Master controlling and communicating system of body Cells communicate via electrical and chemical signals Rapid and specific Usually cause almost

Transcription

1 Master controlling and communicating system of body Cells communicate via electrical and chemical signals Rapid and specific Usually cause almost immediate responses

2 Central nervous system (CNS) Brain and spinal cord of dorsal body cavity Integration and control center Interprets sensory input and dictates motor output Peripheral nervous system (PNS) The portion of the nervous system outside CNS Consists mainly of nerves that extend from brain and spinal cord Spinal nerves to and from spinal cord Cranial nerves to and from brain

3 Central nervous system (CNS) Brain and spinal cord of dorsal body cavity Integration and control center Interprets sensory input and dictates motor output Peripheral nervous system (PNS) The portion of the nervous system outside CNS Consists mainly of nerves that extend from brain and spinal cord Spinal nerves to and from spinal cord Cranial nerves to and from brain

4 Input = sensory information Integration is not the only thing the brain and spinal cord does internal circuits consist of input-integrative-output sections of brain and cord Output = motor effects and hormonal changes, control effects

5 Highly cellular; little extracellular space Tightly packed Two principal cell types Neuroglia small cells that surround and wrap delicate neurons Neurons (nerve cells) excitable cells that transmit electrical signals 5

6 Astrocytes (CNS) Microglial cells (CNS) Ependymal cells (CNS) Oligodendrocytes (CNS) Satellite cells (PNS) Schwann cells (PNS)

7 Most abundant, versatile, and highly branched glial cells Cling to neurons, synaptic endings, and capillaries Functions include Support and brace neurons Play role in exchanges between capillaries and neurons Guide migration of young neurons

8 Control chemical environment around neurons Respond to nerve impulses and neurotransmitters Influence neuronal functioning Participate in information processing in brain 7

9 Branched cells Processes wrap CNS nerve fibers, forming insulating myelin sheaths thicker nerve fibers

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11 Small, ovoid cells with thorny processes that touch and monitor neurons prune synapses, for instance Migrate toward injured neurons Can transform to phagocytize microorganisms and neuronal debris Derived from bone-marrow monocyte precursors but resident micro pop different from blood-derived acute responders

12 Range in shape from squamous to columnar May be ciliated Cilia beat to circulate CSF Line the central cavities of the brain and spinal column Form permeable barrier between cerebrospinal fluid (CSF) in cavities and tissue fluid bathing CNS cells Derived from radial glia early astrocytes, all neural tissue originally from (neuro)epithelium

13 Satellite cells Surround neuron cell bodies in PNS Function similar to astrocytes of CNS Schwann cells (neurolemmocytes) Surround all peripheral nerve fibers and form myelin sheaths in thicker nerve fibers Similar function as oligodendrocytes Vital to regeneration of damaged peripheral nerve fibers

14 Structural units of nervous system Large, highly specialized cells that conduct impulses Extreme longevity ( 100 years or more) Amitotic with few exceptions High metabolic rate requires continuous supply of oxygen and glucose All have cell body and one or more processes 13

15 Biosynthetic center of neuron Synthesizes proteins, membranes, and other chemicals Rough ER (chromatophilic substance or Nissl bodies) Most active and best developed in body Spherical nucleus with nucleolus Some contain pigments 14

16 In most, plasma membrane part of receptive region 14

17 Groups of neuronal cell bodies are called Nuclei (nucleus) in CNS Ganglia (ganglion) in PNS 15

18 Armlike processes extend from body Two types of processes Dendrites Axon Tracts Bundles of neuron processes in CNS Nerves

19 Bundles of neuron processes in PNS 16

20 Nissl substance = rough endoplasmic reticulum Only in cell body and dendrites so axon hillock (and axon) clear in nisslstained microscopy

21 In motor neurons 100s of short, tapering, diffusely branched processes Same organelles as in body Receptive (input) region of neuron Convey incoming messages toward cell body as graded 18

