6 October 2010 Section B: Action Potentials Section C: Synapses Two 1QQs on Friday covering: One covers Action Potential Conduction Velocity Lab Review Questions posted to website! Other covers today’s lecture topics Check your MC grade by Code Number from link on Website

1QQ # 12 for 8:30 class 1.Why doesn’t an action potential reach + 60 mV? a)Voltage- gated Na+ channels open and spontaneously close quickly b)Voltage-gated K+ channels open a little later than the Na+ channels c)Na+ K+ ATPase quickly pumps out the Na+ that enters during an AP d)As the membrane approaches +60 mV, the driving force for Na+ entry is weaker e)6’s are evil numbers and are to be avoided. 2.Which are the accurate statements regarding V-gated K+ channels? a)The more the membrane is depolarized, the more K+ channels will open, and the membrane will depolarize even more, generating a positive feedback cycle. b)These channels inactive after a short open time and can only reopen if the membrane potential returns to negative values. c)These channels are “blocked” by lidocaine, xylocaine, and novocaine. d)These channels open shortly after the V-gated Na+ channels open.

1QQ # 12 for 9:30 class 1.Why doesn’t an action potential reach + 60 mV? a)Voltage- gated Na+ channels would be forced “shut” at + 60 mV b)Voltage-gated K+ channels open a little sooner than the Na+ channels. c)Na+ K+ ATPase quickly pumps out the Na+ that enters during an AP d)As the membrane approaches +60 mV, the driving force for Na+ entry is weaker e)Na+ channels open only briefly and then quickly inactivate. 2.Which are the accurate statements regarding V-gated K+ channels? a)The more the membrane is depolarized, the more K+ channels will open, and the membrane will depolarize even more, generating a positive feedback cycle. b)These channels inactive after a short open time and can only reopen if the membrane potential returns to negative values. c)These channels are “blocked” by lidocaine, xylocaine, and novocaine. d)These channels open shortly after the V-gated Na+ channels open.

Relative permeabilities Duration of AP Refractory periods absolute RP relative RP Properties of V-gated Na and K channels account for the shape of the action potential and the refractory periods. Why does the peak of the action potential not reach E Na ? Rising Phase Falling Phase S 1

Natural ways to Initate an Action Potential Graded depolarization in cell body reach threshold at axon hillock Graded depolarization in in receptive membranes of sensory neurons reach threshold for AP. i.e. nociceptors and stretch receptors. Unstable membrane potential cycles: pacemaker potentials in pacemaker cells of heart, smooth muscles of gut, and medullary neurons for respiratory rhythm. Size of graded potential is proportinal to intensity of the stimulus. But action potentials are “all or nothing” so the size of an AP cannot “code” for stimulus Intensity. So how is the intensity of a stimulus encoded by APs? S 2

What types of ion-channels are labeled in this neuron in red? S 3

Voltage-gated Na+ channel scienceblogs.com/.../upload/2006/03/channel.jpg Tetrodotoxin from ovary of Puffer fish, used in Japanese sushi (fugu) S 4

Axon Hillock Axon The Questions: How does an action potential move along the axon? Why doesn’t the amplitude get smaller with distance? Why is the conduction of an action potential unidirectional? What is the absolute refractory period and is going on with voltage gated sodium channels that accounts for the absolute refractory period? What is the relative refractory period and what is going on with voltage gated sodium channels that accounts for the relative refractory period? S 5

In unmyelinated axons, action potential must be generated at each point along the membrane, a relatively slow process that involves influx of Na+ which sets up positive feedback cycle. In myelinated axons, action potential must be generated only at the nodes of Ranvier, which allows AP to be conducted much faster and with fewer ions moving, and thus less energetically expensive. S 6

Figure 6.23 AP CV (up to 100 m/s) Location of channels Energy Requirements Axon diameter Clustering of V-gated channels at Nodes of Ranvier Reminder: influx of Na+ is very quickly followed by efflux of K+ (not shown above) Saltatory Conduction Refractory period assures unidirectional conduction S 7

Who Cares? Multiple sclerosis and episodic degeneration of myelin by immune disorder. S 8

Important Information S 9

Figure 6.24 Section C: Synapses and Synaptic Transmission S 10

Anatomy of a Chemical Synapse Presynaptic cell Postsynaptic cell S 7 S 11

Anatomy of an Electrical Synapse (aka Gap Junction) Comparison to Chemical Synapses Directionality Response time Sign inversion? Uncommon in human CNS Common in cardiac muscle and some smooth muscle. S 8 S 12

Figure 6.25 Unidirectional Release, diffusion, binding, Post-synaptic Receptor Types: Inotropic or Metabotropic Classification: Excitatory (closer to threshold for AP) Or Inhibitory (stabilizes or hyperpolarizes) S 13

Inotropic receptorMetabotropic receptor Some ion channels are permeable to both Na+ and K+ Types of Acetylcholine Receptor so named for its agonist: Nicotinic AChR and Muscarinic AChR Types of Ligand-Gated Receptors S 14

Synapses named for NT used: -ergic Examples: Cholinergic Adrenergic Serotonergic GABAergic Peptidergic S 15