A theoretical framework is proposed for the analysis of adhesion between cells or of cells to surfaces when the adhesion is mediated by reversible bonds between specific molecules such as antigen and antibody, lectin and carbohydrate, or enzyme and substrate. From a knowledge of the reaction rates for reactants in solution and of their diffusion constants both in solution and on membranes, it is possible to estimate reaction rates for membrane-bound reactants. Two models are developed for predicting the rate of bond formation between cells and are compared with experiments. The force required to separate two cells is shown to be greater than the expected electrical forces between cells, and of the same order of magnitude as the forces required to pull gangliosides and perhaps some integral membrane proteins out of the cell membrane.

https://science.sciencemag.org/content/sci/200/4342/618.full.pdf

Models for the specific adhesion of cells to cells

Tissue culture plastic adhesivity was precisely varied by applying different concentrations of poly(2-hydroxyethyl methacrylate). The extent of cell spreading was thus accurately controlled so that cells cultured on these substrata could be held at any one of a graded series of quantitated cell shapes. Cell shape was found to be tightly coupled to DNA synthesis and growth in nontransformed cells. These findings suggest a mechanism that is important in growth control of mammalian cells, and provide a more fundamental interpretation of such phenomena as density dependent inhibition of cell growth and anchorage dependence.

Role of cell shape in growth control

This paper concerns how motor actions are neurally represented and coded. Action planning and motor preparation can be studied using a specific type of representational activity, motor imagery. A close functional equivalence between motor imagery and motor preparation is suggested by the positive effects of imagining movements on motor learning, the similarity between the neural structures involved, and the similar physiological correlates observed in both imaging and preparing. The content of motor representations can be inferred from motor images at a macroscopic level, based on global aspects of the action (the duration and amount of effort involved) and the motor rules and constraints which predict the spatial path and kinematics of movements. A more microscopic neural account calls for a representation of object-oriented action. Object attributes are processed in different neural pathways depending on the kind of task the subject is performing. During object-oriented action, a pragmatic representation is activated in which object affordances are transformed into specific motor schemas (independently of other tasks such as object recognition). Animal as well as human clinical data implicate the posterior parietal and premotor cortical areas in schema instantiation. A mechanism is proposed that is able to encode the desired goal of the action and is applicable to different levels of representational organization.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0140525X00034026

The representing brain: Neural correlates of motor intention and imagery

This article reviews the electroresponsive properties of single neurons in the mammalian central nervous system (CNS). In some of these cells the ionic conductances responsible for their excitability also endow them with autorhythmic electrical oscillatory properties. Chemical or electrical synaptic contacts between these neurons often result in network oscillations. In such networks, autorhythmic neurons may act as true oscillators (as pacemakers) or as resonators (responding preferentially to certain firing frequencies). Oscillations and resonance in the CNS are proposed to have diverse functional roles, such as (i) determining global functional states (for example, sleep-wakefulness or attention), (ii) timing in motor coordination, and (iii) specifying connectivity during development. Also, oscillation, especially in the thalamo-cortical circuits, may be related to certain neurological and psychiatric disorders. This review proposes that the autorhythmic electrical properties of central neurons and their connectivity form the basis for an intrinsic functional coordinate system that provides internal context to sensory input.

https://science.sciencemag.org/content/sci/242/4886/1654.full.pdf

The intrinsic electrophysiological properties of mammalian neurons: insights into central nervous system function.

Taxol, a potent inhibitor of human HeLa and mouse fibroblast cell replication, blocked cells in the G2 and M phase of the cell cycle and stabilized cytoplasmic microtubules. The cytoplasmic microtubules of taxol-treated cells were visualized by transmission electron microscopy and indirect immunofluorescence microscopy. More than 90% of the cells treated with 10 micro M taxol for 22 hr at 37 degrees C displayed bundles of microtubules that appeared to radiate from a common site (or sites), in addition to their cytoplasmic microtubules. Untreated cells that were kept in the cold (4 degrees C) for 16 hr lost their microtubules, whereas cells that were pretreated with taxol for 22 hr at 37 degrees C continued to display their microtubules and bundles of microtubules in the cold. Taxol inhibited the migration behavior of fibroblast cells, but these cells did not lose their ability to produce mobile surface projections such as lamellipodia and filopodia.

https://www.pnas.org/content/pnas/77/3/1561.full.pdf

Taxol stabilizes microtubules in mouse fibroblast cells.

