What do receptor molecules receive signals from?

Receptor molecules play a crucial role in cellular communication and signal transduction. They are proteins typically located on the cell surface or within cells that bind to specific signaling molecules, such as hormones or neurotransmitters.

The nervous and endocrine systems are the primary sources of these signaling molecules. The nervous system uses neurotransmitters to send signals rapidly across synapses, while the endocrine system relies on hormones that travel through the bloodstream to target distant cells. Receptors specifically recognize and bind to these signals, triggering a response in the target cell. This interaction is fundamental for various physiological processes, including growth, metabolism, and homeostasis.

The muscular, circulatory, and digestive systems primarily do not produce the signaling molecules that receptors respond to in the same way that the nervous and endocrine systems do, making them less directly relevant in this context. Hence, the nervous and endocrine systems are the correct answer regarding the source of signals that receptor molecules receive.