Cell regulation can be thought of as the effector side of cell communication. Most commonly cell regulation occurs by means of extracellular chemical messengers. Which of the following statement(s) is/are true concerning these messengers?

b. The extracellular signal or stimulus is received by a receptor on or in the target cell

c. Neurocrine regulation depends on a physical connection between the neuron and the target cell

Depending on how the extracellular messenger arrives, cell regulation can be classified as paracrine, endocrine, or neurocrine. In paracrine regulation, a chemical messenger or mediator is produced and acts locally. In endocrine regulation, the extracellular messengers (hormones) are released into the blood and act on target cells anywhere on the body that has appropriate receptors. In neurocrine regulation, neurons secrete transmitters into highly localized regions, the synaptic cleft, so that the regulation depends on a physical connection between the neuron and the target cell as well as the presence of a specific receptor. In almost all cases of cell regulation, the extracellular signal or stimulus is restricted to being an informational molecule. This information is received by receptor on or in the target cell, which generally has an affinity for the signal molecule. Most hormones, local mediators, and neurotransmitters are water-soluble and cannot readily cross the plasma membrane.