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1 Lecture #13 – Animal Nervous Systems. 2 Key Concepts: Evolution of organization in nervous systems Neuron structure and function Neuron communication.

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Presentation on theme: "1 Lecture #13 – Animal Nervous Systems. 2 Key Concepts: Evolution of organization in nervous systems Neuron structure and function Neuron communication."— Presentation transcript:

1 1 Lecture #13 – Animal Nervous Systems

2 2 Key Concepts: Evolution of organization in nervous systems Neuron structure and function Neuron communication at synapses Organization of the vertebrate nervous systems Brain structure and function The cerebral cortex Nervous system injuries and diseases???

3 3 All animals except sponges have some kind of nervous system Increasing complexity accompanied increasingly complex motion and activities Nets of neurons  bundles of neurons  cephalization

4 4 First split was tissues; next was body symmetry; echinoderms “went back” to radial symmetry

5 5 Derived radial symmetry and nerve network

6 6 Cephalization The development of a brain Associated with the development of bilateral symmetry Complex, cephalized nervous systems are usually divided into 2 sections  Central nervous system (CNS) integrates information, exerts most control  Peripheral nervous system (PNS) connects CNS to the rest of the body

7 7

8 8 Critical Thinking What is the functional advantage of cephalization???

9 9 Critical Thinking What is the functional advantage of cephalization???

10 10 Cephalization The development of a brain Associated with the development of bilateral symmetry Complex, cephalized nervous systems are usually divided into 2 sections  Central nervous system (CNS) integrates information, exerts most control  Peripheral nervous system (PNS) connects CNS to the rest of the body

11 11 PNS  CNS  PNS

12 12 Specialized neurons support different sections Sensory  Transmit information from the sensory structures that detect the both external and internal conditions Interneurons  Analyze and interpret sensory information, formulate response Motor  Transmit information to effector cells – the muscle or endocrine cells that respond to input

13 13 Critical Thinking Which type of neuron would have the most branched structure???  Sensory neurons  Interneurons  Motor neurons

14 14 Critical Thinking Which type of neuron would have the most branched structure???  Sensory neurons  Interneurons  Motor neurons

15 15 Neuron structure is complex 100 billion nerve cells in the human brain!

16 16

17 17

18 18 Basic Neuron Structure Cell body Dendrites Axons Axon hillock Myelin sheath Synaptic terminal

19 19 Cell Body Contains most cytoplasm and organelles Extensions branch off cell body

20 20 Dendrites Highly branched extensions Receive signals from other neurons

21 21 Axons Usually longer extension, unbranched til end Transmits signals to other cells

22 22 Axon Hillock Enlarged region at base of axon Site where axon signals are generated  Signal is sent after summation

23 23 Myelin Sheath Insulating sheath around axon Also speeds up signal transmission

24 24 Synaptic Terminal End of axon branches Each branch ends in a synaptic terminal  Actual site of between-cell signal generation

25 25 Synapse Site of signal transmission between cells More later…

26 26 Supporting Cells - Glia Maintain structural integrity and function of neurons 10 – 50 x more glia than neurons in mammals Major categories  Astrocytes  Radial glia  Oligodendrocytes and Schwann cells

27 27 Glia – Astrocytes Structural support for neurons Regulate extracellular ion and neurotransmitter concentrations Facilitate synaptic transfers Induce the formation of the blood-brain barrier  Tight junctions in capillaries allow more control over the extracellular chemical environment in the brain and spinal cord

28 28 Glia – Radial Glia Function mostly during embryonic development Form tracks to guide new neurons out from the neural tube (neural tube develops into the CNS) Can also function as stem cells to replace glia and neurons (so can astrocytes)  This function is limited in nature; major line of research

29 29 Glia – Oligodendrocytes (CNS) and Schwann Cells (PNS) Form the myelin sheath around axons Cells are rectangular and tile-shaped, wrapped spirally around the axons High lipid content insulates the axon – prevents electrical signals from escaping Gaps between the cells (Nodes of Ranvier) speed up signal transmission

30 30 The nerve signal is electrical! To understand signaling process, must understand the difference between resting potential and action potential

