Search

The neural mechanisms controlling locomotion have been examined in many organisms (Gray, 1968). Several authors have proposed that these mechanisms can be characterized by a set of unifying principles (e.g., Brown, 1911, 1914; Wiersma and Ikeda, 1964; Wilson, 1966a, b, 1967, 1968c, 1972; Szekeley, 1968; Lundberg, 1969; Hoyle, 1970; Evarts et al., 1971; Evoy and Cohen, 1971; Davis, 1973b; Gurfinkel and Shik, 1973; Stein, 1974, 1976; Grillner, 1975; Shik and Orlovsky, 1976; Wetzel and Stuart, 1976). Such principles are useful in that they can order vast groups of data and provide working hypotheses for the design of new experiments. This chapter will present one version of the unifying principles with emphasis on their joint application to the arthropods and the vertebrates.

Crustacean walking has only recently been subjected to intensive investigation by modern behavioral and neurophysiological techniques (Evoy and Fourtner, 1973a; Barnes, 1975b; Evoy and Ayers, 1980). A large body of data exists in the older literature but it is more descriptive than experimental due to inadequate methodology (see Lochhead, 1961). Moreover, in many of these cases, I have the impression that crustaceans such as decapods have more often been considered as insects with four or five pair of legs than for their own peculiarity. This is unfortunate for they are intriguing in their own right, especially in regard to leg coordination, a subject I will discuss in this paper.

1. Experiments with rock lobsters walking on a treadmill were undertaken to obtain information upon the system controlling the movement of the legs. Results show that the position of the leg is an important parameter affecting the cyclic movement of the walking leg. Stepping can be interrupted when the geometrical conditions for terminating either a return stroke or a power stroke are not fullfilled. 2. The mean value of anterior and posterior extreme positions (AEP and PEP respectively) of the walking legs do not depend on the walking speed (Fig. 1). 3. When one leg is isolated from the other walking legs by placing it on a platform the AEPs and PEPs of the other legs show a broader distribution compared to controls (Figs. 2 and 3). 4. Force measurements (Fig. 4) are in agreement with the hypothesis that the movement of the leg is controlled by a position servomechanism. 5. When one leg stands on a stationary force transducer this leg develops forces which oscillate with the step rhythm of the other legs (Fig. 5). 6. A posteriorly directed influence is found, by which the return stroke of a leg can be started when the anterior leg performs a backward directed movement. 7. Results are compared with those obtained from stick insects. The systems controlling the movement of the individual leg are similar in both, lobster and stick insect but the influences between the legs seem to be considerably different.

In five adult monkeys, electromyograms (EMGs) were recorded from bipolar surface electrodes positioned over the superficial masseter and from bipolar fine-wire electrodes within both the superficial and deep masseter. Relative masseter force was estimated by measuring surface bone strain from the lateral aspect of the zygomatic arch using rosette strain gauges. Multiple step-wise regression procedures demonstrated that peak values of the averaged masseter EMG could often explain a considerable amount of the variation of peak relative masseter force during mastication, i.e. r2 values ranged from 0.23 to 0.96 for the various single-electrode models and R2 values ranged from 0.78 to 0.96 for the various multiple-electrode models. The r2 values for relative masseter force and EMG data from the surface electrodes ranged from 0.69 to 0.96, and, on average, EMG data from surface electrodes provided somewhat more information about overall relative muscle force than data from fine-wire electrodes. The R2 values for a two-electrode model, consisting of data from surface electrodes over the superficial masseter and fine-wire electrodes in the posterior portion of the deep masseter, ranged from 0.78 to 0.95. The latency between the averaged surface EMG and relative muscle force was determined and the data indicated that the surface EMG usually preceded muscle force. This latency tended to decrease gradually throughout the entire power stroke of mastication. At peak values, the surface EMG preceded muscle force by about 22 ms. Towards the end of the power stroke, i.e. the 25% of peak values during unloading, muscle force may actually precede the average EMG.

