CHAPTER 28: THE NERVOUS SYSTEM

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CHAPTER 28 THE NERVOUS SYSTEM
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THE NERVOUS SYTEM HAS 3 MAIN TASKS

• RECEIVE AND INTERPRET INFORMATION ABOUT THE
  INTERNAL AND EXTERNAL ENVIRONMENT
• SENSORY SYSTEM
• ORGANIZE AND CARRY OUT THIS ACTION
• MOTOR SYSTEM
• MAKE DECISIONS ABOUT ACTIONS TO TAKE
• INTEGRATION

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THE NERVOUS SYSTEM IS DIVIDED INTO TWO MAIN PARTS

• CENTRAL NERVOUS SYSTEM
• BRAIN AND SPINAL CORD
• PERIPHERAL NERVOUS SYSTEM
• EVERYTHING THAT LIES OUTSIDE THE CENTRAL NERVOUS
  SYSTEM
• SENSORY NERVES CARRY INFORMATION TOWARDS THE
  CNS FROM SENSORY ORGANS
• MOTOR NERVES CARRY INFORMATION AWAY FROM THE
  CNS TO MUSCLES OR GLANDS
• INTERNEURONS INTEGRATE DATA AND RELAY
  APPROPRIATE SIGNALS TO OTHER INTERNEURONS OR TO
  MOTOR NEURONS

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THE KNEE-JERK REFLEX

• SENSORY RECEPTOR DETECTS A STRETCH IN THE MUSCLE
  AND A SENSORY NEURON CONVEYS INFORMATION TO THE
  CNS
• INFORMATION GOES TO A MOTOR NEURON AND AN
  INTERNEURON
• QUADRACEPS RESPOND TO MOTOR NEURON AND CONTRACT
  AND JERK THE LEG FORWARD
• ANOTHER MOTOR NEURON RESPONDS TO THE INTERNEURONS
  AND INHIBITS THE FLEXOR MUSCLES RESPOND TO
  INTERNEURON SO AS NOT TO RESIST THE ACTION OF THE
  QUADRICEPS

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STRUCTURE OF NEURONS

• SAME ORGANELLES FOUND IN OTHER EUKARYOTIC CELLS
• AND
• CELL BODY CONTAINS THE NUCLEUS
• AXONS LONG NEURITES THAT CONVEY THE
  INFORMATION AWAY FROM THE CELL BODY
• DENDRITES SEVERAL SHORTER NEURTIES THAT CONVEY
  INFORMATION TOWARDS THE CELL BODY

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COMMUNICATION TAKES PLACE AT SYNAPSES

• AN AREA WHERE ONE NEURON COMES CLOSE TO OR
  TOUCHES ANOTHER NEURON
• CHEMICAL SYNAPSES
• NEUROTRANSMITTER IS RELEASED BY ONE NEURON AND
  INFLUENCES ANOTHER
• ELECTRICAL SYNAPSES
• COMMUNICATION TAKES PLACE BY THE FLOW OF
  ELECTRICAL CURRENT

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NONNEURONAL CELLS

• SUPPORTING CELLS THAT PROTECT, INSULATE AND
  REINFORCE THE NEURONS
• CENTRAL NERVOUS SYSTEM
• ASTROCYTES REGULATE IONIC ENVIRONMENT
• MICROGLIAL CELLS - PHAGOCYTES
• OLIGODENDROCYTES FORM MYELIN
• PERIPHERAL NERVOUS SYSTEM
• SCHWANN CELLS FORM MYELIN

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SCHWANN CELLS MAKE MYELIN

• MYELIN
• COMPOSED OF LIPID AND PROTEIN
• ELECTRICALLY INSULATE THE NERVE CELL
• AIDS IN THE CONDUCTION OF NERVE TISSUE
• ACTION POTENTIALS

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MYELIN SHEATH

• CHAIN OF SCHWANN CELLS
• WRAPS AROUND THE AXONS
• NODES OF RANVIER SPACES BETWEEN THE SCHWANN
  CELLS
• ONLY POINTS WHERE SIGNALS CAN BE TRANSMITTED
• SIGNALS JUMP FROM NODE TO NODE

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MULTIPLE SCLEROSIS

• AUTOIMMUNE DISEASE
• DESTRUCTION OF MYELIN SHEATHS
• LOSS OF SIGNAL CONDUCTION, MUSCLE CONTROL AND
  BRAIN FUNCTION

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STRUCTURE OF NEURONS
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HOW NEURONS COMMUNICATE AND MOVE SUBSTANCES
BETWEEN CELLS

• CONTROLLED FLOW OF IONS THROUGH THE NEURONS
  MEMBRANE THAT IS THE BASIS FOR THE CELLS ABILITY
  TO SEND AND RECEIVE MESSAGES
• DIFFERENT CONCENTRATION OF IONS BETWEEN THE
  INSIDE AND OUTSIDE OF THE MEMBRANE
• MEMBRANES ARE SELECTIVELY PERMEABLE
• IONS PASS THROUGH ION CHANNELS IN THE CELL
  MEMBRANE
• SODIUM POTASSIUM PUMP

