Answer to Question 1
Researchers often refer to the neuropeptides as neuromodulators, because they have several Synapses 111 properties that set them apart from other transmitters. Whereas the neuron synthesizes most other neurotransmitters in the presynaptic terminal, it synthesizes neuropeptides in the cell body and then slowly transports them to other parts of the cell. Whereas other neurotransmitters are released at the axon terminal, the neuropeptides are released mainly by dendrites, and also by the cell body and the sides of the axon. A single action potential can release other neurotransmitters, but neuropeptide release requires repeated stimulation. However, after a few dendrites release a neuropeptide, the released chemical primes other nearby dendrites to release the same neuropeptide also, including dendrites of other cells. Thus, neurons containing neuropeptides do not release them often, but when they do, they release substantial amounts. Furthermore, unlike other transmitters that are released immediately adjacent to their receptors, neuropeptides diffuse widely, slowly affecting many neurons in their region of the brain. In that way they resemble hormones. Because many of them exert their effects by altering gene activity, their effects are long-lasting, in the range of 20 minutes or more. Neuropeptides are important for hunger, thirst, and other long-term changes in behavior and experience.
Answer to Question 2
a
Two main types of nerve cells.
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