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Chapter 48/49

Chapter 48/49. Nervous Systems. The human brain contains an estimated 100 billion nerve cells, or neurons Each neuron may communicate with thousands of other neurons Complex information processing network is at work. Different neurons do different things.

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Chapter 48/49

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  1. Chapter 48/49 Nervous Systems

  2. The human brain contains an estimated 100 billion nerve cells, or neurons • Each neuron may communicate with thousands of other neurons • Complex information processing network is at work

  3. Different neurons do different things • Functional magnetic resonance imaging (fMRI) can reconstruct a three-dimensional map of brain activity • The results of brain imaging and other research methods revealed that groups of neurons function in specialized circuits dedicated to different tasks

  4. Nervous systems consist of circuits of neurons and supporting cells • All animals except sponges have some type of nervous system • What distinguishes the nervous systems of different animal groups is… how the neurons are organized into circuits

  5. Nerve net (a) Hydra (cnidarian) Organization of Nervous Systems • The simplest animals with nervous systems, the cnidarians • They have neurons arranged in nerve nets

  6. Radialnerve Nervering (b) Sea star (echinoderm) Sea Stars • Sea stars have a nerve net in each arm connected by radial nerves to a central nerve ring

  7. Eyespot Brain Nerve cord Transversenerve (c) Planarian (flatworm) Flatworms • In relatively simple cephalized animals, such as flatworms a central nervous system (CNS) is evident

  8. Brain Brain Ventralnerve cord Ventral nervecord Segmentalganglia Segmentalganglion (d) Leech (annelid) (e) Insect (arthropod) Annelids and arthropods • Have segmentally arranged clusters of neurons called ganglia • These ganglia connect to the CNS and make up a peripheral nervous system (PNS)

  9. Anteriornerve ring Ganglia Brain Longitudinalnerve cords Ganglia (g) Squid (mollusc) (f) Chiton (mollusc) Molluscs • Nervous systems in molluscs correlate with the animals’ lifestyles • Sessile molluscs have simple systems • More complex molluscs have more sophisticated systems

  10. Brain Sensoryganglion Spinalcord (dorsalnerve cord) (h) Salamander (chordate) Vertebrates • The central nervous system consists of a brain and dorsal spinal cord • The PNS connects to the CNS via nerves

  11. Sensory input Integration Sensor Motor output Effector Central nervoussystem (CNS) Peripheral nervoussystem (PNS) Information Processing • Nervous systems process information in three stages: Sensory input, integration, and motor output

  12. Sensory neurons transmit information from sensors (receptors) that detect external stimuli and internal conditions • Sensory information is sent to the CNS • Where interneurons integrate the information • Motor output leaves the CNS via motor neurons • Which communicate with effector cells

  13. Cells of the Nervous System

  14. Dendrites Cell body Nucleus Synapse Signal direction Axon Axon hillock Presynaptic cell Postsynaptic cell Myelin sheath Synapticterminals Neuron Structure • Schwann cells • Synaptic Terminals • Synapse • Neurotransmitters • Cell body • Dendrites • Axons • Myelin Sheath

  15. Types of neurons • Sensory Neurons • Interneurons • Motor Neurons

  16. Node of Ranvier Layers of myelin Axon Schwann cell Schwann cell Nodes of Ranvier Nucleus of Schwann cell Axon Myelin sheath 0.1 µm Supporting Cells (Glia) • Glia are supporting cells • They are essential for the structural integrity of the nervous system and for the normal functioning of neurons

  17. Ion pumps and ion channels maintain the resting potential of a neuron • Across its plasma membrane, every cell has a voltage called a membrane potential • Resting Membrane Potential: membrane potential of a neuron that is not transmitting signals • The inside of a cell is negative relative to the outside

  18. The concentration of Na+ is higher in the extracellular fluid than in the cytosol • While the opposite is true for K+

  19. A neuron that is not transmitting signals • Contains many open K+ channels and fewer open Na+ channels in its plasma membrane • The diffusion of K+ and Na+ through these channels • Leads to a separation of charges across the membrane, producing the resting potential

  20. Gated Ion Channels • Gated ion channels open or close • In response to membrane stretch or the binding of a specific ligand • In response to a change in the membrane potential

  21. Action Potential • Action potentials are the signals conducted by nerve fibers • If a cell has gated ion channels • Its membrane potential may change in response to stimuli that open or close those channels

  22. Stimuli +50 0 Membrane potential (mV) Threshold –50 Restingpotential Hyperpolarizations –100 0 1 2 3 4 5 Time (msec) (a) Graded hyperpolarizations produced by two stimuli that increase membrane permeability to K+. The larger stimulus producesa larger hyperpolarization. Hyperpolarization Stimuli may trigger an increase in the magnitude of the membrane potential

