Question: how does nerve degeneration cause you to shake uncontrollably surely it should cause paralysis in parts of the body not shaking if the nerves aren't connecting properly? also would the environment in space cause this or cause it to happen less than on earth?

Nina Rzechorzek answered on 20 Jun 2019: last edited 20 Jun 2019 9:17 pm

This is a brilliant question, and a great answer from Marianne. Sometimes we also see excessive shaking in neurodegenerative disorders that are typically associated with slower movements (e.g. Parkinson’s disease) due to a side effect of one the main drugs used to treat it (levodopa). Neurons that normally produce dopamine in the midbrain to enable movement are the neurons that degenerate and die in Parkinson’s disease. Levodopa is a drug that boosts the dopamine levels in the brain but can cause dopamine spikes leading to ‘overactivity’ of this part of the brain and therefore, paradoxically, too much movement. The abnormal movements caused by levodopa are separate from the ‘Parkinson’s tremor’ typically observed in patients at rest. Some neurodegenerative disorders can affect neuronal networks in the brain to the extent that epileptic seizures develop and if these are convulsive seizures, the patient will be observed having ‘fits’ with uncontrollable rhythmic movements. The key thing to remember is that the motor pathways of the nervous system (from the motor cortex in the brain, through the midbrain, down to the brainstem or spinal cord, and into peripheral nerves and muscles) have a carefully balanced set of ‘movement promoting’ and ‘movement preventing’ connections. This is really important for being able to control movement and balance – if all of the connections were positive all the time, we would never stop moving! So it is quite possible to mess up this balance with degeneration of certain connections in the pathways. Generally the last part of the pathway (the peripheral nerves that connect the central nervous system to the muscles) are referred to as the ‘lower motor neuron’ system and this acts to promote muscle contraction and movement – if this part degenerates we see less movement and loss of muscle mass and tone (‘flaccid’ limbs’). By contrast, the motor pathway neurons higher up the chain of command in the central nervous system (brain and spinal cord, the ‘upper motor neurons’) usually act to inhibit the lower motor neurons to control movement. So, when only the upper motor neurons are lost or damaged, we generally see increased/exaggerated movement and increased muscle tone (‘spasticity’). There is some evidence that the anti-gravity experienced in space can have negative effects on the nervous system, which might increase or decrease the capacity for uncontrollable movement, depending on which part(s) were affected most:
https://www.nasa.gov/mission_pages/station/research/experiments/explorer/Investigation.html?#id=240