31 31 Resting Potential All cells have a resting potential  Electrical potential energy – the separation of opposite charges  Due to the unequal distribution of anions and cations on opposite sides of the membrane  Maintained by selectively permeable membranes and by active membrane pumps  Charge difference = one component of the electrochemical gradient that drives the diffusion of all ions across cell membranes

32 32 Neuron Function – Resting Potential Neuron resting potential is ~ -70mV  At resting potential the neuron is NOT actively transmitting signals  Maintained largely because cell membranes are more permeable to K + than to Na + ; more K + leaves the cell than Na + enters  An ATP powered K + /Na + pump continually restores the concentration gradients; this also helps to maintain the charge gradient

33 33 Resting Potential Ion Concentrations 1.Cell membranes are more permeable to K + than to Na + 2.There is more K + inside the cell than outside 3.There is more Na + outside the cell than inside Both ions follow their [diffusion] gradients

34 34 Critical Thinking If both ions follow their diffusion gradients, what is the predictable consequence???

35 35 Critical Thinking If both ions follow their diffusion gradients, what is the predictable consequence???

36 36 Resting Potential Ion Concentrations A dynamic equilibrium is predictable, but is prevented by an ATP powered K + /Na + pump

37 37 Neuron Function – Resting Potential Neuron resting potential is ~ -70mV  At resting potential the neuron is NOT actively transmitting signals  Maintained largely because cell membranes are more permeable to K + than to Na + ; more K + leaves the cell than Na + enters  An ATP powered K + /Na + pump continually restores the concentration gradients; this also helps to maintain the charge gradient

38 38 Resting Potential Ion Concentrations ATP powered pump continually transfers 3 Na + ions out of the cytoplasm for every 2 K + ions it moves back in to the cytoplasm This means that there is a net transfer of + charge OUT of the cell

39 39 Resting Potential Ion Concentrations Thus, the membrane potential is maintained Cl - and large anions also contribute to the net negative charge inside the cell

40 40 Neuron Function – Resting Potential Neuron resting potential is ~ -70mV  At resting potential the neuron is NOT actively transmitting signals  Maintained largely because cell membranes are more permeable to K + than to Na + ; more K + leaves the cell than Na + enters  An ATP powered K + /Na + pump continually restores the concentration gradients; this also helps to maintain the charge gradient  Cl -, other anions, and Ca ++ also affect resting potential REVIEW

41 41 Gated Ion Channels Why Neurons are Different All cells have a membrane potential Neurons can change their membrane potential in response to a stimulus The ability of neurons to open and close ion gates allows them to send electrical signals along the extensions (dendrites and axons)  Gates open and close in response to stimuli Only neurons can do this!

42 42 Gated Ion Channels Why Neurons are Different Gated ion channels manage membrane potential  Stretch gates – respond when membrane is stretched  Ligand gates – respond when a molecule binds (eg: a neurotransmitter)  Voltage gates – respond when membrane potential changes

43 43 Gated Ion Channels Why Neurons are Different Hyperpolarization = inside of neuron becomes more negative Depolarization = inside of neuron becomes more positive  Either can occur, depending on stimulus  Either can be graded – more stimulus = more change in membrane potential Depolarization eventually triggers an action potential = NOT graded

44 44 Depolarization eventually triggers an action potential – action potentials are NOT graded

45 45 Action Potentials ARE the Nerve Signal Triggered whenever depolarization reaches a set threshold potential Action potentials are all-or-none responses of a fixed magnitude  Once triggered, they can’t be stopped  There is no gradation once an action potential is triggered Action potentials are brief depolarizations  1 – 2 milliseconds Voltage gated ion channels control signal

46 46 Critical Thinking If the action potential is of a fixed magnitude, how do we sense different levels of a stimulus???

47 47 Critical Thinking If the action potential is of a fixed magnitude, how do we sense different levels of a stimulus???

48 48 Action Potentials ARE the Nerve Signal Triggered whenever depolarization reaches a set threshold potential Action potentials are all-or-none responses of a fixed magnitude  Once triggered, they can’t be stopped  There is no gradation once an action potential is triggered Action potentials are brief depolarizations  1 – 2 milliseconds Voltage gated ion channels control signal

49 49 Fig ; p. 1019, 7 th Ed.

