Neurobiologists once believed that the workings of the brain were guided exclusively by electrical signals; according to this theory, communication between neurons (brain cells) is possible because electrical impulses travel from one neuron to the next by literally leaping across the synapses (gaps between neurons). But many neurobiologists puzzled over how this leaping across synapses might be achieved, and as early as 1904 some speculated that electrical impulses are transmitted between neurons chemically rather than electrically. According to this alternative theory, the excited neuron secretes a chemical called a neurotransmitter that binds with its corresponding receptor molecule in the receiving neuron. This binding of the neurotransmitter renders the neuron permeable to ions, and as the ions move into the receiving neuron they generate an electrical impulse that runs through the cell; the electrical impulse is thereby transmitted to the receiving neuron. This theory has gradually won acceptance in the scientific community, but for a long time little was known about the mechanism by which neurotransmitters manage to render the receiving neuron permeable to ions. In fact, some scientists remained skeptical of the theory because they had trouble imagining how the binding of a chemical to a receptor at the cell surface could influence the flow of ions through the cell membrane. Recently, however, researchers have gathered enough evidence for a convincing explanation: that the structure of receptors plays the pivotal role in mediating the conversion of chemical signals into electrical activity. The new evidence shows that receptors for neurotransmitters contain both a neurotransmitter binding site and a separate region that functions as a channel for ions; attachment of the neurotransmitter to the binding site causes the receptor to change shape and so results in the opening of its channel component. Several types of receptors have been isolated that conform to this structure, among them the receptors for acetylcholine, gamma-aminobutyric acid (GABA), glycine, and serotonin. These receptors display enough similarities to constitute a family, known collectively as neurotransmitter-gated ion channels. It has also been discovered that each of the receptors in this family comes in several varieties so that, for example, a GABA receptor in one part of the brain has slightly different properties than a GABA receptor in another part of the brain. This discovery is medically significant because it raises the possibility of the highly selective treatment of certain brain disorders. As the precise effect on behavior of every variety of each neurotransmitter-gated ion channel is deciphered, pharmacologists may be able to design drugs targeted to specific receptors on defined categories of neurons that will selectively impede or enhance these effects. Such drugs could potentially help ameliorate any number of debilitating conditions, including mood disorders, tissue damage associated with stroke, or Alzheimer's disease.Q: The author most likely uses the phrase "defined categories of neurons" in lines 55–56 in order to refer to neurons that Answer Choices: (A)possess channels for ions (B)respond to drug treatment (C)contain receptor molecules (D)influence particular brain functions (E)react to binding by neurotransmitters
A: Among A through E, the answer is
(A)possess channels for ions
(B)respond to drug treatment
(C)contain receptor molecules
(D)influence particular brain functions
(E)react to binding by neurotransmitters