Answer:
d. neurotransmitters that excite or inhibit a neural signal across a synapse
Explanation:
Neurotramniters are molecules that transmit chemical messages from nerve cells by traveling across the synapse to target cells. Some of the most important neurotransmitters include acetylcholine, norepinephrine, dopamine, gamma-aminobutyric acid (GABA), glutamate, histamine and serotonin. Norepinephrine is a neurotransmitter synthesized from dopamine which is involved in the regulation of stress response, sleep, and blood pressure. Norepinephrine, also known as noradrenaline, is an important neurotransmitter involved in stress response and exercise. Finally, acetylcholine is a neuromodulator of the autonomic nervous system involved in attention responses, memory and motivation.