Proceedings of the Royal Society of London Series B-Containing Papers of Abiological Character最新文献

A brief introduction to the brain-mind problem leads on to a survey of the neuronal structure of the cerebral cortex. It is proposed that the basic receptive units are the bundles or clusters of apical dendrites of the pyramidal cells of laminae V and III-II as described by Fleischhauer and Peters and their associates. There are up to 100 apical dendrites in these receptive units, named dendrons. Each dendron would have an input of up to 100,000 spine synapses. There are about 40 million dendrons in the human cerebral cortex. A study of the influence of mental events on the brain leads to the hypothesis that all mental events, the whole of the World 2 of Popper, are composed of mental units, each carrying its own characteristic mental experience. It is further proposed that each mental unit, named psychon, is uniquely linked to a dendron. So the mind-brain problem reduces to the interaction between a dendron and its psychon for all the 40 million linked units. In my 1986 paper (Proc. R. Soc. Lond. B 227, 411-428) on the mind-brain problem, there was developed the concept that the operation of the synaptic microsites involved displacement of particles so small that they were within range of the uncertainty principle of Heisenberg. The psychon-dendron interaction provides a much improved basis for effective selection by a process analogous to a quantal probability field. In the fully developed hypothesis psychons act on dendrons in the whole world of conscious experiences and dendrons act on psychons in all perceptions and memories. It is shown how these interactions involve no violation of the conservation laws. There are great potentialities of these unitary concepts, for example as an explanation of the global nature of a visual experience from moment to moment. It would seem that there can be psychons not linked to dendrons, but only to other psychons, creating what we may call a psychon world.

The visual system can extract information about shape from the pattern of light and dark surface shading on an object. Very little is known about how this is accomplished. We have used a learning algorithm to construct a neural network model that computes the principal curvatures and orientation of elliptic paraboloids independently of the illumination direction. Our chief finding is that receptive fields developed by units of such model network are surprisingly similar to some found in the visual cortex. It appears that neurons that can make use of the continuous gradations of shading have receptive fields similar to those previously interpreted as dealing with contours (i.e. 'bar' detectors or 'edge' detectors). This study illustrates the difficulty of deducing neuronal function within a network solely from receptive fields. It is also important to consider the pattern of connections a neuron makes with subsequent stages, which we call the 'projective field'.

ATP-sensitive K(+)-channel currents were recorded from isolated membrane patches and voltage-clamped CRI-G1 insulin-secreting cells. Internal Mg2+ ions inhibited ATP-K+ channels by a voltage-dependent block of the channel current and decrease of open-state probability. The run-down of ATP-K+ channel activity was also shown to be [Mg2+]i dependent, being almost abolished in Mg2(+)-free conditions. Substitution of Mn2+ for Mg2+ did not prevent run-down, nor did the presence of phosphate-donating nucleotides, a protease or phosphatase inhibitor or replacement of Cl- by gluconate.

The responses to single electrical stimuli have been recorded from neurons in the brains of domestic chicks, by using an in vitro preparation consisting of a coronal slice taken from the forebrain. All slices were cut so that they contained the intermediate part of the medial hyperstriatum ventrale (IMHV). When such a slice is bathed in standard Krebs' solution there is no evidence that the excitation produced by a single stimulus can be transmitted more than 1 mm either towards or away from the IMHV. The addition of bicuculline methiodide (more than 3 x 10(-6) M) to Krebs' solution allows the excitation produced by a single stimulus to spread in all directions throughout the dorsal half of a coronal slice. At points remote (more than 1.5 mm) from the stimulated point, the magnitude of the spreading wave of excitation bears an all-or-nothing relation to the strength of stimulus used to excite it. This wave of excitation spreads from the excited point in all directions without attenuation at 0.9 +/- 0.017 (s.d.) m s-1 and consists of a prolonged burst of activity of the invaded neurons. The properties of coronal slices described above are also true of brain slices cut in a parasagittal plane. The spreading response to a single stimulus given in the presence of bicuculline, can be reduced in magnitude by the addition of AP-5 but it still spreads throughout the dorsal part of the slice at the same velocity. The response can be eliminated by the addition of kynurenic acid. The addition of curare to the bathing medium produces similar responses that spread in a similar fashion to those seen under bicuculline. These results suggest that the dorsal part of the forebrain of the domestic chick (in fact, the part derived from the embryological alar plate) contains a network of reciprocally connected local circuits. Transmission throughout the network is normally prevented by active inhibition.

