Lec 7c - Nervous System Synapses - Nurs1230 PDF

Topic 7c: Introduction to the Nervous System CHAPTER 11 IN TEXTBOOK Important Pages and Topics Introduction to the Nervous System (Chapter 11) 1. What is a synapse? P412 2. Describe a typical electrical synapse P413 3. Describe in detail how a chemical synapse works. P413-415 Focus Fig 11.3 4....

Topic 7c: Introduction to the Nervous System CHAPTER 11 IN TEXTBOOK Important Pages and Topics Introduction to the Nervous System (Chapter 11) 1. What is a synapse? P412 2. Describe a typical electrical synapse P413 3. Describe in detail how a chemical synapse works. P413-415 Focus Fig 11.3 4. Explain postsynaptic potentials and synaptic integration. P416-419 5. Explain the term summation and differentiate between spatial and temporal summation. P 418-420 6. What are neurotransmitters, how are they classified in terms or effects and actions and what are 2 types of neurotransmitter receptors? P 420-425 (just what we cover in the notes) 7. Explain neural integration and the basic ways neurons work together to process information P 426- 427 8. Be able to explain the nervous system disorders covered in the student research presentations Introduction to the Synapse Nervous system works because information flows from neuron to neuron Neurons are functionally connected by synapses ◦ Junctions that mediate information transfer from one neuron to another neuron ◦ Or from one neuron to an effector cell https://qph.fs.quoracdn.net/main-qimg-533c30164c41e75c3804a2d192bf446e.webp Introduction to the Synapse Presynaptic neuron: ◦ Neuron conducting impulses toward synapse ◦ Sends information Postsynaptic neuron: ◦ Neuron transmitting electrical signal away from synapse ◦ Receives information ◦ In PNS may be a neuron or effector cell Figure 11.15 Most function as both  Introduction to the Synapse Synaptic connections: ◦ Axodendritic: between axon terminals of one neuron and dendrites of others ◦ Axosomatic: between axon terminals of one neuron and soma (cell body) of others Less common connections: ◦ Axoaxonal (axon to axon) ◦ Dendrodendritic (dendrite to dendrite) ◦ Somatodendritic (dendrite to soma) Figure 11.15 2 Types: Chemical and Electrical  Chemical Synapses Most common type of synapse Specialized for release and reception of chemical neurotransmitters 2 Parts: Axon terminal of presynaptic neuron: contains synaptic vesicles filled with neurotransmitter Receptor region on postsynaptic neuron’s membrane: receives neurotransmitter ◦ Usually on dendrite or cell body https://upload.wikimedia.org/wikipedia/commons/thumb/e/e0/Synapse_Illustration2_tweaked.svg/1280px- Synapse_Illustration2_tweaked.svg.png ◦ Separated by fluid-filled synaptic cleft  Chemical Synapses Electrical impulse changed to chemical across synapse, then back into electrical Transmission across synaptic cleft: ◦ Synaptic cleft prevents nerve impulses from directly passing to neurons ◦ Chemical event ◦ Depends on release, diffusion, and receptor binding of neurotransmitters https://upload.wikimedia.org/wikipedia/commons/thumb/e/e0/Synapse_Illustration2_tweaked.svg/1280px- Synapse_Illustration2_tweaked.svg.png ◦ Ensures unidirectional communication between neurons  Information transfer across chemical synapses Six steps involved: 1. AP arrives at axon terminal of presynaptic neuron 2. Voltage-gated Ca2+ channels open, and Ca2+ enters axon terminal ◦ Ca2+ flows down electrochemical gradient from ECF to inside of axon terminal Focus Figure Page 415 Information transfer across chemical synapses 3. Ca2+ entry causes synaptic vesicles to release neurotransmitter ◦ Ca2+ causes a Calcium sensing protein (synaptotagmin) to react with SNARE proteins > membrane fusion ◦ Exocytosis of neurotransmitter into synaptic cleft ◦ The higher the impulse frequency, the more vesicles exocytose Focus Figure Page 415 Information transfer across chemical synapses 4. Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane ◦ Often chemically gated ion channels Focus Figure Page 415  Information transfer across chemical synapses 5. Binding of neurotransmitter opens ion channels, creating graded potentials ◦ Causes ion channels to open ◦ Causes a graded potential in postsynaptic cell ◦ Can be an excitatory or inhibitory event ◦ Some receptor proteins are also ion channels Focus Figure Page 415  Information transfer across chemical synapses 6. Neurotransmitter effects are terminated ◦ If neurotransmitter is bound, graded potentials continue ◦ In milliseconds, neurotransmitter effect is terminated one of three ways ◦ Reuptake by astrocytes or axon terminal ◦ Degradation by enzymes ◦ Diffusion away from synaptic cleft Focus Figure Page 415 Animation: Chemical Synapses https://www.youtube.com/watch?v=WhowH0kb7n0 Chemical Synapses Synaptic delay: ◦ Time needed for neurotransmitter to be released, diffuse across synapse, and bind to receptors ◦ Can take anywhere from 0.3 to 5.0 ms ◦ Rate-limiting step of neural transmission ◦ AP is very fast, this slows transmission ◦ Not noticeable, because these are still very fast Electrical Synapses Less common than chemical synapses Neurons are electrically coupled: ◦ Joined by gap junctions ◦ Communication very rapid ◦ Unidirectional or bidirectional ◦ Some regions responsible for eye movements or hippocampus in areas involved in emotions and memory ◦ Most abundant in embryonic nervous tissue Pereda 2014 Nature Reviews Neuroscience 15, 250-263 Review Questions What is a synapse? What are 2 of the most common types of synaptic connections? Briefly, through what mechanism (steps) does a pre-synaptic neuron signal to a post-synaptic neuron? What is the role of the neurotransmitter in this process? How are electrical synapses different from chemical synapses? Postsynaptic Potentials Neurotransmitter receptors cause graded potentials that vary in strength based on: ◦ Amount of neurotransmitter released ◦ Time neurotransmitter stays in cleft Depending on effect of chemical synapse, there are two types of postsynaptic potentials ◦ EPSP: excitatory postsynaptic potentials ◦ IPSP: inhibitory postsynaptic potentials Excitatory Synapses and EPSPs Neurotransmitter binding opens chemically gated channels ◦ Allows flow of Na+ and K+ in opposite directions, simultaneously Na+ influx greater than K+ efflux, resulting in local net graded potential depolarization called excitatory postsynaptic potential (EPSP) ◦ Can trigger AP if reaches threshold Focus Figure 11.4  Inhibitory Synapses and IPSPs Neurotransmitter binding to receptor opens channels, causes hyperpolarization Postsynaptic membrane more permeable to K+ or Cl– ◦ If K+ channels open, it moves out of cell ◦ If Cl– channels open, it moves into cell Reduces ability to produce AP Focus Figure 11.4 Integration and Modification of Synaptic Events Summation by the postsynaptic neuron: ◦ A single EPSP cannot induce an AP, but EPSPs can summate (add together) to influence postsynaptic neuron ◦ IPSPs can also summate ◦ Most neurons receive thousands of excitatory and inhibitory inputs ◦ Only if EPSPs predominate and bring to threshold will an AP be generated ◦ Two types of summations: temporal and spatial  Integration and Modification of Synaptic Events Focus Figure 11.4 No summation: No summation occurs when time between EPSPs is too long  Integration and Modification of Synaptic Events Focus Figure 11.4 Temporal summation: One or more presynaptic neurons transmit impulses in rapid-fire order ◦ First impulse produces EPSP, and before it can dissipate another EPSP is triggered, adding on top of first impulse  Integration and Modification of Synaptic Events Focus Figure 11.4 Spatial summation: Postsynaptic neuron is stimulated by large number of terminals simultaneously ◦ Many receptors are activated, each producing EPSPs ◦ May occurs with IPSPs Integration and Modification of Synaptic Events Synaptic potentiation: ◦ Repeated use of synapse increases ability of presynaptic cell to excite postsynaptic neuron ◦ Ca2+ concentration increases in presynaptic terminal, causing release of more neurotransmitter ◦ Leads to more EPSPs in postsynaptic neuron https://qbi.uq.edu.au/files/7776/Long-term-synaptic-plasticity_QBI-the-brain.jpg