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Chapter 28

Chapter 28. Nervous Systems. Can an Injured Spinal Cord Be Fixed? The spinal cord is the central communication conduit between the brain and the rest of the body Injuries to the spinal cord can produce paraplegia or quadriplegia The spinal cord cannot repair itself

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Chapter 28

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  1. Chapter 28 Nervous Systems

  2. Can an Injured Spinal Cord Be Fixed? • The spinal cord is the central communication conduit between the brain and the rest of the body • Injuries to the spinal cord can produce paraplegia or quadriplegia • The spinal cord cannot repair itself • Researchers are working on ways to regenerate or replace damaged nerve cells • Growth factor proteins • Embryonic stem cells

  3. NERVOUS SYSTEM STRUCTURE AND FUNCTION • 28.1 Nervous systems receive sensory input, interpret it, and send out appropriate commands • The nervous system obtains and processes sensory information and sends commands to effector cells • Central nervous system (CNS): brain and spinal cord • Peripheral nervous system (PNS): nerves that carry signals to and from CNS

  4. Nervous tissue • Neuron: nerve cell specialized for carrying signals from one location in the body to another • Nerve: bundle of neuron extensions wrapped in connective tissue • Ganglia: clusters of neuron cell bodies; found in PNS

  5. A nervous system has three interconnected functions • Sensory input: Sensory neurons conduct signals from sensory receptors to integration centers • Integration: Interneurons interpret signals and formulate responses • Motor output: Motor neurons conduct signals from integration centers to effector cells • A reflex is an automatic response to stimuli

  6. LE 28-1a Sensory input Integration Sensory receptor Motor output Brain and spinal cord Effector cells Peripheral nervous system (PNS) Central nervous system (CNS)

  7. LE 28-1b Sensory receptor Sensory neuron Brain Ganglion Motor neuron Spinal cord Quadriceps muscles Interneuron CNS Nerve Flexor muscles PNS

  8. 28.2 Neurons are the functional units of nervous systems • Neurons are cells specialized for carrying signals • Cell body: contains most organelles • Dendrites: highly branched extensions that carry signals from other neurons toward the cell body • Axon: long extension that transmits signals to other cells

  9. Many axons are enclosed by an insulating myelin sheath • Chain of Schwann cells • Nodes of Ranvier: points where signals can be transmitted • Speeds up signal transmission • Supporting cells (glia) are essential for structural integrity and normal functioning of the nervous system

  10. The axon ends in a cluster of branches • Each branch ends in a synaptic terminal • A synapse is a site of communication between a synaptic terminal and another cell

  11. LE 28-2 Signal direction Dendrites Cell Body Cell body SEM 3,600 Node of Ranvier Layers of myelin in sheath Signal pathway Axon Schwann cell Nucleus Nucleus Nodes of Ranvier Schwann cell Myelin sheath Synaptic terminals

  12. NERVE SIGNALS AND THEIR TRANSMISSION • 28.3 A neuron maintains a membrane potential across its membrane • A resting neuron has potential energy • Membrane potential: electrical charge difference across the neuron's plasma membrane • Resting potential: voltage across the plasma membrane of a resting neuron

  13. The resting potential depends on differences in ionic composition inside and outside the cell • More K+ than Na+ diffuses inward through membrane channels • Sodium-potassium pumps actively transport Na+ out of cell and K+ in • The ionic gradient produces a voltage across the membrane • The basis of nervous system signals Animation: Resting Potential Animation: Sodium Potassium Pump

  14. LE 28-3a Voltmeter Plasma membrane Microelectrode outside cell –70 mV Microelectrode inside cell Axon Neuron

  15. LE 28-3b Outside of cell Na+ Na+ K+ Na+ K+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ channel Na+ K+ Plasma membrane Na+-K+ pump K+channel Na+ K+ K+ K+ Protein K+ K+ K+ K+ K+ Na+ Inside of cell

