Search

In the present study, we examined the correlations between the recovery of stepping obtained with intrathecal Brain Derived Neurotrophic Factor (BDNF) delivery via mini-pump to the lumbar spinal cord and the firing of the lumbar spinal interneurons in a feline model of Spinal Cord Injury (SCI).

In spinal cord injury rehabilitation, one of the main methods used involves re-training the patients' spinal circuitry to produce movement. Located in the spinal cord, the central pattern generator (CPG) is the term used to describe the neural circuitry which drives locomotion within vertebrates. Understanding the composition and architecture of the CPG will assist the development of rehabilitation approaches for spinal cord injury.

A closed loop stiffness regulation system is incorporated into a neuroprosthesis for hand grasp for quadriplegic patients. The open loop neuroprosthesis controls the open loop channels of stimulation while a digital computer controls the closed loop channels. An automated tuning procedure adjusts the parameters of the system to provide a stable and rapid response under isometric, compliant, and unloaded conditions.

Conventional wisdom is that proximal muscles will have larger motor units. However, a mechanical perspective suggests that precise control is more important for proximal than for distal muscles. Our study uses an established model of cat hindlimb to study how single motor units may affect force and acceleration at the toe. Motor unit counts were obtained from published data. Moment arms and inertia matrices were used to calculate joint isometric torques, and force or accelerations at the toe. A planar analysis was shown to be a valid approximation (errors < 5%) for study of planar forces in the 7 d.o.f. cat hindlimb model. Relative changes in force or acceleration were generally < 6% per motor unit. Motor unit forces acted along several different directions. In contrast, with the foot in the central workspace, accelerations from different muscles tended to fall along two distinct lines as a result of the effect of the inertia tensor. Percentage changes in muscle force as a result of single motor units were on the average smaller in proximal muscles. We conclude that there is finer-grained control of proximal muscle force, perhaps as a result of motor units of similar sizes in all muscles.

Using a biomechanical model of the cat hindlimb, we studied patterns of endpoint forces created by all muscle combinations of fourteen selected muscles, and compared them to the force patterns produced by intraspinal microstimulation of the lumbar spinal gray matter. We ran the model with two different activation schemes for the muscles. The first run used combinations of the fourteen selected muscles stimulated at the same level of activation. The second run used combinations where muscle forces were normalized to produce the same maximum end-point force. These results were compared to force field patterns obtained experimentally during intraspinal microstimulation. Although there were slight variations in the force patterns produced, both methods converged to four dominant patterns. When muscles in the model were normalized, some force patterns were found that were not observed experimentally. These results show the significance of specific levels of muscle activation to the production of the experimental patterns.

We studied the structure of endpoint forces produced by microstimulation of the cat spinal cord, and from combinations of muscles using a biomechanical model of the cat hindlimb. The forces evoked by microstimulation were of four types. At some stimulation sites, the force patterns exhibited a point of convergence where the active endpoint force was zero. The endpoint forces produced by activating combinations of muscles in the biomechanical model were of types similar to the experimental ones, although they demonstrated points of convergence in locations not observed experimentally. These results suggest that the spinal circuitry uses a subset of the possible muscular combinations.

Recently, methods have been developed that can 'cage' neurotransmitters, limiting their ability to bind with a receptor and rendering them biologically inert until photolytic uncaging. We explored uncaging of glutamate with implanted fiber optics as a stimulation technology. The spinal cord was bathed in caged glutamate at different concentrations, and a photolysis unit projected brief, spatially concentrated bursts of light into the lumbar spinal cord. Forces generated at the ankle were measured in 3 dimensions. Responses were tested at discrete depths in the lumbar cord, with the strongest responses located in the 900 to 1100 /spl mu/m range. Our results indicate feasibility of this approach for engineering a prosthesis. The advantage of this technology is that excitation, inhibition, and modulation of spinal circuits can be controlled. This technique, when used in a paralyzed individual, would allow greater freedom of movement and independence.

The authors studied the mapping and structure of endpoint forces produced by microstimulation of the cat spinal cord. The results demonstrate that stimulation in the dorsal and intermediate aspects of the spinal cord generated organized, convergent force patterns, while stimulation in the ventral aspect of the cord did not. These results suggest that electrical activation of higher-order neurons can be used to coordinate the activation of multiple muscles in multi-joint movements.

An active, closed-loop stiffness regulation system is being incorporated into a neuroprosthesis for hand grasp for quadriplegic patients. The stiffness regulator controls the activation of the thumb in lateral grasp and the fingers in palmar grasp. The opposing digits are fixed in place by an open-loop system that operates in parallel. A single continuous input specifies the size of grasp opening and the grasp force under a wide range of loading conditions. The parameters of the system can be tuned to specify the static input-output properties of the grasp as well as a stable and rapid response under isometric, compliant and unloaded conditions. >

The authors investigated the feasibility of using functional neuromuscular stimulation (FNS) to control wrist angle in C5 tetraplegics. They measured the moments available from the wrist muscles, the moments produced at the wrist by the finger muscles during grasp, and the passive properties of the wrist joint. Furthermore, the authors evaluated three wrist position controllers. They found that the passive and finger muscle moments are significant compared with the moment produced by single wrist muscles and will affect the maximal range of wrist angle obtainable and the stability of the wrist during grasp. Feedback regulation would provide increase wrist stability in the presence of disturbances, with possibly less wrist muscle activation. >

Since individuals with C5/C6 tetraplegia lack voluntary control of the forearm pronators, the authors evaluated the feasibility of restoring forearm pronation/supination control using an electrically activated pronator opposed by voluntary supination. To this end, the authors measured the electrically produced pronation moments of subjects with tetraplegia. The maximal pronation moment achieved by stimulating the pronator quadratus ranged from 30-100 N/spl middot/cm in 3 forearms of 2 subjects. Voluntary control of pronosupination during constant pronator stimulation was achieved by having the subject voluntarily supinate or relax to change the balance of rotational torques acting on the forearm. In all cases, the subjects were able to supinate voluntarily against the continuously stimulated pronator, producing intermediate angles between full pronation and full supination. The authors also observed under some conditions that subjects could voluntarily pronate and supinate even without pronator stimulation. Using a biomechanical model, the authors show how pronation can be initiated from a supinated position using the brachioradialis, with gravity completing the pronation. This method of pronation without stimulation is extremely sensitive to the orientation of the forearm in the gravitational field, and thus is not a widely applicable technique.

We studied the mapping and structure of endpoint forces produced by microstimulation of the cat spinal cord. The forces evoked by microstimulation varied in magnitude and direction as a function of limb configuration. At some stimulation sites, the force patterns exhibited a point of convergence where the net endpoint force was zero. Ipsilateral stimulation in the dorsal aspect of the cord evoked flexion responses that exhibited a convergent point, while extension responses evoked by ipsilateral stimulation were not convergent. Conversely, contralateral stimulation in the dorsal aspect of the cord evoked extension responses that exhibited a convergent point, while flexion responses evoked by contralateral stimulation were not convergent. Stimulation in the ventral aspects of the cord, as well as intramuscular stimulation of single muscles did not produce convergent force patterns. The results demonstrate that stimulation in the dorsal and intermediate aspects of the spinal cord can generate organized, convergent force patterns, while stimulation in the ventral aspect of the cord can not. These results suggest that electrical activation of higher-order neurons may be used to coordinate the activation of the multiple muscles required in multi-joint movements.