The modular operation of the cerebral neocortex considered as the material basis of mental events

Is the cerebral cortex modular?

Two types of modular subunit, differing in size, have been hypothesized to exist in the cerebral cortex. The first, known as a mini-column, consists of a group of 110 +/- 10 cells which form a fascicle about 30 micrograms in diameter oriented perpendicular to the cortical surface. Mini-columns are believed to be organized into larger modular groupings, referred to here as macro-columns, with a diameter of about a millimetre or less. Nicholas Swindale argues in this article that there is very little real evidence in favour of either type of module. As an alternative, he suggests that the diversity of types of columnar organization, both within and between different cortical areas, may reflect the diversity of types of information stored in the cortex. Consequently, columnar organization can be expected to vary within and between species, and even between different individuals of the same species. This new interpretation is in line with current neural network theories, which do not demand the existence of structural modularity, but show how complex forms of organization can result from the existence of simple processing rules between the elements of a structure given complex structured inputs.

Neuronal mechanisms of goal-directed behaviour in monkeys

The present paper is a review of investigations by the author and coworkers on rearrangements of impulse activity of single cortical neurons and subcortical structures in monkeys during the performance of delayed alternation choice. In prefrontal and parietal cortices as well as in the head of the striate nucleus and in the anterior ventral thalamic nucleus spatioselective neurons are found which detect cue location. This spatial selectivity is retained within the entire delay. Neurons of shortterm memory and units involved the realization of a forthcoming goal-directed movement are found in the prefrontal and parietal cortices. Basing on morphophysiological data intracortical reverberation might be considered a principal mechanism of storing the relevant information in shortterm memory. Intracortical neuronal circuits in the prefrontal cortex and long thalamocortical loops of direct connections and feedback in the parietal cortex may ensure this type of reverberation. The above brain structures comprise a dynamic constellation which provides extraction from longterm memory and storing in the shortterm memory of information about the conditioned signal properties and the relevant motor program.

Synaptic connections of axo-axonic (chandelier) cells in human epileptic temporal cortex

The human temporal cortex contains a type of interneuron, identified by Golgi impregnation which, like the axo-axonic or chandelier cells found in animals, establishes Gray's type II synaptic contacts exclusively with the axon initial segments of pyramidal cells. Each terminal segment is composed of 3-12 boutons to form a "chandelier"-like appearance. For the two human axo-axonic cells analysed in this study we could identify 269 and 86 bouton rows respectively, which represents an equivalent number of postsynaptic pyramidal cells. A terminal bouton row from one of these Golgi-impregnated cells was shown to be in synaptic contact with the axon initial segment of a Golgi-impregnated pyramidal cell. The very specific nature of the target of axo-axonic cells, together with their highly divergent axonal arborization, means that they are ideally placed to control the output of a large population of pyramidal cells. Since previous studies in animals have shown the GABAergic nature of axo-axonic cells it is possible that human axo-axonic cells could be involved in the generation of epileptic activity or in the control of its propagation.

Extensive intraneuronal spread of horseradish peroxidase from a focus of vasogenic edema into remote areas of central nervous system. Observations on mouse central nervous system subjected to cortical cold injury

A study was made of the uptake of horseradish peroxidase (HRP) into neurons from a cryogenic cortical lesion in the mouse brain associated with vasogenic edema, following intravenous administration of the tracer. Particular emphasis was placed on the axonal spread of HRP from the primary lesion to other areas of the central nervous system. The distribution of HRP was studied by light microscopy, using highly sensitive histochemical methods, 3-144 h after the onset of the injury. Extravasated HRP was taken up into nerve cell bodies in and around the primary lesion, forming different patterns of labelling: (1) granular, (2) diffuse, and (3) a combination of granular and diffuse staining. Granularity is considered to be the result of HRP accumulation in lysosomes occurring in undamaged or slightly damaged nerve cells, whereas the diffuse, non-granular pattern presumably occurs in severely damaged neurons. Nerve cell bodies containing HRP reaction product were also found in the contralateral cortex, ipsilateral thalamus, substantia nigra, amygdala and ventral tegmental area, presumably a consequence of retrograde axonal transport of the tracer from the primary injury. HRP-containing axons were present in the corpus callosum and in the pyramidal tract of the injured hemisphere all the way down to the cervical spinal cord. Labelling of axonal terminals and preterminal axons in the ipsilateral thalamus, entopeduncular nucleus, subthalamic nucleus, substantia nigra and pons indicated anterograde transport of HRP to these regions. Thus very extensive intraneuronal spread of a macromolecular edema component takes place from a primary focal brain lesion to areas located far away from but neuroanatomically connected to this injured region. The brain thus seems to be affected by focal vasogenic edema in many more ways than are recognized at present.

