Chapter 12 *Lecture PowerPoint

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1 Chapter 12 *Lecture PowerPoint Nervous Tissue *See separate FlexArt PowerPoint slides for all figures and tables preinserted into PowerPoint without notes. Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2 Introduction The nervous system is one of great complexity Nervous system is the foundation of our conscious experience, personality, and behavior Neurobiology combines the behavioral and life sciences 11-2

3 Overview of the Nervous System Expected Learning Outcomes Describe the overall function of the nervous system. Describe its major anatomical and functional subdivisions. 11-3

4 Overview of the Nervous System Endocrine and nervous systems maintain internal coordination Endocrine system: communicates by means of chemical messengers (hormones) secreted into to the blood Nervous system: employs electrical and chemical means to send messages from cell to cell 12-4

5 Overview of the Nervous System Nervous system carries out its task in three basic steps Sense organs receive information about changes in the body and the external environment, and transmit coded messages to the spinal cord and the brain Brain and spinal cord process this information, relate it to past experiences, and determine what response is appropriate to the circumstances Brain and spinal cord issue commands to muscles and gland cells to carry out such a response 12-5

6 Overview of the Nervous System Nervous system has two major anatomical subdivisions Central nervous system (CNS) Brain and spinal cord enclosed in bony coverings Enclosed by cranium and vertebral column Peripheral nervous system (PNS) All the nervous system except the brain and spinal cord; composed of nerves and ganglia Nerve a bundle of nerve fibers (axons) wrapped in fibrous connective tissue Ganglion a knotlike swelling in a nerve where neuron cell bodies are concentrated 12-6

7 Overview of the Nervous System Peripheral nervous system has two major functional subdivisions Sensory (afferent) division: carries sensory signals from various receptors to the CNS Informs the CNS of stimuli within or around the body Somatic sensory division: carries signals from receptors in the skin, muscles, bones, and joints Visceral sensory division: carries signals from the viscera of the thoracic and abdominal cavities Heart, lungs, stomach, and urinary bladder 12-7

8 Overview of the Nervous System Motor (efferent) division carries signals from the CNS to gland and muscle cells that carry out the body s response Effectors: cells and organs that respond to commands from the CNS Somatic motor division: carries signals to skeletal muscles Output produces muscular contraction as well as somatic reflexes involuntary muscle contractions 12-8

9 Overview of the Nervous System Visceral motor division (autonomic nervous system) Carries signals to glands, cardiac muscle, and smooth muscle Involuntary, and responses of this system and its receptors are visceral reflexes Sympathetic division Tends to arouse body for action Accelerating heart beat and respiration, while inhibiting digestive and urinary systems Parasympathetic division Tends to have calming effect Slows heart rate and breathing Stimulates digestive and urinary systems 12-9

10 Subdivisions of the Nervous System Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Central nervous system (CNS) Brain Peripheral nervous system (PNS) Spinal cord Nerves Ganglia Figure

11 Subdivisions of the Nervous System Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Central nervous system Peripheral nervous system Brain Spinal cord Sensory division Motor division Visceral sensory division Somatic sensory division Visceral motor division Somatic motor division Figure 12.2 Sympathetic division Parasympathetic division 12-11

12 Properties of Neurons Expected Learning Outcomes Describe three functional properties found in all neurons. Define the three most basic functional categories of neurons. Identify the parts of a neuron. Explain how neurons transport materials between the cell body and tips of the axon

13 Universal Properties Excitability (irritability) Respond to environmental changes called stimuli Conductivity Neurons respond to stimuli by producing electrical signals that are quickly conducted to other cells at distant locations Secretion When electrical signal reaches end of nerve fiber, a chemical neurotransmitter is secreted that crosses the gap and stimulates the next cell 12-13

14 Functional Classes Three general classes of neurons (sensory, interneuron, motor) based on function Sensory (afferent) neurons Specialized to detect stimuli Transmit information about them to the CNS Begin in almost every organ in the body and end in CNS Afferent conducting signals toward CNS 12-14

15 Functional Classes Three general classes of neurons (cont.) Interneurons (association neurons) Lie entirely within the CNS Receive signals from many neurons and carry out the integrative function Process, store, and retrieve information and make decisions that determine how the body will respond to stimuli 90% of all neurons are interneurons Lie between and interconnect the incoming sensory pathways and the outgoing motor pathways of the CNS 12-15

16 Functional Classes Three general classes of neurons (cont.) Motor (efferent) neuron Send signals out to muscles and gland cells (the effectors) Motor because most of them lead to muscles Efferent neurons conduct signals away from the CNS 12-16

17 Classes of Neurons Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Peripheral nervous system Central nervous system 1 Sensory (afferent) neurons conduct signals from receptors to the CNS. 3 Motor (efferent) neurons conduct signals from the CNS to effectors such as muscles and glands. 2 Inter neurons (association neurons) are confined to the CNS. Figure

18 Structure of a Neuron Soma the control center of the neuron Also called neurosoma, cell body, or perikaryon Has a single, centrally located nucleus with large nucleolus Cytoplasm contains mitochondria, lysosomes, a Golgi complex, numerous inclusions, and extensive rough endoplasmic reticulum and cytoskeleton Cytoskeleton consists of dense mesh of microtubules and neurofibrils (bundles of actin filaments) Compartmentalizes rough ER into dark-staining Nissl bodies Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Dendrites Soma Nucleus Nucleolus Axon Direction of signal transmission (a) Node of Ranvier Terminal arborization Synaptic knobs Trigger zone: Axon hillock Initial segment Axon collateral Figure 12.4a Internodes Myelin sheath Schwann cell 12-18

19 Structure of a Neuron Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Soma the control center of the neuron (cont.) No centrioles: no further cell division Inclusions: glycogen granules, lipid droplets, melanin, and lipofuscin (golden brown pigment produced when lysosomes digest worn-out organelles) Lipofuscin accumulates with age Wear-and-tear granules Most abundant in old neurons Dendrites Soma Nucleus Nucleolus Axon Direction of signal transmission Node of Ranvier Trigger zone: Axon hillock Initial segment Axon collateral Internodes Myelin sheath Schwann cell Terminal arborization Synaptic knobs (a) Figure 12.4a 12-19

