Breathing. It happens so easily and so naturally that most of us take it for granted. The rhythmic movement that sends oxygen into our bloodstream is the rhythm of life. However, for more than 5,000 new cervical SCI patients each year, the precious gift of breath is made possible only with the aid of a mechanical ventilator. The descending connections are disrupted between the brain areas that transmit the signal to breathe and motor neurons in the spinal cord that cause the diaphragm muscle to contract and pump air into the lungs. Yet, there is hope.

At Mayo Clinic, physiologists Gary C. Sieck, Ph.D. and Carlos B. Mantilla, M.D., Ph.D., are working together to improve the efficacy of spared connections between the brain and spinal cord. This area of research is called neuroplasticity, which refers to the brain’s natural ability to strengthen synapses in order to adapt or — in this case — compensate for injury.

“In most patients with cervical SCI, the communication between the respiratory pattern generator in the brain and the spinal cord motor neurons that effect diaphragm contraction is not completely disrupted,” says Dr. Sieck. “If these remaining connections can take over the work of those that were lost, rhythmic activation of the diaphragm muscle may be restored, which means patients on mechanical ventilators could learn to breathe on their own.”

Neuroplasticity and functional recovery of breathing

All muscles are controlled the same way. They have a motor neuron, which makes the muscles move by making contact with a group of muscle fibers. Collectively those muscle fibers affected by one motor neuron are called motor units. Every time a motor neuron becomes active, it activates a group of muscle fibers — the motor unit. This is how the nervous system controls muscles. For almost 30 years, Dr. Sieck — a Mayo Distinguished Investigator — has studied the neural control of the diaphragm muscle, the most important muscle involved in breathing. His more than 150 published papers have characterized the basic elements of neural control called motor units.

“Since the time of Charles Sherrington (British physiologist and Nobel Laureate), it has been known that motor units are the building blocks for neural control of muscles. A motor unit is a single motor neuron in the spinal cord and the group of muscle fibers it innervates. In the diaphragm muscle, like other skeletal muscles, motor units range in their mechanical properties. There are slow twitch motor units that are very fatigue resistant and are ideally suited to accomplish breathing. There are also fast twitch motor units that are more fatigable and thus more appropriate for shorter duration activation of the diaphragm as occurs during sneezing and coughing. The nervous system recruits these motor units to accomplish a range of motor behaviors.”

The mechanisms underlying the development and maintenance of the different types of motor units in respiratory muscles are not well understood. Drs. Sieck and Mantilla also study motor unit diversity that is necessary to accomplish the range of motor behaviors of the diaphragm during breathing, swallowing, and speech. They are investigating the development of respiratory motor units before and after birth — a critical time when breathing muscles transition from the in utero conditions of rest to the air environment, where they must rhythmically and continuously contract for a lifetime.

Brain derived neurotrophic factor

Basic research on neuroplasticity and functional improvement in brain areas responsible for memory and learning has implicated a class of growth promoting compounds called neurotrophins. Neurotrophins are naturally produced chemicals that can be released either by neurons or surrounding cells (such as glial cells) in response to normal stimuli or injury. Although neurotrophins exist naturally within the nervous system, they can also be added artificially as drugs.

“In our studies, we’ve shown that we can strengthen neuromuscular synapses by exogenous treatment with neurotrophins,” says Dr. Sieck. “These substances include brain derived neurotrophic factor (BDNF) and neurotrophin 4 (NT–4), both of which act on a common TrkB receptor, which stands for tropomyosin related kinase.

Our studies have shown that the TrkB receptor is present in phrenic motor neurons and that its expression changes in response to SCI. This is important because these neurotrophins and their TrkB receptor are implicated in neuroplasticity and strengthening synaptic transmission.”

In order to see if neurotrophins are effective in ultimately restoring breathing function, Drs. Sieck and Mantilla conducted animal studies in which a catheter was inserted into the space surrounding the spinal cord.

“We do similar procedures during spinal blocks in anesthesiology,” says Dr. Mantilla. “Both anesthesiologists and pain medicine specialists, routinely perform these procedures during surgery and in patients with chronic pain. In the operating room we infuse anesthetics that cause the patient to become numb so surgery can be performed. In patients with chronic pain, we infuse substances that can provide pain relief while minimizing side effects. In our animal studies, we infused BDNF and NT4 into the cervical spinal cord after SCI. We found that BDNF and NT4 treatment markedly enhanced recovery of rhythmic phrenic nerve and diaphragm muscle activity after SCI. Conversely, if we blocked TrkB receptors at phrenic motor neurons, using pharmaceuticals, a blocking antibody or a silencing RNA to reduce TrkB receptor expression in phrenic motor neurons, we found that spontaneous recovery of breathing after SCI was significantly delayed or completely blocked.”

“To date, no one has used this method to promote functional recovery of breathing after cervical SCI,” says Dr. Sieck. “Our study is the first to show that neurotrophins can enhance functional recovery of breathing. So, we’re not going to make SCI patients walk again, but if you ask these patients the one thing they would like to have back in their life — it would be the freedom to breathe on their own.”

But neurotrophin treatment may have significant side effects, most notably increased pain. “Although we are very excited about our results so far, we recognize that intrathecal neurotrophin treatment is not the final answer,” says Dr. Mantilla. “More recently we’ve explored whether we can enhance TrkB receptor expression in phrenic motor neurons using gene therapy. Such an approach would enhance neurotrophin signaling in phrenic motor neurons while avoiding the negative side effects.”

“Our evidence so far clearly indicates that the neurotrophins and signaling through the TrkB receptor may be involved in promoting functional recovery of breathing in SCI patients,” says Dr. Sieck. “For us, that’s very exciting because it presents a therapeutic strategy that might be important in the future.”

— Nick Charles, February 2009