If a single ion channel record is observed in which two ion channels are never simultaneously open, then it is often of interest to know whether the observations indeed arose from the activity of only one ion channel. This question can be answered if it is possible to calculate the distribution of the duration of runs of single openings in a membrane patch that contains two active channels. If the observed run of single openings is much longer than that expected for a patch with two channels it is likely that only one channel was active. An approximate method is presented for calculating the distribution of the duration of runs of single openings in a patch with two active channels; this method has the advantage that it can be calculated from observable quantities, and requires no knowledge of the details of the ion-channel mechanism or its rate constants. The accuracy of this approximation is tested by exact calculations of the properties of runs of single openings, and of single bursts, for two specific mechanisms and a large range of rate constants. The approximation is good in all cases in which openings occur singly, or in closely spaced bursts. If, as is common in practice, openings occur in clusters that are separated by long shut periods, then overlap of clusters from two different channels may be detected, if no double opening is produced, as a period in the middle of a cluster in which the probability of being open doubles. The results derived here can be applied to such a period to test whether it results from the simultaneous activity of two channels, rather than from a change in the properties of a single channel.

Staining for motor endplates and chemical digestion of five major muscles of the domestic chicken shows that these confirm the short-fibre strap muscle paradigm. The individual fibres are spindle-shaped, terminating in gradually tapering ends. The motor endplates of the individual fibres align in cross-bands along the length of the fascicles. These bands are spaced much more tightly than are comparable bands in mammals; unlike the condition in mammals, many fibres are longer than twice the interband spacing. The spacings between bands differ by more than a factor of five along the length of each muscle. The proportions among bands remain relatively constant. These proportions are not affected by the degree of muscular contraction, nor do they change with ontogeny, suggesting that the arrangement is established before hatching.

Neutron diffraction has been used to determine the preferred orientation of the hydroxyapatite crystals at the lower front edge of the tibia. The bones are compared for two Neolithic Orkney tribes. At Isbister, a muscular tribe on a hilly site engaged in heavy uphill work, whereas a tribe at Quanterness lived on level ground. The two sets are contrasted with the texture of present-day amputations. Values of 16, 8 and 3 are found for an 'index of manoeuvrability' for the Isbister, Quanterness and modern ankles, respectively.

The responses to local stimulation have been recorded from neurons in the intermediate part of the medial hyperstriatum ventrale (IMHV) of the domestic chick, by using an in vitro slice preparation. When the slice is bathed in gassed Krebs' solution, a single stimulus evokes a short-lasting diphasic response. The first phase is negative and lasts some 3 ms, whereas the second, positive phase is often of lower amplitude and usually persists for about 15 ms. The first phase is little altered by perfusion with either Ca2(+)-free Krebs' solution or Krebs' solution containing a high concentration of Mg2+. In contrast, the second phase is abolished by these procedures. The post-synaptic phase is positive when it is recorded anywhere between 0.1-1.25 mm from the stimulated point; however, in the immediate vicinity (0.0-0.1 mm) of the stimulating electrodes, the post-synaptic response is strongly negative. A pair of stimuli has to be separated by at least 10 s to guarantee complete recovery of excitability of the post-synaptic response. The recovery curve for this response shows a refractory period of some 5 ms, a peak of excitability at an interval of about 20 ms, and then a sharp trough of relative inexcitability at about 200 ms. The post-synaptic response is considerably reduced in magnitude and duration by the addition of AP-5 to the perfusion fluid; the remaining post-synaptic response is completely abolished by kynurenic acid. The addition of bicuculline methiodide in concentrations of at least 1 x 10(-6) M increases both the magnitude and duration of the second, positive phase of the response to single stimuli. This extended positive response (which may last from 500-800 ms) is abolished by perfusion with bicuculline dissolved in Ca2(+)-free Krebs' solution. For the entire duration of the extended post-synaptic positive response produced by bicuculline, the irregular discharge of single neurons can be recorded. Like the post-synaptic positive response in Krebs' solution, the much larger response produced by bicuculline shows a very localized negativity beneath the stimulating electrodes and displays an almost identical time-course for the recovery of excitability following a single stimulus. The bicuculline induced positive response is also considerably reduced by the presence of AP-5; the addition of kynurenic acid abolishes the remaining post-synaptic response completely. A post-synaptic response, similar to that produced under bicuculline, can be produced by the addition of a maximally effective dose of d-tubocurarine.(ABSTRACT TRUNCATED AT 400 WORDS)

A critical study has been made of the characteristics of the kinetic components of the sodium gating current in the squid giant axon, of which not less than five can be resolved. In addition to the principal fast component Ig2, there are two components of appreciable size that relax at an intermediate rate, Ig3 alpha and Ig 3 beta. Ig3 alpha has a fast rise, and is present over the whole range of negative test potentials. Ig3 beta is absent below -40 mV, exhibits a delayed onset and disappears on inactivation of the sodium system. There are also two smaller components, Ig1 and Ig4, with very fast and much slower relaxation time constants, respectively.

A series-parallel model of the kinetics of the voltage-gated sodium channel is described. It goes some way towards reconciling the time-courses of the gating and macroscopic sodium currents in the squid giant axon with the molecular structure of the channel.