20 Structure of a Neuron Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Dendrites vast number of branches coming from a few thick branches from the soma Resemble bare branches of a tree in winter Primary site for receiving signals from other neurons The more dendrites the neuron has, the more information it can receive and incorporate into decision making Provide precise pathway for the reception and processing of neural information Dendrites Soma Nucleus Nucleolus Axon Direction of signal transmission Node of Ranvier Terminal arborization Synaptic knobs Trigger zone: Axon hillock Initial segment Axon collateral Internodes Myelin sheath Schwann cell (a) Figure 12.4a 12-20

21 Structure of a Neuron Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Axon (nerve fiber) originates from a mound on one side of the soma called the axon hillock Cylindrical, relatively unbranched for most of its length Axon collaterals branches of axon Branch extensively on distal end Specialized for rapid conduction of nerve signals to points remote to the soma Dendrites Soma Nucleus Nucleolus Axon Direction of signal transmission Node of Ranvier Terminal arborization Trigger zone: Axon hillock Initial segment Axon collateral Internodes Myelin sheath Schwann cell Synaptic knobs (a) Figure 12.4a 12-21

22 Structure of a Neuron Axon (nerve fiber) (cont.) Axoplasm: cytoplasm of axon Axolemma: plasma membrane of axon Only one axon per neuron Schwann cells and myelin sheath enclose axon Distal end, axon has terminal arborization: extensive complex of fine branches Synaptic knob (terminal button) little swelling that forms a junction (synapse) with the next cell Contains synaptic vesicles full of neurotransmitter Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Dendrites Soma Nucleus Nucleolus Axon Direction of signal transmission Node of Ranvier Terminal arborization Synaptic knobs Trigger zone: Axon hillock Initial segment Axon collateral Internodes Myelin sheath Schwann cell (a) Figure 12.4a 12-22

23 Structure of a Neuron Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Multipolar neuron One axon and multiple dendrites Most common Most neurons in the brain and spinal cord Bipolar neuron One axon and one dendrite Olfactory cells, retina, inner ear Unipolar neuron Single process leading away from the soma Sensory from skin and organs to spinal cord Anaxonic neuron Many dendrites but no axon Help in visual processes Dendrites Dendrites Axon Multipolar neurons Dendrites Axon Bipolar neurons Unipolar neuron Dendrites Anaxonic neuron Axon Figure

24 Axonal Transport Many proteins made in soma must be transported to axon and axon terminal To repair axolemma, serve as gated ion channel proteins, as enzymes or neurotransmitters Axonal transport two-way passage of proteins, organelles, and other material along an axon Anterograde transport: movement down the axon away from soma Retrograde transport: movement up the axon toward the soma Microtubules guide materials along axon Motor proteins (kinesin and dynein) carry materials on their backs while they crawl along microtubules Kinesin motor proteins in anterograde transport Dynein motor proteins in retrograde transport 12-24

25 Axonal Transport Fast axonal transport occurs at a rate of 20 to 400 mm/day Fast anterograde transport (up to 400 mm/day) Organelles, enzymes, synaptic vesicles, and small molecules Fast retrograde transport For recycled materials and pathogens rabies, herpes simplex, tetanus, polio viruses Delay between infection and symptoms is time needed for transport up the axon Slow axonal transport or axoplasmic flow 0.5 to 10 mm/day Always anterograde Moves enzymes, cytoskeletal components, and new axoplasm down the axon during repair and regeneration of damaged axons Damaged nerve fibers regenerate at a speed governed by slow axonal transport 12-25

26 Supportive Cells (Neuroglia) Expected Learning Outcomes Name the six types of cells that aid neurons and state their respective functions. Describe the myelin sheath that is found around certain nerve fibers and explain its importance. Describe the relationship of unmyelinated nerve fibers to their supportive cells. Explain how damaged nerve fibers regenerate

27 Supportive Cells (Neuroglia) About 1 trillion (10 12 ) neurons in the nervous system Neuroglia outnumber the neurons by as much as 50 to 1 Neuroglia or glial cells Support and protect the neurons Bind neurons together and form framework for nervous tissue In fetus, guide migrating neurons to their destination If mature neuron is not in synaptic contact with another neuron it is covered by glial cells Prevents neurons from touching each other Gives precision to conduction pathways 12-27

28 Types of Neuroglia Four types occur only in CNS Oligodendrocytes Form myelin sheaths in CNS Each armlike process wraps around a nerve fiber forming an insulating layer that speeds up signal conduction Ependymal cells Line internal cavities of the brain Cuboidal epithelium with cilia on apical surface Secretes and circulates cerebrospinal fluid (CSF) Clear liquid that bathes the CNS 12-28

29 Types of Neuroglia Four types occur only in CNS (cont.) Microglia Small, wandering macrophages formed white blood cell called monocytes Thought to perform a complete checkup on the brain tissue several times a day Wander in search of cellular debris to phagocytize 12-29

30 Types of Neuroglia Four types occur only in CNS (cont.) Astrocytes Most abundant glial cell in CNS Cover entire brain surface and most nonsynaptic regions of the neurons in the gray matter of the CNS Diverse functions Form a supportive framework of nervous tissue Have extensions (perivascular feet) that contact blood capillaries that stimulate them to form a tight seal called the blood brain barrier Convert blood glucose to lactate and supply this to the neurons for nourishment 12-30

31 Types of Neuroglia Cont. Nerve growth factors secreted by astrocytes promote neuron growth and synapse formation Communicate electrically with neurons and may influence synaptic signaling Regulate chemical composition of tissue fluid by absorbing excess neurotransmitters and ions Astrocytosis or sclerosis when neuron is damaged, astrocytes form hardened scar tissue and fill space formerly occupied by the neuron 12-31

32 Types of Neuroglia Two types occur only in PNS Schwann cells Envelope nerve fibers in PNS Wind repeatedly around a nerve fiber Produce a myelin sheath similar to the ones produced by oligodendrocytes in CNS Assist in the regeneration of damaged fibers Satellite cells Surround the neurosomas in ganglia of the PNS Provide electrical insulation around the soma Regulate the chemical environment of the neurons 12-32