22 potentials (short distance signals) 18

23 In many brain areas fine dendrites specialized Collect information with dendritic spines Appendages with bulbous or spiky ends

24 One axon per cell arising from axon hillock Cone-shaped area of cell body In some axons the hillock is short or absent In others most of length of cell Some 1 meter long Long axons called nerve fibers 20

25 Occasional branches (axon collaterals) Branches profusely at end (terminus) Can be 10,000 terminal branches Distal endings called axon terminals or terminal boutons 20

26 Long axons called nerve fibers Occasional branches (axon collaterals) Branches profusely at end (terminus) Can be 10,000 terminal branches Distal endings called axon terminals or terminal boutons 21

27 Conducting region of neuron Generates nerve impulses (series of action potentials) Transmits them along axolemma (neuron cell membrane) to axon terminal Secretory region Neurotransmitters released into extracellular space Either excite or inhibit neurons with which axons in close contact

28 Axon lacks rough ER and Golgi apparatus Relies on cell body to renew proteins and membranes Efficient transport mechanisms Quickly decay if cut or damaged Molecules and organelles are moved along axons by motor proteins and cytoskeletal elements Movement in both directions Anterograde away from cell body Examples: mitochondria, cytoskeletal elements, membrane components, enzymes Retrograde toward cell body 23

29 Examples: organelles to be degraded, signal molecules, viruses, and bacterial toxins 23

30 Composed of myelin Whitish, protein-lipoid substance Segmented sheath around most long or large-diameter axons Myelinated fibers Function of myelin Protects and electrically insulates axon 24

31 Increases speed of nerve impulse transmission Nonmyelinated fibers conduct impulses more slowly 24

32 Formed by Schwann cells Wrap around axon in jelly roll fashion One cell forms one segment of myelin sheath Myelin sheath Concentric layers of Schwann cell plasma membrane around axon

33 Outer collar of perinuclear cytoplasm (formerly called neurilemma) Peripheral bulge of Schwann cell containing nucleus and most of cytoplasm

34 M = myelin Nd = node Ax = axon Nf = neurofilament Mt = microtubule Plasma membranes of myelinating cells have less protein No channels or carriers Good electrical insulators Interlocking proteins bind adjacent myelin membranes Myelin sheath gaps Gaps between adjacent Schwann cells Sites where axon collaterals can emerge Formerly called nodes of Ranvier 27

35 Nonmyelinated fibers Thin fibers not wrapped in myelin; surrounded by Schwann cells but no coiling; one cell may surround 15 different fibers 27

36 Formed by multiple, flat processes of oligodendrocytes, not whole cells Can wrap up to 60 axons at once Myelin sheath gap is present No outer collar of perinuclear cytoplasm Thinnest fibers are unmyelinated Covered by long extensions of

37 adjacent neuroglia White matter Regions of brain and spinal cord with dense collections of myelinated fibers usually fiber tracts Gray matter Mostly neuron cell bodies and nonmyelinated fibers 28

38 Note overlapping ends/edges of myelin layers at nodes 29

39 Note prominent edges of layers at nodes (No) in B Central axon (center picture) 30

40 Grouped by number of processes Three types Multipolar 3 or more processes 1 axon, others dendrites Most common; major neuron in CNS Bipolar 2 processes 1 axon and 1 dendrite Rare, e.g., Retina and olfactory mucosa Unipolar 1 short process

41 Divides T-like both branches now considered axons Distal (peripheral) process associated with sensory receptor Proximal (central) process enters CNS 31

42 By shape of processes no practical use, but helpful in understanding circuits in the nervous system

43 Grouped by direction in which nerve impulse travels relative to CNS Three types Sensory (afferent) Motor (efferent) Interneurons

44 Sensory Motor Transmit impulses from sensory receptors toward CNS Almost all are Unipolar Cell bodies in ganglia in PNS Carry impulses from CNS to effectors Multipolar Most cell bodies in CNS (except some autonomic neurons) Interneurons (association neurons)

