11 - Dr. Jerry Cronin

PowerPoint Lecture Slides prepared by Janice Meeking, Mount Royal College CHAPTER 11 Fundamentals of the Nervous System and Nervous Tissue: Part C Copyright 2010 Pearson Education, Inc. The Synapse A junction that mediates information transfer from one neuron: To another neuron, or To an effector cell

Copyright 2010 Pearson Education, Inc. The Synapse Presynaptic neuronconducts impulses toward the synapse Postsynaptic neurontransmits impulses away from the synapse PLAY Animation: Synapses Copyright 2010 Pearson Education, Inc. Types of Synapses Axodendriticbetween the axon of one neuron and the dendrite of another Axosomaticbetween the axon of one neuron and the soma of another

Less common types: Axoaxonic (axon to axon) Dendrodendritic (dendrite to dendrite) Dendrosomatic (dendrite to soma) Copyright 2010 Pearson Education, Inc. Axodendritic synapses Dendrites Axosomatic synapses Cell body Axoaxonic synapses (a) Axon

Axon Axosomatic synapses (b) Copyright 2010 Pearson Education, Inc. Cell body (soma) of postsynaptic neuron Figure 11.16 Electrical Synapses Less common than chemical synapses Neurons are electrically coupled (joined by gap junctions) Communication is very rapid, and may be unidirectional or bidirectional Are important in: Embryonic nervous tissue

Some brain regions Copyright 2010 Pearson Education, Inc. Chemical Synapses Specialized for the release and reception of neurotransmitters Typically composed of two parts Axon terminal of the presynaptic neuron, which contains synaptic vesicles Receptor region on the postsynaptic neuron Copyright 2010 Pearson Education, Inc. Synaptic Cleft Fluid-filled space separating the presynaptic and postsynaptic neurons Prevents nerve impulses from directly passing from one neuron to the next

Copyright 2010 Pearson Education, Inc. Synaptic Cleft Transmission across the synaptic cleft: Is a chemical event (as opposed to an electrical one) Involves release, diffusion, and binding of neurotransmitters Ensures unidirectional communication between neurons PLAY Animation: Neurotransmitters Copyright 2010 Pearson Education, Inc. Information Transfer AP arrives at axon terminal of the presynaptic neuron and opens voltage-gated Ca2+

channels Synaptotagmin protein binds Ca2+ and promotes fusion of synaptic vesicles with axon membrane Exocytosis of neurotransmitter occurs Copyright 2010 Pearson Education, Inc. Information Transfer Neurotransmitter diffuses and binds to receptors (often chemically gated ion channels) on the postsynaptic neuron Ion channels are opened, causing an excitatory or inhibitory event (graded potential) Copyright 2010 Pearson Education, Inc. Chemical synapses transmit signals from

one neuron to another using neurotransmitters. Presynaptic neuron Presynaptic neuron Postsynaptic neuron 1 Action potential arrives at axon terminal. 2 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal. Mitochondrion Ca2+ Ca2+

Ca2+ 3 Ca2+ entry causes neurotransmittercontaining synaptic vesicles to release their contents by exocytosis. Axon terminal Ca2+ Synaptic cleft Synaptic vesicles 4 Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the

postsynaptic membrane. Postsynaptic neuron Ion movement Enzymatic degradation Graded potential Reuptake Diffusion away from synapse 5 Binding of neurotransmitter opens ion channels, resulting in graded potentials. 6 Neurotransmitter effects are

terminated by reuptake through transport proteins, enzymatic degradation, or diffusion away from the synapse. Copyright 2010 Pearson Education, Inc. Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters. Presynaptic neuron Presynaptic neuron Postsynaptic neuron

1 Action potential arrives at axon terminal. Mitochondrion Ca2+ Ca2+ Axon terminal Ca2+ Ca2+ Synaptic cleft Synaptic vesicles Postsynaptic neuron

Copyright 2010 Pearson Education, Inc. Figure 11.17, step 1 Chemical synapses transmit signals from one neuron to another using neurotransmitters. Presynaptic neuron Presynaptic neuron Postsynaptic neuron 1 Action potential arrives at axon terminal. 2 Voltage-gated Ca2+ channels open and Ca2+

enters the axon terminal. Mitochondrion Ca2+ Ca2+ Axon terminal Ca2+ Ca2+ Synaptic cleft Synaptic vesicles Postsynaptic neuron Copyright 2010 Pearson Education, Inc.

Figure 11.17, step 2 Chemical synapses transmit signals from one neuron to another using neurotransmitters. Presynaptic neuron Presynaptic neuron Postsynaptic neuron 1 Action potential arrives at axon terminal. 2 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal.

Mitochondrion Ca2+ Ca2+ 3 Ca2+ entry causes neurotransmittercontaining synaptic vesicles to release their contents by exocytosis. Axon terminal Ca2+ Ca2+ Synaptic cleft Synaptic vesicles

Postsynaptic neuron Copyright 2010 Pearson Education, Inc. Figure 11.17, step 3 Chemical synapses transmit signals from one neuron to another using neurotransmitters. Presynaptic neuron Presynaptic neuron Postsynaptic neuron 1 Action potential

arrives at axon terminal. 2 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal. Mitochondrion Ca2+ Ca2+ 3 Ca2+ entry causes neurotransmittercontaining synaptic vesicles to release their contents by exocytosis. 4 Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane. Copyright 2010 Pearson Education, Inc.