33 Neuroglial Cells of CNS Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Capillary Neurons Astrocyte Oligodendrocyte Perivascular feet Myelinated axon Ependymal cell Myelin (cut) Cerebrospinal fluid Microglia Figure

34 Glial Cells and Brain Tumors Tumors masses of rapidly dividing cells Mature neurons have little or no capacity for mitosis and seldom form tumors Brain tumors arise from: Meninges (protective membranes of CNS) Metastasis from nonneuronal tumors in other organs Often glial cells that are mitotically active throughout life Gliomas grow rapidly and are highly malignant Blood brain barrier decreases effectiveness of chemotherapy Treatment consists of radiation or surgery 12-34

35 Myelin Myelin sheath an insulating layer around a nerve fiber Formed by oligodendrocytes in CNS and Schwann cells in PNS Consists of the plasma membrane of glial cells 20% protein and 80% lipid Myelination production of the myelin sheath Begins at week 14 of fetal development Proceeds rapidly during infancy Completed in late adolescence Dietary fat is important to CNS development 12-35

36 Myelin In PNS, Schwann cell spirals repeatedly around a single nerve fiber Lays down as many as a hundred layers of its own membrane No cytoplasm between the membranes Neurilemma: thick, outermost coil of myelin sheath Contains nucleus and most of its cytoplasm External to neurilemma is basal lamina and a thin layer of fibrous connective tissue endoneurium 12-36

37 Myelin Sheath in PNS Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Axoplasm Schwann cell nucleus Axolemma Neurilemma Figure 12.4c (c) Myelin sheath Nodes of Ranvier and internodes 12-37

38 Myelin In CNS oligodendrocytes reach out to myelinate several nerve fibers in its immediate vicinity Anchored to multiple nerve fibers Cannot migrate around any one of them like Schwann cells Must push newer layers of myelin under the older ones; so myelination spirals inward toward nerve fiber Nerve fibers in CNS have no neurilemma or endoneurium 12-38

39 Myelin Many Schwann cells or oligodendrocytes are needed to cover one nerve fiber Myelin sheath is segmented Nodes of Ranvier: gap between segments Internodes: myelin-covered segments from one gap to the next Initial segment: short section of nerve fiber between the axon hillock and the first glial cell Trigger zone: the axon hillock and the initial segment Play an important role in initiating a nerve signal 12-39

40 Myelination in CNS Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Oligodendrocyte Myelin Nerve fiber Figure 12.7b (b) 12-40

41 Myelination in PNS Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Schwann cell Axon Basal lamina Endoneurium Nucleus (a) Figure 12.7a Neurilemma Myelin sheath 12-41

42 Diseases of the Myelin Sheath Degenerative disorders of the myelin sheath Multiple sclerosis Oligodendrocytes and myelin sheaths in the CNS deteriorate Myelin replaced by hardened scar tissue Nerve conduction disrupted (double vision, tremors, numbness, speech defects) Onset between 20 and 40 and fatal from 25 to 30 years after diagnosis Cause may be autoimmune triggered by virus 12-42

43 Diseases of the Myelin Sheath Degenerative disorders of the myelin sheath (cont.) Tay Sachs disease: a hereditary disorder of infants of Eastern European Jewish ancestry Abnormal accumulation of glycolipid called GM 2 in the myelin sheath Normally decomposed by lysosomal enzyme Enzyme missing in individuals homozygous for Tay Sachs allele Accumulation of ganglioside (GM 2 ) disrupts conduction of nerve signals Blindness, loss of coordination, and dementia Fatal before age

44 Unmyelinated Nerve Fibers Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Neurilemma Myelin sheath Unmyelinated nerve fibers Myelinated axon Schwann cell cytoplasm Basal lamina Neurilemma Unmyelinated axon (c) 3 µm c: The McGraw-Hill Companies, Inc./Dr. Dennis Emery, Dept. of Zoology and Genetics, Iowa State University, photographer Figure 12.7c Figure 12.8 Schwann cell Basal lamina Schwann cells hold 1 to 12 small nerve fibers in grooves on the surface Membrane folds once around each fiber overlapping itself along the edges Mesaxon neurilemma wrapping of unmyelinated nerve fibers 12-44

45 Conduction Speed of Nerve Fibers Speed at which a nerve signal travels along a nerve fiber depends on two factors Diameter of fiber Presence or absence of myelin Signal conduction occurs along the surface of a fiber Larger fibers have more surface area and conduct signals more rapidly Myelin further speeds signal conduction 12-45

46 Conduction Speed of Nerve Fibers Conduction speed Small, unmyelinated fibers: 0.5 to 2.0 m/s Small, myelinated fibers: 3 to 15.0 m/s Large, myelinated fibers: up to 120 m/s Slow signals supply the stomach and dilate pupil where speed is less of an issue Fast signals supply skeletal muscles and transport sensory Signals for vision and balance 12-46

47 Regeneration of Nerve Fibers Regeneration of a damaged peripheral nerve fiber can occur if: Its soma is intact At least some neurilemma remains Fiber distal to the injury cannot survive and degenerates Macrophages clean up tissue debris at the point of injury and beyond Soma swells, ER breaks up, and nucleus moves off center Due to loss of nerve growth factor from neuron s target cell Axon stump sprouts multiple growth processes Severed distal end continues to degenerate 12-47

48 Regeneration of Nerve Fibers Regeneration tube formed by Schwann cells, basal lamina, and the neurilemma near the injury Regeneration tube guides the growing sprout back to the original target cells and reestablishes synaptic contact Nucleus returns to normal shape Regeneration of damaged nerve fibers in the CNS cannot occur at all 12-48

49 Regeneration of Nerve Fiber Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Endoneurium Neuromuscular junction Myelin sheath Muscle fiber 1 Normal nerve fiber Local trauma 2 Injured fiber Macrophages 3 Degeneration of severed fiber 4 Early regeneration Growth processes Degenerating terminal Degenerating Schwann cells Degenerating axon Schwann cells Regeneration tube Atrophy of muscle fibers Denervation atrophy of muscle due to loss of nerve contact by damaged nerve Retraction of growth processes Growth processes 5 Late regeneration 6 Regenerated fiber Regrowth of muscle fibers Figure

