Explaining behavior at the level of the neuron

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Explaining behavior at the level of the neuron

• Your brain and the rest of your nervous system is
  made up of neurons.
• Neurons are brain cells
• All neurons are separated from one another, but
  communicate electrochemically.

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The neuron

• The neuron consists of three parts
• The cell body - contains the nucleus and much of
  the machinery that keeps a neuron alive and
  working.
• The dendrites - widely branching structures that
  receive transmissions from other neurons.
• The axon - a single, long, thin, straight fiber
  with branches near its tip

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Myelin sheath

• The axon is coated in an insulating substance
  known as Myelin.
• Myelin allows for faster transmission of impulses
  along an axon.
• Myelin has breaks in it known as the Nodes of
  Ranvier
• Once an impulse reaches the end of an axon (the
  terminal buttons), molecules are released that
  can either excite or inhibit the receiving cell.

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Resting Potential

• Normally there is an electrical polarization
  across the membrane of an axon.
• This means that there is a negative charge on the
  inside of the cell and a positive charge on the
  outside.
• At resting potential the inside of the neuron is
  at -70 millivolts.

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Four Factors Determine the Ionic Distribution
That Underlies the Resting Potential

• Differential Permeability of the Membrane
• The Sodium/Potassium Pump
• Diffusion
• Electrostatic Pressure

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Differential Permeability of the Membrane

• Ions pass through membrane at special pores
  called ion channels
• When neurons are at rest, the membrane is
• extremely resistant to the passage of Sodium
  (Na) ions
• only slightly resistant to the passage of
  Potassium (K) ions and Chloride (Cl-) ions

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The sodium potassium pump

• There are little pumps that pump Sodium (NA) out
  of the cell, and potassium (K) in.
• The pumps move 3 Sodium molecules out for every 2
  potassium molecules they move in
• Sodium and potassium both have a 1 charge
• more Sodium is being moved out than potassium is
  being moved in
• the build up of Sodium on the outside of the
  membrane makes it positive and the inside
  negative.

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Diffusion also known as -Random Motion

• Ions in solution are in random motion
• Thus, any time that there is an accumulation of a
  particular class of ions in one area,
• the probability is increased that random motion
  will move ions out of this area (because there
  are more ions available to leave)
• the probability is decreased that random motion
  will move more ions into the area (because there
  are fewer ions available to come in)

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Electrostatic Pressure

• Like charges repel and opposite charges attract
• Therefore electrostatic pressure disperses any
  accumulation of positive or negative charges in
  an area

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Generation of Action Potentials

• action potentials (APs neuron firing) are
  triggered at the axon hillock when a neuron is
  depolarized to the point that the membrane
  potential at the axon hillock reaches about -65
  mV
• this is the threshold of excitation for many
  neurons
• they are all-or-none (they occur full blown or
  not at all)

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How does an impulse travel down an axon?

• The action potential is an excitation that
  travels along an axon at a constant strength, no
  matter how far it must travel.
• It is slower then a straight electrical impulse,
  but has the advantage of maintaining its
  strength no matter how far it must travel.

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Travel of the action potential

• When a neuron fires, certain gates open up that
  allow Sodium to flow in.
• When sodium flows in the electrical charge is
  neutralized across the membrane.
• Then the sodium channels close, and potassium
  channels open, allowing potassium to leave the
  cell.
• This returns the cell to its resting potential
  (-70 mv).

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Travel of the action potential.

• The axon only has these Sodium gates at breaks in
  the myelin sheath called the Nodes of Ranvier.
• The sodium gates are voltage dependent - that is
  they open up when the voltage across the membrane
  drops
• Thus, the action potential moves like a wave
• jumping from one Node of Ranvier to the next down
  to the end of the axon.
• Saltatory Conduction

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What happens when an action potential reaches the
end of an axon?

• The end of an axon has several branching areas
  called the terminal buttons
• Each edge of the terminal button is called the
  presynaptic membrane.
• The presynaptic membrane is separated from the
  other neuron by what is called the synaptic cleft.

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The Synapse

• The axon that has fired releases a chemical into
  the synaptic cleft.
• This chemical crosses the gap and binds to what
  is called the postsynaptic membrane.
• The chemicals are called neurotransmitters.
• They bind at the postsynaptic membrane at what
  are called receptors.

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Neurotransmitters

• There are many types of neurotransmitters
• Acetylcholine
• Serotonin
• Dopamine

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Dopamine

• Dopamine is one neurotransmitter that has been
  associated with many neurological disorders
• Parkinsons disease.
• Muhammed Ali
• Schizophrenia
• Sometimes these diseases can be treated by
  increasing dopamine levels in the brain

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Behavior and the Nervous System

• Psychologists distinguish between the central
  nervous system and the peripheral nervous system.
• Central nervous system consists of the brain and
  the spinal cord
• Peripheral nervous system is composed of bundles
  of axons between the spinal cord and the rest of
  the body.

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Peripheral nervous system

• The peripheral system can be further divided
• Somatic nervous system nerves that communicate
  with the skin and muscles.
• Autonomic nervous system nerves that
  communicate with the heart, stomach, and other
  organs.

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Autonomic nervous system

• The autonomic nervous system is a system that we
  do not have as much control over.
• It largely controls things we wouldnt want to
  have to think about
• breathing
• heart rate

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Divisions of the autonomic nervous system

• The sympathetic system - controls fight or flight
  - increases heart and breathing rate.
• The parasympathetic system - decreases heart
  rate, controls digestion, basically runs the body
  during normal functioning.

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Organization and functioning of the brain

• The cerebral cortex - the outer surface of the
  brain. The wrinkled area.
• Right and left hemisphere
• crosses over - communicates via the corpus
  callosum

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The four lobes of the cerebral cortex

• Frontal Lobe - thought to be involved in planning
  and working memory
• primary motor cortex
• Parietal Lobe - body sensations
• primary somatosensory cortex
• Occipital lobe - vision
• Temporal lobe - hearing - advanced visual
  processing - emotion

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What if we cut the corpus callosum?

• Travel from the eye to the brain is divided.
• Information from the left side of each eye
  travels to the left hemisphere of the brain
• Information from the right side of each eye
  travels to the right hemisphere of the brain.

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Hemisphere division continued

• The left hemisphere of the brain controls speech
  for most people
• Most people are only able to describe information
  that reaches the left hemisphere.
• However information that reaches the right
  hemisphere quickly crosses the corpus callosum
  to the left hemisphere, so that it can be
  described verbally.

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What if the corpus callosum is cut

• In one experiment researchers showed a woman with
  a severed corpus callosum a picture of a naked
  woman to the left field of vision to the right
  of her face
• When asked what she had seen, she laughed and
  said a nude picture.

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Continued.

• When the nude picture was shown only in her right
  field of vision (left of her face), she laughed
  and turned a little red in the face.
• When asked why she was laughing, she said I dont
  know, oh that silly machine.
• The right side of the brain knew what it saw and
  caused her to laugh, the left side of the brain
  heard the laughter, and tried to interpret why it
  occurred.

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continued

• If she had been allowed to point at several
  alternatives with her left hand (controlled by
  the right hemisphere), she would have been able
  to correctly point at the picture she had seen,
  even though she would say that she didnt know
  what she had seen.

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Unified consciousness?

• We all experience a unified consciousness.
• That is, we experience a single self.
• The split brain experiments show that that
  unified consciousness depends on the two
  hemispheres being able to communicate.
• If the corpus callosum is severed then each
  hemisphere begins to act and experience things
  independently of the other.