Neurons and their Actions

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Neurons and their Actions

• Get ready to sense, receive, and react.

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Things that make you go hmmm

• Average number of neurons in the human brain 100
  billion
• Average number of neurons in an octopus brain
  300 billion
• Rate of neuron growth during development of a
  fetus (in the womb) 250,000 neurons/minute
• Longest axon of a neuron around 15 feet (Giraffe
  primary afferent axon from toe to neck)
• Velocity of a signal transmitted through a
  neuron 1.2 to 250 miles/hour

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Neuron

• A nerve cell consists of many different parts.

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Parts of a Neuron

• Cell Body Life support center of the neuron.
• Dendrites Branching extensions at the cell body.
  Receive messages from other neurons.
• Axon Long single extension of a neuron, covered
  with myelin MY-uh-lin sheath to insulate and
  speed up messages through neurons.
• Terminal Branches of axon Branched endings of an
  axon that transmit messages to other neurons.

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Synapse

• Synapse SIN-aps a junction between the axon tip
  of the sending neuron and the dendrite or cell
  body of the receiving neuron. This tiny gap is
  called the synaptic gap or cleft.

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Types of Neurons

• Most communicate within the central nervous
  system (CNS - brain spinal cord) with rare
  exceptions.
• There are three major types of neurons upon which
  information travels. The information travels from
  the Sensory Neurons to the Interneurons, and then
  finally to the Motor Neurons.

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Sensory Neurons

• bring information from sensory receptors to the
  central nervous system. Brings information from
  the eyes, ears, etc., as well as from within the
  body like the stomach.

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Interneurons

• neurons in the brain and spinal cord that serve
  as an intermediary between sensory and motor
  neurons. They carry info around the brain for
  processing.

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Motor Neurons

• carry the information from the CNS to the
  appropriate muscles to carry out behaviors.

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Follow the Neuron Path

• For example, if you hold your hand over a hot
  flame, the information about "heat" travels from
  your hand on the sensory neurons, to the
  Interneurons where it is brought to the
  appropriate brain region to process the
  information (now you know it is "hot") and make a
  decision about a corresponding action (too hot,
  let's move the hand). The information then
  travels on the Motor Neurons from the brain to
  the hand so that your muscles move the hand from
  the hot flame. See how easy that is?

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Neural Impulse (A Neuron Firing)

• Defined as the electrical and chemical
  transmission of information from one neuron to
  another.
• It takes the same path all the time - it is a
  process of conducting information from a stimulus
  by the dendrite of one neuron and carrying it
  through the axon and on to the next neuron. Let's
  look at all the elements that are involved in the
  neural impulse

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Change of the Neuron

• inside the neuron, the ions are mostly negatively
  charged. Outside the neuron, the ions are mostly
  positively charged. In this state (with mostly
  negative charge inside and positive charge on the
  outside) the neuron is said to be Polarized
• When a neuron fires (neuron impulse), that all
  changes Just watch!

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Ions

•  Definition n  a particle that is electrically
  charged (positive or negative) an atom or
  molecule or group that has lost or gained one or
  more electrons
• We have positively () and negatively (-) charged
  particles called ions.
• Neural Impulse
• Sodium (Na)
• Potassium (K-)

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Selectively Permeable Membrane

• The outer membrane of the neuron is not
  impermeable, but instead selectively allows some
  ions to pass back and forth. The way it selects
  is easy - it has pores that are only so big. So,
  only very small ions can fit through. Any large
  ions simply can't pass through the small pores.

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Lights, Stimulus, Action

• The following slides go through the stages of a
  neuron firing, or a neural impulse, with other
  neurons or muscle fibers.
• You will want to write down these steps as a
  chain of events that goes in a circular motion.

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Resting Potential

• while the neuron is Polarized, it is in a stable,
  negatively charged, inactive state The charge is
  approx. -70 millivolts, and it means that the
  neuron is ready to fire (receive and send
  information).

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Stimulus

• eventually, some stimulation occurs (ex. hand to
  close to a flame), and the information is brought
  into the body by a sensory receptor and brought
  to the dendrites of a neuron.

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Threshold

• a dividing line that determines if a stimulus is
  strong enough to warrant action. If the threshold
  is reached, an action potential will occur.
• For example, how hot does a stove have to be for
  a person to pull away. When you have reached
  that level of uncomfortable temperature, then you
  have reached the threshold.

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All or None Law

• all-or-none law - a neural impulse will either
  occur or not. There is no in between. Once the
  threshold is reached, there is no going back, the
  neural impulse will begin and will go through the
  complete cycle.

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Action Potential

• Once the stimulus reaches a certain threshold the
  neural membrane opens at one area and allows the
  positively charged ions to rush in and the
  negative ions to rush out. The charge inside the
  neuron then rises to approx. 40 mv. This only
  occurs for a brief moment, but it is enough to
  create a domino effect.

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Absolute Refractory Period

• after the action potential occurs, there is a
  brief period during which the neuron is unable to
  have another action potential. Then the charge
  inside the neuron drops to about -90 mv
  (refractory period) before restoring itself to
  normal.
• Analogy the absolute refractory period is like a
  gun. After you shoot it, you must reload it
  before you can shoot again. The charge dropping
  in the neuron is its way of reloading.

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Repolarization

• the neuron tries to quickly restore it's charge
  by pumping out the positively charged ions and
  bringing back the negative ones.
• occurs fast enough to allow up to 1,000 action
  potentials per second.

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The Firing of a Neuron
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Speed of an Action Potential

• Neural impulses can travel from 10120 meters/sec,
  or 2-270 miles/hour.
• The speed of light is 186,000 (1.86 X 105) miles
  per second.
• The speed of sound, at sea-level, is 968 ft/sec.
• The speed of a garden snail is 0.03 mph.
• What should you learn from this? Neurons are
  slower than the energy neurons use, and faster
  than anything that uses neurons to move.

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Neuron Activity (10 minutes)

• You have 8 minutes to organize yourselves as a
  neuron.
• You must all have a role as a part of the neuron
  (more than one person can make one part of a
  neuron).
• After 8 minutes of preparation you must show me
  through your movements a neuron firing.
• Before you fire you must identify which part of
  the neuron you represent.