50 Nerve Growth Factor Nerve growth factor (NGF) a protein secreted by a gland, muscle, and glial cells and picked up by the axon terminals of the neurons Prevents apoptosis (programmed cell death) in growing neurons Enables growing neurons to make contact with their target cells Isolated by Rita Levi-Montalcini in 1950s Won Nobel prize in 1986 with Stanley Cohen Use of growth factors is now a vibrant field of research Figure

51 Electrophysiology of Neurons Expected Learning Outcomes Explain why a cell has an electrical charge difference (voltage) across its membrane. Explain how stimulation of a neuron causes a local electrical response in its membrane. Explain how local responses generate a nerve signal. Explain how the nerve signal is conducted down an axon

52 Electrophysiology of Neurons Galen thought that the brain pumped a vapor called psychic pneuma through hollow nerves and squirted it into the muscles to make them contract René Descartes in the seventeenth century supported this theory Luigi Galvani discovered the role of electricity in muscle contraction in the eighteenth century Camillo Golgi developed an important method for staining neurons with silver in the nineteenth century 12-52

53 Electrophysiology of Neurons Santiago Ramón y Cajal set forth the neuron doctrine: nervous pathway is not a continuous wire or tube, but a series of cells separated by gaps called synapses Neuron doctrine brought up two key questions How does a neuron generate an electrical signal? How does it transmit a meaningful message to the next cell? 12-53

54 Electrical Potentials and Currents Electrophysiology cellular mechanisms for producing electrical potentials and currents Basis for neural communication and muscle contraction Electrical potential a difference in the concentration of charged particles between one point and another Electrical current a flow of charged particles from one point to another In the body, currents are movements of ions, such as Na + or K +, through gated channels in the plasma membrane Gated channels are opened or closed by various stimuli Enables cell to turn electrical currents on and off 12-54

55 Electrical Potentials and Currents Living cells are polarized Resting membrane potential (RMP) charge difference across the plasma membrane About 70 mv in a resting, unstimulated neuron Negative value means there are more negatively charged particles on the inside of the membrane than on the outside 12-55

56 The Resting Membrane Potential Resting membrane potential (RMP) exists because of unequal electrolyte distribution between extracellular fluid (ECF) and intracellular fluid (ICF) RMP results from the combined effect of three factors Ions diffuse down their concentration gradient through the membrane Plasma membrane is selectively permeable and allows some ions to pass easier than others Electrical attraction of cations and anions to each other 12-56

57 The Resting Membrane Potential Potassium ions (K + ) have the greatest influence on RMP Plasma membrane is more permeable to K + than any other ion Leaks out until electrical charge of cytoplasmic anions attracts it back in and equilibrium is reached and net diffusion of K + stops K + is about 40 times as concentrated in the ICF as in the ECF Cytoplasmic anions cannot escape due to size or charge (phosphates, sulfates, small organic acids, proteins, ATP, and RNA) 12-57

58 The Resting Membrane Potential Membrane much less permeable to high concentration of sodium (Na + ) found outside the cell Some leaks and diffuses into the cell down its concentration gradient Na + is about 12 times as concentrated in the ECF as in the ICF Resting membrane is much less permeable to Na + than to K

59 The Resting Membrane Potential Na + /K + pumps out 3 Na + for every 2 K + it brings in Works continuously to compensate for Na+ and K+ leakage, and requires great deal of ATP 70% of the energy requirement of the nervous system Necessitates glucose and oxygen be supplied to nerve tissue (energy needed to create the resting potential) Pump contributes about 3 mv to the cell s resting membrane potential of 70 mv 12-59

60 The Resting Membrane Potential Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ECF Figure Na m Eq/L K + 4 m Eq/L ICF Na + channel K + channel Na + 12 m Eq/L K m Eq/L Large anions that cannot escape cell Na + concentrated outside of cell (ECF) K + concentrated inside cell (ICF) 12-60

61 Local Potentials Local potentials disturbances in membrane potential when a neuron is stimulated Neuron response begins at the dendrite, spreads through the soma, travels down the axon, and ends at the synaptic knobs 12-61

62 Local Potentials When neuron is stimulated by chemicals, light, heat, or mechanical disturbance Opens the Na + gates and allows Na + to rush into the cell Na + inflow neutralizes some of the internal negative charge Voltage measured across the membrane drifts toward zero Depolarization: case in which membrane voltage shifts to a less negative value Na + diffuses for short distance on the inside of the plasma membrane producing a current that travels toward the cell s trigger zone; this short-range change in voltage is called a local potential 12-62

63 Local Potentials Differences of local potentials from action potentials Graded: vary in magnitude with stimulus strength Stronger stimuli open more Na + gates Decremental: get weaker the farther they spread from the point of stimulation Voltage shift caused by Na + inflow diminishes rapidly with distance Reversible: when stimulation ceases, K + diffusion out of cell returns the cell to its normal resting potential Either excitatory or inhibitory: some neurotransmitters (glycine) make the membrane potential more negative hyperpolarize it so it becomes less sensitive and less likely to produce an action potential 12-63

64 Local Potentials Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Dendrites Soma Trigger zone Axon Current ECF Ligand Receptor Plasma membrane of dendrite Na + ICF Figure

65 Action Potentials Action potential more dramatic change produced by voltage-regulated ion gates in the plasma membrane Only occur where there is a high enough density of voltage-regulated gates Soma (50 to 75 gates per m 2 ); cannot generate an action potential Trigger zone (350 to 500 gates per m 2 ); where action potential is generated If excitatory local potential spreads all the way to the trigger zone and is still strong enough when it arrives, it can open these gates and generate an action potential 12-65

66 Action Potentials Action potential is a rapid up-and-down shift in the membrane voltage Sodium ions arrive at the axon hillock Depolarize the membrane at that point Threshold: critical voltage to which local potentials must rise to open the voltage-regulated gates 55 mv When threshold is reached, neuron fires and produces an action potential More and more Na + channels open in the trigger zone in a positive feedback cycle creating a rapid rise in membrane voltage, called spike 12-66

