1. Copyright © 2010 Pearson Education, Inc. The Synapse A junction that mediates information transfer from one neuron: To another neuron, or To an effector cell (muscle, secretory…) Presynaptic neuron—conducts impulses toward the synapse (sending side) Postsynaptic neuron—transmits impulses away from the synapse (receiving side)

2. Copyright © 2010 Pearson Education, Inc. Types of Synapses Axodendritic—axon of one to dendrite of another Axosomatic—axon of one to soma of another Less common types: Axoaxonic (axon to axon) Dendrodendritic (dendrite to dendrite) Dendrosomatic (dendrite to soma)

3. Copyright © 2010 Pearson Education, Inc. Figure 11.16 Dendrites Cell body Axon Axodendritic synapses Axosomatic synapses Cell body (soma) of postsynaptic neuron Axon (b) Axoaxonic synapses Axosomatic synapses (a)

4. Copyright © 2010 Pearson Education, Inc. 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

5. 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

6. Copyright © 2010 Pearson Education, Inc. Synaptic Cleft Fluid-filled space between presynaptic and postsynaptic neurons, found at chemical synapses Transmission across the synaptic cleft Is a chemical event (not electrical) Involves release, diffusion, and binding of neurotransmitters Provides unidirectional communication between neurons

7. Copyright © 2010 Pearson Education, Inc. Synaptic Transmission Summary Action potential arrives at presynaptic neuron’s axon terminal and opens voltage-gated calcium channels Calcium enters neuron terminal and causes synaptic vesicle fusion with cell membrane Neurotransmitter exocytosis occurs Neurotransmitter diffuses across cleft and binds to receptors on postsynaptic neuron Ion channels in membrane of post-synaptic cell open, causing excitation or inhibition (graded potential) Neurotransmitter diffuses away from receptors as it is broken down in the cleft and/or taken back up by pre- synaptic neuron

8. Copyright © 2010 Pearson Education, Inc. Action potential arrives at axon terminal. Voltage-gated Ca 2+ channels open and Ca 2+ enters the axon terminal. Ca 2+ entry causes neurotransmitter- containing synaptic vesicles to release their contents by exocytosis. Chemical synapses transmit signals from one neuron to another using neurotransmitters. Ca 2+ Synaptic vesicles Axon terminal Mitochondrion Postsynaptic neuron Presynaptic neuron Synaptic cleft Ca 2+ Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane. Postsynaptic neuron 1 2 3 4 Figure 11.17, step 4

9. Copyright © 2010 Pearson Education, Inc. Figure 11.17, step 5 Ion movement Graded potential Binding of neurotransmitter opens ion channels, resulting in graded potentials. 5

10. Copyright © 2010 Pearson Education, Inc. Figure 11.17, step 6 Reuptake Enzymatic degradation Diffusion away from synapse Neurotransmitter effects are terminated by reuptake through transport proteins, enzymatic degradation, or diffusion away from the synapse. 6

11. Copyright © 2010 Pearson Education, Inc. Termination of Neurotransmitter Effects Neurotransmitter effect is terminated in a few milliseconds by Degradation by enzymes Reuptake by astrocytes or axon terminal Diffusion away from synaptic cleft

12. Copyright © 2010 Pearson Education, Inc. Synaptic Delay Neurotransmitter must be released, diffuse across synapse, and bind to receptors Synaptic delay = time needed to do this (0.3-5.0 ms) Synaptic delay is the rate-limiting step of neural transmission (in short neurons at least)

13. 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 EPSP—excitatory postsynaptic potentials IPSP—inhibitory postsynaptic potentials

14. Copyright © 2010 Pearson Education, Inc. Table 11.2 (1 of 4)

15. Copyright © 2010 Pearson Education, Inc. Table 11.2 (2 of 4)

16. Copyright © 2010 Pearson Education, Inc. Table 11.2 (3 of 4)

17. Copyright © 2010 Pearson Education, Inc. Table 11.2 (4 of 4)

18. Copyright © 2010 Pearson Education, Inc. 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 than 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

