Lecture 5.3 (Synaptic Transmission) - 112524 DRAFT PDF

PSIO 201 – Human Anatomy & Physiology I 1 https://makingsenseofalzheimers.org/illuminating-action-potential/ Where We’ve Been Action potentials (APs) Generation of the AP Change in MP...

PSIO 201 – Human Anatomy & Physiology I 1 https://makingsenseofalzheimers.org/illuminating-action-potential/ Where We’ve Been Action potentials (APs) Generation of the AP Change in MP & ion channels Depolarization Repolarization Hyperpolarization Refractory periods AP conduction Ch. 12 2 http://www.beggslab.com/neuroscience-500-notes.html  Lecture 5.2 Learning Objectives List in order and describe the principal events associated with an action potential. Define the phrase ‘all-or-none’ in the context of the neuronal action potential. List and describe the events that result in the propagation of the action potential. Describe the factors that influence the speed of propagation of the action potential. Describe the importance of myelination and the function of oligodendrocytes and Schwann cells. Compare and contrast ‘continuous’ and ‘saltatory’ conduction. Describe the basis of absolute and relative refractory periods and explain their importance. 3 Action Potentials Starts at trigger zone, reaches end of axon What is the molecular basis of an action potential? How do neurons use action potentials to spread a signal along the length of an axon (conduction)? 4  Action Potential Generation Major events of an action potential Fig. 12.14 5 Action Potentials Key point: Very few ions actually move to trigger an action potential 1/1,000,000 K+ ions leave axon during falling phase of one action potential A neuron could fire >1000 times without significantly affecting ion gradients 6 Action Potentials Starts at trigger zone, reaches end of axon What is the molecular basis of an action potential? How do neurons use action potentials to spread a signal along the length of axons (conduction)? 7 Signal Conduction in Axons Continuous conduction Unmyelinated nerve fibers (axons) Slower Computer comparison: space bar on your keyboard Section 1 → Section 2 → 3 → 4 → 5… Saltatory conduction Myelinated nerve fibers (axons) Faster Computer comparison: Tab key on your keyboard Section 1 → Section 5 → 10 → 15 → 20… 8 Continuous vs. Saltatory Conduction Unmyelinated neurons Myelinated neurons Fig. 12.17, 12.18 9 Refractory Periods Plasma membrane resists immediate restimulation due to refractory period → unidirectional AP flow 10  Where We’re Going Action Potentials Factors affecting speed I need 24 volunteers that like to eat Starbursts for a demo in one minute Synaptic Transmission Structure of a synapse Types of synapses https://64.media.tumblr.com/tumblr_mej1jnxj0s1qf252bo1_540.jpg Ch. 12 11  Lecture 5.3 Learning Objectives Describe the factors that influence the speed of propagation of the action potential. Draw a picture showing the structure of a synapse. Label the principal structures on the pre-synaptic neuron and post-synaptic neuron. Include an astrocyte in your picture. List and describe (in order) the physiological events associated with synaptic transmission. Define graded potential and describe the difference between EPSPs and IPSPs. Define spatial and temporal summation and describe how they contribute to neuron activation. 12 Special Topic: Conduction Velocity Size matters for conduction velocity Diameter of the axon affects velocity of AP Resistance (due to walls of axon) slows down conduction Non-biological, non-electrical example: straw diameter Bubble tea straw – Larger diameter → less fluid hitting resistance (friction of straw walls) → greater conduction velocity of fluid 13 https://www.bubbletea.com.au/site/images/cache/strawheader2.800.gif Conduction Velocity Size matters for conduction velocity Diameter of the axon affects velocity of AP Resistance (due to walls of axon) slows down conduction Non-biological, non-electrical example: straw diameter Larger axon diameter = less resistance to current flow = faster signal propagation Resistance due to charged particles in & out of axon that affect the spread of the electrical signal Some organisms have axons up to 1 mm in diameter → faster continuous conduction Squid, fish, & earthworms 14 How Does Myelination Affect AP Speed? Unmyelinated neurons Myelinated neurons Fig. 12.17, 12.18 15 In-Class Demonstration 16 In-Class Demonstration 17 Action Potential Questions? Starts at trigger zone, reaches end of axon What is the molecular basis of an action potential? How do neurons use action potentials to spread a signal along the length of axons (conduction)? 