of the contacts of a connection varied between 80 and 585 ,um (mean, 147 /sm; median, 105 ,um). The correlation between EPSP amplitude and the number of morphologically determined synaptic contacts or the mean geometric distances from the soma was only weak (correlation coefficients were 0-2 and 0-26, respectively).7. Compartmental models constructed from camera lucida drawings of eight target neurones showed that synaptic contacts were located at mean electrotonic distances between 0 07 and 0 33 from the soma (mean, 0-13). Simulations of unitary EPSPs, assuming quantal conductance changes with fast rise time and short duration, indicated that amplitudes of quantal EPSPs at the soma were attenuated, on average, to < 10% of dendritic EPSPs and varied in amplitude up to 10-fold depending on the dendritic location of synaptic contacts. The inferred quantal peak conductance increase varied between 1.5 and 5.5 nS (mean, 3 nS).8. The combined physiological and morphological measurements in conjunction with EPSP simulations indicated that the 20-fold range in efficacy of the synaptic connections between thick tufted pyramidal neurones, which have their synaptic contacts preferentially located on basal and apical oblique dendrites, was due to differences in transmitter release probability of the projecting neurones and, to a lesser extent, to differences in the number of release sites per bouton or quantal size. 9. The continuum of efficacies in their synaptic connections implies that layer 5 pyramidal neurones can be recruited to ensemble electrical activity via their axon collaterals if as few as five of the strongly and reliably connected neighbouring neurones are active synchronously, whereas coincident APs of as many as 100 of the weakly connected pyramidal neurones are necessary.

Gene-targeted mice lacking the L-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor subunit GluR-A exhibited normal development, life expectancy, and fine structure of neuronal dendrites and synapses. In hippocampal CA1 pyramidal neurons, GluR-A-/- mice showed a reduction in functional AMPA receptors, with the remaining receptors preferentially targeted to synapses. Thus, the CA1 soma-patch currents were strongly reduced, but glutamatergic synaptic currents were unaltered; and evoked dendritic and spinous Ca2+ transients, Ca2+-dependent gene activation, and hippocampal field potentials were as in the wild type. In adult GluR-A-/- mice, associative long-term potentiation (LTP) was absent in CA3 to CA1 synapses, but spatial learning in the water maze was not impaired. The results suggest that CA1 hippocampal LTP is controlled by the number or subunit composition of AMPA receptors and show a dichotomy between LTP in CA1 and acquisition of spatial memory.

In rodents, whisker-related patterns of neuronal architecture are present at the level of the brainstem trigeminal nuclei, the ventrobasal thalamus and the somatosensory cortex. In 1970, Woolsey & van der Loos described neuronal clusters in layer (L) 4 of the mouse somatosensory cortex that mirror the topography of the whisker pad on the animal's muzzle. These cell clusters were named 'barrels' and it is now established that each cortical barrel represents a principal whisker on the contralateral side of the face in a one-to-one relationship (Woolsey & van der Loos, 1970;Welker, 1976;Wallace, 1987;Agmon & Connors, 1991; for a review see Jones & Diamond, 1995). The columnar organisation and receptive field properties of these neurones appear to be comparatively simple. This should facilitate elucidation of cortical signal flow at the cellular level. Afferent nerve fibres from the thalamic 'relay nuclei' terminate in L4 of the sensory cortices (

Whole‐cell voltage recordings were obtained from 64 synaptically coupled excitatory layer 4 (L4) spiny neurones and L2/3 pyramidal cells in acute slices of the somatosensory cortex (‘barrel’ cortex) of 17‐ to 23‐days‐old rats. Single action potentials (APs) in the L4 spiny neurone evoked single unitary EPSPs in the L2/3 pyramidal cell with a peak amplitude of 0.7 ± 0.6 mV. The average latency was 2.1 ± 0.6 ms, the rise time was 0.8 ± 0.3 ms and the decay time constant was 12.7 ± 3.5 ms. The percentage of failures of an AP in a L4 spiny neurone to evoke a unitary EPSP in the L2/3 pyramidal cell was 4.9 ± 8.8 % and the coefficient of variation (c.v.) of the unitary EPSP amplitude was 0.27 ± 0.13. Both c.v. and percentage of failures decreased with increased average EPSP amplitude. Postsynaptic glutamate receptors (GluRs) in L2/3 pyramidal cells were of the N‐methyl‐d‐aspartate (NMDA) receptor (NMDAR) and the non‐NMDAR type. At −60 mV in the presence of extracellular Mg2+ (1 mm), 29 ± 15 % of the EPSP voltage‐time integral was blocked by NMDAR antagonists. In 0 Mg2+, the NMDAR/AMPAR ratio of the EPSC was 0.50 ± 0.29, about half the value obtained for L4 spiny neurone connections. Burst stimulation of L4 spiny neurones showed that EPSPs in L2/3 pyramidal cells depressed over a wide range of frequencies (1–100 s−1). However, at higher frequencies (30 s−1) EPSP summation overcame synaptic depression so that the summed EPSP was larger than the first EPSP amplitude in the train. The number of putative synaptic contacts established by the axonal collaterals of the L4 projection neurone with the target neurone in layer 2/3 varied between 4 and 5, with an average of 4.5 ± 0.5 (n= 13 pairs). Synapses were established on basal dendrites of the pyramidal cell. Their mean geometric distance from the pyramidal cell soma was 67 ± 34 μm (range, 16–196 μm). The results suggest that each connected L4 spiny neurone produces a weak but reliable EPSP in the pyramidal cell. Therefore transmission of signals to layer 2/3 is likely to have a high threshold requiring simultaneous activation of many L4 neurons, implying that L4 spiny neurone to L2/3 pyramidal cell synapses act as a gate for the lateral spread of excitation in layer 2/3.

The three-dimensional morphology of the axosomatic synaptic structures between a calyx of Held and a principal neuron in the medial nucleus of the trapezoid body (MNTB) in the brainstem of young postnatal day 9 rats was reconstructed from serial ultrathin sections. In the apposition zone between the calyx and the principal neuron two types of membrane specializations were identified: synaptic contacts (SCs) with active zones (AZs) and their associated postsynaptic densities (PSDs) constituted approximately 35% (n = 554) of the specializations; the remaining 65% (n = 1010) were puncta adherentia (PA). Synaptic contacts comprised approximately 5% of the apposition area of presynaptic and postsynaptic membranes. A SC had an average area of 0.100 microm(2), and the nearest neighbors were separated, on average, by 0.59 microm. Approximately one-half of the synaptic vesicles in the calyx were clustered within a distance of 200 nm of the AZ membrane area, a cluster consisting of approximately 60 synaptic vesicles (n = 52 SCs). Approximately two synaptic vesicles per SC were "anatomically docked." Comparing the geometry of the synaptic structure with its previously studied functional properties, we find that during a single presynaptic action potential (AP) (1) approximately 35% of the AZs release a transmitter quantum, (2) the number of SCs and anatomically docked vesicles is comparable with the low estimates of the readily releasable pool (RRP) of quanta, and (3) the broad distribution of PSD areas [coefficient of variation (CV) = 0.9] is likely to contribute to the large variability of miniature EPSC peaks. The geometry of the reconstructed synapse suggests that each of the hundreds of SCs is likely to contribute independently to the size and rising phase of the EPSC during a single AP.