45 Lie between motor and sensory neurons Shuttle signals through CNS pathways; most are entirely within CNS 99% of body's neurons Most confined in CNS 34

46 Neurons are highly excitable Respond to adequate stimulus by generating an action potential (nerve impulse) Impulse is always the same regardless of stimulus

47 Opposite charges attract each other Energy is required to separate opposite charges [e.g. across a membrane] Energy is liberated when the charges move toward one another If opposite charges are separated, the system has potential energy 36

48 Voltage is a measure of potential energy generated by separated charge Measured between two points in Volts (V) or Millivolts (mv) Called potential difference or potential Remember the charge difference (potential) across plasma membranes! Greater charge difference between points = higher voltage

49 Current is flow of electrical charge (Ions) between two points Can be used to do work Resistance is hindrance to charge flow Insulator substance with high electrical resistance Conductor substance with low electrical resistance 37

50 Gives relationship of voltage, current, resistance Current (I) = voltage (V) / resistance (R) Hence: Current is directly proportional to voltage No net current flow between points with same potential Current inversely related to resistance

51 Large proteins serve as selective membrane ion channels Two main types of ion channels Leakage (nongated) channels Always open Gated Part of protein changes shape to open/close channel

52 Three types Chemically gated (ligand-gated) channels Open with binding of a specific neurotransmitter Voltage-gated channels Open and close in response to changes in membrane potential Mechanically gated channels Open and close in response to physical deformation of receptors, as in sensory receptors

53 When gated channels are open Ions diffuse quickly across membrane along electrochemical gradients Along chemical concentration gradients from higher concentration to lower concentration Along electrical gradients toward opposite electrical charge Ion flow creates an electrical current and voltage changes across membrane

54 Potential difference across membrane of resting cell Approximately 70 mv in neurons (cytoplasmic side of membrane negatively charged relative to outside) Actual voltage difference varies from - 40 mv to -90 mv Membrane termed polarized 42

55 Generated by: Differences in ionic makeup of ICF and ECF Differential permeability of the plasma membrane 42

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57 ECF has higher concentration of Na + than ICF Balanced chiefly by chloride ions (Cl - ) ICF has higher concentration of K + than ECF Balanced by negatively charged proteins K + plays most important role in membrane potential 44

58 Impermeable to large anionic proteins Slightly permeable to Na + (through leakage channels) Sodium diffuses into cell down concentration gradient 25 times more permeable to K + than sodium (more leakage channels) Potassium diffuses out of cell down concentration gradient Quite permeable to Cl 45

59 More potassium diffuses out than sodium diffuses in Cell more negative inside Establishes resting membrane potential Sodium-potassium pump stabilizes resting membrane potential Maintains concentration gradients for Na + and K + 3 Na + pumped out of cell; two K + pumped in

60 Changes in membrane potential used as signals to receive, integrate, and send information Membrane potential changes when Concentrations of ions across membrane change Membrane permeability to ions changes

61 Changes produce two types signals Graded potentials Incoming signals operating over short distances Action potentials Long-distance signals of axons

62 Terms describing membrane potential changes relative to resting membrane potential Depolarization Decrease in membrane potential (toward zero and above) Inside of membrane becomes less negative than resting membrane potential Increases probability of producing a nerve impulse

63 Terms describing membrane potential changes relative to resting membrane potential Hyperpolarization An increase in membrane potential (away from zero) Inside of cell more negative than resting membrane potential) Reduces probability of producing a nerve impulse

64 Short-lived, localized changes in membrane potential Magnitude varies with stimulus strength Stronger stimulus more voltage changes; farther current flows Either depolarization or hyperpolarization

65 Triggered by stimulus that opens gated ion channels Current flows but dissipates quickly and decays Graded potentials are signals only over short distances 51

66 Short-lived, localized changes in membrane potential Magnitude varies with stimulus strength Stronger stimulus more voltage changes; farther current flows Either depolarization or hyperpolarization