Development, morphology and topography of chandelier cells in the auditory cortex of the cat

Using the Golgi method we have studied the development, morphology and topographic distribution of chandelier cells in the auditory cortex of the cat. Kittens from 9 to 42 days old were used. Chandelier cells can be recognized in 9-day-old kittens as a distinct neuronal variety. In the 15- and 23-day-old kittens chandelier cells develop axonal complexes elongated vertically. In the 42-day-old kitten, they seem to have attained their adult morphology. They are not distributed uniformly and their predominance in certain areas suggest that they may have a relation with callosal projecting pyramidal cells.

Somatostatin and the cerebral cortex

A unique subset of interneurons which are rich in immunoreactive somatostatin (IRS) exists in the cerebral cortex. The regulation of IRS secretion by these cells is reviewed. Acetylcholine, glutamic acid and several neuropeptides including VIP, CCK, and metenkephalin have been identified as IRS secretagogues. The types of molecules which stimulate IRS release, the electrophysiologic effects of somatostatin, and the recognition of abnormal IRS levels in human CNS diseases were all used to formulate a working model of the role of the somatostatinergic cell in ongoing cerebral cortical function.

Two neuronal systems are involved in a classical conditioning in the rat

Unit activity of the hippocampus, the centrum-medianum-parafascicularis and medialis dorsalis nuclei of the thalamus, was recorded in chronic rats during a classical conditioning; neocortical electroencephalographic and somatic responses were also recorded. Conditioned unit responses of the different groups of neurons were compared according to the precocity of their appearance, their stability, their latency and the differentiation between the positive (reinforced) and the negative (non-reinforced) conditioned stimuli. Conditioned unit responses of type I hippocampal neurons (probably pyramids) and of neurons located in the centrum-medianum-parafascicularis nucleus did not differentiate between the positive and negative conditioned stimuli; they progressed rapidly, then declined and disappeared. They were contemporary with an initial conditioning stage which was characterized by undifferentiated arousal responses (desynchrony of electroencephalographic activity) to both conditioned stimuli. Conditioned unit responses of type II hippocampal neurons (probably granule cells or interneurons of the fascia dentata) and of neurons located in the medialis dorsalis nucleus progressed slowly and differentiated between the conditioned stimuli during a late conditioning stage which was characterized by the regression of arousal responses and the differentiation of somatic responses. These data strongly suggest that two neuronal systems, each having a different role, are involved in classical conditioning in the rat.

Coordination: a vector-matrix description of transformations of overcomplete CNS coordinates and a tensorial solution using the Moore-Penrose generalized inverse

