Alan Crowe

Further analyses of reflex movements in the hind limbs of the terrapin,Pseudemys scripta elegans

Summary1.Studies of the control of reflex (scratching) movements of the rear limb of the terrapin were made on spinal preparations and chronic preparations where the spine was transected at the D3–D4 levels.2.The mechanism in the spinal cord which controls this movement does not obviously compensate either for external frictional forces acting on the limb or to effects of gravity if the preparation is placed in a non-horizontal position.3.EMG patterns are not modified if an obstacle prevents the limb from reaching its target.4.Peripheral information is not continuously used to control this movement. The control mechanism is probably a generator which can adjust to different stimulus places.5.Extensor activity can be inhibited by afferent inflow from the limb.6.Generation of new cycles of movement in response to a single stimulus can be inhibited by afferent inflow.

Proprioceptive accuracy in idiopathic scoliosis.

Defects in proprioceptive Postural control have been linked to the etiology of idiopathic scoliosis. In particular, a rearrangement of the internal representation of the body has been proposed in these cases. In this study, upper–extremity proprioceptive accuracy was compared among the following groups: 1) patients with idiopathic scoliosis (n = 25); 2) subjects with nonprogressive spinal asymmetry detected by screening in school (n = 23); 3) subjects undergoing behavioral training for nocturnal enuresis (n = 17); and 4) normal subjects (n = 134). A significant inaccuracy was found among the righthanded subjects of the scoliosis and spinal asymmetry groups as compared to the normal group. It is postulated that proprioceptive dysfunction, or borderline function, is a causative factor of spinal asymmetry, which is often observed in early adolescence and which in some cases may be progressive.

Simulation of quadrupedal locomotion using a rigid body model

Locomotion of the horse is simulated using a mathematical model based on rigid body dynamics. A general method to generate the equations of motion for a two-dimensional rigid body model with an arbitrary number of hinge joints is presented and a numerical solution method, restricted to tree-structured models, is described. Joint movements originating from muscular forces or moments are simulated, but the method also allows that parts of the model follow strictly the pattern of kinematic data. Moment-generators with first-order linear feedback were used as a rotational muscle-equivalent. Ground-hoof interaction forces are approximated by a viscoelastic model and pseudo-Coulomb friction in vertical and horizontal directions respectively. Results of model simulations are compared to experimentally recorded data. Subsequently, adjustments are made to improve the agreement between simulation and experimental results.

PROPRIOCEPTIVE ACCURACY IN TWO DIMENSIONS

Slow arm movements were made over a smooth horizontal table at shoulder height. With visual cues excluded, target position was indicated by the index finger of the nonmoving arm touching the underside of the table. 11 students (mean age 21.9 yr.) and 24 children (mean age 10.3 yr.) were compared. Both groups showed an ‘overlap effect’: movements with the right hand went too far to the left, while movements with the left hand went too far to the right. The children as a group were significantly less accurate than the students and showed a significant asymmetry in that movements with the dominant hand were more accurate than those with the nondominant hand.

Active tension changes in frog skeletal muscle during and after mechanical extension

Isolated frog sartorious muscle at 4°C has been used to study the phenomenon whereby tetanically stimulated muscle, subjected to a mechanical extension, yields an active tension which is greater than that obtained during an isometric contraction in which the muscle is stretched prior to stimulation. The excess tension magnitude and decay with time depend upon the total muscle length. After stimulation ceases the excess tension does not persist over a time appreciably longer than that needed for the normal isometric tension to disappear. No persistent excess tension could be seen from stretches applied after stimulation had ceased but before the active tension had fully decayed.

Studies on the excitability of the central program generator in the spinal cord of the terrapin Pseudemys scripta elegans

We present observations on the multicyclic scratch reflex in spinal terrapins as produced by electrical stimuli applied to the shell at the specific regions at which a mechanical stimulus produces the reflex. EMGs and hip and knee movements are recorded. The responses to the electrical stimuli are similar to the responses to mechanical stimuli. There is a three phase EMG pattern (Stein and Grossman, 1980), to which the movement pattern is related (Bakker and Crowe, 1982). A response may consist of a series of up to 25 movement cycles with a total time course of up to about 30 sec. The initial cycles of a response are relatively fast (less than 1 sec), but the cycles at the expiration of the response may have a duration of 2-3 sec. A single electrical stimulus pulse is often insufficient to trigger a series response. Instead, a weak EMG burst of a few tenths of a second duration, together with a slight movement, is often seen. However, a second pulse can set the cycle series in motion even after an interval of 40 sec between the pulses. A further booster stimulus pulse given while a reflex response is taking place can increase the speed of the movement. If the booster pulse is given just after cessation of reflex activity it can restart the activity, but this second cycle series is often shorter than the first one. The results indicate that the excitability of the central program generator is not constant. Long duration changes in the excitability are produced within the spinal cord.

Simulation studies of contracting skeletal muscles during mechanical stretch.

A form of the sliding filament model is presented to simulate the experimentally observed phenomena when a contracting muscle is subjected to mechanical stretches. It is assumed that the cross bridges can be extended to provide extra tension and that they can be broken if a critical limit of extension is exceeded. Passive components, including non-linear parallel elasticity are incorporated in the model. Such effects as short-range stiffness, the slip effect and excess tension after mechanical stretch can be simulated. The simulations are compared with our own experimental tests on frog sartorious muscle and with previously reported results from other muscle preparations.

Information transmission in non-visual fingertip matching along a horizontal track in the median plane

Non-visually triggered arm movements over a horizontal table at shoulder height were analysed by an Information Theory approach according to a method suggested by Sakitt et al. (1983) and Sakitt (1980). The movement track was along the subjects median line and was indicated by a vertical metal ridge fixed to the table. The observer passively moved the subjects left index finger along the left side of the ridge to the target position. The blindfolded subject then had to move his right index finger along the right side of the ridge to match the left finger position. Direct contact between the two fingers was prevented by the ridge. We compared our results, which involve the transmission of information through the arm and shoulder joints of both arms, whith those of Sakitt et al. which involved just one elbow joint. We supplemented our experimental results with simulations and show that the value for the transmitted information, obtained using the method of analysis suggested by Sakitt et al., is very dependent upon the number of trials, and number and spacing of the targets. Sakitt et al. suggest that the Information Theory approach permits easy comparison between different tasks and different observers. Our results suggest that comparisons should be made with caution.

Multichannel transmission of proprioceptive input to motoneurons

A model has been developed to simulate the parallel channels of muscle spindles and their monosynaptic connections to a homonymous motoneuron in the turtle. Input to the model is muscle length and β stimulation, output is motoneuronal membrane potential. Quality of transmission is greatly dependent upon dispersive properties of the system. The contributions of different dispersive features are compared and also cumulative effects are considered. Reference is made to conditions which are found in actual movements.

Simulation of chelonian hind-limb reflex movements

A model of the hind limb of the terrapin, devoid of sensory feedback, but which is capable of producing realistic reflex movements is presented. It is shown that very small adjustments of the activation pattern of the muscles (the input of the model) are sufficient to correct the movement for different starting positions or to different targets. Mechanical disturbances of the movement can also be simulated. Comparisons with experimental tests with the same sorts of disturbance were done to try and determine if the real system possesses feedback which tries to adjust to the disturbance. Since the simulations of disturbed movements predict fairly well the experimental movements we are drawn to the conclusion that the movement takes place by means of a pattern generator and no compensation against disturbances is present.