67 Triggered by stimulus that opens gated ion channels Current flows but dissipates quickly and decays Graded potentials are signals only over short distances 52

68 Short-lived, localized changes in membrane potential Magnitude varies with stimulus strength Stronger stimulus more voltage changes; farther current flows Either depolarization or hyperpolarization

69 Triggered by stimulus that opens gated ion channels Current flows but dissipates quickly and decays Graded potentials are signals only over short distances 53

70 Graded potentials are produced by neurotransmitters from axons onto dendrites and cell bodies. 54

71 The cell body is covered by synapses Effects can be viewed in two ways: the somatic potentials add up to bolster or inhibit axon hillock action potentials or local membrane ionic concentrations and potentials propagate or retard spread of the graded potentials from the dendrites to the axon hillock 55

72 Principle way neurons send signals Principal means of long-distance neural communication Occur only in muscle cells and axons of neurons Brief reversal of membrane potential with a change in voltage of ~100 mv Do not decay over distance as graded potentials do

73 Each Na + channel has two voltage-sensitive gates Activation gates Closed at rest; open with depolarization allowing Na + to enter cell Inactivation gates Open at rest; block channel once it is open to prevent more Na + from entering cell

74 Each K + channel has one voltage-sensitive gate Closed at rest Opens slowly with depolarization

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76 All gated Na + and K + channels are closed Only leakage channels for Na + and K + are open This maintains the resting membrane potential

77 Depolarizing local currents open voltage-gated Na + channels Na + rushes into cell Na + influx causes more depolarization which opens more Na + channels ICF less negative At threshold ( 55 to 50 mv) positive feedback causes opening

78 of all Na + channels a reversal of membrane polarity to +30mV Spike of action potential 61

79 Repolarizing phase Na + channel slow inactivation gates close Membrane permeability to Na + declines to resting state AP spike stops rising Slow voltage-gated K + channels open K + exits the cell and internal negativity is restored

80 Some K + channels remain open, allowing excessive K + efflux Inside of membrane more negative than resting state This causes hyperpolarization of the membrane (slight dip below resting voltage) Na + channels begin to reset

81 Repolarization resets electrical conditions, not ionic conditions After repolarization Na + /K + pumps (thousands of them in an axon) restore ionic conditions

82 Not all depolarization events produce APs For axon to "fire", depolarization must reach threshold That voltage at which the AP is triggered At threshold: Membrane has been depolarized by 15 to 20 mv Na + permeability increases Na + influx exceeds K + efflux The positive feedback cycle begins An AP either happens completely, or it does not happen at all = The All-or- None Phenomenon

83 Propagation allow AP to serve as a signaling device Na + influx causes local currents Local currents cause depolarization of adjacent membrane areas in direction away from AP origin (toward axon's terminals) Local currents trigger an AP there This causes the AP to propagate

84 AWAY from the AP origin Since Na + channels closer to AP origin are inactivated no new AP is generated there 66

85 Once initiated an AP is self-propagating In nonmyelinated axons each successive segment of membrane depolarizes, then repolarizes Propagation in myelinated axons differs

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87 All action potentials are alike and are independent of stimulus intensity How does CNS tell difference between a weak stimulus and a strong one? Strong stimuli cause action potentials to occur more frequently

88 # Of impulses per second or frequency of APs CNS determines stimulus intensity by the frequency of impulses Higher frequency means stronger stimulus 69

89 When voltage-gated Na + channels open neuron cannot respond to another stimulus Absolute refractory period Time from opening of Na + channels until resetting of the channels Ensures that each AP is an all-or-none event Enforces one-way transmission of nerve impulses Relative refractory period Follows absolute refractory period Most Na + channels have returned to their resting state Some K + channels still open Repolarization is occurring Threshold for AP generation is elevated Inside of membrane more negative than resting state Only exceptionally strong stimulus could stimulate an AP

