Cns2007

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    09-Dec-2014
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Transcript of Cns2007

  • 1. ORGANIZATION
    • Summary
  • Central nervous system
    • Brain
    • Spinal cord
  • Peripheral nervous system
    • Sensory division
    • Motor division
      • Somatic nervous system
      • Autonomic nervous system
        • Sympathetic division
        • Parasympathetic division

2. HISTOLOGY

  • Nervous system consists mainly of nervous tissue
  • Highly cellular
    • e.g., 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
    • Pseudounipolar neurons
      • Originate as bipolar neurons
      • Two processes converge and fuse
    • 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

35. Classification of neurons by shape 36. NEURON CLASSIFICATION

  • Functional Classification
  • Sensory (afferent) neurons
    • Transmit impulsestowardCNS
      • From sensory receptors or internal organs
    • Most are unipolar
    • Cell bodies are located outside CNS
  • Motor (efferent) neurons
    • Carry impulsesaway fromCNS
      • 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

37. 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

38. 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

39. ION CHANNELS

  • Channels are specific as to what type of ions are allowed to pass
    • e.g., K +channels allow only K +to pass
  • Ions moving through open channels follow their electrochemical gradients
    • Electrical current is generated
    • Voltage changes across the membrane

40. MEMBRANE POTENTIALS

  • A voltage exists across the plasma membrane
    • Due to separation of oppositely charged ions
  • Potential difference in a resting membrane is termed itsresting membrane potential
    • ~ -70 mV in a restingneuron
    • Membrane ispolarized

41. MEMBRANE POTENTIALS

  • Resting potential exists across the membrane
    • Majority of Na +outside of cell
    • Majority of K +inside of cell
  • Resting membrane
    • Only slightly permeable to Na +
    • 75 times more permeable to K +
    • How do these ions cross the membrane?

42. 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

43. MEMBRANE POTENTIALS

  • Changes in membrane potentials are caused by three events
  • Depolarization
    • Inside of membrane becomes less negative
    • Nerve impulsesmorelikely to be produced
  • Repolarization
    • Membrane returns to resting membrane potential
  • Hyperpolarization
    • Inside of membrane becomes more negative than the resting potential
    • Nerve impulseslesslikely to be produced

44. 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 stimuluslarger voltage change
    • Larger voltage changefarther current flows

45. MEMBRANE POTENTIALS

  • Graded Potentials
  • Triggered by change in neurons environment
    • Change causes gated ion channels to open
  • Small area of neurons plasma membrane becomes depolarized (by this stimulus)
  • Current flows on both sides of the membrane
    • + moves toward andvise versa

46. 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

47. MEMBRANE POTENTIALS

  • Graded Potentials
  • Act as signals over very short distances
  • Important in initiating action potentials

48. MEMBRANE POTENTIALS

  • Action Potentials
  • Principal means by which neurons communicate
    • Brief reversal of membrane potential
      • Total amplitude of ~ 100 mV (-70+30)
    • Depolarization followed by repolarization, then brief period of hyperpolarization
    • Time for entire event is only a few milliseconds
  • Events in generation and transmission of an action potential identical between neurons and skeletal muscle cells

49. ACTION POTENTIALS 50. ACTION POTENTIALS

  • Not all local depolarizations produce action potentials
  • Depolarization must reach threshold values
    • Brief, weak stimuli produce subthreshold depolarizations that are not translated into nerve impulses
    • Stronger threshold stimuli produce depolarizing event