Effects of metaphase inhibitors (colcemid, colchicine, vinblastine) on mouse and human embryonic, fibroblast-like cells growing on glass and on an oriented substrate (fish scale) were studied. All three inhibitors caused similar changes in the form of interphase cells and inhibited their directional locomotion. The effects of two inhibitors (colcemid and vinblastine) were found to be completely reversible. Microcinematographic studies have shown that the most conspicuous change of locomotory behaviour induced by colcemid was the disappearance of non-active stable parts of the cell edge; in normal cells only the leading part of the edge was actively moving, while in colcemid-treated cells all parts of the edge eventually became active. Activation of the whole edge made these cells unable to perform directional translocation. It is suggested that colcemid and other metaphase inhibitors prevent stabilization of the non-active state of the cell surface. The possible role of this suggested colcemid-sensitive stabilization mechanism in the normal locomotory behaviour of fibroblasts is discussed. Electron-microscopic examination has shown that microtubules disappeared from the cytoplasm of colcemid-treated, mouse, fibroblast-like cells. The formation of microtubules as the possible structural basis of the stabilization of the non-active state of the cell surface is discussed.

Effect of colcemid on the locomotory behaviour of fibroblasts.

On the role of central program and afferent inflow in the control of scratching movements in the cat.

Origin of modulation in neurones of the ventral spinocerebellar tract during locomotion

Recording of neurones of the dorsal spinocerebellar tract during evoked locomotion

1. In decerebrate, curarized cats, stimulation of the cervical spinal cord evoked fictitious scratching (9), i.e., periodical activity of the hindlimb motoneurons with a discharge pattern typical of actual scratching (cycle duration about 250 ms, flexor phase about 200 ms, extensor phase about 50 ms). During fictitious scratching, extra-cellular records were obtained from 182 spinal neurons located in different regions of the gray matter cross section (except for the motor nuclei), from segments L4 and L5. 2. The firing rate of 73% of neurons was rhythmically modulated in relation with the scratch cycle. Most of the modulated neurons fired in bursts and were silent between bursts. They were located mainly in Rexed's (22) layer VII. 3. Burst onsets ("switchings on" of the neurons) were distributed rather evenly throughout the scratch cycle except for a small maximum at the very beginning of the cycle (the cycle was assumed to start with the termination of the extensor phase). Burst terminations ("switchings off") in the overwhelming majority of the neurons were distributed over the last-third part of the cycle. As a result, those neurons which began to fire earlier in the cycle usually had longer bursts, compared to the neurons which began to fire later. Besides, since there were very few switchings off in the first half of the cycle, the number of simultaneously active neurons increased during the first half of the cycle, reached the maximum somewhat later than the middle of the cycle, and considerably decreased by the end of the cycle. 4. With more intensive scratching, the firing rate in the bursts considerably increased in all neurons tested, while the duration of the scratch cycle changed only slightly. 5. A correlation between the burst position in the cycle and the behavior during the latent period of scratching (when stimulation of the cervical spinal cord had already been started but rhythmical oscillations had not yet appeared) was found in many neurons. Most of the neurons which began to fire at the beginning of the scratch cycle and had long bursts were tonically activated during the latent period. On the contrary, most of the neurons which fired in short bursts at the end of the cycle were either inhibited or not affected during this period. 6. A correlation betwen the burst position in the cycle and the frequency pattern was found in many neurons. In most of the neurons which began to fire in the first half of the cycle (except for the very beginning), the discharge rate increased in the course of the burst. In the remaining neurons, the discharge rate changed only slightly during the burst. 7. Hypotheses concerning organization of the spinal mechanism of scratching are discussed.

I. M. Gelfand

Papers

Effect of colcemid on the locomotory behaviour of fibroblasts.

On the role of central program and afferent inflow in the control of scratching movements in the cat.

Origin of modulation in neurones of the ventral spinocerebellar tract during locomotion

Recording of neurones of the dorsal spinocerebellar tract during evoked locomotion

Generation of scratching. I. Activity of spinal interneurons during scratching