1. The Nervous System Chapter 44

2. 2 Nervous System Organization All animals must be able to respond to environmental stimuli -Sensory receptors = Detect stimulus -Motor effectors = Respond to it -The nervous system links the two -Consists of neurons and supporting cells

3. 3 Nervous System Organization Vertebrates have three types of neurons -Sensory neurons (afferent neurons) carry impulses to central nervous system (CNS) -Motor neurons (efferent neurons) carry impulses from CNS to effectors (muscles and glands) -Interneurons (association neurons) provide more complex reflexes and associative functions (learning and memory)

4. 4

5. 5 Nervous System Organization The CNS consists of the brain and spinal cord The peripheral nervous system (PNS) consists of sensory and motor neurons -Somatic NS stimulates skeletal muscles -Autonomic NS stimulates smooth and cardiac muscles, as well as glands -Sympathetic and parasympathetic NS -Counterbalance each other

6. 6 PNS CNS Brain and Spinal Cord Sympathetic nervous system "fight or flight" Parasympathetic nervous system "rest and repose" Somatic nervous system (voluntary) Sensory neurons registering external stimuli Autonomic nervous system (involuntary) Sensory Pathways Motor Pathways central nervous system (CNS) peripheral nervous system (PNS) Sensory neurons registering external stimuli

7. 7 Nervous System Organization Neurons have the same basic structure -Cell body = Enlarged part containing nucleus -Dendrites = Short, cytoplasmic extensions that receive stimuli -Axon = Single, long extension that conducts impulses away from cell body

8. 8 Nervous System Organization

9. 9 Neurons are supported both structurally and functionally by cells called neuroglia -Schwann cells and oligodendrocytes produce myelin sheaths surrounding axons -In the CNS, myelinated axons form white matter -Dendrites/cell bodies form gray matter -In the PNS, myelinated axons are bundled to form nerves

10. 10 Nervous System Organization

11. 11 Nerve Impulse Transmission A potential difference exists across every cell’s plasma membrane -Negative pole = Cytoplasmic side -Positive pole = Extracellular fluid side When a neuron is not being stimulated, it maintains a resting potential -Ranges from -40 to -90 millivolts (mV) -Average about -70 mV

12. 12 Nerve Impulse Transmission The inside of the cell is more negatively charged than the outside because of: 1. Sodium-potassium pump = Brings two K + into cell for every three Na + it pumps out 2. Ion leakage channels = Allow more K + to diffuse out than Na + to diffuse in

13. 13 1. Carrier in membrane binds intracellular sodium. 2. ATP phosphorylates protein with bound sodium. 3. Phosphorylation causes conformational change in protein, reducing its affinity for Na +. The Na + then diffuses out. 4. This conformation has higher affinity for K +. Extracellular K + binds to exposed sites. 5. Binding of potassium causes dephosphorylation of protein. Extracellular Intracellular ATP ADP PiPi P + K+K+ Na + 6. Dephosphorylation of protein triggers change to original conformation, with low affinity for K +. K + diffuses into the cell, and the cycle repeats. PiPi PiPi PiPi

14. 14 Nerve Impulse Transmission There is a buildup of positive charge outside and negative charge inside the membrane -This electrical potential is an attractive force to bring K + ions back into the cell -Balance between diffusional and electrical forces leads to the equilibrium potential The resting membrane potential can be viewed using a voltmeter and two electrodes

15. 15 Nerve Impulse Transmission

16. 16 Nerve Impulse Transmission There are two types of potentials: -Graded potentials and action potentials Graded potentials are small transient changes in membrane potential due to activation of gated ion channels -Most are closed in the normal resting cell

17. 17 Nerve Impulse Transmission Chemically-gated or ligand-gated channels -Ligands are hormones or neurotransmitters -Induce opening and cause changes in cell membrane permeability

18. 18 Nerve Impulse Transmission Depolarization makes the membrane potential more positive, whereas a hyperpolarization makes it more negative -These small changes result in graded potentials -Can reinforce or negate each other Summation is the ability of graded potentials to combine

19. 19 Nerve Impulse Transmission

20. 20 Nerve Impulse Transmission Action potentials result when depolarization reaches the threshold potential The action potential is caused by voltage- gated ion channels -Two different channels are used: -Voltage-gated Na + channels -Voltage-gated K + channels

21. 21 Nerve Impulse Transmission When the threshold voltage is reached, sodium channels open rapidly -Transient influx of Na + causes the membrane to depolarize In contrast, potassium channel opens slowly -Efflux of K + repolarizes the membrane

