1. Human Anatomy & Physiology Ninth Edition PowerPoint ® Lecture Slides prepared by Barbara Heard, Atlantic Cape Community College C H A P T E R © 2013 Pearson Education, Inc.© Annie Leibovitz/Contact Press Images Fundamentals of the Nervous System and Nervous Tissue: Part C 11

2. © 2013 Pearson Education, Inc. The Synapse Nervous system works because information flows from neuron to neuron Neurons functionally connected by synapses –Junctions that mediate information transfer From one neuron to another neuron Or from one neuron to an effector cell

3. © 2013 Pearson Education, Inc. PLAY Animation: Synapses Important Terminology Presynaptic neuron –Neuron conducting impulses toward synapse –Sends the information Postsynaptic neuron (in Pns may be a neuron, muscle cell, or gland cell) –Neuron transmitting electrical signal away from synapse –Receives the information Most function as both

4. © 2013 Pearson Education, Inc. Figure 11.16 Synapses. Axodendritic synapses Dendrites Cell body Axoaxonal synapses Axon Axosomatic synapses Axon Axosomatic synapses Cell body (soma) of postsynaptic neuron

5. © 2013 Pearson Education, Inc. PLAY Animation: Neurotransmitters Synaptic Cleft Transmission across synaptic cleft –Chemical event (as opposed to an electrical one) –Depends on release, diffusion, and receptor binding of neurotransmitters –Ensures unidirectional communication between neurons

6. © 2013 Pearson Education, Inc. Information Transfer Across Chemical Synapses Neurotransmitter diffuses across synapse Binds to receptors on postsynaptic neuron –Often chemically-gated ion channels Ion channels are opened Causes an excitatory or inhibitory event (graded potential) Neurotransmitter effects terminated

7. © 2013 Pearson Education, Inc. Termination of Neurotransmitter Effects Within a few milliseconds neurotransmitter effect terminated in one of three ways –Reuptake By astrocytes or axon terminal –Degradation By enzymes –Diffusion Away from synaptic cleft

8. © 2013 Pearson Education, Inc. Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters. Presynaptic neuron Action potential arrives at axon terminal. Voltage-gated Ca 2+ channels open and Ca 2+ enters the axon terminal. Ca 2+ entry causes synaptic vesicles to release neurotransmitter by exocytosis Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane. Mitochondrion Axon terminal Synaptic cleft Synaptic vesicles Postsynaptic neuron Ion movement Graded potential Enzymatic degradation Reuptake Postsynaptic neuron Diffusion away from synapse Binding of neurotransmitter opens ion channels, resulting in graded potentials. Neurotransmitter effects are terminated by reuptake through transport proteins, enzymatic degradation, or diffusion away from the synapse. Presynaptic neuron 1 2 3 4 5 6

9. © 2013 Pearson Education, Inc. Synaptic Delay Time needed for neurotransmitter to be released, diffuse across synapse, and bind to receptors –0.3–5.0 ms Synaptic delay is rate-limiting step of neural transmission

10. © 2013 Pearson Education, Inc. Postsynaptic Potentials Neurotransmitter receptors cause graded potentials that vary in strength with –Amount of neurotransmitter released and –Time neurotransmitter stays in area

11. © 2013 Pearson Education, Inc. Table 11.2 Comparison of Graded Potentials and Action Potentials (4 of 4)

12. © 2013 Pearson Education, Inc. Postsynaptic Potentials Types of postsynaptic potentials –EPSP—excitatory postsynaptic potentials –IPSP—inhibitory postsynaptic potentials

13. © 2013 Pearson Education, Inc. Excitatory Synapses and EPSPs Neurotransmitter binding opens chemically gated channels Allows simultaneous flow of Na + and K + in opposite directions Na + influx greater than K + efflux  net depolarization called EPSP (not AP) EPSP help trigger AP if EPSP is of threshold strength – Can spread to axon hillock, trigger opening of voltage-gated channels, and cause AP to be generated

14. © 2013 Pearson Education, Inc. Figure 11.18a Postsynaptic potentials can be excitatory or inhibitory. 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 Na + and K + to pass through simultaneously. Threshold Stimulus +30 0 –55 –70 Time (ms) 10 2030 Membrane potential (mV) Excitatory postsynaptic potential (EPSP)