90 Conduction velocities of neurons vary widely Rate of AP propagation depends on Axon diameter Larger diameter fibers have less resistance to local current flow so faster impulse conduction Degree of myelination Continuous conduction in nonmyelinated axons is slower than saltatory conduction in myelinated axons

91 Myelin sheaths insulate and prevent leakage of charge Saltatory conduction (possible only in myelinated axons) is about 30 times faster Voltage-gated Na + channels are located at myelin sheath gaps APs generated only at gaps Electrical signal appears to jump rapidly from gap to gap

92 Nerve fibers classified according to Diameter Degree of myelination Speed of conduction

93 Group A fibers Large diameter, myelinated somatic sensory and motor fibers of skin, skeletal muscles, joints Transmit at 150 m/s Group B fibers Intermediate diameter, lightly myelinated fibers Transmit at 15 m/s

94 Group C fibers Smallest diameter, unmyelinated ANS fibers Transmit at 1 m/s 74

95 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 75

96 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 Axodendritic between axon terminals of one neuron and dendrites of others Axosomatic between axon terminals of one neuron and soma of others Less common types: Axoaxonal (axon to axon) Dendrodendritic (dendrite to dendrite) Somatodendritic (dendrite to soma)

97 Less common than chemical synapses Neurons electrically coupled (joined by gap junctions that connect cytoplasm of adjacent neurons) Communication very rapid May be unidirectional or bidirectional Synchronize activity More abundant in: Embryonic nervous tissue Nerve impulse remains electrical 77

98 Specialized for release and reception of chemical neurotransmitters Typically composed of two parts Axon terminal of presynaptic neuron Contains synaptic vesicles filled with neurotransmitter Neurotransmitter receptor region on postsynaptic neuron's 78

99 membrane Usually on dendrite or cell body Two parts separated by synaptic cleft Fluid-filled space Electrical impulse changed to chemical across synapse, then back into electrical 78

100 Electron microscopy of axodendritic synapse 79

101 30 50 nm wide (~1/1,000,000 th of an inch) Prevents nerve impulses from directly passing from one neuron to next 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 80

102 AP arrives at axon terminal of presynaptic neuron Causes voltage-gated Ca 2+ channels to open Ca 2+ floods into cell Soluble NSF Attachment Protein = SNAP - receptor = SNARE Synaptotagmin protein binds Ca 2+ and promotes fusion of synaptic vesicles with axon 81

103 membrane Exocytosis of neurotransmitter into synaptic cleft occurs Higher impulse frequency more released 81

104 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

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106 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 clef

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110 Time needed for neurotransmitter to be released, diffuse across synapse, and bind to receptors ms Synaptic delay is rate-limiting step of neural transmission Neurotransmitter receptors cause graded potentials that vary in strength with Amount of neurotransmitter released and Time neurotransmitter stays in area Types of postsynaptic potentials EPSP excitatory postsynaptic potentials IPSP inhibitory postsynaptic potentials

111 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

112 opening of voltage-gated channels, and cause AP to be generated 89

113 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

114 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 Temporal summation One or more presynaptic neurons transmit impulses in rapid-fire order Spatial summation Postsynaptic neuron stimulated simultaneously by large number of terminals at same time

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118 Repeated use of synapse increases ability of presynaptic cell to excite postsynaptic neuron Ca 2+ concentration increases in presynaptic terminal and postsynaptic neuron Brief high-frequency stimulation partially depolarizes postsynaptic neuron Chemically gated channels (NMDA receptors) allow Ca 2+ entry Ca 2+ activates kinase enzymes that promote more effective responses to subsequent stimuli 95

119 Astrocyte has some role in Long-Term-Potentiation 96

120 Excitatory neurotransmitter release by one neuron inhibited by another neuron via an axoaxonal synapse Less neurotransmitter released Smaller EPSPs formed 97

121 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 98

122 Acetylcholine (ACh) First identified; best understood Released at neuromuscular junctions,by some ANS neurons, by some CNS neurons Synthesized from acetic acid and choline by enzyme choline acetyltransferase Degraded by enzyme acetylcholinesterase (AChE) 99

