Who Needs Emotions? The Brain Meets the Robot

organization of motivational–emotional systems 47
Walker, Brooks, & Holden–Dye, 1996). Neurotransmitters are released
from axon terminals, cross the synapse, and bind to postsynaptic receptor
sites to effect a cascade of intracellular biochemical events. Uptake sites
on presynaptic terminals are proteins that regulate the synaptic level of
neurotransmitter by binding released transmitter and transporting it back
into the terminal. These molecules play a role in adaptive behaviors to a
surprisingly conserved degree across species and phyla. Subjective states
in humans which are associated with such feelings as joy, fear, anxiety, and
maternal love are derived from the actions of truly primordial chemical
systems. Following the origins of bacterial life, eukaryotic cells appeared
approximately 2 billion years ago, primitive multicelled organisms appeared
around 800 million years ago, and vertebrates are estimated to have diverged
from invertebrates around 500–600 million years ago. All extant mammals,
birds, and reptiles are derived from stem reptiles that lived approximately
200–300 million years ago. Neurotransmitter development followed this
evolutionary path. All neurons, throughout the animal kingdom, contain
at least one releasable substance (usually an amine, peptide, amino acid,
or acetylcholine) and utilize either ligand-gated ion channels or second
messengers such as G proteins, AMP, phospholipase C, and calcium to
communicate their signal postsynaptically. Second-messenger systems
appeared quite early in evolution, perhaps to add a longer time scale and
greater flexibility in neural communication.* For example, the yeast alphamating
factor (a peptide pheromone) is a member of the G protein–coupled
receptor superfamily (Darlison & Richter, 1999), and G protein–coupled
receptors are found throughout arthropods, flatworms, and mollusks
(Walker, Brooks, & Holden-Dye, 1996).† Calcium, a ubiquitous second
messenger, plays this role even in bacteria (Tisa & Adler, 1992). Ligandgated
channels, complex membrane-bound proteins that allow fast chemical
transmission via gating of the flow of cations and anions in and out of the
cell (such as that involving g-aminobutyric acid, acetylcholine, and
glutamate), are present in all animal species studied thus far. Chemical
compounds can act in several ways: strictly as transmitters that convey
specific information via their effect on postsynaptic receptors, as modulators
of the postsynaptic receptor so as to alter other incoming signals, or as
signals acting at sites distal from release sites, thus acting as neurohormones.
*Ligand-gated ion channels are proteins that allow rapid flux of ions such as
sodium or potassium in and out of the neuron, depending on the binding of neurotransmitter
to its receptor. Second messengers are molecules that aid in the transduction
of the chemical signal to an electrical signal.
†G protein–coupled receptors are receptors for neurotransmitters that utilize
specific membrane-bound proteins—G proteins—that activate certain critical intracellular
second messenger enzymes, such as cyclic AMP.