Paired intracellular recordings with biocytin labelling were made in slices of adult rat somatosensory and visual cortex and in cat visual cortex to examine the properties of synaptic connections made by layer 6 pyramidal cells, to determine whether cortico-cortical pyramids more commonly provide input to other layer 6 pyramids than cortico-thalamic cells, and whether these connections exhibit paired pulse and brief train depression. Pyramidal cells with cortico-cortical like morphology were 2-4 times more likely to innervate other pyramidal cells than were cortico-thalamic like cells, but less likely to innervate inhibitory interneurons. The excitatory postsynaptic potentials elicited by presynaptic, phasically firing cortico-cortical pyramids in all classes of postsynaptic infragranular layer pyramidal cells exhibited strong, presynaptically mediated paired pulse and brief train depression. Those with larger paired pulse ratios also exhibited post-tetanic potentiation, but this was accompanied by stronger paired pulse and brief train depression. Both the firing characteristics and the outputs of cortico-cortical pyramidal cells display pronounced phasic characteristics, indicating that these cells respond most effectively to and preferentially pass on information related to novelty.

The properties of the connections made by the axons of pyramidal cells with cortico-thalamic (CT)-like morphology with a range of postsynaptic layer 6 targets were studied with dual intracellular recordings in slices of adult rat and cat neocortex. The cells were filled with biocytin and identified morphologically and, where appropriate, immunofluorescently. CT-like pyramids contacted interneurons with a very high probability (up to 1:2) but contacted other layer 6 pyramidal cells only rarely (approximately 1:80). The excitatory postsynaptic potentials (EPSPs) that they elicited both in pyramidal cells and in a variety of types of interneurons (including those immunopositive for parvalbumin and for somatostatin) facilitated, the second EPSP being larger than the first over a range of interspike intervals. Facilitation was not, however, maximal at the shortest intervals; in fact, depression was apparent at some connections at short interspike intervals. Facilitation in the majority of connections peaked at intervals of 25-35 ms and then declined slowly. Nor did these connections display the augmentation typical of many other strongly facilitating connections. Third EPSPs were smaller on average than second EPSPs, and fourth and subsequent EPSPs could be depressed (relative to first EPSPs). The properties of the outputs of these CT-like pyramidal cells are therefore quite distinct from those of other pyramidal cells, both within layer 6 and in other layers, possibly reflecting their unique role as both first order thalamo-cortical recipient and cortico-thalamic output neurons.

SUMMARYBush crickets have long, thin hind legs but jump and kick rapidly. The mechanisms underlying these fast movements were analysed by correlating the activity of femoral muscles in a hind leg with the movements of the legs and body captured in high-speed images.A female Pholidoptera griseoaptera weighing 600 mg can jump a horizontal distance of 300 mm from a takeoff angle of 34° and at a velocity of 2.1 m s-1, gaining 1350μJ of kinetic energy. The body is accelerated at up to 114 m s-2, and the tibiae of the hind legs extend fully within 30 ms at maximal rotational velocities of 13 500 deg. s-1. Such performance requires a minimal power output of 40 mW. Ruddering movements of the hind legs may contribute to the stability of the body once the insect is airborne. During kicking, a hind tibia is extended completely within 10 ms with rotational velocities three times higher at 41 800 deg. s-1.Before a kick, high-speed images show no distortions of the hind femoro-tibial joints or of the small semi-lunar groove in the distal femur. Both kicks and jumps can be generated without full flexion of the hind tibiae. Some kicks involve a brief, 40-90 ms, period of co-contraction between the extensor and flexor tibiae muscles, but others can be generated by contraction of the extensor without a preceding co-contraction with the flexor. In the latter kicks, the initial flexion of the tibia is generated by a burst of flexor spikes, which then stop before spikes occur in the fast extensor tibiae(FETi) motor neuron. The rapid extension of the tibia can follow directly upon these spikes or can be delayed by as long as 40 ms. The velocity of tibial movement is positively correlated with the number of FETi spikes.The hind legs are 1.5 times longer than the body and more than four times longer than the front legs. The mechanical advantage of the hind leg flexor muscle over the extensor is greater at flexed joint angles and is enhanced by a pad of tissue on its tendon that slides over a protuberance in the ventral wall of the distal femur. The balance of forces in the extensor and flexor muscles, coupled with their changing lever ratio at different joint positions,appears to determine the point of tibial release and to enable rapid movements without an obligatory co-contraction of the two muscles.

We attempt to summarize the properties of cortical synaptic connections and the precision with which they select their targets in the context of information processing in cortical circuits. High-frequency presynaptic bursts result in rapidly depressing responses at most inputs onto spiny cells and onto some interneurons. These 'phasic' connections detect novelty and changes in the firing rate, but report frequency of maintained activity poorly. By contrast, facilitating inputs to interneurons that target dendrites produce little or no response at low frequencies, but a facilitating-augmenting response to maintained firing. The neurons activated, the cells they in turn target and the properties of those synapses determine which parts of the circuit are recruited and in what temporal pattern. Inhibitory interneurons provide both temporal and spatial tuning. The 'forward' flow from layer-4 excitatory neurons to layer 3 and from 3 to 5 activates predominantly pyramids. 'Back' projections, from 3 to 4 and 5 to 3, do not activate excitatory cells, but target interneurons. Despite, therefore, an increasing complexity in the information integrated as it is processed through these layers, there is little 'contamination' by 'back' projections. That layer 6 acts both as a primary input layer feeding excitation 'forward' to excitatory cells in other layers and as a higher-order layer with more integrated response properties feeding inhibition to layer 4 is discussed.