  23. Stimuli +50 0 Membrane potential (mV) –50 Threshold Restingpotential Depolarizations –100 0 1 2 3 4 5 Time (msec) (b) Graded depolarizations produced by two stimuli that increase membrane permeability to Na+.The larger stimulus produces alarger depolarization. Depolarization • stimuli may trigger a reduction in the magnitude of the membrane potential

  24. Hyperpolarization and depolarization • Are both called graded potentials because the magnitude of the change in membrane potential varies with the strength of the stimulus

  25. Stronger depolarizing stimulus +50 Actionpotential 0 Membrane potential (mV) Threshold –50 Restingpotential –100 0123456 Time (msec) (c) Action potential triggered by a depolarization that reaches the threshold. Production of Action Potentials • In most neurons, depolarizations are graded only up to a certain membrane voltage, called the threshold • A stimulus strong enough to produce a depolarization that reaches the threshold triggers a different type of response, called an action potential

  26. Action Potential • An action potential • Is a brief all-or-none depolarization of a neuron’s plasma membrane • Is the type of signal that carries information along axons

  27. Step 1…Sodium gates • Both voltage-gated Na+ channels and voltage-gated K+ channels • Are involved in the production of an action potential • When a stimulus depolarizes the membrane • Na+ channels open, allowing Na+ to diffuse into the cell

  28. Step 2…Potassium gates • As the action potential subsides • K+ channels open, and K+ flows out of the cell • A refractory period follows the action potential • During which a second action potential cannot be initiated

  29. –  – –  – –  – –  – +  + +  + +  + +  + +  + +  + +  + +  + –  – –  – –  – –  – 3 4 Falling phase of the action potential 3 +  + +  + +  + +  + 2 4 –  – –  – –  – –  – 5 1 1 Depolarization 2 +  + +  + +  + +  + Activationgates Extracellular fluid Potassiumchannel –  – –  – –  – –  – +  + Plasma membrane –  – 5 Inactivationgate Resting state 1 Generation of an action potential Na+ Na+ Na+ Na+ K+ K+ Rising phase of the action potential Depolarization opens the activation gates on most Na+ channels, while the K+ channels’ activation gates remain closed. Na+ influx makes the inside of the membrane positive with respect to the outside. The inactivation gates on most Na+ channels close, blocking Na+ influx. The activation gates on mostK+ channels open, permitting K+ effluxwhich again makesthe inside of the cell negative. +50 Actionpotential Na+ Na+ 0 Membrane potential (mV) Threshold Threshold –50 K+ Resting potential –100 Time A stimulus opens the activation gates on some Na+ channels. Na+ influx through those channels depolarizes the membrane. If the depolarization reaches the threshold, it triggers an action potential. Na+ Na+ Na+ +  +  +  +  +  +  +  + +  + +  + K+ –  –  –  –  –  –  –  – –  – –  – Undershoot Both gates of the Na+ channelsare closed, but the activation gates on some K+channels are still open. As these gates close onmost K+ channels, and the inactivation gates open on Na+ channels, the membrane returns toits resting state. Cytosol Sodiumchannel K+ The activation gates on the Na+ and K+ channelsare closed, and the membrane’s resting potential is maintained.

  30. – + + + + + + – – + + + + + + Axon Actionpotential 1 + + An action potential is generated as Na+ flows inward across the membrane at one location. – – – – – – Na+ – – – – – – + + – – + + + + + + Actionpotential 2 K+ The depolarization of the action potential spreads to the neighboring region of the membrane, re-initiating the action potential there. To the left of this region, the membrane is repolarizing as K+ flows outward. – – + – – – + – Na+ – – – – – – + + – – + + + + + + K+ Actionpotential 3 K+ The depolarization-repolarization process isrepeated in the next region of the membrane. In this way, local currents of ions across the plasma membrane cause the action potential to be propagated along the length of the axon. – – – – + + + + – + + + + – – – Na+ – – – + + – + + – + + – – – + + K+ Conduction of Action Potentials An electrical current depolarizes the neighboring region of the axon membrane

  31. Conduction Speed • The speed of an action potential • Increases with the diameter of an axon • In vertebrates, axons are myelinated • Also causing the speed of an action potential to increase

  32. Schwann cell Depolarized region(node of Ranvier) Myelin sheath – –– – – – ++ + Cell body ++ ++ + Axon – – – ++ + – – – Saltatory Conduction • Action potentials in myelinated axons • Jump between the nodes of Ranvier in a process called saltatory conduction

  33. Postsynapticneuron Synapticterminalof presynapticneurons 5 µm Synapse • Electrical synapse • Electrical current flows directly from one cell to another via a gap junction • Chemical synapse • A presynaptic neuron releases chemical neurotransmitters, which are stored in the synaptic terminal

