Which of the following is not correct about a second messenger system? A. The activating hormone interacts with a receptor site on the plasma membrane.  B. The activating hormone activates an enzyme, most often adenylate cyclase C. Activated adenylate cyclase catalyzes the transformation of AMP to cyclic AMP D. Cyclic AMP causes the activation of protein kinase E. None of these are correct.

How is an extracellular signal converted to an intracellular signal in signal transduction? Give a specific example.

You learned in this chapter that Na+/K+ active transport pumps in the plasma membrane of the axons are responsible for creating the imbalance between Na+ and K+ inside and outside of the neuron that produces the resting membrane potential. In early research studying the role of ions and the involvement of active transport of ions in neural signaling, investigators used the giant axon of a squid as a model. The diameter of a giant axon is far greater than that of a mammalian axon, which enabled researchers to isolate it easily and use it in in vitro experiments. In one early experiment, researchers investigated the active transport of Na+ out of the axon in response to the presence of cyanide. Experimentally they hooked up a section of axon to a syringe, immersed the axon in artificial seawater, introduced radioactive 22Na (as 22NaCl) into the axon, and then quantified the transport of 22Na out through the axons plasma membrane. The rate of 22Na transport out of the axon was determined by measuring the radioactivity released into the fluid sur- rounding the axon over a period of time. The Figure shows the results of the experiment. What is the effect of cyanide on Na1 transport out of the squid axon? How do the data show the effect? Source: P. C. Caldwell et al. 1960. The effects of injecting energy-rich phosphate compounds on the active transport of ions in the giant axons of Loligo. The Journal of Physiology 152:561590. Cengage Learning 2017

What distinguishes a steroid receptor from a receptor tyrosine kinase receptor or a G-protein-coupled receptor?