In this brief review of inter-laminar synaptic connections in neocortex a case is made for the hypothesis that axons select their targets, or neurones select their inputs with great specificity. A large part of the data discussed was obtained from dual intracellular recordings in slices of adult neocortex. The neurones innervated in a given layer by a given type of axon were not found to be a random selection of the cells in that layer. Rather in a layer to layer specific fashion, certain types of target cells are densely innervated while others are not contacted or receive only weak inputs. In another layer, the same axons may select other targets.

The spike activity of neuromodulatory dorsal unpaired median (DUM) neurons was analyzed during a pilocarpine-induced motor pattern in the locust. Paired intracellular recordings were made from these octopaminergic neurons during rhythmic activity in hindleg motor neurons evoked by applying pilocarpine to an isolated metathoracic ganglion. This motor pattern is characterized by two alternating phases: a levator phase, during which levator, flexor, and common inhibitor motor neurons spike, and a depressor phase, during which depressor and extensor motor neurons spike. Three different subpopulations of efferent DUM neurons could be distinguished during this rhythmical motor pattern according to their characteristic spike output. DUM 1 neurons, which in the intact animal do not innervate muscles involved in leg movements, showed no change apart from a general increase in spike frequency. DUM 3 and DUM 3,4 neurons produced the most variable activity but received frequent and sometimes pronounced hyperpolarizations that were often common to both recorded neurons. DUM 5 and DUM 3,4,5 neurons innervate muscles of the hindleg and showed rhythmical excitation leading to bursts of spikes during rhythmic activity of the motor neurons, which innervate these same muscles. Sometimes the motor output was coordinated across both sides of the ganglion so that there was alternating activity between levators of both sides. In these cases, the spikes of DUM 5 and DUM 3,4,5 neurons and the hyperpolarization of DUM 3 and DUM 3,4 neurons occurred at particular phases in the motor pattern. Our data demonstrate a central coupling of specific types of DUM neurons to a rhythmical motor pattern. Changes in the spike output of these particular efferent DUM neurons parallel changes in the motor output. The spike activity of DUM neurons thus may be controlled by the same circuits that determine the action of the motor neurons. Functional implications for real walking are discussed.

A group of six dorsal unpaired median (DUM) neurons of the suboesophageal ganglion (SOG) of locusts was studied with neuroanatomical and electrophysiological techniques. The neurons are located posteriorly in the SOG and have axons that descend into the ganglia of the ventral nerve cord, some as far as the terminal abdominal ganglion. Within thoracic ganglia the neurons have profuse dendritic ramifications in many neuropiles, including ventral sensory neuropiles. Based on their projection patterns three different morphological types of neurons can be distinguished. These neurons receive excitatory inputs through sensory pathways that ascend from the thoracic ganglia and are activated by limb movements. They may be involved in the modulation of synaptic transmission in thoracic ganglia.

SUMMARYThe Thailand winged stick insect (Sipyloidea sp.) flees rapidly from a disturbance by jumping forwards when stimulated on the abdomen and backwards when stimulated on the head. The mechanisms underlying these fast movements were analysed by measuring movements of the body and legs from images captured at 250 Hz.A forward jump of both adults and nymphs involves movements of the abdomen and the middle and hind pairs of legs. The abdomen is raised and swung forwards by flexion at the joint with the metathorax and at the joint between the meso- and metathorax. At the same time, the tibiae of the hind and middle legs are extended and their femora depressed. The femoro-tibial joints of the legs are not fully flexed before a jump, and no structures in these joints appear to store muscular energy. The whole jumping sequence takes approximately 100 ms and results in take-off angles of 10-35° at velocities of 0.6-0.8 m s-1 and with an acceleration of 10 m s-2. The abdominal angular velocity was 2000° s-1and the tip of the abdomen moved at linear velocities of some 1 m s-1, while the maximum rate of tibial extension was 4000°s-1.Rapid backward movements result either in the collapse of the body onto the ground, with a displacement away from the stimulus of approximately half a body length, or in the propulsion of the insect off its perch. Neither movement involves curling of the abdomen.From a horizontal posture, the forward jumps result in a displacement of a few body lengths. More lift can be generated in adults by elevating the hind wings as the abdomen is swung forwards and depressing them as the legs lose contact with the ground. In this way, jumps can lead directly to flapping flight. Take-off into flight can, however, be achieved without the abdominal movements seen during jumping.From a vertical posture, a forward jump propels the insect upwards and backwards before it falls to the ground horizontally displaced from its perch. Backward movements result in the insect falling with little horizontal displacement from its perch.