Neuronal organisms express their function, such as a movement, by multicomponental actions. Thus, the problem of how the central nervous system (CNS) coordinates the elements of a single action is fundamental to our understanding of brain function. Coordinated activation of multijointed "limbs" has also become an acute problem in modern multivariable control theory and engineering, such as robotics. Thus, a coherent interdisciplinary approach is expected, one that arrives at concepts and formalisms applicable to this problem both in living and man-made organisms. By treating coordination with coordinates, tensor network theory of the CNS, which explains transformations through the neuronal networks of natural non-orthogonal coordinates that are intrinsic to living organisms, may successfully integrate the diverse approaches to this general problem. A link between tensor network theory of the CNS and multivariable control engineering can be established if the latter is formulated in generalized non-orthogonal coordinates, rather than in conventional Cartesian expressions. In general terms, the problem of coordinating an overcomplete (more than necessary) number of components of an action can be resolved by a three-step tensorial scheme. A key operation is a covariant-to-contravariant transformation executed by the Moore-Penrose generalized inverse when, in an overcomplete manifold, the covariant metric tensor is singular. In the neuronal organization of the CNS, it is assumed that the cerebellum plays this role of acting as a contravariant metric. A quantitative example is also provided, in order to demonstrate the viability of the numerical and network-implementations.

Calcium in long-term potentiation as a model for memory

The granule, CA1 and CA3 cells of the hippocampus have been much investigated during the last decade because there is superimposed on the standard features of synaptic transmission a very prolonged potentiation lasting for weeks that is called long-term potentiation. Evidently long-term potentiation is a promising candidate in the construction of a model for memory. The thesis here developed is that the influx of calcium ions across the membrane of the granule pyramidal cells plays the key role in the generation of long-term potentiation. The proposal makes it possible to account for the necessity of strong repetitive synaptic stimulation, preferably in bursts so as to optimize the conditions for the calcium influx. Studies on hippocampal slices with variations in the synaptic inputs in the granule cells give evidence of cooperativity, which is interpreted in relation to the threshold membrane depolarization for calcium influx. It is conjectured that the large increase of calcium in the granule and pyramidal cells results in the combination with the specific protein, calmodulin, to form a second messenger system, which produces metabolic changes leading to an increase in receptors of the postsynaptic membrane of the spine synapses, i.e. the postsynaptic densities, to the synaptic transmitter, glutamate. For example, Ca2+ could activate calcium-dependent kinases in the postsynaptic density resulting in the modification of protein components by phosphorylation. Other postsynaptic factors contributing to long-term potentiation are presumed to be protein synthesis with spine swelling and increased transport up the dendritic microtubules. There is discussion of the evidence for the alternative hypothesis that long-term potentiation is primarily presynaptic, being due to an increased output of transmitter. A unifying hypothesis is formulated, namely, that the primary event in long-term potentiation is in the increased sensitivity of the postsynaptic densities to the transmitter, and that, secondarily, this induces an increased output of transmitter from the presynaptic terminals by a trophic action across the synaptic cleft. It is shown how the proposed combination of calcium with calmodulin will account for the hypothesis of Marr that cognitive memory is due to conjunction potentiation. Furthermore, the Marr-Albus hypothesis for cerebellar learning is accounted for if the calcium-calmodulin messenger system causes the observed depression of the transmitter sensitivity of the spine synapses on Purkynĕ cells.

Hierarchial thermodynamic approach to the brain

A thermodynamic approach to understanding the brain is presented. Thermodynamics may be extended to "higher than molecule" levels. The role of the concept of entropy in thermodynamic-based nonthermodynamic systems has been discussed. Static and dynamic structures, cooperative and competitive mechanisms and some of their neurobiological applications are discussed. Postsynaptic membrane noises, dynamic synaptic activity, synaptogenesis, plasticity, retinotopy and perception have been considered as self-organizing neural structures appearing at different hierarchical levels.

Melanin: the organizing molecule

The hypothesis is advanced that (neuro)melanin (in conjunction with other pigment molecules such as the isopentenoids) functions as the major organizational molecule in living systems. Melanin is depicted as an organizational "trigger" capable of using established properties such as photon-(electron)-phonon conversions, free radical-redox mechanisms, ion exchange mechanisms, and semiconductive switching capabilities to direct energy to strategic molecular systems and sensitive hierarchies of protein enzyme cascades. Melanin is held capable of regulating a wide range of molecular interactions and metabolic processes primarily through its effective control of diverse covalent modifications. To support the hypothesis, established and proposed properties of melanin are reviewed (including the possibility that (neuro)melanin is capable of self-synthesis). Two "melanocentric systems"--key molecular systems in which melanin plays a central if not controlling role--are examined: 1) the melanin-purine-pteridine (covalent modification) system and 2) the APUD (or diffuse neuroendocrine) system. Melanin's role in embryological organization and tissue repair/regeneration via sustained or direct current is considered in addition to its possible control of the major homeostatic regulatory systems--autonomic, neuroendocrine, and immunological.