The types of movement in animals and plants may be categorized as growth, change of shape, and locomotion, which is the movement of the body as a whole. Surface reaction locomotion depends on the forces applied at a surface and includes locomotion in cracks, burrows and on flat, sloping and inverted surfaces. This requires the animal or machine (hereafter collectively referred to as a mobile) to exert, against the surface, a force having a component in the direction of motion (but opposite in sign, so that the propulsive force is a backward thrust). In general, there are four kinds of force involved (including propulsive thrust) : 1) propulsion: thrust against the substrate in the direction of motion, 2) adhesion to allow anchorage for propulsive thrust, 3) adhesion to resist non-propulsive forces tending to detach the mobile from the substrate, 4) support to resist any forces tending to press the mobile against the substrate.

We would like to understand what a coupled vectorial process (CVP) is and how it works. Progress toward this goal demands that the right questions be asked and answers to them found; we should know what questions need to be answered in order that we can say that we understand how a CVP works. It is frequently suggested that coupling in active transport and other CVPs results from changing affinities for ligands (e.g., Ca2+, Na+, H+), “energized states,” and particular steps with “energy coupling” (e.g., a “power stroke”). Such concepts may be useful if they are clearly defined, but can slow progress if they give a sense that a mechanism is understood when it is not.

This muscle has some physiological properties resembling those of jaw-opening muscles. Biomechanical analysis showed that, because of the anatomical peculiarities of the feeding apparatus in caviid rodents, the action of the muscle changes from closing to opening as the mandibular condyles are protruded in the groove-like mandibular fossa. This unusual changing function may be useful in producing the ventral, medial and anterior power stroke of mastication.

The following results were obtained by earlier authors when investigating the leg coordination of walking crustaceans (Decepoda): 1) After a leg is amputated, its stump moves in anti-phase with the next posterior intact leg. This corresponds to the coordination of intact animals. The stump, however, moves in-phase with the next anterior intact leg which contrasts with the coordination of intact animals (Clarac and Chasserat, 1979; Clarac, 1981). 2) Different results have been reported for the relation between the return stroke duration and step period: some authors found a significant dependency (e.g. MacMillan, 1975), others found none (e.g. Ayers and Davis, 1977). The calculation presented here shows, that these results can be described by a model incorporating the following assumptions: A) The forces developed by both, return stroke and power stroke muscles depend upon the load under which the leg walks. B) The influences which produce the coordinating effects found by Clarac and Chasserat for amputees also exist in intact animals and their strength depends upon the intensity of the motor output of the controlling leg. Within the model the selection of protraction or retraction is made at a "central unit" which calculates a value corresponding to the sum of graded inputs from several sources. The resulting fluctuation in this value might be considered analogous to graded oscillations recorded from central non-spiking interneurons. Qualitatively the model describes similar results obtained from insects.

Of the many important concepts that have stemmed from the prolific research of Professor C. A. G. Wiersma, perhaps the one that has had the greatest impact on comparative neurobiology is that of the “command” neuron. The term has a long history of application in the invertebrates (for recent reviews, see Bowerman and Larimer, 1976a; Davis, 1976; Kennedy and Davis, 1977) and has recently found its way into the mammalian literature (Grillner, 1975). As evidenced at this symposium, however, the concept has also stirred considerable debate. Given the importance of the topic and the incompleteness of existing data on such neurons, this debate is both inevitable and desirable. As I hope to show in this chapter, such debate can help to clarify our thinking on the subject of the command neuron and can also illuminate fruitful areas for future research.

We have performed thin-section electron microscopy on muscle fibers fixed in different mechanically monitored states, in order to identify structural changes in myosin crossbridges associated with force production and maintenance. Tension and stiffness of fibers from glycerinated Lethocerus flight muscle were monitored during a sequence of conditions using AMPPNP and then AMPPNP plus increasing concentrations of ethylene glycol, which brought fibers through a graded sequence from rigor relaxation. Two intermediate crossbridge forms distinct from the rigor or relaxed forms were observed. The first was produced by AMPPNP at 20 degrees C, which reduced isometric tension 60 to 70% below rigor level without reducing rigor stiffness. Electron microscopy of these fibers showed that, in spite of the drop in tension, no obvious change from the 45 degrees crossbridge angle characteristic of rigor occurred. However, the thick filament ends of the crossbridges were altered from their rigor positions, so that they now marked a 14.5 nm repeat, and formed four separate origins at each crossbridge level. The bridges were also less slewed and bent than rigor bridges, as seen in transverse sections. The second crossbridge form was seen in glycol-AMPPNP at 4 degrees C, just below the glycol concentration that produced mechanical relaxation. These fibers retained 90% of rigor stiffness at 40 Hz oscillation, but would not bear sustained tension. Stiffness was also high in the presence of calcium at room temperature under similar conditions. Electron microscopy showed crossbridges projecting from the thick filaments at an angle that centered around 90 degrees, rather than the 45 degree angle familiar from rigor. This coupling of relaxed appearance with persistent stiffness suggests that the 90 degree form may represent a weakly attached crossbridge state like that proposed to precede force development in current models of the crossbridge power stroke.