67 Action Potentials When rising membrane potential passes 0 mv, Na + gates are inactivated Begin closing; when all closed, the voltage peaks at +35 mv Membrane now positive on the inside and negative on the outside Polarity reversed from RMP depolarization By the time the voltage peaks, the slow K + gates are fully open K + repelled by the positive intracellular fluid now exit the cell Their outflow repolarizes the membrane; shifts the voltage back to negative numbers returning toward RMP 12-67

68 Action Potentials K + gates stay open longer than the Na + gates Slightly more K + leaves the cell than Na + entering Drops the membrane voltage 1 or 2 mv more negative than the original RMP negative overshoot hyperpolarization or afterpotential Na + and K + switch places across the membrane during an action potential 12-68

69 mv Action Potentials Only a thin layer of the cytoplasm next to the cell membrane is affected In reality, very few ions are involved Action potential is often called a spike, as it happens so fast +35 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display (a) Local potential Depolarization Threshold Resting membrane potential Time 5 Repolarization 6 Action potential 7 Hyperpolarization Figure 12.13a 12-69

70 mv Action Potentials Characteristics of action potential versus a local potential Follows an all-or-none law If threshold is reached, neuron fires at its maximum voltage If threshold is not reached, it does not fire Nondecremental: do not get weaker with distance Irreversible: once started goes to completion and cannot be stopped +35 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display (a) Local potential Depolarization Threshold Resting membrane potential Time 5 Repolarization Figure 12.13a 6 Action potential 7 Hyperpolarization 12-70

71 mv mv Action Potential vs. Local Potential Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display Spike 0 Depolarization Threshold Repolarization Action potential Local potential 2 1 Resting membrane potential 6 7 Hyperpolarization 70 Hyperpolarization (a) Time (b) ms Figure 12.13a,b 12-71

72 mv mv mv mv Sodium and Potassium Channels Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Na + K + K + channel Na + channel Na + and K + channels closed 70 Resting membrane potential 2 Na + channels open, Na + enters cell, K + channels beginning to open 70 Depolarization begins 3 Na + channels closed, K + 4 channels fully open, K + 70 leaves cell 35 0 Depolarization ends, repolarization begins Na + channels closed, K + channels closing Repolarization complete Figure

73 mv The Refractory Period Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. During an action potential and for a few milliseconds after, it is difficult or impossible to stimulate that region of a neuron to fire again Absolute refractory period Relative refractory period Refractory period the period of resistance to stimulation Time Threshold Figure Resting membrane potential 12-73

74 mv The Refractory Period Two phases of the refractory period Absolute refractory period No stimulus of any strength will trigger AP As long as Na + gates are open From action potential to RMP Relative refractory period Only especially strong stimulus will trigger new AP Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Threshold K + gates are still open and any effect of Time incoming Na + is opposed Figure by the outgoing K + 0 Absolute refractory period Relative refractory period Resting membrane potential 12-74

75 mv The Refractory Period Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The refractory period refers only to a small patch of the neuron s membrane at one time Absolute refractory period Relative refractory period Other parts of the neuron can be stimulated while the small part is in refractory period Time Figure Threshold Resting membrane potential 12-75

76 Signal Conduction in Nerve Fibers For communication to occur, the nerve signal must travel to the end of the axon Unmyelinated fiber has voltage-regulated ion gates along its entire length Action potential from the trigger zone causes Na + to enter the axon and diffuse into adjacent regions beneath the membrane 12-76

77 Signal Conduction in Nerve Fibers The depolarization excites voltage-regulated gates immediately distal to the action potential Na + and K + gates open and close producing a new action potential By repetition the membrane distal to that is excited Chain reaction continues to the end of the axon 12-77

78 Signal Conduction in Nerve Fibers Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Dendrites Cell body Axon Signal Action potential in progress Refractory membrane Excitable membrane Figure

79 Signal Conduction in Nerve Fibers Voltage-gated channels needed for APs Fewer than 25 per m 2 in myelin-covered regions (internodes) Up to 12,000 per m 2 in nodes of Ranvier Fast Na + diffusion occurs between nodes Signal weakens under myelin sheath, but still strong enough to stimulate an action potential at next node Saltatory conduction nerve signal seems to jump from node to node Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Figure 12.17a (a) Na + inflow at node generates action potential (slow but non decremental) Na + diffuses along inside of axolemma to next node (fast but decremental) Excitation of voltageregulated gates will generate next action potential here 12-79

80 Signal Conduction in Nerve Fibers Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display (b) Action potential in progress Refractory membrane Excitable membrane Much faster than conduction in unmyelinated fibers 12-80

81 Synapses Expected Learning Outcomes Explain how messages are transmitted from one neuron to another. Give examples of neurotransmitters and neuromodulators and describe their actions. Explain how stimulation of a postsynaptic cell is stopped

82 Synapses A nerve signal can go no further when it reaches the end of the axon Triggers the release of a neurotransmitter Stimulates a new wave of electrical activity in the next cell across the synapse Synapse between two neurons First neuron in the signal path is the presynaptic neuron Releases neurotransmitter Second neuron is postsynaptic neuron Responds to neurotransmitter 12-82

83 Synapses Presynaptic neuron may synapse with a dendrite, soma, or axon of postsynaptic neuron to form axodendritic, axosomatic, or axoaxonic synapses A neuron can have an enormous number of synapses Spinal motor neuron covered by about 10,000 synaptic knobs from other neurons 8,000 ending on its dendrites 2,000 ending on its soma In the cerebellum of brain, one neuron can have as many as 100,000 synapses 12-83

84 The Discovery of Neurotransmitters Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Soma Synapse Axon Presynaptic neuron Directionof signal transmission Postsynaptic neuron (a) Axodendritic synapse Figure Axosomatic synapse Axoaxonic synapse (b) 12-84

85 The Discovery of Neurotransmitters Synaptic cleft gap between neurons was discovered by Ramón y Cajal through histological observations Otto Loewi, in 1921, demonstrated that neurons communicate by releasing chemicals chemical synapses He flooded exposed hearts of two frogs with saline Stimulated vagus nerve of the first frog and the heart slowed Removed saline from that frog and found it slowed heart of second frog Named it Vagusstoffe ( vagus substance ) Later renamed acetylcholine, the first known neurotransmitter 12-85