A new type of specific interneuron in the monkey hippocampus forming synapses exclusively with the axon initial segments of pyramidal cells

By means of Golgi staining and gold-toning, we have found an interneuron in the pyramidal cell layer of the hippocampus which forms synapses exclusively on the axon initial segments of pyramidal neurons. An individual initial segment receives up to 30 symmetrical synapses from one axo-axonic cell. Each axo-axonic cell is in synaptic contact with the axon initial segments of several hundred pyramidal neurons. The interneuron is thus ideally situated to synchronize the output of a large population of pyramidal cells and so might be involved in the generation of rhythmic activity and in epileptogenesis.

The initiation of voluntary movements by the supplementary motor area

The hypothesis is formulated that in all voluntary movements the initial neuronal event is in the supplementary motor areas (SMA) of both cerebral hemispheres. Experimental support is provided by three lines of evidence. 1. In voluntary movements many neurones of the SMA are activated probably up to 200 ms before the pyramidal tract discharge. 2. Investigations of regional cerebral blood flow by the radioactive Xenon technique reveal that there is neuronal activity in the SMA of both sides during a continual series of voluntary movements, and that this even occurs when the movement is thought of, but not executed. 3. With voluntary movement there is initiation of a slow negative potential (the readiness potential, RP) at up to 0.8 s before the movement. The RP is maximum over the vertex, i.e. above the SMA, and is large there even in bilateral Parkinsonism when it is negligible over the motor cortex. An account is given of the SMA, particularly its connectivities to the basal ganglia and the cerebellum that are active in the preprogramming of a movement. The concept of motor programs is described and related to the action of the SMA. It is proposed that each mental intention acts on the SMA in a specific manner and that the SMA has an 'inventory' and the 'addresses' of stored subroutines of all learnt motor programs. Thus by its neuronal connectivities the SMA is able to bring about the desired movement. There is a discussion of the manner in which the mental act of intention calls forth neural actions in the SMA that eventually lead to the intended movement. Explanation is given on the basis of the dualist-interactionist hypothesis of mind-brain liaison. The challenge is to the physicalists to account for the observed phenomena in voluntary movement.

Uptake of macromolecules into neurons from a focal vasogenic cerebral edema and subsequent axonal spread to other brain regions. A preliminary study in the mouse with horseradish peroxidase as a tracer

Intravenously (i.v.) injected horseradish peroxidase (HRP) which has leaked out of the vessels in a cryogenic cortical injury of adult mice is taken up into a large number of neurons resulting in two different forms of labeling. Diffuse neuronal labeling of the type previously reported in many conditions with vasogenic brain edema occurred particularly within the primary lesion. The other and more frequent type, here called granular neuronal labeling, was present in a wide zone immediately outside the injury. Such neurons contained HRP in numerous cytoplasmic granules and had the same characteristics as normal neurons accumulating HRP after retrograde axonal transport. By using highly sensitive histochemical methods for demonstration of HRP we could also follow bundles of labeled axons out from the primary lesion. Some of them passed the corpus callosum to the fronto-parietal cortex of the contralateral hemisphere. With this report we would like to put emphasize on certain phenomena occurring in neurons which previously have not been particularly recognized in studies on vasogenic brain edema. It can be assumed that in a focal brain lesion components from the edematous fluid and other "would substances" can be taken up into nerve cell processes and then be intracellularly transported in different directions. In this way, nerve cell populations located in other brain areas and even in the contralateral hemisphere may be influenced by components from the primary injury.