  16. 28.4 A nerve signal begins as a change in the membrane potential • Electrical changes make up an action potential, a nerve signal that carries information along an axon • Stimulus raises voltage from resting potential to threshold • Action potential is triggered; membrane polarity reverses abruptly • Membrane repolarizes; voltage drops • Voltage undershoots and then returns to resting potential

  17. Cause of electrical changes of an action potential • Movement of K+ and Na+ across the membrane • Controlled by the opening and closing of voltage-gated channels Animation: Action Potential

  18. LE 28-4-5 Na+ K+ Na+ K+ Additional Na+ channels open, K+ channels are closed; interior of cell becomes more positive. Na+ channels close and inactivate. K+ channels open, and K+ rushes out; interior of cell more negative than outside. Na+ +50 Action potential 0 Membrane potential (mV) The K+ channels close relatively slowly, causing a brief undershoot. Na+ Threshold 50 A stimulus opens some Na+ channels; if threshold is reached, action potential is triggered. Resting potential 100 Time (msec) Neuron interior Neuron interior Resting state: voltage-gated Na+ and K+ channels closed; resting potential is maintained. Return to resting state.

  19. 28.5 The action potential propagates itself along the neuron • An action potential transmits a signal in a domino effect • Na+ channels open, Na+ rushes inward • K+ channels open, K+ diffuses outward; Na+ channels are closed and inactivated • Membrane returns to resting potential

  20. Action potentials are propagated only from cell body to synaptic cleft • Cannot be generated where K+ is leaving axon and Na+ channels are inactivated • Action potentials are all-or-none events • Same events occur no matter how strong or weak the stimulus • Intensity of stimulus determines frequency of action potentials

  21. LE 28-5 Axon Action potential Axon segment Na+ Action potential K+ Na+ K+ Action potential K+ Na+ K+

  22. 28.6 Neurons communicate at synapses • The transmission of signals occurs at synapses • Junction between synaptic terminal and another cell • Electrical synapse • Electrical current passes directly from one neuron to the next • Receiving neuron stimulated quickly and at same frequency as sending neuron • Found in human heart and digestive tract

  23. Chemical synapse • Action potential arrives in sending neuron • Vesicle containing neurotransmitter fuses with plasma membrane • Neurotransmitter is released into synaptic cleft • Neurotransmitter binds to receptor on receiving neuron • Following events vary with different types of chemical synapses

  24. LE 28-6 Sending neuron Action potential arrives Vesicles Axon of sending neuron Synaptic terminal Synapse Vesicle fuses with plasma membrane Neurotransmitter is released into synaptic cleft Synaptic cleft Receiving neuron Neurotrans- mitter binds to receptor Receiving neuron Neurotransmitter molecules Ion channels Neurotransmitter Neurotransmitter broken down and releases Receptor Ions Ion channel opens Ion channel closes

  25. Animation: Synapse

  26. 28.7 Chemical synapses make complex information processing possible • A neuron may receive information from hundreds of other neurons via thousands of synaptic terminals • Some neurotransmitters excite the receiving cell • Other neurotransmitters inhibit the receiving cell's activity by decreasing its ability to develop action potentials • If excitatory signals are strong enough to initiate an action potential, a neuron will transmit a signal

  27. LE 28-7 Synaptic terminals Dendrites Inhibitory Excitatory Myelin sheath Receiving cell body Axon Synaptic terminals SEM 5,500

  28. 28.8 A variety of small molecules function as neurotransmitters • Many small, nitrogen-containing molecules serve as neurotransmitters • Acetylcholine • Important in brain and at synapses between motor neurons and muscles • Biogenic amines • Important in central nervous system • Seratonin, dopamine

  29. Amino acids • Important in central nervous system • Peptides • Substance P, endorphins influence perception of pain • Dissolved gases • NO functions during sexual arousal

  30. CONNECTION • 28.9 Many drugs act at chemical synapses • Many psychoactive drugs act at synapses and affect neurotransmitter action • Caffeine • Nicotine • Alcohol • Psychoactive prescription drugs • Stimulants • THC (marijuana) • Opiates

