Query: Certain chemicals induce neuromodulation.

Relevant passages:
Acetylcholine <=SEP=> Acetylcholine ( ACh ) is an organic chemical that functions in the brain and body of many types of animals , including humans , as a neurotransmitter -- a chemical released by nerve cells to send signals to other cells . Its name is derived from its chemical structure : it is an ester of acetic acid and choline . Parts in the body that use or are affected by acetylcholine are referred to as cholinergic . Substances that interfere with acetylcholine activity are called anticholinergics .   Acetylcholine is the neurotransmitter used at the neuromuscular junction -- in other words , it is the chemical that motor neurons of the nervous system release in order to activate muscles . This property means that drugs that affect cholinergic systems can have very dangerous effects ranging from paralysis to convulsions . Acetylcholine is also used as a neurotransmitter in the autonomic nervous system , both as an internal transmitter for the sympathetic nervous system and as the final product released by the parasympathetic nervous system .   In the brain , acetylcholine functions as a neurotransmitter and as a neuromodulator . The brain contains a number of cholinergic areas , each with distinct functions . They play an important role in arousal , attention , memory and motivation .   Partly because of its muscle-activating function , but also because of its functions in the autonomic nervous system and brain , a large number of important drugs exert their effects by altering cholinergic transmission . Numerous venoms and toxins produced by plants , animals , and bacteria , as well as chemical nerve agents such as Sarin , cause harm by inactivating or hyperactivating muscles via their influences on the neuromuscular junction . Drugs that act on muscarinic acetylcholine receptors , such as atropine , can be poisonous in large quantities , but in smaller doses they are commonly used to treat certain heart conditions and eye problems . Scopolamine , which acts mainly on muscarinic receptors in the brain , can cause delirium and amnesia . The addictive qualities of nicotine derive from its effects on nicotinic acetylcholine receptors in the brain .

Neuromodulation <=SEP=> Neuromodulation is the physiological process by which a given neuron uses one or more chemicals to regulate diverse populations of neurons . This is in contrast to classical synaptic transmission , in which one presynaptic neuron directly influences a single postsynaptic partner . Neuromodulators secreted by a small group of neurons diffuse through large areas of the nervous system , affecting multiple neurons . Major neuromodulators in the central nervous system include dopamine , serotonin , acetylcholine , histamine , and norepinephrine .   Neuromodulation can be conceptualized as a neurotransmitter that is not reabsorbed by the pre-synaptic neuron or broken down into a metabolite . Such neuromodulators end up spending a significant amount of time in the cerebrospinal fluid ( CSF ) , influencing ( or `` modulating '' ) the activity of several other neurons in the brain . For this reason , some neurotransmitters are also considered to be neuromodulators , such as serotonin and acetylcholine .   Neuromodulation is often contrasted with classical fast synaptic transmission . In both cases the transmitter acts on local postsynaptic receptors , but in neuromodulation , the receptors are typically G-protein coupled receptors while in classical chemical neurotransmission , they are ligand-gated ion channels . Neurotransmission that involves metabotropic receptors ( like G-protein linked receptors ) often also involves voltage-gated ion channels , and is relatively slow . Conversely , neurotransmission that involves exclusively ligand-gated ion channels is much faster .  A related distinction is also sometimes drawn between modulator and driver synaptic inputs to a neuron , but here the emphasis is on modulating ongoing neuronal spiking versus causing that spiking .