This is my friend, Bertie. He’s my brother from another mother. His mum and my girl are the greatest of friends, so naturally we are too. As you can see, Bert is in a wheelchair. He accidently herniated a disc in his spine, which compressed the spinal cord and caused weakness in his hind limbs. Luckily though, Bert has been diligent with his physical rehabilitation and he’s getting better every day. Unfortunately, some spinal cord injuries (SCI) are more severe than this and leave patients unable to produce voluntary movements at all. Thankfully, there are scientists working very hard to help those patients with SCI’s who experience minimal benefits from typical physical therapies.
Spinal cord injuries can disrupt the communication between the brain and the rest of the body. Depending on the severity, this can lead to neurological dysfunction, such as changes in muscle strength, loss of sensation, and can even affect bodily functions such as bladder control and blood pressure regulation. SCI’s can be classified as complete, meaning that all sensation and the ability to control movement is lost below the level of the spinal cord injury, or incomplete, where some motor control and sensory function is preserved. Typically, the main medical practices available to help restore physical functioning are activity-based rehabilitation programs, but not all SCI patients respond well to these therapies.
This prompted the investigation into alternative approaches, such as using exoskeletons to support body weight and using electrical stimulation of the nerves and spinal cord to help patients maintain active movements during training. Maintaining these active movements during training helps engage the nervous system and promotes the reorganization of the pathways involved in movement. One technique, proven to be beneficial in rats and monkeys with SCI’s, is electrical stimulation of the spinal cord. Specifically, epidural electrical stimulation (EES), which is the application of a current to the spinal cord that helps produce muscle activity. The theory behind this technique is that it allows the brain to exploit any pathways that may still be functional to produce movements in the paralyzed limbs.
But how does it work?
There are circuits—a collection of interconnected neurons—located in the spinal cord that use sensory information, such as information about how a muscle is stretched, to create rhythmic patterns of movement. These circuits are called Central Pattern Generators (CPGs) and they are involved in rhythmic actions such as walking, swimming, and even breathing. As we take a step, the muscles of the trailing leg that are used to swing the leg forward (hip flexor muscles) are stretched. This activates specialized sensory receptors in the hip flexor muscles called muscle spindles, which cause the hip flexor muscles to contract and swing the leg forward to take another step. Similarly, there are specialised receptors (Golgi Tendon Organs, GTO’s) that detect muscle force and load information, such as how much body load is applied to the ground. As we take a step forward and shift our weight onto the leading leg, the GTO’s of that leg detect an increase in load. This causes the muscles of that leg to contract to ensure that the limb doesn’t collapse under the extra weight. This happens at each step cycle and results in the rhythmic pattern of walking. What’s incredible is that we seem to do this automatically and don’t have to concentrate on each leg as it moves.
A recent study (2018) found that applying EES to the spinal nerves carrying sensory information into the spinal cord can mimic normal sensory input (e.g., muscle length and limb position) and activate the circuitry involved in movement. That is, the stimulation can indirectly engage motor neurons in the spinal cord that connect to the muscles of the legs. This study recruited individuals who sustained a SCI more than four years ago and had permanent motor deficits or complete limb paralysis despite participating in extensive rehabilitation. They used targeted electrical stimulation to enable voluntary control of walking in these patients. During over-ground training, participants used a gravity-assist device which helped support body weight. The EES was then applied in real time to ensure that the motor neurons were activated at the right time during the intended movement. This precise timing helps maximize the effect of the stimulation by ensuring the muscles are activated at the right time to mimic the rhythmic pattern of walking. For instance, the muscles of the weight-bearing leg need to be activated to support the body’s weight during the walking cycle.
What were the outcomes of this research?
In one week, the patients already showed improvements in their locomotor performance during the rehabilitation. More impressively, after only a few months, participants could voluntarily walk or cycle with the assistance of EES and had regained voluntary control of paralyzed muscles, even without stimulation. Thus, combined with the gravity-assist device and the over-ground training, EES promotes the reorganization of spared spinal cord pathways and improves walking in patients with SCI’s. This is an incredible advancement in the field of neuroscience and physical rehabilitation and is hopefully an indication of more great things to come.
Thanks to a team of dedicated specialists and a suitable rehabilitation program, Bert is still living his best life. However, without research to help us better understand how the nervous system controls movement, and how we may bypass deficits that are present, these amazing feats may not be possible. Fortunately, there are dedicated researchers and clinicians who are committed to this cause and to improving the quality of life of thousands of people, and pups, with SCI’s.
Video and photo credits go to my friend, Bertie, and his incredible mum, Amy Robinson. You can follow their adventures on Instagram @bimmersandbertie.
Wagner, F. B. et al. Targeted neurotechnology restores walking in humans with spinal cord injury. Nature 563 (7729), 65-71 (2018).