123 Biogenic amines Catecholamines Dopamine, norepinephrine (NE), and epinephrine Synthesized from amino acid tyrosine Broadly distributed in brain Play roles in emotional behaviors and biological clock Some ANS motor neurons (especially NE) Imbalances associated with 100

124 mental illness 100

125 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 101

126 Production involves same chemical processes on different starting substances. 102

127 Amino acids Peptides (neuropeptides) Glutamate Aspartate Glycine GABA gamma ( )- aminobutyric acid Substance P 103

128 Purines ATP! Adenosine Mediator of pain signals Endorphins Beta endorphin, dynorphin and enkephalins Act as natural opiates; reduce pain perception Gut-brain peptides Somatostatin and cholecystokinin Potent inhibitor in brain Caffeine blocks adenosine receptors Act in both CNS and PNS Produce fast or slow responses Induce Ca 2+ influx in astrocytes 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 103

129 damage in stroke patients, smooth muscle relaxation in intestine H 2 S acts directly on ion channels to alter function 103

130 Endocannabinoids all are eicosanoids (oxidation products of 20- carbon fatty acids including prostaglandins, hormones, and messenger molecules) 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 104

131 Endocannabinoid receptors CB1 and CB2 105

132 Great diversity of functions Can classify by Effects excitatory versus inhibitory Actions direct versus indirect 106

133 Effects - excitatory versus inhibitory Neurotransmitter effects can be excitatory (depolarizing) and/or inhibitory (hyperpolarizing) Effect determined by receptor to which it binds GABA and glycine usually inhibitory Glutamate usually excitatory Acetylcholine and NE bind to at least two receptor types with opposite effects ACh excitatory at neuromuscular junctions in skeletal muscle ACh inhibitory in cardiac muscle 107

134 Great diversity of functions Can classify by Actions direct versus indirect Direct action Neurotransmitter binds to and opens ion channels Promotes rapid responses by altering membrane potential Examples: ACh and amino acids 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

135 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 Responses are indirect, complex, slow, and often prolonged Transmembrane protein complexes Cause widespread metabolic changes Examples: muscarinic ACh receptors, receptors that bind biogenic amines and neuropeptides Second messengers Open or close ion channels Activate kinase enzymes Phosphorylate channel proteins Activate genes and induce protein synthesis

136 Types Channel-linked receptors Mediate fast synaptic transmission 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

137 Actions direct versus indirect G protein-linked receptor Oversee slow synaptic responses Responses are indirect, complex, slow, and often prolonged Transmembrane protein complexes Cause widespread metabolic changes Examples: muscarinic ACh receptors, receptors that bind biogenic amines and neuropeptides Neurotransmitter binds to G protein linked receptor G protein is activated Activated G protein controls production of second messengers, e.g., Cyclic AMP, cyclic GMP, diacylglycerol, or Ca 2+ Second messengers Open or close ion channels Activate kinase enzymes

138 Phosphorylate channel proteins Activate genes and induce protein synthesis 111

139 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 112

140 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 113

141 in pool Discharge zone neurons most closely associated with incoming fiber Facilitated zone neurons farther away from incoming fiber 113

142 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

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144 Circuits Patterns of synaptic connections in neuronal pools Four types of circuits Diverging Converging Reverberating Parallel after-discharge

145 Circuits Patterns of synaptic connections in neuronal pools Four types of circuits Diverging Converging Reverberating Parallel after-discharge

146 Circuits Patterns of synaptic connections in neuronal pools Four types of circuits Diverging Converging Reverberating Parallel after-discharge

147 Circuits Patterns of synaptic connections in neuronal pools Four types of circuits Diverging Converging Reverberating Parallel after-discharge

148 Patterns of Neural Processing: Serial Processing Occur over pathways called reflex arcs Five components: receptor, sensory neuron, CNS integration center, motor neuron, effector 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

149 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 121

150 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

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