Chapter Two Nerve Cells and Nerve Impulses

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Chapter TwoNerve Cells and Nerve Impulses
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Cells of the Nervous System

• Neurons and Glia
• The Structures of an Animal Cell
• Membrane-a structure that separates the inside of
  the cell from the outside
• Nucleus-the structure that contains the
  chromosomes
• Mitochondrion-structure where the cell performs
  metabolic activities
• Ribosomes-sites at which the cell synthesizes new
  protein molecules
• Endoplasmic reticulum-a network of thin tubes
  that transport newly synthesized proteins to
  other locations

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Figure 2.3  The membrane of a neuronEmbedded in
the membrane are protein channels that permit
certainions to cross through the membrane at a
controlled rate.
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Figure 2.2  An electron micrograph of parts of a
neuron from the cerebellum of a mouseThe
nucleus, membrane, and other structures are
characteristic of most animal cells. The plasma
membrane is the border of the neuron.
Magnification approximately 3 23,000.
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Cells of the Nervous System

• Neurons and Glia
• The Structure of a Neuron
• Dendrites-branching fibers that get narrower as
  they extend from the cell body toward the
  periphery information receiver
• Dendritic spines-short outgrowths that increase
  the surface area available for synapses
• Cell body-contains the nucleus and other
  structures found in most cells
• Axon-thin fiber of constant diameter, in most
  cases longer then the dendrites
  information-sender
• Myelin sheath-insulating material covering the
  axons speed up communication in the neuron
• Presynaptic terminal-the point on the axon that
  releases chemicals

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Figure 2.5  The components of a vertebrate motor
neuronThe cell body of a motor neuron is located
in the spinal cord. The various parts are not
drawn to scale in particular, a real axon is
much longer in proportion to the size of the
soma.
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Cells of the Nervous System

• Neurons and Glia
• Terms associated with Neurons
• Motor neuron-receives excitation from other
  neurons and conducts impulses from its soma in
  the spinal cord to muscle of gland cells
• Sensory neuron-specialized at one end to be
  highly sensitive to a particular type of
  stimulation
• Local neuron-small neuron with no axon or a very
  short one
• Efferent axon-carries information away from the
  structure
• Afferent axon-brings information into a structure
• Intrinsic/interneuron-the cells dendrites and
  axons are entirely contained within a single
  structure

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Figure 2.6  A vertebrate sensory neuronNote that
the soma is located in a stalk off the main trunk
of the axon. (As in Figure 2.5, the various
structures are not drawn to scale.)
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Figure 2.8  Cell structures and axonsIt all
depends on the point of view. An axon from A to B
is an efferent axon from A and an afferent axon
to B, just as a train from Washington to New York
is exiting Washington and approaching New York.
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Cells of the Nervous System

• Neurons and Glia
• Glia-supportive cells in the nervous system
• Types
• Astrocytes-star-shaped glia that wrap around the
  presynaptic terminals of several axons
• Radial Glia-a type of astrocyte that guides the
  migration of neurons and the growth of their
  axons and dendrites during embryonic development
• Oligodendrocytes-located in the CNS and provide
  myelin sheaths for axons
• Schwann Cells-located in the PNS and provide
  myelin sheaths for axons

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Figure 2.11 (a)  Shapes of some glia
cells.Oligodendrocytes produce myelin sheaths
that insulate certain vertebrate axons in the
central nervous system Schwann cells have a
similar function in the periphery. The
oligodendrocyte is shown here forming a segment
of myelin sheath for two axons in fact, each
oligodendrocyte forms such segments for 30 to 50
axons. Astrocytes pass chemicals back and forth
between neurons and blood and among various
neurons in an area. Microglia proliferate in
areas of brain damage and remove toxic materials.
Radial glia (not shown here) guide the migration
of neurons during embryological development.
Glia have other functions as well.
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The Blood-Brain Barrier

• Why we need a blood-brain barrier
• To keep out harmful substances such as viruses,
  bacteria, and harmful chemicals
• How the blood-brain barrier works
• Tight Gap Junctions
• What can pass the blood-brain barrier
• Passive Transport-require no energy to pass
• Small uncharged molecules-oxygen and carbon
  dioxide
• Molecules that can dissolve in the fats of the
  capillary walls
• Active Transport-require energy to pass
• Glucose, amino acids, vitamins and hormones