19. Copyright © 2010 Pearson Education, Inc. Figure 11.18a 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 pas- sage of Na + and K +. Time (ms) (a) Excitatory postsynaptic potential (EPSP) Threshold Stimulus Membrane potential (mV)

20. Copyright © 2010 Pearson Education, Inc. Inhibitory Synapses and IPSPs Neurotransmitter binds to and opens channels for K + or Cl – Causes hyperpolarization (inside of cell becomes more negative) Reduces the postsynaptic neuron’s ability to produce an action potential

21. Copyright © 2010 Pearson Education, Inc. Figure 11.18b 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. Time (ms) (b) Inhibitory postsynaptic potential (IPSP) Threshold Stimulus Membrane potential (mV)

22. Copyright © 2010 Pearson Education, Inc. Integration: Summation One EPSP cannot induce an action potential EPSPs can sum to reach threshold IPSPs and EPSPs can cancel each other out

23. Copyright © 2010 Pearson Education, Inc. Integration: Summation Temporal summation One presynaptic neuron sends several or many impulses in a short time to the postsynaptic neuron Spatial summation Multiple presynaptic neurons stimulate the post-synaptic neuron simultaneously

24. Copyright © 2010 Pearson Education, Inc. Figure 11.19a, b Threshold of axon of postsynaptic neuron Excitatory synapse 1 (E 1 ) Excitatory synapse 2 (E 2 ) Inhibitory synapse (I 1 ) Resting potential E1E1 E1E1 E1E1 E1E1 (a)No summation: 2 stimuli separated in time cause EPSPs that do not add together. (b) Temporal summation: 2 excitatory stimuli close in time cause EPSPs that add together. Time E1E1 E1E1

25. Copyright © 2010 Pearson Education, Inc. Figure 11.19c, d E 1 + E 2 I1I1 E 1 + I 1 (d)Spatial summation of EPSPs and IPSPs: Changes in membane potential can cancel each other out. (c) Spatial summation: 2 simultaneous stimuli at different locations cause EPSPs that add together. Time E1E1 E2E2 I1I1 E1E1

26. Copyright © 2010 Pearson Education, Inc. Neurotransmitters Most neurons make two or more neurotransmitters, which are released at different stimulation frequencies 50 or more neurotransmitters have been identified Classified by chemical structure and by function

27. Copyright © 2010 Pearson Education, Inc. Chemical Classes of Neurotransmitters Acetylcholine (Ach) Released at neuromuscular junctions and some autonomic neurons Synthesized in the pre-synaptic neuron Degraded by acetylcholinesterase (AChE)

28. Copyright © 2010 Pearson Education, Inc. Chemical Classes of Neurotransmitters Biogenic amines include: Norepinephrine (NE) Epinephrine Serotonin Many others Broadly distributed in the brain Play roles in emotional behaviors and the biological clock

29. Copyright © 2010 Pearson Education, Inc. Chemical Classes of Neurotransmitters Amino acids include: GABA—Gamma (  )-aminobutyric acid Glutamate Many others

30. Copyright © 2010 Pearson Education, Inc. Functional Classification of Neurotransmitters Excitatory (depolarizing) and/or inhibitory (hyperpol.) Determined by receptor type on postsynaptic neuron GABA usually inhibitory Glutamate, epinephrine usually excitatory Acetylcholine Excitatory at neuromuscular junctions in skeletal muscle Inhibitory in cardiac muscle

31. Copyright © 2010 Pearson Education, Inc. Neurotransmitter Actions Direct action Neurotransmitter binds to channel-linked receptor and opens ion channels Promotes rapid responses Examples: ACh; glutamate and GABA at some of their synapses

32. Copyright © 2010 Pearson Education, Inc. Neurotransmitter Actions Indirect action Neurotransmitter binds to a G protein-linked receptor and acts through an intracellular second messenger Promotes long-lasting effects Examples: Norepinephrine, epinephrine, serotonin; glutamate and GABA at some of their synapses