15. © 2013 Pearson Education, Inc. Inhibitory Synapses and IPSPs Reduces postsynaptic neuron's ability to produce an action potential –Makes membrane more permeable to K + or Cl – If K + channels open, it moves out of cell If Cl - channels open, it moves into cell –Therefore neurotransmitter hyperpolarizes cell Inner surface of membrane becomes more negative AP less likely to be generated

16. © 2013 Pearson Education, Inc. Figure 11.18b Postsynaptic potentials can be excitatory or inhibitory. Threshold Stimulus +30 0 –55 –70 Time (ms) 10 2030 Membrane potential (mV) An IPSP is a local hyperpolarization of the postsynaptic membrane that drives the neuron away from AP threshold. Neurotransmitter binding opens K + or Cl – channels. Inhibitory postsynaptic potential (IPSP)

17. © 2013 Pearson Education, Inc. Synaptic Integration: Summation A single EPSP cannot induce an AP EPSPs can summate to influence postsynaptic neuron IPSPs can also summate Most neurons receive both excitatory and inhibitory inputs from thousands of other neurons –Only if EPSP's predominate and bring to threshold  AP

18. © 2013 Pearson Education, Inc. Neurotransmitters Language of nervous system 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 Classified by chemical structure and by function

19. © 2013 Pearson Education, Inc. Classification of Neurotransmitters: Chemical Structure Acetylcholine (ACh) –First identified; best understood –Released at neuromuscular junctions,by some ANS neurons, by some CNS neurons –Synthesized from acetic and choline by enzyme choline acetyltransferase –Degraded by enzyme acetylcholinesterase (AChE)

20. © 2013 Pearson Education, Inc. Classification of Neurotransmitters: Chemical Structure Biogenic amines Catecholamines –Dopamine, norepinephrine (NE), and epinephrine –Synthesized from amino acid tyrosine Indolamines –Serotonin and histamine –Serotonin synthesized from amino acid tryptophan; histamine synthesized from amino acid histidine Broadly distributed in brain –Play roles in emotional behaviors and biological clock Some ANS motor neurons (especially NE) Imbalances associated with mental illness

21. © 2013 Pearson Education, Inc. Classification of Neurotransmitters: Chemical Structure Gases and lipids - gasotransmitters Nitric oxide (NO), carbon monoxide (CO), hydrogen sulfide gases (H 2 S) Bind with G protein–coupled receptors in the brain Lipid soluble Synthesized on demand NO involved in learning and formation of new memories; brain damage in stroke patients, smooth muscle relaxation in intestine H 2 s acts directly on ion channels to alter function

22. © 2013 Pearson Education, Inc. Classification of Neurotransmitters: Chemical Structure –Endocannabinoids Act at same receptors as THC (active ingredient in marijuana) –Most common G protein-linked receptors in brain Lipid soluble Synthesized on demand Believed involved in learning and memory May be involved in neuronal development, controlling appetite, and suppressing nausea

23. © 2013 Pearson Education, Inc. Classification of Neurotransmitters: Direct versus Indirect Actions Direct action –Neurotransmitter binds to and opens ion channels –Promotes rapid responses by altering membrane potential –Examples: ACh and amino acids

24. © 2013 Pearson Education, Inc. Figure 11.20 Direct neurotransmitter receptor mechanism: Channel-linked receptors. Ion flow blocked Closed ion channel Ligand Ions flow Open ion channel

25. © 2013 Pearson Education, Inc. Classification of Neurotransmitters: Direct versus Indirect 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

26. © 2013 Pearson Education, Inc. G Protein-Linked Receptors: Mechanism Second messengers –Open or close ion channels –Activate kinase enzymes –Phosphorylate channel proteins –Activate genes and induce protein synthesis

27. © 2013 Pearson Education, Inc. Figure 11.21 Indirect neurotransmitter receptor mechanism: G protein–linked receptors. Recall from Chapter 3 that G protein signaling mechanisms are like a molecular relay race. Ligand (1st messenger) Receptor G proteinEnzyme 2nd messenger Neurotransmitter (1st messenger) binds and activates receptor. Receptor G protein Receptor activates G protein. G protein activates adenylate cyclase. Adenylate cyclase Adenylate cyclase converts ATP to cAMP (2nd messenger). cAMP changes membrane permeability by opening or closing ion channels. cAMP activates enzymes. Closed ion channelOpen ion channel cAMP activates specific genes. 1 234 5a 5b Active enzyme Nucleus 5c GDP Slide 8

