Muscle paralysis is usually a very severe symptom related to diseases of neural or muscle system. To understand the causes of paralysis, we need to know more about will-controlled movement. The process begins in neurons of the cerebral cortex, the signal is transmitted through brain into spinal cord and peripheral nerves (exiting spinal cord) lead the signal towards muscle fibers. The transmission of information from nerve to muscle happens in the so-called neuromuscular junction. Neuroelectric signal causes certain local chemical changes leading to muscle irritation and contraction.
The basic scheme of signal transmission is: brain => spinal cord => peripheral nerves => neuromuscular junction => muscle. Damage of any part of this pathway leads to failure of signal transmission, which is perceived as a paralysis of certain muscle or muscles.
Damage to brain tissue caused by virtually any process can lead to disruption of muscles. This includes in particular stroke, brain trauma, brain infections (such as tick-borne encephalitis), brain metastases, brain tumors, and multiple sclerosis. Congenital disorder of the brain related to paralysis is present in cerebral palsy.
Spinal cord disorders
Spinal cord injuries, multiple sclerosis (that damages both the neurons of brain and spinal cord), spinal disc herniation, spinal cord tumors, spina bifida – these all can cause muscle paralysis. Relatively rare but fatal is disease known as amyotrophic lateral sclerosis that is related to necrosis of brain and spinal cord neurons.
Peripheral nerves disorders
Peripheral nerves can be directly damaged by accidents or medical procedures. This is typical especially for nerves controlling the vocal cords during operations in neck area (like thyroid gland operations). Other causes of nerve damage fall into group of polyneuropathies. Polyneuropathy usually does not lead to significant locomotive disruption; it is rather associated with unpleasant tingling, burning or pain in the peripheral parts of the body. A rare acute polyneuropathy known as Guillain-Barré syndrome causes progressive paralysis.
Note: The most common and fortunately only temporary paralysis of nerves is caused by local effect of longer physical pressure, most often during sleep. After waking up, one can not move the affected limb but the condition is fortunately self-limiting. Each of us has certainly experienced this few times.
Neuromuscular junction disorders
Neuromuscular junction is affected for example in Myasthenia gravis or Lambert-Eaton syndrome. Some toxins have also negative effect on neuromuscular transmission such as snake poisons, curare, botulinum toxin, etc.).
The term paralysis is often understood as total loss of voluntary muscle control but this condition has more forms that differ in severity. "Paresis" means partial paralysis when voluntary control is partially preserved, while "plegia" is a total loss of voluntary locomotion in certain muscle or muscle group. We use term paraplegia / paraparesis in paralysis of lower limbs, hemiplegia / hemiparesis in paralysis of the left or right half of the body and quadriplegia / quadruparesis in paralysis of all limbs. Paralysis of ocular muscles results in double vision, muscle paralysis of a vocal cord causes hoarseness (and shortness of breath by damage of both vocal cords nerves) and respiratory muscle paralysis is related to severe shortness of breath.
If there is paralysis present, neurological examination should be performed. According to physical examination including evaluation of muscle strength and reflex reactions the neurologist can often exactly determine the type and location of damage to the neuromuscular pathway. Imaging methods such as computed tomography and magnetic resonance imaging are valuable to display both brain and spinal cord. Transmission of electric signal in peripheral nerves and in the neuromuscular junction can be examined and evaluated by electromyography.