20. which of the following statements regarding human genetic disorders is accurate? a. nondisjunction is responsible for causing huntington's disease. b. disorders in individual genes can cause disease or provide individuals with a genetic advantage. c. disorders in individual genes are responsible for down's syndrome. d. nondisjunction can lead to an incorrect number of sex chromosomes.

the answer that best describes the relative refractory period in the neuron is the resting membrane potential is restored, but concentrations of sodium and potassium are not restored to their original state.

the refractory period is the time during which another stimulus given to the neuron will not lead to a second potential. thus, because na+ channels are inactivated during this time, additional depolarizing stimuli do not lead to new action potentials.  

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voluntary muscles are controlled by the cerebellum.