50 50 Voltage Gate Activity 1.Resting Potential – Na + and K + activation gates closed; Na + inactivation gate open on most channels 2.Depolarization – Na + activation gates begin to open – Na + begins to enter cell 3.Rising Phase – threshold is crossed, Na + floods into the cell, raising the membrane potential to ~ +35mV

51 51

52 52 1.Resting Potential – Na + and K + activation gates closed; Na + inactivation gate open on most channels

53 53 Voltage Gate Activity 1.Resting Potential – Na + and K + activation gates closed; Na + inactivation gate open on most channels 2.Depolarization – Na + activation gates begin to open – Na + begins to enter cell 3.Rising Phase – threshold is crossed, Na + floods into the cell, raising the membrane potential to ~ +35mV

54 54

55 55 2. Depolarization – Na + activation gates begin to open – Na + begins to enter cell

56 56 Voltage Gate Activity 1.Resting Potential – Na + and K + activation gates closed; Na + inactivation gate open on most channels 2.Depolarization – Na + activation gates begin to open – Na + begins to enter cell 3.Rising Phase – threshold is crossed, Na + floods into the cell, raising the membrane potential to ~ +35mV

57 57

58 58 3. Rising Phase – threshold is crossed, Na + floods into the cell, raising the membrane potential to ~ +35mV

59 59 Voltage Gate Activity 4.Falling Phase – Na + inactivation gates close, K + activation gates open – Na + influx stops, K + efflux is rapid 5.Undershoot – K + activation gates close, but not until membrane potential has gone a little bit below resting potential 6.Refractory Period – the Na + inactivation gates remain closed during stages 4 and 5, limiting the maximum frequency of action potentials

60 60 Membrane repolarizes

61 61 4. Falling Phase – Na + inactivation gates close, K + activation gates open – Na + influx stops, K + efflux is rapid

62 62 Voltage Gate Activity 4.Falling Phase – Na + inactivation gates close, K + activation gates open – Na + influx stops, K + efflux is rapid 5.Undershoot – K + activation gates close, but not until membrane potential has gone a little bit below resting potential 6.Refractory Period – the Na + inactivation gates remain closed during stages 4 and 5, limiting the maximum frequency of action potentials

63 63

64 64 5. Undershoot – K + activation gates close, but not until membrane potential has gone a little bit below resting potential

65 65 Voltage Gate Activity 4.Falling Phase – Na + inactivation gates close, K + activation gates open – Na + influx stops, K + efflux is rapid 5.Undershoot – K + activation gates close, but not until membrane potential has gone a little bit below resting potential 6.Refractory Period – the Na + inactivation gates remain closed during stages 4 and 5, limiting the maximum frequency of action potentials

66 66 6. Refractory Period – the Na + inactivation gates remain closed during stages 4 and 5, limiting the maximum frequency of action potentials

67 67 Fig , 7 th Ed.

68 68 Conduction of Action Potential Electrical signal moves along the axon by depolarizing adjacent regions of the membrane past the threshold The depolarization effect is NOT directional – the cytoplasm becomes more + in both directions

69 69 Critical Thinking If the depolarizing effect is bilateral, why does the signal travel in one direction only???

70 70 Critical Thinking If the depolarizing effect is bilateral, why does the signal travel in one direction only???

71 71 Conduction of Action Potential Electrical signal moves along the axon by depolarizing adjacent regions of the membrane past the threshold Depolarization zone travels in one direction only due to the refractory period ( Na + gates locked)

72 72 Speed! Diameter of axon  Larger = less resistance  faster signal  Found in invertebrates  Max speed ~ 100 m/second Nodes of Ranvier  Signal jumps from node to node  Found in vertebrates  Saves space – 2,000 myelinated axons can fit in the same space as one giant axon  Max speed ~ 120 m/second

73 73 Synapses – the gaps between cells Electrical synapses occur at gap junctions  Action potential is transmitted directly from cell to cell  Especially important in rapid responses such as escape movements  Also with controlling heart beat (but with specialized muscle tissue) Most synapses are chemical  The signal is converted from electrical  chemical  electrical  Neurotransmitters cross the synapse and carry the signal to the receiving cell

