The modular organization of nervous systems is a widely documented principle of design for both vertebrate and invertebrate brains of which the columnar organization of the neocortex is an example. The classical cytoarchitectural areas of the neocortex are composed of smaller units, local neural circuits repeated iteratively within each area. Modules may vary in cell type and number, in internal and external connectivity, and in mode of neuronal processing between different large entities; within any single large entity they have a basic similarity of internal design and operation. Modules are most commonly grouped into entities by sets of dominating external connections. This unifying factor is most obvious for the heterotypical sensory and motor areas of the neocortex. Columnar defining factors in homotypical areas are generated, in part, within the cortex itself. The set of all modules composing such an entity may be fractionated into different modular subsets by different extrinsic connections. Linkages between them and subsets in other large entities form distributed systems. The neighborhood relations between connected subsets of modules in different entities result in nested distributed systems that serve distributed functions. A cortical area defined in classical cytoarchitectural terms may belong to more than one and sometimes to several distributed systems. Columns in cytoarchitectural areas located at some distance from one another, but with some common properties, may be linked by long-range, intracortical connections.

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The columnar organization of the neocortex.

To be successful, precision manipulation of small objects requires a refined coordination of forces excerted on the object by the tips of the fingers and thumb. The present paper deals quantitatively with the regulation of the coordination between the grip force and the vertical lifting force, denoted as the load force, while small objects were lifted, positioned in space and replaced by human subjects using the pinch grip. It was shown that the grip force changed in parallel with the load force generated by the subject to overcome various forces counteracting the intended manipulation. The balance between the two forces was adapted to the friction between the skin and the object providing a relatively small safety margin to prevent slips, i.e. the more slippery the object the higher the grip force at any given load force. Experiments with local anaesthesia indicated that this adaptation was dependent on cutaneous afferent input. Afferent information related to the frictional condition could influence the force coordination already about 0.1 s after the object was initially gripped, i.e. approximately at the time the grip and load forces began to increase in parallel. Further, “secondary”, adjustments of the force balance could occur later in response to small short-lasting slips, revealed as vibrations in the object. The new force balance following slips was maintained, indicating that the relationship between the two forces was set on the basis of a memory trace. Its updating was most likely accounted for by tactile afferent information entering intermittently at inappropriate force coordination, e.g. as during slips. The latencies between the onset of such slips and the appearance of the adjustments (0.06–0.08 s) clearly indicated that the underlying neural mechanisms operated highly automatically.

Roles of glabrous skin receptors and sensorimotor memory in automatic control of precision grip when lifting rougher or more slippery objects

Current models of motor cortical plasticity, developed in studies on experimental animals, emphasize the importance of the conjoint activity of somatosensory afferents and intrinsic motor cortical circuits. The hypothesis that an enduring change in excitability in the cortical output circuitry can be induced in the human motor cortex by a paired-stimulation protocol was tested. Low-frequency median nerve stimulation was paired with transcranial magnetic stimulation (TMS) over the optimal cranial site for stimulating the abductor pollicis brevis muscle (APB). This protocol induced an increase in the amplitudes of the motor evoked potentials (MEPs) in the resting APB as well as a prolongation of the silent period measured in the precontracted APB following TMS; amplitudes of MEPs measured in voluntary contraction remained unchanged. Experiments testing the excitability of spinal motoneurons using F-wave studies and electrical stimulation of the brainstem suggested that the site of the plastic changes was within the motor cortex. The increases in resting amplitudes and silent period duration were conditionally dependent on the timing between the afferent and the magnetic stimulation in that they were present when events elicited by afferent and magnetic stimulation were synchronous at the level of the motor cortex. Plasticity induced by paired stimulation evolved rapidly (within 30 min), was persistent (minimum duration 30-60 min) yet reversible, and was topographically specific. This combination of features and the similarity to properties of induced enduring changes in synaptic efficacy, as elucidated in animal studies, leads us to propose that the induced plasticity may represent a signature of associative long-term potentiation of cortical synapses or closely related neuronal mechanisms in the human cortex.

http://www.tmslab.org/tms%20articles%20research/022.pdf

Induction of plasticity in the human motor cortex by paired associative stimulation.

A scheme for understanding the organization of human postural movements is developed in the format of a position paper. The structural characteristics of the body and the geometry of muscular actions are incorporated into a three-dimensional graphical representation of human movement mechanics in the sagittal plane. A series of neural organizational hypotheses limit a theoretically infinite number of combinations of muscle contractions and associated movement trajectories for performing postural corrections: (1) Controls are organized to use the minimum number of muscles; (2) frequently performed movements are organized to require a minimum of neural decision-making.These hypotheses lead to the prediction that postural movements are composed of muscle contractile strategies derived from a limited set of distinct contractile patterns. The imposition of two mechanical constraints related to the configuration of support and to requirements for body stability with respect to gravity predict the conditions under which individual movement strategies will be deployed.A complementary organizational scheme for the senses is developed. We show that organization of postural movements into combinations of distinct strategies simplifies the interpretation of sensory inputs. The fine-tuning of movement strategies can be accomplished by breaking down the complex array of feedback information into a series of scalar quantities related to the parameters of the movement strategies. For example, the magnitude, aim, and curvature of the movement trajectory generated by an individual strategy can be adjusted independently.The second half of the report compares theoretical predictions with a series of actual experimental observations on normal subjects and patients with known sensory and motor disorders. Actual postural movements conform to theoretical predictions about the composition of individual movement strategies and the conditions under which each strategy is used. Observations on patients suggest how breakdowns in individual steps within the logical process of organization can lead to specific movement abnormalities.Discussion focuses on the areas needing further experimentation and on the implications of the proposed organizational scheme. We conclude that although our organizational scheme is not new in demonstrating the need for simplifying the neural control of movement, it is perhaps original in imposing discrete logical control upon a continuous mechanical system. The attraction of the scheme is that it provides a framework compatible with both mechanical and physiological information and amenable to experimental testing.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0140525X00020008

