Chemical messengers are involved in communication in maintaining homeostasis in an animal’s body, and in the body’s response to various stimuli.
The development of most animals commences with fertilization and the subsequent division of the zygote. Further development then depends on continued cell proliferation, growth, and differentiation. The integration of these events, as well as the communication and coordination of physiological processes, such as metabolism, respiration, excretion, movement, and reproduction, depends on chemical messengers — molecules that specialized cells synthesize and secrete.
Chemical messengers can be categorized as follows: Fig. 3.1
- Local Chemical Messengers:
Many cells secrete chemicals that alter physiological conditions in the immediate vicinity. Most of these chemicals act on adjacent cells and do not accumulate in the blood. Vertebrate examples include some of the chemicals called lumones that the gut produces and that help regulate digestion. In a wound, most cells secrete a substance called histamine that participates in the inflammatory response.
Neurons secrete chemicals called neurotransmitters (e.g., nitric oxide and – acetylcholine) that act on immediately adjacent target cells. These chemical messengers reach high concentrations in the synaptic cleft, act quickly, and are actively degraded and recycled.
Some specialized neurones (called neurosecretory cells) secrete neuropeptides (neurohormones). The blood or other body fluids transport neuropeptides to nonadjacent target cells, where neuropeptides exert their effects. In mammals, for example, certain nerve cells in the hypothalamus release a nebropeptide that causes the pituitary gland to release the hormone oxytocin, which induces powerful uterine contractions during the delivery of offspring.
Endocrine glands or cells secret horm6nes that the bloodstream transports to nonadjacent target cells.
Pheromones are chemical messengers released to the exterior of one animal that affect the behavior of another individual of the same species.