The method consists of two parts; (a) the determination of airplane position at the end of the catapult power stroke and (b) the determination of the motion of the airplane subsequent to leaving the catapult. The method is oriented toward use of a small digital computer; however, the calculations could be performed with only the use of a desk calculator.

General principles of neuronal organization and ontogeny in locomotor systems are explored, using the lobster swimmeret system as a model. Centrally, this motor system consists of command interneurones that activate segmental ganglionic oscillators that in turn drive the swimmeret motoneurones. Several sensory receptors excite the command interneurones, and in some cases the corresponding receptive fields have been mapped on the periphery. Swimmeret motoneurones are normally recruited in order of increasing size. The motoneurones follow this size principle also in their effects on muscles (smaller motoneurones have less effect) and in their normal discharge patterns (smaller motoneurones show less adaptation).

The role of the neck in the movement of the giraffe was studied by examining motion picture sequences of wild giraffe. Unlike in the horse, the forequarters in the giraffe are stronger than the hindquarters. When beginning a galloping or a walking stride, the neck moves forward in line with the power stroke of the forelegs, thus decreasing the weight on the hind legs and moving the center of gravity of the animal forward. At the end of each stride, when the hoofs again touch the ground, the neck moves backward in order to decrease the forward momentum of the giraffe and enable him to keep his balance. When the angle of the neck on the back is plotted against time, the graph is like a sine curve. The movements of a giraffe climbing a fence, getting up from lying down and raising himself from the drinking position are considered briefly.

We propose that on binding to actin at the start of the power stroke the myosin cross-bridge takes on the rigor configuration at the actin interface. Starting from the pre-power stroke state, this can be achieved by a small movement (16° rotation) of the lower 50K domain without twisting the central β-sheet or opening switch-1 or switch-2. The movement of the lower 50K domain puts a strain on the W-helix. This strain tries to twist the β-sheet, which could drive the power stroke. This would provide a coupling between actin binding and the execution of the power stroke. During the power stroke the β-sheet twists, moving the P-loop away from switch-2, which opens the nucleotide binding pocket and separates ADP from Pi. The power stroke is different from the recovery stroke because the upper and lower 50K domains are tethered in the rigor configuration.

Biomolecular nanomotors have provided the inspiration for the design and construction of artificial nanoscale motors and machines based on several types of molecule including DNA. However, no synthetic nano-motors have yet been constructed from building blocks of protein-based material even though biomotors themselves are proteins. The HFSP smotor group (1) are in the process of developing a bottom-up approach to the understanding of biomotors by designing and constructing synthetic protein motors and numerically simulating their kinetic properties. One such concept is the “tripedal tumbleweed” motor, which is described in (1). In this context we present the results of numerical simulations for a bipedal motor with two connected peptide legs and with some of the properties of the tumbleweed motor. This motor walks on a one-dimensional track of periodically arranged binding sites. The two “feet” at the end of the legs represent different ligand-gated binding proteins which can only bind to their specific binding sites on the track when the related ligands are themselves bound to the binding proteins. The sequence of binding sites on the track is AB-AB-AB…. and the motor is powered by a temporally periodic sequence of composite washes which modulate the ligand concentrations and the leg angles. The washes cause the motor to undergo directed motion by a hand-over-hand mechanism on a track with asymmetric spacing between the AB and the BA binding sites. We will show simulation results for both two-dimensional and three-dimensional motor action of our bipedal motor which will include stepping diagrams, stall forces and first passage times for a range of parameters. This motor has the following properties observed for biomolecular motors: binding, power stroke and diffusional search. Extensions of the model will also be discussed. 1. E. Bromley et al. HFSP Journal (2009)