Section C: Synapses Section D: Structure of the Nervous System 7 October 2008 Section C: Synapses Section D: Structure of the Nervous System

Fates of neurotransmitters: 1) Diffusion away from synapse, Vesicle release proportional to Ca++ influx (High f AP leads to residual Ca++ in terminal) Figure 6.27 Fates of neurotransmitters: 1) Diffusion away from synapse, 2) Enzymatic degradation (e.g. AChE and MAO) 3) Reuptake into presynaptic terminal (e.g. SSRI) SSRIs: Lexapro, Prozac, Paxil, Zoloft Tetanus toxin & Botulinum toxin disrupt SNARE function.

Figure 6.28 Some ion Channels that allow flux of Na+ and K+ simultaneously e.g. nicotinic Acetylcholine Receptor (nAChR) EPSPs :which ion moving in which direction? Duration of PSP vs AP Synaptic delay

EK+ IPSPs :which ion moving in which direction? Figure 6.29 IPSPs :which ion moving in which direction? Some IPSPs result in no change in membrane potential by opening Chloride channels that stabilize membrane potential at resting value (Nernst Potential for Cl- = -70mV) or in cells that actively transport Cl- out. EK+

Summation and Synaptic Integration Figure 6.31 Summation and Synaptic Integration Different times Different locations Each IPSP hyperpolarizes by 5 mV. Each EPSP depolarizes by 5 mV. If 4 inhibitory synapses are active at the same time, how many excitatory synapses must be active simultaneously to exceed threshold (-55 mV) if the resting membrane potential is -70mV?

Priority by proximity Demo: Eunice is the axon hillock! Diagram on Board: Degree of depolarization above threshold is proportional to frequency of action potentials.

Figure 6.33 Who Cares? Presynaptic Facilitation Presynaptic Inhibition Diagram of descending pathway from periaqueductal gray matter to suppress afferent pain via presynaptic inhibition. Example: cut to abdomen. Mechanism: vary Ca++ entry in presynaptic terminal B.

Figure 6.34 Recall AChE and MAO & SSRI Most neurotransmitters are synthesize in the axon terminal. Exceptions: Peptide NTs originate in cell body, move in vesicles by fast orthograde axonal transport to axon terminal. Figure 6.34 Recall AChE and MAO & SSRI