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NERVOUS SYSTEM

NERVOUS SYSTEM. NERVOUS FUNCTIONS. Body’s master controlling and communicating system Three functions Sensory input Gathers information from sensory receptors Integration Processes and interprets sensory input Motor output Activates effector organs to cause a response.

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NERVOUS SYSTEM

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  1. NERVOUS SYSTEM

  2. NERVOUS FUNCTIONS • Body’s master controlling and communicating system • Three functions • Sensory input • Gathers information from sensory receptors • Integration • Processes and interprets sensory input • Motor output • Activates effector organs to cause a response

  3. Nervous System Organization

  4. ORGANIZATION Two Principal Parts of the System • Central nervous system (CNS) • Brain and spinal cord • Integrating and command center • Interprets sensory input • Dictates motor responses • Peripheral nervous system (PNS) • Nerves extending from brain and spinal cord • Carry impulses to and from the CNS

  5. PERIPHERAL DIVISIONS Two Functional Subdivisions of the PNS • Sensory division • “afferent division” • Nerve fibers conveying impulses to the CNS • Somatic afferent fibers convey impulses from the skin, muscles, and joints • Visceral afferent fibers convey impulses from visceral organs • Motor division • , “efferent division” • Nerve fibers conveying impulses from the CNS

  6. ORGANIZATION

  7. HISTOLOGY • Nervous system consists mainly of nervous tissue • Highly cellular • e.g., <20% extracellular space in CNS • Two principal cell types • Neurons • Excitable nerve cells that transmit electrical signals • Supporting cells • Smaller cells surrounding and wrapping neurons • “Neuroglia”

  8. NEURONS • Nerve cells • Structural units of nervous system • Billions are present in nervous system • Conduct messages throughout body • Nerve impulses • Extreme longevity • Can function optimally for entire lifetime • Amitotic • Ability to divide is lost in mature cells • Cannot be replaced if destroyed • Some (very few) exceptions • e.g., stem cells present in olfactory epithelium can produce new neurons • Stem cell research shows great promise in repairing damaged neurons • High metabolic rate • Require large amounts of oxygen and glucose

  9. Cell Body Dendrites MyelinSheath Axon Neurons Dendrites of another neuron Axon of anotherneuron

  10. Collins I4 lines • Based on the diagram, what do you think each part does to receive and pass along an impulse toward the brain

  11. Agenda11/3/11 – Day 1 • Take more notes • HW- vocab

  12. NEURONS • Generally large, complex cells • Structures vary, but all neurons have the same basic structure • Cell body • Slender processes extending from cell body • Plasma membrane is site of signaling

  13. NEURON CELL BODY • Most neuron cell bodies are located in the CNS • Protected by bones of skull or vertebral column • Clusters of cell bodies in the CNS are termed “nuclei” • Clusters of cell bodies in the PNS are termed “ganglia”

  14. NEURON CELL BODY • Major biosynthetic (control) center of neuron • Other usual organelles present except CENTRIOLES -Why? • What do centrioles do?

  15. NEURON PROCESSES • Extend from the neuron’s cell body • Two types of neuron processes • Dendrites • Axons

  16. NEURON PROCESSES Typical Dendrite • Short, slender, branching extensions of cell body • Generally hundreds clustering close to cell body • Most cell body organelles also present in dendrites • Main receptive / input regions • Large surface area for receiving signals from other neurons • Convey incoming messages toward cell body

  17. NEURON PROCESSES Typical Axon • Single axon per neuron • The axon forms from the narrowing of the cell body. The region between the large cell body and the axon is the “axon hillock” • Sometimes very short • Sometimes very long • e.g., axons controlling big toe are 3 – 4 feet long

  18. NEURON PROCESSES Typical Axon • Single axon may branch along length • “Axon collaterals” extend from neurons at ~ 90o angles • Usually branches profusely at end • 10,000 or more terminal branches is common • Distal endings termed “axonal terminals”

  19. NEURON PROCESSES Typical Axon • Conducting component of neuron • Generates nerve impulse • Transmits nerve impulses away from cell body towards the axonal terminals

  20. NEURON PROCESSES Typical Axon terminal • Axonal terminals are secretory component of neuron • Sequence of events • Signal reaches terminals • Membranes of vesicles fuse with plasma membrane • Neurotransmitters released • Neurotransmitters interact with either other neurons or effector cells • Excite or inhibit

  21. VocabularyEither in flash card form OR in list • CNS • PNS • Neuron • Stimulus • Afferent division • Efferent division • neuroglia • Amitotic • Dendrite • Cell body • Axon • Axon terminal • Ganglia • Nuclei (in terms of clusters)

  22. Collins I2 lines • What is the difference between the PNS and the CNS?

  23. Agenda11/4/11 -- Day 2 • Remember quiz 11/9 • Take notes • Complete labeling and coloring of neuroglia • HW-complete ALL vocab terms

  24. MYELIN SHEATH • Whitish, fatty covering the axons of many neurons • Protects and electrically insulates fibers • Increases speed of nerve impulse transmission • Some axons and all dendrites are unmyelinated

  25. MYELIN SHEATH • In PNS, Schwann cells Continually wrap around the axon of a neuron • Result is many concentric layers of plasma membrane surrounding the axon • Thickness depends on number of wrappings • Nucleus and most of cytoplasm exist as a bulge external to the myelin sheath

  26. Myelin sheath and schwann cells Node of Ranvier Schwann Cells

  27. MYELIN SHEATH • Adjacent Schwann cells on axon do not touch each other • Gaps in sheath occur at regular intervals • “Nodes of Ranvier” • Axon collaterals can emerge at these nodes