22. 22 Nerve Impulse Transmission The action potential has three phases: -Rising, falling and undershoot Action potentials are always separate, all-or- none events with the same amplitude -Do not add up or interfere with each other The intensity of a stimulus is coded by the frequency, not amplitude, of action potentials

23. 23 Membrane potential (mV) 2. Rising Phase Stimulus causes above threshold voltage +50 0 12 3 –70 Time (ms) 1. Resting Phase Equilibrium between diffusion of K + out of cell and voltage pulling K + into cell Voltage-gated potassium channel Potassium channel Voltage-gated sodium channel Potassium channel gate closes Sodium channel activation gate closes. Inactivation gate opens. Sodium channel activation gate opens 3. Top curve Maximum voltage reached Potassium gate opens 4. Falling Phase Potassium gate open Undershoot occurs as excess potassium diffuses out before potassium channel closes Equilibrium restored Na + channel inactivation gate closes K+K+ Na + Na + channel inactivation gate closed

24. 24 Nerve Impulse Transmission Each action potential, in its rising phase, reflects a reversal in membrane polarity -Positive charges due to influx of Na + can depolarize the adjacent region to threshold -And so the next region produces its own action potential -Meanwhile, the previous region repolarizes back to the resting membrane potential

25. 25 Cell membrane Cytoplasm resting repolarized depolarized + + + + + + + + + – – – – – – – – – + + + + + + + + + – – – – – – – – – – – + + + + + + + + + – – – – – – – + + + + + + + – – – – – – – – – + + + + – – + + + + + – – + + – – – – – + + + + + – – + + – – – – – + + – – + + + + – – – + + – – – – + + + – – + + – – – + + + + – – + + + – – – – + + + + + + + – – – – – – – – – + + – – + + + + + + + + + – – – – – – – Na + K+K+ K+K+ K+K+ K+K+ K+K+ K+K+

26. 26 Nerve Impulse Transmission Two ways to increase velocity of conduction: 1. Axon has a large diameter -Less resistance to current flow -Found primarily in invertebrates 2. Axon is myelinated -Action potential is only produced at the nodes of Ranvier -Impulse jumps from node to node -Saltatory conduction

27. 27 Nerve Impulse Transmission

28. 28 Synapses Synapses are intercellular junctions -Presynaptic cell transmits action potential -Postsynaptic cell receives it Two basic types: electrical and chemical Electrical synapses involve direct cytoplasmic connections between the two cells formed by gap junctions -Relatively rare in vertebrates

29. 29 Synapses Chemical synapses have a synaptic cleft between the two cells -End of presynaptic cell contains synaptic vesicles packed with neurotransmitters

30. 30 Synapses Action potential triggers influx of Ca 2+ -Synaptic vesicles fuse with cell membrane -Neurotransmitter is released by exocytosis -Diffuses to other side of cleft and binds to chemical- or ligand-gated receptor proteins -Neurotransmitter action is terminated by enzymatic cleavage or cellular uptake

31. 31 Synapses

32. 32 Neurotransmitters Acetylcholine (ACh) -Crosses the synapse between a motor neuron and a muscle fiber -Neuromuscular junction

33. 33 Neurotransmitters Acetylcholine (ACh) -Binds to ligand-gated receptor in the postsynaptic membrane -Produces a depolarization called an excitatory postsynaptic potential (EPSP) -Stimulates muscle contraction -Acetylcholinesterase (AChE) degrades ACh -Causes muscle relaxation

34. 34 Neurotransmitters Amino acids -Glutamate is the major excitatory neurotransmitter in the vertebrate CNS -Glycine and GABA (  -aminobutyric acid) are inhibitory neurotransmitters -Open ligand-gated channels for Cl – -Produce a hyperpolarization called an inhibitory postsynaptic potential (IPSP)

35. 35 Neurotransmitters

36. 36 Neurotransmitters Biogenic amines -Epinephrine (adrenaline) and norepinephrine are responsible for the “fight or flight” response -Dopamine is used in some areas of the brain that control body movements -Serotonin is involved in the regulation of sleep

37. 37 Neurotransmitters Neuropeptides -Substance P is released from sensory neurons activated by painful stimuli -Intensity of pain perception depends on enkephalins and endorphins Nitric oxide (NO) -A gas ; produced as needed from arginine -Causes smooth muscle relaxation

