Biomedical subjects

Tom Matheson

Publications and source records attributed to Tom Matheson.

5 recordsLinked to original sources

PubMed2006

Co-contraction and passive forces facilitate load compensation of aimed limb movements.

Vertebrates and arthropods are both capable of load compensation during aimed limb movements, such as reaching and grooming. We measured the kinematics and activity of individual motoneurons in loaded and unloaded leg movements in an insect. To evaluate the role of active and passive musculoskeletal properties in aiming and load compensation, we used a neuromechanical model of the femur-tibia joint that transformed measured extensor and flexor motoneuron spikes into joint kinematics. The model comprises three steps: first, an activation dynamics module that determines the time course of isometric force; second, a pair of antagonistic muscle models that determine the joint torque; and third, a forward dynamics simulation that calculates the movement of the limb. The muscles were modeled in five variants, differing in the presence or absence of force-length-velocity characteristics of the contractile element, a parallel passive elastic element, and passive joint damping. Each variant was optimized to yield the best simulation of measured behavior. Passive muscle force and viscous joint damping were sufficient and necessary to simulate the observed movements. Elastic or damping properties of the active contractile element could not replace passive elements. Passive elastic forces were similar in magnitude to active forces caused by muscle contraction, generating substantial joint stiffness. Antagonistic muscles co-contract, although there was no motoneuronal coactivation, because of slow dynamics of muscle activation. We quantified how co-contraction simplified load compensation by demonstrating that a small variation of the motoneuronal input caused a large change in joint torque.

Animals↗

PubMed2004

A posture optimization algorithm for model-based motion capture of movement sequences.

We have developed and evaluated a new optical motion capture approach that is suitable for a wide range of studies in neuroethology and motor control. Based on the stochastic search algorithm of Simulated Annealing (SA), it utilizes a kinematic body model that includes joint angle constraints to reconstruct posture from an arbitrary number of views. Rather than tracking marker trajectories in time, the algorithm minimizes an error function that compares predicted model projections to the recorded views. Thus, each video-frame is analyzed independently from other frames, enabling the system to recover from incorrectly analyzed postures. The system works with standard computer and video equipment. Its accuracy is evaluated using videos of animated locust leg movements, recorded by two orthogonal views. The resulting joint angle RMS errors range between 0.7 degrees and 4.9 degrees, limited by the pixel resolution of the digital video. 3D-movement reconstruction is possible even from a single view. In a real experimental application, stick insect walking sequences are analyzed with leg joint angle deviations between 0.5 degrees and 3.0 degrees. This robust and accurate performance is reached in spite of marker fusions and occlusions, simply by exploiting the natural constraints imposed by a kinematic chain and a known experimental setup.

Algorithms↗

PubMed2003

Cell biology in the Antarctic: studying life in the freezer.

Many classical biologists working in Antarctica have 'gone molecular' to study the physiological basis for life on or below the ice. Investigating the remarkable adaptations of specific cells, organelles and molecules to this extreme environment can provide new perspectives on the processes studied in conventional experimental organisms.

Animals↗

PubMed2003

Load compensation in targeted limb movements of an insect.

The task of a multi-jointed limb making an aimed movement towards a target requires that the movement is regulated against external perturbations such as changing load. In particular, loading one part of a limb leads to altered static forces on all proximal segments, and to additional dynamic joint interaction forces when the limb moves. We have addressed the question of load compensation in an insect preparation in which a locust makes aimed scratching movements with a hind leg in response to tactile stimulation of a wing. We show that loading the femur or tibia with the equivalent of 8.5 times the mass of the tibia (corresponding to an increase of up to 11.6 times the rotational moment of inertia at the femur-tibia joint) does not impair the animal's ability to make well-coordinated, aimed movements of that leg towards different targets. The kinematics of the movements are the same, and animals aim the same part of their distal tibia at the target, regardless of loading. The movements are carried out with equal accuracy and at the same initial velocity under all load conditions. Because loading of the leg does not change the behavioural performance, there is no indication of a change in aiming strategy. This implies high leg joint stiffness and/or the existence of high gain proprioceptive control loops. We have previously shown that in the unloaded condition, movements elicited by stimuli to different places on the wing are driven by a single underlying movement pattern that shifts depending on stimulus location along the wing surface. Our present data show that leg proprioceptive inputs are also integrated into the leg motor networks, rendering hind limb targeting robust against large changes in moment of inertia.

Animals↗

PubMed2002

Metathoracic neurons integrating intersegmental sensory information in the locust.

This paper describes the morphology and physiology of five types of local interneurons and three types of ascending intersegmental interneurons in the locust metathoracic ganglion that are points of convergence of sensory information from the wings. Four types of spiking local interneurons are members of a population with somata at the ventral midline. They are depolarised by stimulation of a metathoracic wing nerve, suggesting that they encode a sensory representation of this appendage. Some are also depolarised with short latencies following stimulation of a mesothoracic wing nerve, indicating that they collate intersegmental as well as local information. All the local interneurons have branches in the anterior ventral association centre or around the roots of the nerve that carries wing sensory neurons. This distinguishes them from other interneurons in the population. A fifth type of local interneuron that has unusual bilateral branching and is not a member of this population is described for the first time. The ascending interneurons are members of three populations. Neurons of each population have a characteristic pattern of responses to stimulation of the mesothoracic or metathoracic wing nerves, and some respond to tactile stimulation or movements of a hind leg. These latter interneurons thus collate information from both wings and legs. All three types of intersegmental interneurons have branches in the anterior ventral association centre or around the roots of the wing nerve. The responses of the interneurons described here shed new light on both local and intersegmental network function in this model system.

Animals↗