How Do Motoneurons Interact with Muscle Fibers?
A motoneuron is simply a nerve with a motor function. It extends out from the spinal cord to innervate skeletal muscle fibers. The drive to breathe resides in the medulla, which is located in the lower brain, while phrenic motoneurons that control the diaphragm are located in the lower cervical spinal cord. So when a patient sustains a severe injury to the upper cervical spinal cord, the connection between the two vital locations is disrupted, and the patient stops breathing. Our rhythmic pattern of inspiration means that phrenic motoneurons are active almost half the day, all day long, every day, making them some of the most active neurons in the body. “When SCI imposes sudden and total inactivity, phrenic motoneurons, diaphragm muscle fibers and the synapses between them display plasticity – each adapts to the change in activity,” explains Dr. Sieck.