Axon terminal Ca2+ Ca2+ Synaptic cleft Synaptic vesicles Postsynaptic neuron Figure 11.17, step 4 Ion movement Graded potential 5 Binding of neurotransmitter

opens ion channels, resulting in graded potentials. Copyright 2010 Pearson Education, Inc. Figure 11.17, step 5 Enzymatic degradation Reuptake Diffusion away from synapse 6 Neurotransmitter effects are terminated by reuptake through transport proteins, enzymatic degradation, or diffusion away from the synapse. Copyright 2010 Pearson Education, Inc. Figure 11.17, step 6

Chemical synapses transmit signals from one neuron to another using neurotransmitters. Presynaptic neuron Presynaptic neuron Postsynaptic neuron 1 Action potential arrives at axon terminal. 2 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal. Mitochondrion

Ca2+ Ca2+ Ca2+ 3 Ca2+ entry causes neurotransmittercontaining synaptic vesicles to release their contents by exocytosis. Axon terminal Ca2+ Synaptic cleft Synaptic vesicles 4 Neurotransmitter

diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane. Postsynaptic neuron Ion movement Enzymatic degradation Graded potential Reuptake Diffusion away from synapse 5 Binding of neurotransmitter opens ion channels, resulting in

graded potentials. 6 Neurotransmitter effects are terminated by reuptake through transport proteins, enzymatic degradation, or diffusion away from the synapse. Copyright 2010 Pearson Education, Inc. Figure 11.17 Termination of Neurotransmitter Effects Within a few milliseconds, the neurotransmitter effect is terminated Degradation by enzymes Reuptake by astrocytes or axon terminal Diffusion away from the synaptic cleft Copyright 2010 Pearson Education, Inc.

Synaptic Delay Neurotransmitter must be released, diffuse across the synapse, and bind to receptors Synaptic delaytime needed to do this (0.3 5.0 ms) Synaptic delay is the rate-limiting step of neural transmission Copyright 2010 Pearson Education, Inc. Postsynaptic Potentials Graded potentials Strength determined by:

Amount of neurotransmitter released Time the neurotransmitter is in the area Types of postsynaptic potentials 1. EPSPexcitatory postsynaptic potentials 2. IPSPinhibitory postsynaptic potentials Copyright 2010 Pearson Education, Inc. Copyright 2010 Pearson Education, Inc. Table 11.2 (1 of 4) Copyright 2010 Pearson Education, Inc.

Table 11.2 (2 of 4) Copyright 2010 Pearson Education, Inc. Table 11.2 (3 of 4) Copyright 2010 Pearson Education, Inc. Table 11.2 (4 of 4) Excitatory Synapses and EPSPs Neurotransmitter binds to and opens chemically gated channels that allow simultaneous flow of Na+ and K+ in opposite directions Na+ influx is greater that K+ efflux, causing a net depolarization EPSP helps trigger AP at axon hillock if EPSP is of threshold strength and opens the

voltage-gated channels Copyright 2010 Pearson Education, Inc. Membrane potential (mV) Threshold An EPSP is a local depolarization of the postsynaptic membrane that brings the neuron closer to AP threshold. Neurotransmitter binding opens chemically gated ion channels, allowing the simultaneous passage of Na+ and K+. Stimulus Time (ms) (a) Excitatory postsynaptic potential (EPSP)

Copyright 2010 Pearson Education, Inc. Figure 11.18a Inhibitory Synapses and IPSPs Neurotransmitter binds to and opens channels for K+ or Cl Causes a hyperpolarization (the inner surface of membrane becomes more negative) Reduces the postsynaptic neurons ability to produce an action potential Copyright 2010 Pearson Education, Inc. Membrane potential (mV) Threshold An IPSP is a local

hyperpolarization of the postsynaptic membrane and drives the neuron away from AP threshold. Neurotransmitter binding opens K+ or Cl channels. Stimulus Time (ms) (b) Inhibitory postsynaptic potential (IPSP) Copyright 2010 Pearson Education, Inc. Figure 11.18b Integration: Summation A single EPSP cannot induce an action potential EPSPs can summate to reach threshold IPSPs can also summate with EPSPs,

canceling each other out Copyright 2010 Pearson Education, Inc. Integration: Summation Temporal summation One or more presynaptic neurons transmit impulses in rapid-fire order Spatial summation Postsynaptic neuron is stimulated by a large number of terminals at the same time Copyright 2010 Pearson Education, Inc. E1 E1 Threshold of axon of

postsynaptic neuron Resting potential E1 E1 Time (a) No summation: 2 stimuli separated in time cause EPSPs that do not add together. E1 E1 Time (b) Temporal summation: 2 excitatory stimuli close in time cause EPSPs that add together.

Excitatory synapse 1 (E1) Excitatory synapse 2 (E2) Inhibitory synapse (I1) Copyright 2010 Pearson Education, Inc. Figure 11.19a, b E1 E1 E2 I1 E1 + E2 Time (c) Spatial summation: 2 simultaneous stimuli at

different locations cause EPSPs that add together. Copyright 2010 Pearson Education, Inc. I1 E1 + I1 Time (d) Spatial summation of EPSPs and IPSPs: Changes in membane potential can cancel each other out. Figure 11.19c, d

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