Which category of signal exerts its effects on target cells by binding to membrane-bound receptor proteins

Neurohormones

Explanation:

Neurohormones are chemicals produced by neuroendocrine cells which behave like hormones. They are released into the bloodstream unlike neurotransmitters that are released into the synaptic cleft.

The same neurohorome can have both functions (hormonal and neurotransmitter). Hormones are chemicals produced by the endocrine glands that move through the body in the bloodstream. They control many biological processes, including muscle growth, heart rate, hunger, and the menstrual cycle. A neurotransmitter is a biomolecule that enables neurotransmission, i.e. the transmission of information from one neuron (a type of cell in the nervous system) to another neuron, a muscle cell or a gland, via the synapse that separates them. The neurotransmitter is released from the synaptic vesicles at the end of the presynaptic neuron, towards the synapse, through the synaptic space and acts on the specific cell receptors of the target cell.

So, a neurohormone is a biomolecule that, like the neurotransmitter and the hormone, is responsible for an exchange of information between cells.  The neuroendocrine cells that secretes de neurohormones are, in turn, half neurons, half endocrines, and are found both in the Nervous System and in other parts of the body. They bind to receptors located on the membranes of other cells. Unlike neurotransmitters that interact only with other neurons, neurohormones also interact with other cells. When a neurohormone enters a cell and binds to its receptor, it causes the receptor to change shape, allowing the neurohormone-receptor complex to enter the nucleus and regulate gene activity. The binding exposes regions of the receptor that can bind to specific DNA sequences. These sequences are found next to certain genes in the cell's DNA, and when the receptor binds to them, it alters their transcription levels.

The most obvious case is catecholamines, formed in the adrenal glands by chromaffin cells, which are modified neurons and they are released directly into the blood. In turn, they are also produced by neurons and act at the synapses.

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