Answer:
a. Brain and spinal cord
Step-by-step explanation:
The brain and spinal cord are essential components of the nervous system and damage to any of the two can cause many problems in reasoning, balance, stability of a body, among others.
Russel probably has a disease associated with damage to the brain and spinal cord that does not allow his body to stabilize, causing constant tremors in his limbs. In addition to causing cognitive defects and failures in cognitive and motor activities.