<span>The best describes the first reaction the neuron has is </span>Ions flow into the neuron.
When a neuron sense a stimulus, it will open ion canal and allow ion to flow into neuron causing a change in membrane potential. This will activate a cascade of reaction which needed to sent message to other neurons.
The levels of cell organization go
1. cells
2. tissues
3. organs
4. organ systems
5. organisms
so the answer is D, organ level
