Smooth, voluntary movement is a result of a complex set of pathways working correctly and in synchronicity.  A voluntary message/action potential starts in the primary motor cortex in the brain (grey matter in the precentral gyri of the cerebral cortex of the brain) and travels along the white matter tracts down – through the subcortical fibers, brainstem, and into the spinal cord. At the level of the desired muscle, the motor message/action potential leaves the CNS (spinal cord) and relays its message to the corresponding peripheral nerve of the PNS.  The peripheral nerve then synapses with its desired muscle target, at the neuromuscular junction (NMJ), using the neurotransmitter acetylcholine. Upon Ach binding to its receptor on the skeletal muscle, electrical excitation occurs allowing for muscle contraction to happen.  Any interruption in any of these aforementioned steps will lead to problems with neuromuscular control.

Issues with the generation of  motor message/action potential generation can be caused by direct damage to the cerebral cortex,  whether by physical trauma (traumatic brain injury) or lack of blood flow (stroke).

Issues with conduction of the motor message along a nerve can be caused by loss of myelin in the white matter and/or death of the axon. This causes a ‘short circuiting’ of action potential such that the message never reaches the PNS and muscles.  Demyelination of CNS is a hallmark of multiple sclerosis whereas widespread neuronal damage of the PNS is commonly seen with diabetic neuropathy.

Problems at the pivotal neuromuscular junction can be caused by autoimmune condition which attacks the acetylcholine receptors on muscle, resulting in lack of muscle stimulation.  Myasthenia Gravis, although rare, presents as difficulties in mobility even with functional nerves and muscles since the NMJ is impaired.

Muscle damage is common with movement and is considered as muscle strain (esp with tendons) or sprain (as with ligaments that join the bones over a joint).  Muscles and tendons have an incredible ability to heal, such that impaired mobility is relatively shortlived.  However, the genetic condition muscular dystrophy has a faulty dystrophin protein in its muscles, resulting in all muscles being damaged with each use causing scarring and progressive loss of mobility.

Clinical presentation of neuromuscular disorders is wide and varied, dependent on the location and nature of the pathology along the CNS, PNS, NMJ, and/or muscles.  However, the end result is the same: impairment to voluntary movement. A highly knowledgeable team of health care providers is needed to work collaboratively to help isolate the location of the pathology and diagnose the disorder:  electroneurophysiologists being such a provider.  Similarly that team will include professionals such as orthotists to help create a workable management plan to help the patient maintain the best mobility and stability possible.

Credits

Authors: Dr. Jennifer Kong (BCIT & UBC), Valerie Swanston (UBC medical student), Mena Burr (UBC medical student), Carter Allen (UBC Nursing Student), Eva M. Su (UBC BMLSc student)

Author of questions and exercises: Carter Allen (UBC Nursing student), Avah Zamani Farahani (UBC BMLSc student), & Saba Saberi (UBC BMLSc student)

Orthotist:  Yvonne Jeffreys (Prosthetic & Orthotics, BCIT)

Vaso Obradovic, (Electroneurophysiology, BCIT)

Lindsay Mazepa,  (Electroneurophysiology, BCIT)

Videoproducer:  Lindsay Belloc (BCIT)