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IONIC COMPOSITION

• POTASSIUM CONCENTRATION (K) IS HIGH
  INTRACELLULARLY AND LOW EXTRACELLULARLY
• SODIUM (Na) CONCENTRATION IS LOW INTRACELLULARLY
  AND HIGH EXTRACELLULARLY

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MEMBRANE POTENTIAL

• ACROSS THE MEMBRANE OF EACH CELL, THERE IS A
  SMALL DIFFRENCE IN ELECTRICAL POTENTIAL DUE TO
  THE FACT THAT THAT THE INSIDE IS NEGATIVELY
  CHARGED COMPARED TO THE OUTSIDE
• THE VOLTAGE ACROSS THE PLASMA MEMBRANE OF A
  RESTING NEURON IS CALLED THE RESTING POTENTIAL
  -70 mV
• (-70 millivolts)
• THE RESTING MEMBRANE POTENTIAL CAN BE MEASURED
  WITH A VOLTMETER

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THE INSIDE OF THE MEMBRANE IS NEGATIVE COMPARED
TO THE OUTSIDE
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THE MEMBRANE POTENTIAL IS DUE TO THE MOVEMENT OF
IONS ACROSS MEMBRANES

• DIFFERENCES IN IONIC CONCENTRATION LEADS TO
  DIFFUSION
• MOVEMENT FROM HIGH TO LOW CONCENTRATION
• ELECTRICAL POTENTIAL DIFFERENCE
• IONS MOVE DUE TO DIFFERENCES IN CHARGE (NOT SIZE)
• IONS MOVE TO SIDE OF MEMBRANE WITH OPPOSITE
  CHARGE
• IONS MOVE ACROSS MEMBRANES DUE TO ION PUMPS

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Na/K PUMPS

• MAINTAINS A HIGH CONCENTRATION OF K INSIDE A
  CELL AND A HIGH CONCENTRATION OF Na OUTSIDE THE
  CELL
• THE Na/K PUMP WORKS BY PUMPING OUT 3 Na IONS
  FOR EACH 2 K PUMPED IN

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Na/K PUMP MAINTAINS LOW Na BUT HIGH K INSIDE
THE CELL
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ACTION POTENTIALS

• DEFINITIONS
• ACTION POTENTIAL NERVE SIGNAL
• STIMULUS ANY FACTOR THAT CAUSES A NERVE SIGNAL
  TO BE GENERATED
• DEPOLARIZE TO PUSH THE MEMBRANE POTENTIAL TO
  MORE POSITIVE VALUES
• HYPERPOLARIZE - MAKE THE MEMBRANE POTENTIAL MORE
  NEGATIVE

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ACTION POTENTIALS

• STIMULATED BY DEPOLARIZATION
• OCCURS WHEN THE MEMBRANE POTENTIAL IS PUSHED
  PASSED A CERTAIN THRESHOLD LEVEL
• ALL OR NONE PHENOMENON
• ONCE GENERATED, IT DOES NOT DEPEND ON THE SIZE OF
  THE STIMULUS

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• RESTING POTENTIAL IS DUE TO THE MOVEMENT OF K
  IONS
• THE ACTION POTENTIAL OCCURS BECAUSE OF
  DEPOLARIZATION AND IS DUE TO AN INCREASE IN
  PERMEABILITY TO Na IONS

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6 DIFFERENT STAGES IN THE ACTION POTENTIAL

• YOU SHOULD BE ABLE TO IDENTIFY WHAT HAPPENS AT
  EACH STAGE

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RESTING STATE NA AND K CHANNELS CLOSED
(1)INSIDE OF MEMBRANE IS NEGATIVE IN RELATION TO
THE OUTSIDE
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A STIMULUS CAUSES SOME Na CHANNELS TO OPEN
DEPOLARIZATIONACTION POTENTIAL TRIGGERED (2)
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MORE Na CHANNELS OPEN, GREATER DEPOLARIZATION (3)
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Na CHANNELS CLOSE AND INACTIVATE K CHANNELS
OPEN AND K RUSHES OUTHYPERPOLARIZATION (4)
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K CHANNELS CLOSE SLOWLY CAUSING A BRIEF
UNDERSHOOT (5) RETURN TO RESTING STATE (6)
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SUMMARY
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THE ACTION POTENTIAL PROPAGATES ALONG THE NEURON
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OTHER FACTS ABOUT ACTION POTENTIALS

• ACTION POTENTIALS ARE GENERATED IN ONLY ONE
  DIRECTION
• REMEMBER Na CHANNELS CLOSE AND INACTIVATE
  THEREFORE, THE ACTION POTENTIAL CANNOT GO IN BOTH
  DIRECTIONS
• ACTION POTENTIALS ARE ALL OR NONE EVENTS
• IT DOES NOT DEPEND ON THE STRENGTH OF THE STIMULUS