86 The Discovery of Neurotransmitters Electrical synapses do exist Some neurons, neuroglia, and cardiac and single-unit smooth muscle Gap junctions join adjacent cells Ions diffuse through the gap junctions from one cell to the next Advantage of quick transmission No delay for release and binding of neurotransmitter Cardiac and smooth muscle and some neurons Disadvantage is they cannot integrate information and make decisions Ability reserved for chemical synapses in which neurons communicate by releasing neurotransmitters 12-86

87 Synaptic Knobs Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Axon of presynaptic neuron Synaptic knob Soma of postsynaptic neuron Omikron/Science Source/PhotoResearchers, Inc. Figure

88 Structure of a Chemical Synapse Synaptic knob of presynaptic neuron contains synaptic vesicles containing neurotransmitter Many docked on release sites on plasma membrane Ready to release neurotransmitter on demand A reserve pool of synaptic vesicles located further away from membrane Postsynaptic neuron membrane contains proteins that function as receptors and ligandregulated ion gates 12-88

89 Structure of a Chemical Synapse Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Microtubules ofcytoskeleton Axon of presynaptic neuron Mitochondria Postsynaptic neuron Synaptic knob Synaptic vesicles containing neurotransmitter Synaptic cleft Postsynaptic neuron Neurotransmitter receptor Neurotransmitter release Presynaptic neurons have synaptic vesicles with neurotransmitter and postsynaptic have receptors and ligand-regulated ion channels 12-89

90 Neurotransmitters and Related Messengers More than 100 neurotransmitters have been identified Fall into four major categories according to chemical composition Acetylcholine In a class by itself Formed from acetic acid and choline Amino acid neurotransmitters Include glycine, glutamate, aspartate, and -aminobutyric acid (GABA) 12-90

91 Cont. Neurotransmitters and Related Monoamines Messengers Synthesized from amino acids by removal of the COOH group Retaining the NH 2 (amino) group Major monoamines Epinephrine, norepinephrine, dopamine (catecholamines) Histamine and serotonin Neuropeptides 12-91

92 Neurotransmitters and Related Messengers Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Acetylcholine C H 3 H 3 C N + C H 2 C H 2 O O C C H 3 C H 3 O C HO O C HO Amino acids CH 2 CH 2 CH 2 NH GABA O C CH NH HO Glycine O CH CH C NH OH 2 Asparticacid O O C C H CH 2 CH 2 C HO O H NH 2 Glutamic acid Monoamines Catecholamines OH HO CH CH 2 NH CH 2 HO Epinephrine OH HO CH 2 HO CH 2 NH 2 Norepinephrine HO CH 2 CH 2 NH 2 HO Dopamine HO CH 2 CH 2 NH 2 N Serotonin N N CH 2 CH 2 NH 2 Histamine ß-endorphin Neuropeptides Met Phe Gly Gly Try Enkephalin Substance P Ary Try Lys Pro Glu Glu Phe Phe Gly Leu Met Glu Lys Ser Glu Thr Ser Met Phe Asp Tyr Gly SO 4 Gly Try Thr Ala Pro Asn Lys Leu Val Leu Phe Thr Met Gly Phe Trp Met Asp Cholecystokinin Ile Ile Lys Asn Ala Tyr Lys Glu Lys Gly Figure

93 Neurotransmitters and Related Messengers Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Neuropeptides Neuropeptides are chains of 2 to Met Phe 40 amino acids Gly Gly Try Beta-endorphin and substance P Enkephalin Ary Try Act at lower concentrations than Lys Pro Glu Glu Phe Phe other neurotransmitters Asp Tyr Met Gly Longer lasting effects Met Thr Ser Phe Gly SO Stored in axon terminal as larger 4 Glu Gly Try secretory granules (called dense- Lys core vesicles) Ser ß-endorphin Glu Some function as hormones or Thr neuromodulators Ile Lys Ile Ala Pro Asn Some also released from digestive Lys Leu tract Val Leu Phe Thr Gut brain peptides cause food cravings Figure Phe Gly Leu Met Substance P Phe Trp Met Asp Cholecystokinin Lys Asn Ala Tyr Tyr Lys Glu Lys Gly 12-93

94 Synaptic Transmission Neurotransmitters Synthesized by the presynaptic neuron Released in response to stimulation Bind to specific receptors on the postsynaptic cell Alter the physiology of that cell 12-94

95 Synaptic Transmission A given neurotransmitter does not have the same effect everywhere in the body Multiple receptor types exist for a particular neurotransmitter 14 receptor types for serotonin Receptor governs the effect the neurotransmitter has on the target cell 12-95

96 Synaptic Transmission Neurotransmitters are diverse in their action Some excitatory Some inhibitory Some the effect depends on what kind of receptor the postsynaptic cell has Some open ligand-regulated ion gates Some act through second-messenger systems 12-96

97 Synaptic Transmission Three kinds of synapses with different modes of action Excitatory cholinergic synapse Inhibitory GABA-ergic synapse Excitatory adrenergic synapse Synaptic delay time from the arrival of a signal at the axon terminal of a presynaptic cell to the beginning of an action potential in the postsynaptic cell 0.5 ms for all the complex sequence of events to occur 12-97

98 An Excitatory Cholinergic Synapse Cholinergic synapse employs acetylcholine (ACh) as its neurotransmitter ACh excites some postsynaptic cells Skeletal muscle Inhibits others 12-98

99 An Excitatory Cholinergic Synapse Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Presynaptic neuron Figure Ca 2+ 3 Presynaptic neuron 1 2 ACh Na + 4 K + 5 Postsynaptic neuron 12-99

100 An Inhibitory GABA-ergic Synapse GABA-ergic synapse employs -aminobutyric acid as its neurotransmitter Nerve signal triggers release of GABA into synaptic cleft GABA receptors are chloride channels Cl enters cell and makes the inside more negative than the resting membrane potential Postsynaptic neuron is inhibited, and less likely to fire