The organization of human postural movements: A formal basis and experimental synthesis

Anterograde and retrograde transport methods were used to study the corticocortical connectivity of areas 3a, 3b, 1, 2, 5, 4 and 6 of the monkey cerebral cortex. Fields were identified by cytoarchitectonic features and by thalamic connectivity in the same brains. Area 3a was identified by first recording a short latency group I afferent evoked potential. Attempts were made to analyze the data in terms of: (1) routes whereby somatic sensory input might influence the performance of motor cortex neurons; (2) possible multiple representations of the body surface in the component fields of the first somatic sensory area (SI). Apart from vertical interlaminar connections, two types of intracortical connectivity are recognized. The first, regarded as "non-specific," consists of axons spreading out in layers I, III and V-VI from all sides of an injection of isotope; these cross architectonic borders indiscrimininately. They are not unique to the regions studied. The second is formed by axons entering the white matter and re-entering other fields. In these, they terminate in layers I-IV in one or more mediolaterally oriented strips of fairly constant width (0.5--1 mm) and separated by gaps of comparable size. Though there is a broadly systematic topography in these projections, the strips are probably best regarded as representing some feature other than receptive field position. Separate representations are nevertheless implied in area 3b, in areas 1 and 2 (together), in areas 3a and 4 (together) and in area 5; with, in each case, the representations of the digits pointed at the central sulcus. Area 3b is not connected with areas 3a or 4, but projects to a combined areas 1 and 2. Area 1 is reciprocally connected with area 3a and area 2 reciprocally with area 4. The connectivity of area 3a, as conventionally identified, is such that it is probably best regarded not as an entity, but as a part of area 4. Areas identified by others as area 3a should probably be regraded as parts of area 3b. Parts of area 5 that should be more properly considered as area 2, and other parts that receive thalamic input not from the ventrobasal complex but from the lateral nuclear complex and anterior pulvinar, are also interconnected with area 4. More posterior parts of area 5 are connected with laterally placed parts of area 6. A more medial part of area 6, the supplementary motor area, occupies a pivotal position in the sensory-motor cortex, for it receives fibers from areas 3a, 4, 1, 2 and 5 (all parts), and projects back to areas 3a, 4 and 5.

Intracortical connectivity of architectonic fields in the somatic sensory, motor and parietal cortex of monkeys

1.

Peripheral inputs to cortical efferent zones projecting to distal forelimb muscles were examined in Cebus monkey by using one microelectrode both for low intensity intracortical stimulation and for recording cellular discharges.




2.

Efferent zones within the motor cortex received afferent inputs from skin, joint and muscle receptors.




3.

A given efferent zone received afferent inputs from receptors in muscles as well as joints involved in the movement produced by microstimulation within that zone.




4.

Cells activated by tactile stimuli had receptive fields almost exclusively on the glabrous volar surface of the hand and lay within cortical efferent zones projecting to finger flexor muscles.




5.

The region of low threshold efferent zones which projected to distal forelimb muscles was found to coincide with a region receiving short latency afferent input from group II cutaneous and deep afferents. Group I muscle afferents projected to an adjacent area of the cortex.




6.

The organization of motor cortex in cat and monkey was compared with particular reference to input-output relations.

Peripheral Afferent Inputs to the Forelimb Area of the Monkey Motor Cortex: Input-Output Relations

The functional organization of a cortical efferent system controlling contralateral distal forelimb muscles was examined in monkeys using the method of intracortical microstimulation (ICMS). The results obtained are:

Topographical organization of cortical efferent zones projecting to distal forelimb muscles in the monkey.

1.

Spread of synaptic effects within the gray substance of the motor cortex was examined in cats by delivering intracortical microstimulation (ICMS) through a tungsten electrode and recording postsynaptic potentials (p.s.p.'s) intracellularly from neighbouring cells.




2.

ICMS of 4 μa within layer I did not produce any p.s.p.'s in the surrounding neurons.




3.

ICMS of 4 μa at layers II, III and IV produced monosynaptic p.s.p.'s only in adjacent neurons. Their horizontal and vertical spread was limited to within 0.5 mm of the stimulating site. Ascending spread from the stimulating sites was not observed in these layers.




4.

Polysynaptic effects from layers II, III and IV were found at a distance up to 1.0 mm from the stimulating site both in horizontal and vertical directions. Inhibitory spread was wider than excitatory.




5.

ICMS of 4 μa in the deep layers (V and VI) produced both mono and poly synaptic activation of cells in these same layers as well as in the superficial layers (II–III). The effects of ICMS were mixed, but were predominantly inhibitory.




6.

The possible synaptic organization within the cortex is discussed and it is concluded that these neuronal connections as a whole constitute a columnar organization within the cortex with feedback circuits built into the system.

Spread of mono- and polysynaptic connections within cat's motor cortex

Functional role of afferent inputs to the monkey motor cortex

27 cells in the S I group I projection area of the cerebral cortex, identified as activated by group I afferent fibres by electrical stimulation of the deep radial nerve, were examined by natural stimulation in 4 awake cats.

Ingmar Rosén

Papers

Peripheral Afferent Inputs to the Forelimb Area of the Monkey Motor Cortex: Input-Output Relations

Topographical organization of cortical efferent zones projecting to distal forelimb muscles in the monkey.

Spread of mono- and polysynaptic connections within cat's motor cortex

Functional role of afferent inputs to the monkey motor cortex

Natural stimulation of group I activated cells in the cerebral cortex of the awake cat.