38. 38 Synaptic Integration Integration of EPSPs (depolarization) and ISPSs (hyperpolarization) occurs on the neuronal cell body -Small EPSPs add together to bring the membrane potential closer to the threshold -IPSPs subtract from the depolarizing effect of EPSPs -And will therefore deter the membrane potential from reaching threshold

39. 39 Synaptic Integration

40. 40 Synaptic Integration There are two ways that the membrane can reach the threshold voltage -Spatial summation -Many different dendrites produce EPSPs -Temporal summation -One dendrite produces repeated EPSPs

41. 41 Drug Addiction Prolonged exposure to a stimulus may cause cells to lose the ability to respond to it -This process is called habituation -The cell decreases the number of receptors because there is an abundance of neurotransmitters

42. 42 Drug Addiction Cocaine affects neurons in the brain’s “pleasure pathways” (limbic system) -Binds dopamine transporters and prevents the reuptake of dopamine -Dopamine survives longer in the synapse and fires pleasure pathways more and more -Prolonged exposure triggers the limbic system neurons to reduce receptor numbers -The cocaine user is now addicted

43. 43 Receptor protein Drug molecule Synapse 1. Reuptake of neuro- transmitter by transporter at a normal synapse. 2. Drug molecules block transporter and cause overstimulation of the postsynaptic membrane. 3. Neuron adjusts to overstimulation by decreasing the number of receptors. 4. Decreased number of receptors make the synapse less sensitive when the drug is removed. Neurotransmitter Transporter protein Transporter protein Dopamine Cocaine Receptor protein

44. 44 Drug Addiction Nicotine binds directly to a specific receptor on postsynaptic neurons of the brain -Brain adjusts to prolonged exposure by “turning down the volume” in two ways: 1. Making fewer nicotine receptors 2. Altering the pattern of activation of the nicotine receptors

45. 45 The Central Nervous System Sponges are only major phylum without nerves Cnidarians have the simplest nervous system -Neurons linked to each other in a nerve net -No associative activity Free-living flatworms (phylum Platyhelminthes) are simplest animals with associative activity -Two nerve cords run down the body -Permit complex muscle control

46. 46 Human Cerebrum Cerebellum Spinal cord Cervical nerves Thoracic nerves Lumbar nerves Femoral nerve Sciatic nerve Tibial nerve Sacral nerves Nerve net Nerve cords Associative neurons Brain Giant axon Mollusk Echinoderm Central nervous system Peripheral nerves Brain Ventral nerve cords Radial nerve Nerve ribs Cnidarian Flatworm Earthworm Arthropod

47. 47 Vertebrate Brains All vertebrate brains have three basic divisions: -Hindbrain or rhombencephalon -Midbrain or mesencephalon -Forebrain or prosencephalon In fishes, -Hindbrain = Largest portion -Midbrain = Processes visual information -Forebrain = Processes olfactory information

48. 48 Vertebrate Brains Olfactory bulb CerebrumThalamus Optic tectum Cerebellum Spinal cord Medulla oblongata Pituitary Hypothalamus Optic chiasm Forebrain (Prosencephalon) Midbrain (Mesencephalon) Hindbrain (Rhombencephalon)

49. 49 Vertebrate Brains The relative sizes of different brain regions have changed as vertebrates evolved -Forebrain became the dominant feature

50. 50 Vertebrate Brains Forebrain is composed of two elements: -Diencephalon -Thalamus: Integration and relay center -Hypothalamus: Participates in basic drives & emotions; controls pituitary gland -Telencephalon (“end brain”) -Devoted largely to associative activity -Called the cerebrum in mammals

51. 51 Cerebrum The increase in brain size in mammals reflects the great enlargement of the cerebrum -Split into right and left cerebral hemispheres, which are connected by a tract called the corpus callosum -Each hemisphere receives sensory input from the opposite side -Hemispheres are divided into: frontal, parietal, temporal and occipital lobes

52. 52 Cerebrum

53. 53 Cerebrum Cerebral cortex -Outer layer of the cerebrum -Contains about 10% of all neurons in brain -Highly convoluted surface -Increases threefold the surface area of the human brain -Divided into three regions, each with a specific function

54. 54 Cerebrum Cerebral cortex -Primary motor cortex: Movement control -Primary somatosensory cortex: Sensory control -Association cortex: Higher mental functions Basal ganglia -Aggregates of neuron cell bodies -Form islands of grey matter within the cerebrum’s white matter