28. © 2013 Pearson Education, Inc. Basic Concepts of Neural Integration Neurons function in groups Groups contribute to broader neural functions There are billions of neurons in CNS –Must be integration so the individual parts fuse to make a smoothly operating whole

29. © 2013 Pearson Education, Inc. Organization of Neurons: Neuronal Pools Functional groups of neurons –Integrate incoming information received from receptors or other neuronal pools –Forward processed information to other destinations Simple neuronal pool –Single presynaptic fiber branches and synapses with several neurons in pool –Discharge zone—neurons most closely associated with incoming fiber –Facilitated zone—neurons farther away from incoming fiber

30. © 2013 Pearson Education, Inc. Presynaptic (input) fiber Facilitated zone Discharge zoneFacilitated zone Figure 11.22 Simple neuronal pool.

31. © 2013 Pearson Education, Inc. Types of Circuits Circuits –Patterns of synaptic connections in neuronal pools Four types of circuits –Diverging –Converging –Reverberating –Parallel after-discharge

32. © 2013 Pearson Education, Inc. Figure 11.23a Types of circuits in neuronal pools. Input Diverging circuit One input, many outputs An amplifying circuit Example: A single neuron in the brain can activate 100 or more motor neurons in the spinal cord and thousands of skeletal muscle fibers Many outputs

33. © 2013 Pearson Education, Inc. Figure 11.23b Types of circuits in neuronal pools. Converging circuit Many inputs, one output A concentrating circuit Example: Different sensory stimuli can all elicit the same memory Input 1 Input 2Input 3 Output

34. © 2013 Pearson Education, Inc. Figure 11.23c Types of circuits in neuronal pools. Reverberating circuit Signal travels through a chain of neurons, each feeding back to previous neurons An oscillating circuit Controls rhythmic activity Example: Involved in breathing, sleep-wake cycle, and repetitive motor activities such as walking Output Input

35. © 2013 Pearson Education, Inc. Figure 11.23d Types of circuits in neuronal pools. Output Input Parallel after-discharge circuit Signal stimulates neurons arranged in parallel arrays that eventually converge on a single output cell Impulses reach output cell at different times, causing a burst of impulses called an after-discharge Example: May be involved in exacting mental processes such as mathematical calculations

36. © 2013 Pearson Education, Inc. Patterns of Neural Processing: Serial Processing Input travels along one pathway to a specific destination System works in all-or-none manner to produce specific, anticipated response Example – spinal reflexes –Rapid, automatic responses to stimuli –Particular stimulus always causes same response –Occur over pathways called reflex arcs Five components: receptor, sensory neuron, CNS integration center, motor neuron, effector

37. © 2013 Pearson Education, Inc. Figure 11.24 A simple reflex arc. Interneuron Spinal cord (CNS) Stimulus Receptor Sensory neuron Integration center Motor neuron Effector Response 1 2 3 4 5

38. © 2013 Pearson Education, Inc. 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 Example: a sensed smell may remind one of an odor and any associated experiences

39. © 2013 Pearson Education, Inc. Developmental Aspects of Neurons Nervous system originates from neural tube and neural crest formed from ectoderm The neural tube becomes CNS –Neuroepithelial cells of neural tube proliferate to form number of cells needed for development –Neuroblasts become amitotic and migrate –Neuroblasts sprout axons to connect with targets and become neurons

40. © 2013 Pearson Education, Inc. Axonal Growth: Finding the Target Growth cone at tip of axon interacts with its environment via: –Cell surface adhesion proteins (laminin, integrin, and nerve cell adhesion molecules or N-CAMs) which provide anchor points –Neurotropins that attract or repel the growth cone –Nerve growth factor (NGF) which keeps neuroblast alive Once finds target must find right place to form synapse –Astrocytes provide physical support and cholesterol essential for construction of synapses

41. © 2013 Pearson Education, Inc. Figure 11.25 A neuronal growth cone.

42. © 2013 Pearson Education, Inc. Cell Death About 2/3 of neurons die before birth –If do not form synapse with target –Many cells also die due to apoptosis (programmed cell death) during development