  34. Postsynaptic cell Presynapticcell Na+ Neuro-transmitter Synaptic vesiclescontainingneurotransmitter K+ Presynapticmembrane Postsynaptic membrane Ligand-gatedion channel Voltage-gatedCa2+ channel Ca2+ Postsynaptic membrane 3 Synaptic cleft Ligand-gatedion channels 5 4 6 1 2 Neurotransmitter release • When an action potential reaches a terminal the final result is the release of neurotransmitters into the synaptic cleft

  35. Synaptic Transmission • Neurotransmitters bind to ligand-gated ion channels • Binding causes the ion channels to open, generating a postsynaptic potential and generation of action potential • Fate of neurotransmitter in the cleft • Diffuses out of the synaptic cleft • Taken up by surrounding cells and degraded by enzymes • Degraded in the cleft

  36. Neurotransmitters • Different neurons may release different neurotransmitters • The same neurotransmitter can produce different effects in different types of cells

  37. Neurotransmitters

  38. Neurotransmitters • Acetylcholine • Is one of the most common neurotransmitters in both vertebrates and invertebrates • Can be inhibitory or excitatory • Biogenic amines • Include epinephrine, norepinephrine, dopamine, and serotonin • Are active in the CNS and PNS • Amino acids and peptides • Are active in the brain • Gases such as nitric oxide and carbon monoxide • Are local regulators in the PNS

  39. Central nervous system (CNS) Peripheral nervous system (PNS) Brain Cranial nerves Spinal cord Ganglia outside CNS Spinal nerves Vertebrate Nervous System • Regionally specialized • In all vertebrates, the nervous system shows a high degree of cephalization and distinct CNS and PNS components

  40. Central Nervous System • The brain provides the integrative power • That underlies the complex behavior of vertebrates • The spinal cord integrates simple responses to certain kinds of stimuli • And conveys information to and from the brain

  41. Peripheral Nervous System • The PNS transmits information to and from the CNS • And plays a large role in regulating a vertebrate’s movement and internal environment • The cranial nerves originate in the brain • And terminate mostly in organs of the head and upper body • The spinal nerves originate in the spinal cord • And extend to parts of the body below the head

  42. Peripheral nervous system Somatic nervous system Autonomic nervous system Sympathetic division Parasympathetic division Enteric division PNS components • The somatic nervous system • The autonomic nervous system

  43. Somatic and Autonomic • The somatic nervous system • Carries signals to skeletal muscles • The autonomic nervous system • Regulates the internal environment, in an involuntary manner • Is divided into the sympathetic, parasympathetic, and enteric divisions

  44. Sympathetic division Parasympathetic division Action on target organs: Action on target organs: Dilates pupil of eye Constricts pupil of eye Location of preganglionic neurons: brainstem and sacral segments of spinal cord Location of preganglionic neurons: thoracic and lumbar segments of spinal cord Inhibits salivary gland secretion Stimulates salivary gland secretion Sympathetic ganglia Neurotransmitter released by preganglionic neurons: acetylcholine Constricts bronchi in lungs Relaxes bronchi in lungs Neurotransmitter released by preganglionic neurons: acetylcholine Cervical Accelerates heart Slows heart Inhibits activity of stomach and intestines Thoracic Stimulates activity of stomach and intestines Location of postganglionic neurons: in ganglia close to or within target organs Location of postganglionic neurons: some in ganglia close to target organs; others in a chain of ganglia near spinal cord Inhibits activity of pancreas Stimulates activity of pancreas Stimulates glucose release from liver; inhibits gallbladder Stimulates gallbladder Lumbar Neurotransmitter released by postganglionic neurons: acetylcholine Neurotransmitter released by postganglionic neurons: norepinephrine Stimulates adrenal medulla Promotes emptying of bladder Inhibits emptying of bladder Promotes erection of genitalia Promotes ejaculation and vaginal contractions Sacral Synapse Sympathetic and parasympathetic divisions • They have antagonistic effects on target organs

  45. The sympathetic division • Correlates with the “fight-or-flight” response • The parasympathetic division • Promotes a return to self-maintenance functions • The enteric division • Controls the activity of the digestive tract, pancreas, and gallbladder

  46. The Brainstem • The brainstem consists of three parts: the medulla oblongata, the pons, and the midbrain

  47. Brainstem • The medulla oblongata • Contains centers that control several visceral functions • The pons • Also participates in visceral functions • The midbrain • Contains centers for the receipt and integration of several types of sensory information • All three areas are center of reflex actions

  48. Eye Input from ears Reticular formation Input from touch, pain, and temperature receptors Arousal and Sleep • A diffuse network of neurons called the reticular formation is present in the core of the brainstem • A part of the reticular formation, the reticular activating system (RAS) regulates sleep and arousal

  49. The Cerebellum • Important for coordination and error checking during motor, perceptual, and cognitive functions • Also involved in learning and remembering motor skills

  50. The Diencephalon • The embryonic diencephalon develops into three adult brain regions • The epithalamus, thalamus, and hypothalamus

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