55. 55 Cerebrum

56. 56 Cerebrum

57. 57 Other Brain Structures Thalamus -Integrates visual, auditory and somatosensory information Hypothalamus -Integrates visceral activities -Controls pituitary gland -Forms limbic system, with hippocampus and amygdala -Responsible for emotional responses

58. 58 Complex Functions of the Brain Sleep and arousal -One section of reticular formation controls consciousness and alertness -Reticular-activating system controls both sleep and the waking state -Brain state can be monitored by means of an electroencephalogram (EEG) -Records electrical activity

59. 59 Complex Functions of the Brain Language -Left hemisphere is “dominant” hemisphere -Adept at sequential reasoning Spatial recognition -Right hemisphere is adept at spatial reasoning -Primarily involved in musical ability

60. 60 Complex Functions of the Brain

61. 61 Complex Functions of the Brain Memory -Appears dispersed across the brain -Short-term memory is stored in the form of transient neural excitations -Long-term memory appears to involve structural changes in neural connections

62. 62 Complex Functions of the Brain Alzheimer disease is a condition where memory and thought become dysfunctional -Two causes have been proposed 1. Nerve cells are killed from the outside in -External protein:  -amyloid 2. Nerve cells are killed from the inside out -Internal proteins: tau ( 

63. 63 Spinal Cord The spinal cord is a cable of neurons extending from the brain down through the backbone -Enclosed and protected by the vertebral column and the meninges

64. 64 Spinal Cord It serves as the body’s “information highway” -Relays messages between the body and the brain It also functions in reflexes -The knee-jerk reflex is monosynaptic -However, most reflexes in vertebrates involve a single interneuron

65. 65 Quadriceps muscle (effector) Spinal cord Dorsal root ganglion Gray matter White matter Monosynaptic synapse Sensory neuro Nerve fiber Stretch receptor (muscle spindle) Skeletal muscle Stimulus Response Motor neuron

66. 66 Effector (muscle) Dorsal Ventral Spinal Cord Interneuron Cell body in dorsal root ganglion Gray matter White matter Motor neuron Sensory neuron Receptor in skin Stimulus

67. 67 The Peripheral Nervous System The PNS consists of nerves and ganglia -Nerves are bundles of axons bound by connective tissue -Ganglia are aggregates of neuron cell bodies

68. 68 The Peripheral Nervous System Sensory neurons: -Axons enter the dorsal surface of the spinal cord and form dorsal root of spinal nerve -Cell bodies are grouped outside the spinal cord in dorsal root ganglia Motor neurons: -Axons leave from the ventral surface and form ventral root of spinal nerve -Cell bodies are located in the spinal cord

69. 69 The Somatic Nervous System Somatic motor neurons stimulate the skeletal muscles to contract -In response to conscious command or reflex actions The antagonist of the muscle is inhibited by hyperpolarization (IPSPs) of spinal motor neurons

70. 70 The Autonomic Nervous System Composed of the sympathetic and parasympathetic divisions, plus the medulla oblongata In both, efferent motor pathway has 2 neurons -Preganglionic neuron: exits the CNS and synapses at an autonomic ganglion -Postganglionic neuron: exits the ganglion and regulates visceral effectors -Smooth or cardiac muscle or glands

71. 71 Viscera Autonomic ganglion Postganglionic neuron Autonomic motor reflex Interneuron Dorsal root ganglion Preganglionic neuron Sensory neuron Spinal cord

72. 72 The Autonomic Nervous System Sympathetic division -Preganglionic neurons originate in the thoracic and lumbar regions of spinal cord -Most axons synapse in two parallel chains of ganglia right outside the spinal cord Parasympathetic division -Preganglionic neurons originate in the brain and sacral regions of spinal cord -Axons terminate in ganglia near or even within internal organs

73. 73 Constrict Secrete saliva Dilate Stop secretion Dilate bronchioles Speed up heartbeat Increase secretion Empty colon Increase motility Empty bladder Slow down heartbeat Constrict bronchioles Sympathetic ganglion chain Stomach Secrete adrenaline Decrease secretion Decrease motility Retain colon contents Delay emptying Adrenal gland Bladder Small intestine Large intestine Spinal cord ParasympatheticSympathetic

74. 74 The Autonomic Nervous System Autonomic effects are mediated by the action of G protein-coupled receptors -The receptor is activated by binding to its ligand (Ach, for example) -The G protein is activated -It activates the effector protein

75. 75 The Autonomic Nervous System G protein activates Ligand (signal) Ion channel closed Ion channel open    GDP G protein    GTP K+K+ K+K+ K+K+ K+K+