101 An Excitatory Adrenergic Synapse Adrenergic synapse employs the neurotransmitter norepinephrine (NE), also called noradrenaline NE and other monoamines, and neuropeptides, act through second-messenger systems such as cyclic AMP (camp) Receptor is not an ion gate, but a transmembrane protein associated with a G protein Slower to respond than cholinergic and GABA-ergic synapses Has advantage of enzyme amplification single molecule of NE can produce vast numbers of product molecules in the cell

102 An Excitatory Adrenergic Synapse Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Presynaptic neuron Postsynaptic neuron Neurotransmitter receptor Norepinephrine Adenylate cyclase G protein camp Ligandgated channels opened 5 Na Enzyme activation Metabolic changes 6 4 Multiple possible effects 7 Genetic transcription Postsynaptic potential Figure Enzyme synthesis

103 Cessation of the Signal Mechanisms to turn off stimulation to keep postsynaptic neuron from firing indefinitely Neurotransmitter molecule binds to its receptor for only 1 ms or so Then dissociates from it If presynaptic cell continues to release neurotransmitter One molecule is quickly replaced by another and the neuron is restimulated

104 Cessation of the Signal Stop adding neurotransmitter and get rid of that which is already diffusion Neurotransmitter escapes the synapse into the nearby ECF Astrocytes in CNS absorb it and return it to neurons Reuptake Synaptic knob reabsorbs amino acids and monoamines by endocytosis Break neurotransmitters down with monoamine oxidase (MAO) enzyme Some antidepressant drugs work by inhibiting MAO Degradation in the synaptic cleft Enzyme acetylcholinesterase (AChE) in synaptic cleft degrades ACh into acetate and choline Choline reabsorbed by synaptic knob

105 Neuromodulators Neuromodulators hormones, neuropeptides, and other messengers that modify synaptic transmission May stimulate a neuron to install more receptors in the postsynaptic membrane adjusting its sensitivity to the neurotransmitter May alter the rate of neurotransmitter synthesis, release, reuptake, or breakdown Enkephalins a neuromodulator family Small peptides that inhibit spinal interneurons from transmitting pain signals to the brain

106 Neuromodulators Nitric oxide (NO) simpler neuromodulator A lightweight gas released by the postsynaptic neurons in some areas of the brain concerned with learning and memory Diffuses into the presynaptic neuron Stimulates it to release more neurotransmitter One neuron s way of telling the other to give me more Some chemical communication that goes backward across the synapse

107 Neural Integration Expected Learning Outcomes Explain how a neurons decides whether or not to generate action potentials. Explain how the nervous system translates complex information into a simple code. Explain how neurons work together in groups to process information and produce effective output. Describe how memory works at cellular and molecular levels

108 Neural Integration Synaptic delay slows the transmission of nerve signals More synapses in a neural pathway, the longer it takes for information to get from its origin to its destination Synapses are not due to limitation of nerve fiber length Gap junctions allow some cells to communicate more rapidly than chemical synapses

109 Neural Integration Then why do we have synapses? To process information, store it, and make decisions Chemical synapses are the decision-making devices of the nervous system The more synapses a neuron has, the greater its information-processing capabilities Pyramidal cells in cerebral cortex have about 40,000 synaptic contacts with other neurons Cerebral cortex (main information-processing tissue of your brain) has an estimated 100 trillion (10 14 ) synapses Neural integration the ability of your neurons to process information, store and recall it, and make decisions

110 Postsynaptic Potentials Neural integration is based on the postsynaptic potentials produced by neurotransmitters Typical neuron has a resting membrane potential of 70 mv and threshold of about 55 mv

111 Postsynaptic Potentials Excitatory postsynaptic potential (EPSP) Any voltage change in the direction of threshold that makes a neuron more likely to fire Usually results from Na + flowing into the cell cancelling some of the negative charge on the inside of the membrane Glutamate and aspartate are excitatory brain neurotransmitters that produce EPSPs

112 Postsynaptic Potentials Inhibitory postsynaptic potential (IPSP) Any voltage change away from threshold that makes a neuron less likely to fire Neurotransmitter hyperpolarizes the postsynaptic cell and makes it more negative than the RMP making it less likely to fire Produced by neurotransmitters that open ligand-regulated chloride gates Causing inflow of Cl making the cytosol more negative

113 Cont. Postsynaptic Potentials Glycine and GABA produce IPSPs and are inhibitory Acetylcholine (ACh) and norepinephrine are excitatory to some cells and inhibitory to others Depending on the type of receptors on the target cell ACh excites skeletal muscle, but inhibits cardiac muscle due to the different type of receptors

114 mv mv Postsynaptic Potentials Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display EPSP Threshold Resting membrane potential Repolarization Depolarization (a) Stimulus Time IPSP Threshold Resting membrane potential 80 Hyperpolarization Figure (b) Stimulus Time

115 Summation, Facilitation, and Inhibition One neuron can receive input from thousands of other neurons Some incoming nerve fibers may produce EPSPs while others produce IPSPs Neuron s response depends on whether the net input is excitatory or inhibitory Summation the process of adding up postsynaptic potentials and responding to their net effect Occurs in the trigger zone

116 Summation, Facilitation, and Inhibition The balance between EPSPs and IPSPs enables the nervous system to make decisions Temporal summation occurs when a single synapse generates EPSPs so quickly that each is generated before the previous one fades Allows EPSPs to add up over time to a threshold voltage that triggers an action potential Spatial summation occurs when EPSPs from several different synapses add up to threshold at an axon hillock Several synapses admit enough Na + to reach threshold Presynaptic neurons cooperate to induce the postsynaptic neuron to fire

117 Temporal and Spatial Summation Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1 Intense stimulation by one presynaptic neuron 2 EPSPs spread from one synapse to trigger zone 3 Postsynaptic neuron fires (a) Temporal summation 3 Postsynaptic neuron fires 1 Simultaneous stimulation by several presynaptic neurons 2 EPSPs spread from several synapses to trigger zone (b) Spatial summation Figure

118 mv Summation of EPSPs Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display Action potential 40 Threshold 60 EPSPs 80 Stimuli Time Resting membrane potential Figure Does this represent spatial or temporal summation?

