b Neurotransmitter Indirect effects by G proteins Examples: E, NE, dopamine, histamine, GABA Activates enzymes that change cell metabolism and activity G protein (inactive) Receptor Opens ion channels cAMP ATP G protein (active) Adenylate cyclase Figure 12-17b Mechanisms of Neurotransmitter Function.
Figure 12-17c Mechanisms of Neurotransmitter Function. c Nitric oxide Indirect effects by intracellular enzymes Examples: Nitric oxide, carbon monoxide Opens ion channels Production of secondary messengers Activation of enzymes Changes in cell metabolism and activity
Figure 12-18a Temporal and Spatial Summation. a Temporal Summation. Temporal summation occurs on a membrane that receives two depolarizing stimuli from the same source in rapid succession. The effects of the second stimulus are added to those on the first. Initial segment Threshold reached FIRST STIMULUS SECOND STIMULUS ACTION POTENTIAL PROPAGATION Action potential is generated Second stimulus arrives and is added to the first stimulus First stimulus arrives 3 2 1
Figure 12-20a Presynaptic Inhibition and Presynaptic Facilitation. Action potential arrives GABA release Inactivation of calcium channels Ca 2+ 2. Less calcium enters 1. Action potential arrives 3. Less neurotransmitter released 4. Reduced effect on postsynaptic membrane Presynaptic inhibition a