Ans. is 'b' i.e., Dopamine is neurotransmitter Peripheral chemoreceptors The peripheral chemoreceptors which regulate respiration are located in the carotid body and aoic bodies. These bodies are located in the connective tissue associated with the vessel wall, at the bifurcation of the common carotid, and on the arch of aoa, respectively. The characteristic cells of both these structures are called glomus cells. Type I glomus cells have a high dopamine content, which they possibly employ as a neurotransmetter. Peripheral chemoreceptors convey information to the DRG of neurons in medulla, for which purpose the afferent neurons from the carotid bodies pass through glossopharyngeal nerve and from the aoic bodies pass through vagus nerve. The most potent natural stimulus for peripheral chemoreceptors is low aerial P02 (hypoxic hypoxia). The other stimuli which activate peripheral chemoreceptors are high aerial PCO2, and an increase in aerial hydrogen ion concentration (acidosis or low pH). The response to activation of peripheral chemoreceptors is an increase in pulmonary ventilation through an increase in the rate and depth of breathing. The blood flow to peripheral chemoreceptors, on per unit mass basis, is the highest to any tissue in the body. With a blood flow of 2000 ml/min/100 gm tissue, the carotid and aoic bodies, inspite of their high metabolic rate, hardly remove any oxygen from the blood supplies to them. That is why the aeriovenous oxygen difference is negligible in these bodies. Thus these structures are idealy built to sense change in aerial P02. It is good to keep in mind that stimulation of peripheral chemoreceptors not only stimulates medullary respiratory centers but also medullary vasomotor center. Therefore, the response to their stimulation is tachycardia, vasoconstriction and increase BP along with hyperventilation.