119 Summation, Facilitation, and Inhibition Neurons routinely work in groups to modify each other s action Facilitation a process in which one neuron enhances the effect of another one Combined effort of several neurons facilitates firing of postsynaptic neuron Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Signal in presynaptic neuron Signal in presynaptic neuron Figure Signal in inhibitory neuron No activity in inhibitory neuron No neurotransmitter release here Neurotransmitter Inhibition of presynaptic neuron IPS P Neurotransmitter Excitation of postsynaptic neuron S + EPSP R No neurotransmitter release here No response in postsynaptic neuron S R (a) (b)

120 Summation, Facilitation, and Inhibition Presynaptic inhibition process in which one presynaptic neuron suppresses another one Opposite of facilitation Reduces or halts unwanted synaptic transmission Neuron I releases inhibitory GABA Prevents voltage-gated calcium channels from opening in synaptic knob and presynaptic neuron releases less or no neurotransmitter Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Figure Signal in presynaptic neuron Signal in inhibitory neuron Signal in presynaptic neuron No activity in inhibitory neuron No neurotransmitter release here Neurotransmitter Inhibition of presynaptic neuron IPS P Neurotransmitter Excitation of postsynaptic neuron S + EPSP R No neurotransmitter release here No response in postsynaptic neuron S R (a) (b)

121 Neural Coding Neural coding the way in which the nervous system converts information to a meaningful pattern of action potentials Qualitative information depends upon which neurons fire Labeled line code: each nerve fiber to the brain leads from a receptor that specifically recognizes a particular stimulus type

122 Neural Coding Quantitative information information about the intensity of a stimulus is encoded in two ways One depends on the fact that different neurons have different thresholds of excitation Stronger stimuli causes a more rapid firing rate Excitement of sensitive, low-threshold fibers gives way to excitement of less sensitive, high-threshold fibers as intensity of stimuli increases

123 Cont. Neural Coding Other way depends on the fact that the more strongly a neuron is stimulated, the more frequently it fires CNS can judge stimulus strength from the firing frequency of afferent neurons

124 Neural Coding Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Action potentials 2 g 5 g 10 g 20 g Time Figure

125 Neural Pools and Circuits Neural pools neurons function in large groups, each of which consists of millions of interneurons concerned with a particular body function Control rhythm of breathing Moving limbs rhythmically when walking Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Input neuron Figure Facilitated zone Discharge zone Facilitated zone

126 Neural Pools and Circuits Information arrives at a neural pool through one or more input neurons Branch repeatedly and synapse with numerous interneurons in the pool Some input neurons form multiple synapses with a single postsynaptic cell Can produce EPSPs in all points of contact with that cell Through spatial summation, make it fire more easily than if they synapsed with it at only one point

127 Neural Pools and Circuits Cont. Within the discharge zone of an input neuron That neuron acting alone can make the postsynaptic cells fire In a broader facilitated zone, it synapses with still other neurons in the pool Fewer synapses on each of them Can only stimulate those neurons to fire with the assistance of other input neurons

128 Neural Pools and Circuits Diverging circuit One nerve fiber branches and synapses with several postsynaptic cells One neuron may produce output through hundreds of neurons Converging circuit Input from many different nerve fibers can be funneled to one neuron or neural pool Opposite of diverging circuit

129 Neural Pools and Circuits Cont. Reverberating circuits Neurons stimulate each other in linear sequence but one cell restimulates the first cell to start the process all over Diaphragm and intercostal muscles Parallel after-discharge circuits Input neuron diverges to stimulate several chains of neurons Each chain has a different number of synapses Eventually they all reconverge on a single output neuron After-discharge continued firing after the stimulus stops

130 Neural Pools and Circuits Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Diverging Converging Input Output Output Input Reverberating Parallel after-discharge Figure Input Output Input Output

131 Memory and Synaptic Plasticity Physical basis of memory is a pathway through the brain called a memory trace or engram Along this pathway, new synapses were created or existing synapses modified to make transmission easier Synaptic plasticity: the ability of synapses to change Synaptic potentiation: the process of making transmission easier Kinds of memory Immediate, short- and long-term memory Correlate with different modes of synaptic potentiation

132 Immediate Memory Immediate memory the ability to hold something in your thoughts for just a few seconds Essential for reading ability Feel for the flow of events (sense of the present) Our memory of what just happened echoes in our minds for a few seconds Reverberating circuits

133 Short-Term Memory Short-term memory (STM) lasts from a few seconds to several hours Quickly forgotten if distracted Calling a phone number we just looked up Reverberating circuits Facilitation causes memory to last longer Tetanic stimulation: rapid arrival of repetitive signals at a synapse Causes Ca 2+ accumulation and postsynaptic cell more likely to fire Posttetanic potentiation: to jog a memory Ca 2+ level in synaptic knob stays elevated Little stimulation needed to recover memory

134 Long-Term Memory Types of long-term memory Declarative: retention of events that you can put into words Procedural: retention of motor skills Physical remodeling of synapses New branching of axons or dendrites Molecular changes long-term potentiation Changes in receptors and other features increase transmission across experienced synapses Effect is longer-lasting

135 Long-Term Memory Molecular changes are called long-term potentiation Method described Receptors on synaptic knobs are usually blocked by Mg 2+ ions When they bind glutamate and receive tetanic stimuli, they repel Mg 2+ and admit Ca 2+ into the dendrite; Ca 2+ acts as second messenger More synaptic knob receptors are produced Synthesizes proteins involved in synapse remodeling Releases nitric oxide that triggers more neurotransmitter release at presynaptic neuron

136 100,000 deaths/year Alzheimer Disease 11% of population over 65; 47% by age 85 Memory loss for recent events, moody, combative, lose ability to talk, walk, and eat Show deficiencies of acetylcholine (ACh) and nerve growth factor (NGF) Diagnosis confirmed at autopsy Atrophy of gyri (folds) in cerebral cortex Neurofibrillary tangles and senile plaques Formation of β-amyloid protein from breakdown product of plasma membranes Treatment halt β-amyloid production Research halted due to serious side effects Give NGF or cholinesterase inhibitors

137 Alzheimer Disease Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display Shrunken gyri Wide sulci Neurons with neurofibrillary tangles Senile plaque (b) b: Simon Fraser/Photo Researchers, Inc. (a) a: Custom Medical Stock Photo Figure 12.31a Figure 12.31b