74 74 Chemical Synapses A multi-stage process  Neurons synthesize neurotransmitters, isolated into synaptic vesicles located at the synaptic terminal  The action potential triggers the release of neurotransmitters into the synapse  Neurotransmitters diffuse across the synapse  Neurotransmitter binds to a receptor, stimulating a response (more later)

75 75 Chemical Synapses 1.Action potential depolarizes membrane at synaptic terminal 2.Depolarization in this region opens Ca ++ channels 3.Influx of Ca ++ stimulates synaptic vesicles to fuse with neuron cell membrane 4.Neurotransmitters are released by exocytosis 5.Neurotransmitters bind to the receiving cell membrane

76 76 Chemical Synapses

77 77 Chemical Synapses 1.Action potential depolarizes membrane at synaptic terminal 2.Depolarization in this region opens Ca ++ channels 3.Influx of stimulates synaptic vesicles to fuse with neuron cell membrane 4.Neurotransmitters are released by exocytosis 5.Neurotransmitters bind to the receiving cell membrane REVIEW

78 78 Chemical Synapses Direct synaptic transmission  Neurotransmitter binds directly to ligand-gated channels  Channel opens for Na +, K + or both Indirect synaptic transmission  Neurotransmitter binds to a receptor on the membrane (not to a channel protein)  Signal transduction pathway is initiated  Second messengers eventually open channels  Slower but amplified response

79 79 Chemical synapses allow more complicated signals Electrical signals pass unmodified at electrical synapses Chemical signals are modified during transmission  Type of neurotransmitter varies  Amount of neurotransmitter released varies  Some receptors promote depolarization; some promote hyperpolarization  Signals are summed over both time and space  Remember that many, many neurons are responding to any given stimulus

80 80 Chemical synapses allow more complicated signals Responses are summed at the axon hillock  Action potential is generated and sent down axon; or not

81 81 Chemical synapses allow more complicated signals Summation is over both time and space Excitory and inhibitory signals can “cancel” each other

82 82 Neurotransmitters – review text and table, but don’t memorize Table 48.1, 7 th ed.

83 83 CNS Organization in Vertebrates Brain – integrates Spinal cord – 1 o transmits Both derived from hollow, dorsal embryonic nerve cord  Hollow remnants remain in ventricles of brain and central canal of spinal cord  Spaces are filled with cerebrospinal fluid that helps circulate nutrients, hormones, wastes, etc  Fluid also cushions CNS Axons are aggregated = white matter

84 84

85 85 PNS Organization in Vertebrates Major role – transmitting information from sensory structures to the CNS; and from the CNS to effector structures  Nerves always in left/right pairs that serve both sides of the body

86 86 PNS Organization in Vertebrates Cranial nerves originate in brain and connect to the head and upper body  Some have only sensory neurons (eyes, nose) Spinal nerves originate in spinal cord and connect to the rest of the body  Contain both sensory and motor neurons

87 87 Critical Thinking Can the eyes do anything besides see??? Can the nose do anything besides smell??? Can the ears do anything besides hear???

88 88 Critical Thinking Can the eyes do anything besides see??? Can the nose do anything besides smell??? Can the ears do anything besides hear???

89 89 PNS Organization in Vertebrates Cranial nerves originate in brain and connect to the head and upper body  Some have only sensory neurons (eyes, nose) Spinal nerves originate in spinal cord and connect to the rest of the body  Contain both sensory and motor neurons

90 90 PNS – Sub-divisions All work together to maintain homeostasis and respond to external stimuli

91 91 PNS - Somatic Nerves that transmit signals to and from skeletal muscles Respond primarily to external stimuli Largely under voluntary control

92 92 PNS - Autonomic Nerves that control the internal environment Respond to both internal and external signals Largely under involuntary control Three sub-divisions  Sympathetic – stress responses  Parasympathetic – opposes sympathetic  Enteric – controls digestive system

93 93 PNS – Autonomic

94 94 Autonomic - Sympathetic Activates flight or fight responses Promotes functions that increase sensory perception and ATP levels Inhibits non-essential functions such as digestion and urination

95 95 Autonomic – Parasympathetic Returns body systems to base-line function Promotes digestion and other normal functions Usually antagonistic to sympathetic division

96 96 Autonomic – Enteric Specifically controls the digestive system Regulated by both the sympathetic and parasympathetic divisions

97 97 Brain Development


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