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Figure 2.13  The blood-brain barrierMost large
molecules and electrically charged molecules
cannot cross from the blood to the brain. A few
small uncharged molecules such as O2 and CO2 can
cross so can certain fat-soluble molecules.
Active transport systems pump glucose and certain
amino acids across the membrane.
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Nourishment of Vertebrate Neurons

• Glucose-primary energy source for the brain
• Oxygen-needed to metabolize glucose
• Thiamine-necessary for the use of glucose

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The Nerve Impulse

• The Resting Potential of the Neuron
• Resting potential-results from a difference in
  distribution of various ions between the inside
  and outside of the cell
• (-70mV)
• Measurement of the Resting Membrane Potential
• Microelectrodes
• Why a Resting Potential?
• Prepares neuron to respond rapidly to a stimulus

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Figure 2.14  Methods for recording activity of a
neuron(a) Diagram of the apparatus and a sample
recording. (b) A microelectrode and stained
neurons magnified hundreds of times by a light
microscope. (Fritz Goro)
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The Nerve Impulse

• The Forces Behind the Resting Potential
• Selective Permeability-the membrane allows some
  molecules to pass more freely than others
• The Forces
• Sodium-Potassium Pump-actively transports three
  sodium ions out of the cell while simultaneously
  drawing two potassium ions into the cell
• Concentration Gradients-difference in
  distribution for various ions between the inside
  and outside of the membrane
• Electrical Gradient-the difference in positive
  and negative charges across the membrane

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Figure 2.16  The sodium and potassium gradients
for a resting membraneSodium ions are more
concentrated outside the neuron potassium ions
are more concentrated inside. However, because
the body has far more sodium than potassium, the
total number of positive charges is greater
outside the cell than inside. Protein and
chloride ions (not shown) bear negative charges
inside the cell. At rest, very few sodium ions
cross the membrane except by the sodium-potassium
pump. Potassium tends to flow into the cell
because of an electrical gradient but tends to
flow out because of the concentration gradient.
Animation
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The Action Potential

• Important Definitions
• Hyperpolarization-increasing the negative charge
  inside the neuron
• Depolarization-decreasing the negative charge
  inside the neuron
• Threshold of excitation-Any stimulation beyond a
  certain level producing a sudden, massive
  depolarization of the membrane
• Action Potential-rapid depolarization and slight
  reversal of the usual polarization

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Molecular Basis of the Action Potential

• Sodium channels open once threshold is reached
  causing an influx of sodium
• Potassium channels open as the action potential
  approaches its peak allowing potassium to flow
  out of the cell
• Cell overshoots resting membrane potential

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Figure 2.17  The movement of sodium and potassium
ions during an action potentialNote that sodium
ions cross during the peak of the action
potential and that potassium ions cross later in
the opposite direction, returning the membrane to
its original polarization.
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The Action Potential

• The All-or-None Law
• The size, amplitude, and velocity of an action
  potential are independent of the intensity of the
  stimulus that initiated it.

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The Action Potential

• The Refractory Potential
• Defined-During this time the cell resists the
  production of further action potentials
• Two Refractory Periods
• Absolute Refractory Periods
• The sodium gates are firmly closed
• The membrane cannot produce an action potential,
  regardless of the stimulation.
• Relative Refractory Periods
• The sodium gates are reverting to their usual
  state, but the potassium gates remain open.
• A stronger than normal stimulus can result in an
  action potential.

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Propagation of the Action Potential

• Axon Hillock-where the action potential begins
• Terminal Buttons-the end point for the action
  potential
• The action potential flows toward the terminal
  and does not reverse directions because the area
  where the action potential just came from are
  still in refractory

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The Myelin Sheath and Saltatory Conduction

• Myelin Sheaths increase the speed of neural
  transmission
• Nodes of Ranvier-Short areas of the axon that
  are unmyelinated
• Saltatory Conduction-jumping action of actions
  potentials from node of Ranvier to node of Ranvier

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Figure 2.20  Saltatory conduction in a myelinated
axonAn action potential at the node triggers
flow of current to the next node, where the
membrane regenerates the action potential.
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Signaling Without Action Potentials
Depolarizations and hyperpolarizations of
dendrites and cell bodies Small Local
neurons-produce graded potentials (membrane
potentials that vary in magnitude and do not
follow the all-or-none law)