  28. MYELIN SHEATH • In CNS, there are both myelinated and unmyelinated axons • Oligodendrocytes, not Schwann cells, form CNS myelin sheaths • Numerous processes that can coil around numerous (up to 60) axons at once

  29. NEUROGLIA • “Nerve glue” • Six types of small cells associated with neurons • 4 in CNS • 2 in PNS • Several functions • Supportive scaffolding for neurons • Electrical isolation of neurons • Neuron health and growth

  30. CNS NEUROGLIA • Astrocytes • Microglia • Ependymal cells • Oligodendrocytes

  31. CNS NEUROGLIA Astrocytes • Anchor neurons to capillary blood supply • Facilitate nutrient delivery to neurons • (blood  astrocyte  neuron)

  32. CNS NEUROGLIA Microglia • Small ovoid cells; thorny looking • Transform into macrophage • Phagocytize microorganisms, debris • (Cells of immune system cannot enter the CNS)

  33. CNS NEUROGLIA Oligodendrocytes • Wrap processes tightly around thicker neuron fibers in CNS • Makes “Myelin sheath” • Insulating covering

  34. CNS NEUROGLIA Ependymal Cells • Line central cavities of brain and spinal cord • Many are ciliated • Beating helps circulate cerebrospinal fluid cushioning brain and spinal cord

  35. PNS NEUROGLIA Schwann cells • Surround and form myelin sheaths around larger neurons of PNS • Functionally similar to oligodendrocytes

  36. PNS NEUROGLIA • Satellite cells • Surround cell bodies of PNS ganglia

  37. HW- Vocab Terms • Myelin sheath • Schwann cells • Nodes of ranvier • Oligodendrocytes • Neuroglea • Astrocyte • Microglia • Ependymal cell • Satalite cell

  38. MYELIN SHEATH • White matter • Regions of the brain and spinal cord containing dense collections of myelinated fibers • Gray matter • Regions of the brain and spinal cord containing mostly nerve cell bodies and unmyelinated fibers

  39. NEURON CLASSIFICATION • Structural classification based upon number of processes • Multipolar neurons • Bipolar neurons • Unipolar neurons • Functional classification based upon direction nerve impulse travels • Sensory (afferent) neurons • Motor (efferent) neurons • Interneurons (association neurons)

  40. NEURON CLASSIFICATION Structural Classification • Multipolar neurons • Three or more processes • Most common neuron type in humans • (> 99% of neurons) • Bipolar neurons • Two processes – axon and dendrite • Found only in some special sense organs • e.g., retina of eye • Act as receptor cells • Unipolar neurons • Single short process • Process divides into proximal and distal branches • Distal process often associated with a sensory receptor • “Peripheral process” • Central process enters CNS • Most are sensory neurons in PNS

  41. Classification of neurons by shape

  42. NEURON CLASSIFICATION Functional Classification • Sensory (afferent) neurons • Transmit impulses toward CNS • From sensory receptors or internal organs • Most are unipolar • Cell bodies are located outside CNS • Motor (efferent) neurons • Carry impulses away from CNS • Toward effector organs • Multipolar • Cell bodies generally located in the CNS • Interneurons • a.k.a., association neurons • Lie between motor and sensory neurons in neural pathways • Shuttle signals through CNS pathways where integration occurs • > 99% of neurons in body • Most are multipolar • Most are confined within the CNS

  43. NEUROPHYSIOLOGY • Neurons are highly irritable • Responsive to stimuli • Response to stimulus is action potential • Electrical impulse carried along length of axon • Always the same regardless of stimulus • The underlying functional feature of the nervous system

  44. ION CHANNELS Plasma membranes contain various ion channels • Passive channels (leakage channels) • Always open • Active channels (gated channels) • Ligand-gated channels • Open when specific chemical binds • Voltage-gated channels • Open and close in response to membrane potential • Mechanically-gated channels • Open in response to physical deformation of receptor • e.g., touch and pressure receptors

  45. MEMBRANE POTENTIALS • A voltage exists across the plasma membrane • Due to separation of oppositely charged ions • Potential difference in a resting membrane is termed its “resting membrane potential” • ~ -70 mV in a resting neuron • Membrane is “polarized”

  46. MEMBRANE POTENTIALS • Neurons use changes in membrane potentials as signals • Used to receive, integrate, and send signals • Changes in membrane potentials produced by • Anything changing membrane permeability to ions • Anything altering ion concentrations • Two types of signals • Graded potentials • Short-distance signals • Action potentials • Long-distance signals

  47. MEMBRANE POTENTIALS Graded Potentials • Short-lived local changes in membrane potential • Either depolarizations or hyperpolarizations • Cause current flows that decrease in magnitude with distance • Magnitude of potential dependent upon stimulus strength • Stronger stimulus  larger voltage change • Larger voltage change  farther current flows

  48. MEMBRANE POTENTIALS Graded Potentials • Triggered by change in neuron’s environment • Change causes gated ion channels to open • Small area of neuron’s plasma membrane becomes depolarized (by this stimulus) • Current flows on both sides of the membrane • + moves toward – and vise versa

  49. MEMBRANE POTENTIALS Graded Potentials • Inside cell: + ions move away from depolarized area • Outside cell: + ions move toward depolarized area • (+ and – ions switch places) • Membrane is leaky • Most of the charge is quickly lost through membrane • Current dies out after traveling a short distance

  50. MEMBRANE POTENTIALS Graded Potentials • Act as signals over very short distances • Important in initiating action potentials

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