Integration and Modification of Synaptic Events Synaptic Potentiation: Potentiation can cause Ca2+ voltage gates to open on postsynaptic neuron Ca2+ activates kinase enzymes, leading to more effective response to subsequent stimuli Long-term potentiation: https://qbi.uq.edu.au/files/7776/Long-term-synaptic-plasticity_QBI-the-brain.jpg learning and memory Integration and Modification of Synaptic Events Presynaptic inhibition: ◦ Release of excitatory neurotransmitter by one neuron is inhibited by another neuron via an axoaxonal synapse ◦ Less neurotransmitter is released, leading to smaller EPSPs http://humanphysiology.academy/Neurosciences%202015/Images/1/presynaptic%20inhibition%20droualb_faculty_mjc_edu.jpeg Neurotransmitters How nervous system communicates - 50 or more neurotransmitters have been identified Most neurons make two or more neurotransmitters ◦ Neurons can exert several influences ◦ Usually released at different stimulation frequencies ◦ Neuotransmitters can produce different effects depending on type of receptor they bind Classified by: ◦ Chemical structure ◦ Function (we will focus on function) Table 11.4 (Page 421-422) has a detailed overview Classification of Neurotransmitters by Function Neurotransmitters exhibit a great diversity of functions Functions can be grouped into two classifications: ◦ Effects ◦ Actions https://ib.bioninja.com.au/_Media/types-of-neurotransmitters_med.jpeg Neurotransmitter Effects Excitatory versus Inhibitory: Neurotransmitter effects can be excitatory (depolarizing) and/or inhibitory (hyperpolarizing) Effect determined by receptor to which it binds ◦ Eg. GABA and glycine are usually inhibitory ◦ Eg. Glutamate is usually excitatory Neurotransmitter Actions Direct action: Neurotransmitter binds directly to, and opens, ion channels ◦ Promotes rapid responses by altering membrane potential ◦ Eg. Acetylcholine (Ach) https://sites.google.com/site/limbicsystembytaylorgallman/_/rsrc/1401407275001/drugs-affecting-neurotransmitters/acetylcholine-and- nicotine/acetylcholine-and-receptor.jpg Neurotransmitter Actions Indirect action: Neurotransmitter acts through intracellular second messengers, usually G protein pathways: ◦ Broader, longer-lasting effects similar to hormones ◦ Biogenic amines, neuropeptides, and dissolved gases  Neurotransmitter Receptors Channel-linked receptors: ◦ Ligand-gated ion channels ◦ Action is immediate and brief ◦ Excitatory receptors are channels for small cations ◦ Na+ influx contributes most to depolarization ◦ Inhibitory receptors allow Cl– influx that causes hyperpolarization Figure 11.16  Neurotransmitter Receptors G protein–linked receptors: ◦ Responses are indirect, complex, slow, and often prolonged ◦ Involves transmembrane protein complexes ◦ Uses second messenger system (as we covered) ◦ Cause widespread metabolic changes Figure 11.17 ◦ Eg. Muscarinic ACh receptors Neural Integration Neural integration: neurons functioning together in groups Groups contribute to broader neural functions There are billions of neurons in CNS > Required to smoothly operate Patterns of Neural Processing Serial processing: ◦ Input travels along one pathway to a specific destination ◦ One neuron stimulates next one, which stimulates next one, etc. ◦ All-or-none manner to produce specific ◦ Reflex – consistent, anticipated response ◦ Eg. Spinal reflex Figure 11.19 Patterns of Neural Processing Parallel processing: ◦ Input travels along several pathways ◦ Different parts of circuitry deal simultaneously with the information ◦ One stimulus promotes numerous responses ◦ Important for higher-level mental functioning ◦ Eg. A sensed smell may remind one of an odor and any associated experiences https://www.d.umn.edu/~jfitzake/Lectures/DMED/SensoryPhysiology/GeneralPrinciples/Figures/Divergence.gif Review Questions What are the 2 different types of post-synaptic potentials? How does summation occur? How is long term memory formed (the general process)? What are 2 different types of neurotransmitter receptors? What is the difference between serial and parallel processing?