Neuron Structure and Firing

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Neuron Structure and Firing

• Anna Fure
• February 12, 2008

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Divisions of the Nervous System

• Central Nervous System (CNS) consists of the
  brain and the spinal cord
• Peripheral Nervous System (PNS) is found outside
  of the brain and spinal cord and consists of the
  nerves and most of the sensory organs

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Neurons

• Neuron? individual cellular units that make up
  the nervous system
• The information-processing and information-transmi
  tting element of the nervous system
• Neurons differ from other cells in the body with
  their ability to carry messages

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3 Different Neurons

• Sensory Neuron? detects changes in the external
  or internal environment
• Located in the PNS
• Carries information from PNS to CNS
• Motor Neuron? controls the contraction of a
  muscle or the secretion of a gland
• Located in the CNS
• Carries information from the CNS to the PNS
• Interneuron? located entirely within the CNS
  located between 2 neurons
• Local interneuron? forms circuits with nearby
  neurons and analyzes small pieces of information
• Relay interneuron? connects circuits of local
  interneurons in one region of the brain with
  those in other regions

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Basic Structure

• Soma? Cell body of a neuron, which contains the
  nucleus
• Nucleus? structure in the central region of a
  cell that contains the chromosomes
• Dendrites? branched, treelike structure attached
  to the soma of a neuron
• Receives information from the terminal buttons of
  other neurons
• Information shared between neurons is transmitted
  across the synapse
• Synapse? junction between the terminal button of
  an axon and the membrane of another neuron

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Basic Structure

• Axon? long, thin, cylindrical structure that
  conveys information from the soma of a neuron to
  its terminal buttons
• Covered by a myelin sheath
• Basic message is called an axon potential
• Terminal Buttons? the bud at the end of a branch
  of an axon
• Forms synapses with another neuron
• Sends information to other neurons
• Specific function when an action potential
  traveling down the axon reaches the terminal
  buttons, they secrete a neurotransmitter
• Neurotransmitter? chemical that either excites or
  inhibits the receiving cell and thus helps to
  determine whether an action potential occurs in
  its axons

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Basic Structure of a Neuron
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Types of Neurons

• Multipolar Neuron
• 1 axon and multiple dendrites attached to the
  soma
• Most common type of neuron found in the CNS

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Types of Neurons

• Bipolar Neuron
• 1 axon and 1 dendrite located at opposite ends of
  the soma
• Usually sensory neurons
• Primarily found in sensory systems (i.e. vision
  and audition)

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Types of Neurons

• Unipolar Neuron
• 1 axon attached to the soma
• Axon divides, with one branch receiving sensory
  information and the other sending the info into
  the CNS

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Supporting Cells

• Glia? supporting cells of the central nervous
  system
• Surround neurons and hold them in place
• Controls neurons supply of nutrients and some of
  the chemicals they need to exchange messages with
  other neurons
• Insulate neurons from one another so that neural
  messages do not get scrambled
• Destroy and remove carcasses of neurons that are
  killed by disease or injury

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Supporting Cells

• 3 types of Glia
• Astrocytes? provide support for neurons of the
  CNS, provide nutrients and other substances, and
  regulate the chemical composition of the
  extracellular fluid clean up debris from dead
  neurons within the brain
• Oligodendrocytes? provides support to axons
  produces the myelin sheath
• Myelin Sheath? surrounds the axon and insulates
  them, preventing messages from spreading between
  adjacent axons
• Node of Ranvier? naked portion of a myleinated
  axon
• Microglia? breaks down dead and dying neurons
  protect brain from invading microorganisms

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Supporting Cells

• Schwann Cell? cell in the PNS that is wrapped
  around a myelinated axon, providing one segment
  of its myelin sheath
• Perform same actions in PNS as the
  oligodendrocytes perform in the CNS
• Provides myelin for only one axon
• Entire Schwann Cell surrounds the axon

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Neural Impulse

• Neural Impulse? chemical electrical feature of
  neurons
• 3 main components
• Resting Potential
• Membrane Potential
• Action Potential

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Resting Potential

• Resting Potential? the membrane potential of a
  neuron when it is not being altered by excitatory
  or inhibitory postsynaptic potentials
• Approx. -70 mV

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Membrane Potential

• Membrane Potential? the electrical charge across
  a cell membrane
• The difference in electrical potential inside and
  outside of the cell
• Charge inside the membrane -70 mV
• Axon is negatively charged with respect to the
  outside
• Depolarization? reduction (toward 0) of the
  membrane potential of a cell from its normal
  resting potential
• Hyperpolarization? increase in the membrane
  potential of a cell, relative to the normal
  resting potential

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Membrane Potential

• Electrical charge is the result of a balance
  between two opposing forces
• Diffusion
• Electrostatic Pressure
• Diffusion? Movement of molecules from regions of
  high concentration to regions of low concentration

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Membrane Potential

• Electrostatic Pressure? the attractive force
  between atomic particles charged with opposite
  signs or repulsive force between atomic particles
  charged with the same sign
• Electrolyte? an aqueous solution of a material
  that ionizesnamely a soluble acid, base, or salt
• Ion? a charged molecule
• Cations positively charged
• Anions negatively charged

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Membrane Potential

• Ions are contained in the intracellular and
  extracellular fluid
• Intracellular fluid? fluid contained within the
  cell
• Organic anions (A-) found ONLY inside cell
• Unable to pass through the membrane of the axon
• Potassium (K) found predominantly inside cell
• Concentrated within the axon
• Diffusion tends to push out of cell
• Electrostatic pressure tends to force the cation
  inside
• Two opposing forces balance, and potassium ions
  tend to remain in the cell

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Membrane Potential

• Extracellular fluid? Body fluids located outside
  the cells
• Chloride (Cl-) found predominantly outside the
  cell
• Diffusion pushes ion inward
• Electrostatic pressure pushes the anion outward
• Two opposing forces balance, and chloride remains
  outside the cell
• Sodium (Na) found predominantly outside the
  cell
• Diffusion pushes ion into the cell
• Electrostatic pressure does not prevent the it
  from entering the cell
• Sodium-potassium pump continuously pushes sodium
  out of the axon

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Membrane Potential

• Sodium-Potassium Pump? protein found in the
  membrane of all cells that extrudes sodium ions
  from the cell and transports potassium ions into
  the cell
• Pushes 3 sodium ions out for every 2 potassium
  ions they push in

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Action Potential

• Action Potential? the brief electrical impulse
  that provides the basis for conduction of
  information along an axon
• Constitutes the message carried by the axon from
  the cell body to the terminal buttons
• Threshold of excitation? the value of the
  membrane potential that must be reached to
  produce an action potential
• Neuron firing refers to the production of action
  potentials

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Action Potential

• All-or-none law? the principle that once an
  action potential is triggered in an axon, it is
  propagated, without decrement, to the end of the
  fiber
• If the threshold is reached, cell fires and has
  40 mV all the way down the axon
• If threshold is not reached, the cell does not
  fire
• Rate Law? principle that variations in intensity
  of a stimulus or other information being
  transmitted in an axon are represented by
  variation in the rate at which the axon fires

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Nature of the Neural Impulse

• Resting potential (-70 mV)
• Excitatory messages move the membrane potential
  toward the threshold of excitation (-65 mV) and
  depolarization
• When the cell is depolarized, sodium rushes into
  the neuron
• Sudden influx in positively charged ions triggers
  the axon potential
• Potassium channels open and potassium moves to
  the outside of the neuron
• Sodium-Potassium Pump attempts to recover the
  resting potential
• Hyperpolarization of cell
• Extra potassium diffuses
• Resting potential is restored (-70 mV)