  31. AN OVERVIEW OF ANIMAL NERVOUS SYSTEMS • 28.10 Nervous system organization usually correlates with body symmetry • Sponges have no nervous system • Radially symmetrical animals • Nervous system arranged in a weblike system of neurons called a nerve net • Though uncentralized, not simple

  32. Most bilaterally symmetrical animals • Tendency to move through environment headfirst • Cephalization, concentration of the nervous system in the head region • Centralization, presence of a central nervous system

  33. LE 28-10a Nerve net Neuron Hydra (cnidarian)

  34. LE 28-10b Eyespot Brain Nerve cord Transverse nerve Flatworm (planarian)

  35. LE 28-10c Brain Ventral nerve cord Segmental ganglion Leech (annelid)

  36. LE 28-10d Brain Ventral nerve cord Ganglia Insect (arthropod)

  37. LE 28-10e Brain Giant axon Squid (mollusc)

  38. 28.11 Vertebrate nervous systems are highly centralized and cephalized • Peripheral nervous system (PNS) includes cranial and spinal nerves and ganglia • Central nervous system (CNS) made up of spinal cord and brain • Spinal cord • Inside vertebral column • Conveys information from brain • Integrates simple responses

  39. Brain, the master control center • Homeostatic centers keep body functioning smoothly • Sensory centers integrate data from sense organs • Can include centers of emotion and intellect • Sends motor commands to muscles

  40. LE 28-11a Central nervous system (CNS) Peripheral nervous system (PNS) Brain Cranial nerves Spinal cord Ganglia outside CNS Spinal nerves

  41. Vast network of blood vessels services the CNS • Blood-brain barrier maintains a stable chemical environment for the brain • Ventricles in brain are continuous with central canal of spinal cord • Filled with cerebrospinal fluid • Protected by meninges (layers of connective tissue) • Two distinct areas in CNS • White matter: axons • Gray matter: nerve cell bodies and dendrites

  42. LE 28-11b Cerebrospinal fluid Dorsal root ganglion (part of PNS) Brain Meninges Gray matter White matter Spinal nerve (part of PNS) Central canal Ventricles Spinal cord (cross section) Central canal of spinal cord Spinal cord

  43. 28.12 The peripheral nervous system of vertebrates is a functional hierarchy • The PNS has two functional components • Somatic nervous system • Carries signals to and from skeletal muscles • Responds mainly to external stimuli • Autonomic nervous system • Regulates internal environment • Controls smooth and cardiac muscle, various organs; involuntary

  44. LE 28-12 Peripheral nervous system Autonomic nervous system Somatic nervous system Enteric division Parasympathetic division Sympathetic division

  45. 28.13 Opposing actions of sympathetic and parasympathetic neurons regulate the internal environment • The autonomic nervous system has two sets of neurons with opposing effects • Parasympathetic division • Primes the body for activities that gain and conserve energy for the body • Sympathetic division • Prepares the body for intense, energy-consuming activities

  46. The enteric division is regulated by the sympathetic and parasympathetic divisions • Controls the digestive process • Sympathetic and parasympathetic neurons emerge from different regions of the CNS • Use different neurotransmitters • Somatic and autonomic components of PNS cooperate to maintain homeostasis

  47. LE 28-13 Parasympathetic division Sympathetic division Brain Eye Dilates pupil Constricts pupil Salivary glands Stimulates saliva production Inhibits saliva production Lung Dilates bronchi Constricts bronchi Accelerates heart Slows heart Heart Adrenal gland Spinal cord Stimulates epinephrine and norepi- nephrine release Liver Stomach Stimulates stomach, pancreas, and intestines Stimulates glucose release Pancreas Inhibits stomach, pancreas, and intestines Intestines Bladder Stimulates urination Inhibits urination Promotes ejacu- lation and vaginal contractions Promotes erection of genitals Genitalia

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