Signal processing in the axon initial segment

The axon initial segment (AIS) is a specialized membrane region in the axon of neurons where action potentials are initiated. Crucial to the function of the AIS is the presence of specific voltage-gated channels clustered at high densities, giving the AIS unique electrical properties. Here we review recent data on the physiology of the AIS. These data indicate that the role of the AIS is far richer than originally thought, leading to the idea that it represents a dynamic signal processing unit within neurons, regulating the integration of synaptic inputs, intrinsic excitability, and transmitter release. Furthermore, these observations point to?a critical role of the AIS in disease.     

Laminar distribution of neurons with different types of receptive fields in the rabbit visual cortex

Investigation of receptive fields of 232 primary visual cortical neurons in rabbits by the use of shaped visual stimuli showed that 21.1% are unselective for stimulus orientation, and 34.1% have simple, 16.4% complex, and 18.5% hypercomplex receptive fields, and 9.9% have other types. Neurons with different types of receptive fields also differed in spontaneous activity, selectivity for rate of stimulus movement, and acuteness of orientational selectivity. Neurons not selective to orientation were found more frequently in layer IV than in other layers, and very rarely in layer VI. Cells with simple receptive fields were numerous in all layers but predominated in layer VI. Neurons with complex receptive fields were rare in layer IV and more numerous in layers V and VI. Neurons with hypercomplex receptive fields were found frequently in layers II + III and IV, rarely in layers V and VI. Spontaneous unit activity in layer II + III was lowest on average, and highest in layer V. Acuteness or orientational selectivity of neurons with simple and complex receptive fields in layers II + III and V significantly exceeded the analogous parameter in layers IV and VI.A. N. Severtsov Institute of Evolutionary Morphology and Ecology of Animals, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 17, No. 1, pp. 19–27, January–February, 1985.     

Convergence of visual and auditory stimuli on the neurons of the visual cortex in the conscious rabbit

We investigated the visual-cortex neurons of the conscious rabbit during simultaneous stimulation with a clicking sound and a light flash (complex) and during separate application of these stimuli. We tested the development of the reflex with time and of the sound-light association during prolonged rhythmic application of the sound and light. Fifty visual-cortex neurons were studied; 20% of the cells responded with a specific phased reaction and 16% exhibited a specific response to the complex different from the responses to each of its components. Development of a sound-light association was observed in 18% of the cells and a temporal reflex was induced in 25%. In most cases, the conditioned reaction evoked was similar to some informational element in the neuronal response to the complex.M. V. Lomonosov Moscow State University. Institute of Cybernetics, Academy of Sciences of the GruzSSR. Translated from Neirofiziologiya, Vol. 2, No. 4, pp. 391–398, July–August, 1970.     

Computation of color and brightness differences by neurons in the rabbit visual cortex

Changes in activity of 54 neurons in the rabbit visual cortex evoked by the replacement of eight color and eight achromatic stimuli in pairs were analyzed. The diffused stimuli generated by color SVGA monitor were used in the experiments. The earliest response of phasic neurons (50-90 ms after the replacement) was strongly correlated with differences between stimuli in color or intensity. This response ("the signal of differences") was used as a basis of a matrix (8 x 8) constructed for each neuron. Such matrices included mean numbers of spikes per second in responses to changes of different stimuli pairs. All matrices were subjected to factor analysis, and the basic axes (the main factors) of sensory spaces were revealed. It was found that 16 neurons (30%) detected only achromatic differences between stimuli. Perceptual spaces of these neurons were two-dimensional with brightness and darkness orthogonal axes. The spaces of 12 neurons (22%) were four-dimensional with two chromatic and two achromatic axes. The structure of the perceptual space reconstructed from neuronal spikes was similar to the space calculated from the early VEP components recorded under similar conditions and to another space reconstructed on the basis of rabbit's instrumental learning. The fundamental coincidence of color spaces revealed by different methods may reflect the general principle of vector coding in the visual system and suggests the coexistence of two independent cortical mechanisms of the detection of chromatic and achromatic differences.     

Limited plasticity of the rabbit visual cortex and hippocampal neurons in the oddball test

The activity of 41 visual cortex and 20 hippocampal neurons from field CA1 was registered in experiments using oddball-stimulation with different color stimuli varied in intensity. 34% cortical and 37% hippocampal neurons demonstrated plasticity reactions. The significant increase of latest phases of neuronal activity (200-500 and 200-1000 ms after stimulation for cortical neurons and 300-550 ms for hippocampal neurons) was shown in responses to rare deviant stimuli, which had a less intensity than frequently standards. The quantity of the earliest neuronal phase of activity (40-120 ms after stimulation) was stabilized in responses to deviants and standards during the experiment. We propose that such increase of the latest phases of neuronal activity (the limited plasticity) may reflect the mechanisms of orienting reaction.     

The cell adhesion molecule neurofascin stabilizes axo-axonic GABAergic terminals at the axon initial segment

Cell adhesion molecules regulate synapse formation and maintenance via transsynaptic contact stabilization involving both extracellular interactions and intracellular postsynaptic scaffold assembly. The cell adhesion molecule neurofascin is localized at the axon initial segment of granular cells in rat dentate gyrus, which is mainly targeted by chandelier cells. Lentiviral shRNA-mediated knockdown of neurofascin in adult rat brain indicates that neurofascin regulates the number and size of postsynaptic gephyrin scaffolds, the number of GABA(A) receptor clusters as well as presynaptic glutamate decarboxylase-positive terminals at the axon initial segment. By contrast, overexpression of neurofascin in hippocampal neurons increases gephyrin cluster size presumably via stimulation of fibroblast growth factor receptor 1 signaling pathways.