in parallel supporting and opposing pathways.

Long-term adaptation in lobster motor neurons and compensation of transmitter release by synergistic inputs

Earlier studies with crayfish have shown that chronic increases in neural activity, by electrical stimulation, cause a long-lasting reduction in the amount of transmitter released at low stimulus frequencies or at the beginning of a stimulus train. When such chronic stimulation is applied to phasic extensor motor neurons of the lobster abdomen, a similar change in transmitter release is apparent, as indicated by a decrease in excitatory postsynaptic potential (EPSP) size at 0.1 Hz. However, the EPSPs from unstimulated axons which innervate the same target muscle from a different nerve increase in size. Thus, activity-dependent reduction in transmitter release at one set of synapses appears to be compensated for by increased synaptic efficacy from less active synergistic inputs. The mechanism of such compensation is not known.

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.

Functional organization of crayfish abdominal ganglia: I. The flexor systems

For insect ganglia, Altman (Advances in Physiological Science, Vol. 23. Neurobiology of Invertebrates. New York: Pergamon Press, pp. 537-555, '81) proposed that individual neuropils control different motor activities. A corollary of this hypothesis is that motor neurons involved in many behavioral functions should branch in more neuropils than those active in fewer behaviors. In crayfish, the abdominal fast-flexor muscles are active only during the generation of the powerstroke for tailflips, whereas the slow-flexor muscles are involved in the maintenance of body posture. The slow flexors are thus active in many of the crayfish's behavioral activities. To test the generality of Altman's idea, we filled groups of crayfish fast-flexor and slow-flexor motor neurons with cobalt chloride and described their shapes with respect to the ganglionic structures through which they pass. Individual fast flexors were also filled intracellularly with HRP. Ganglia containing well-filled neurons were osmicated, embedded in plastic, and sectioned. Unstained sections were examined by light microscopy and pertinent sections were photographed. We found that the paths of the larger neurites were invariant, that the dendritic domains of fast and slow motor neurons occupied distinctive sets of neuropils, and that dendrites of slow motor neurons branched in more ganglionic structures than did those of fast motor neurons. These results are consistent with Altman's hypothesis.