18 The Synapse Electrical signal moves down the neuron to axon terminals Local potential → action potential → synapse Connection between a presynaptic neuron & a postsynaptic cell (often another neuron) Typical neuron has between 1,000 to 10,000 axon terminals 19  The Synapse Electrical synapse Uncommon Pass signal (ions) through gap junctions (ex: connexons) → extremely rapid signaling CNS, glial cells, muscle, pancreatic beta cells Chemical synapse Majority of synapses Uses neurocrine signals Fig. 12.21 20 http://kin450-neurophysiology.wikispaces.com/file/view/Chemical_vs_Electrical.jpg/141266021/Chemical_vs_Electrical.jpg  Chemical Synapses Chemical signals (neurocrines) released by neurons onto target Neurotransmitters Act at a synapse Usually act only on postsynaptic cell Quick response Neuromodulators Act at both synaptic & non-synaptic sites Modulate response for groups of neurons Slower & longer response Neurohormones Released into the blood & widely circulated Longest, slowest response 21  Neurotransmitters Structural classification Acetylcholine Amino acids Monoamines Purines Gases Neuropeptides Table 12.3, Fig. 12.21 22 Neurotransmitters Functional classification Type of effect Excitatory → depolarization Example: glutamate Inhibitory → hyperpolarization Example: GABA, glycine 23 Neurotransmitters Functional classification Type of effect Excitatory → depolarization Example: glutamate Inhibitory → hyperpolarization Example: GABA, glycine Type of action Direct → affect ion channels Example: ACh Indirect → affect second messenger pathways Example: Dopamine 24  Neurotransmitters In case you’re curious, no need to memorize 25  Nerve Cell Communication Receptor for neurocrines is equally important in determining a cell’s response Major neurocrine receptor types Ligand-gated ion channels Ionotropic receptors Move ions → change membrane potential → potentials Rapid responses G protein-coupled receptors (GPCRs) Metabotropic receptors Activate signal transduction cascades Slower responses 26 Signal Transduction Ligand binds to a receptor protein triggering a cascade of intracellular events → specific cellular response 27  Generic Chemical Synapse Presynaptic neuron Axon terminal Synaptic vesicles Neurotransmitters Synaptic cleft Enzymes Extracellular matrix proteins Glial cell processes Postsynaptic neuron Neurotransmitter receptors Response Fig. 12.21 28  Excitatory Cholinergic Synapse Cholinergic = uses ACh as the NT Could be excitatory or inhibitory Step 1 Action potential arrives at axon terminal Opens voltage-gated Ca2+ channels Fig. 12.23 29  Excitatory Cholinergic Synapse Step 2 Ca2+ influx causes vesicle exocytosis ACh released into synaptic cleft Step 3 Not really a separate step, happening all of the time Empty vesicle re-filled with ACh Fig. 12.23 30  Excitatory Cholinergic Synapse Step 4 ACh moves across synaptic cleft and binds to ligand-gated channels Opens ion channels → Na+ & K+ flow Step 5 Na+ influx → depolarization (postsynaptic potential) Fig. 12.23 31  Excitatory Cholinergic Synapse Step 6 NT degradation Ex: AChE breaks down ACh into acetate + choline Fig. 12.23 32  Excitatory Cholinergic Synapse Step 6 NT degradation Ex: AChE breaks down ACh into acetate + choline Step 7 NT reuptake Ex: Choline is reabsorbed into presynaptic → new ACh Fig. 12.23 33  Excitatory Cholinergic Synapse Step 6 NT degradation Ex: AChE breaks down ACh into acetate + choline Step 7 NT reuptake Ex: Choline is reabsorbed into presynaptic → new Ach Step 8 NT diffuses away from synapse Astrocytes/satellite cells absorb stray NT Fig. 12.23 34  Inhibitory GABA-ergic Synapse Most steps are similar to previous example Use GABA as NT instead Binds to ionotropic receptors Primary target: Cl- channels What type of stimulus would this be to the postsynaptic cell? Fig. 12.21 35  Inhibitory GABA-ergic Synapse Most steps are similar to previous example Use GABA as NT instead Binds to ionotropic receptors Cl- channels Cl- influx → hyperpolarization Fig. 12.21 36