Neurons: The Messengers

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Neurons The Messengers

• About 100 billion neurons (nerve cells) in the
  human brain
• Neurons have many of the same features as other
  cells
• Nucleus
• Cytoplasm
• Cell membrane
• What makes neurons unique is their shape and
  function

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Neural Communication

• The bodys information system is built from
  billions of interconnected cells called neurons.

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Neuron

• A nerve cell, or a neuron, 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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Types of Neurons

• Sensory neurons
• Carry information from sensory systems to the
  brain
• Also referred to as afferent
• Motor neurons
• Carry information from the brain to muscles and
  glands
• Also referred to as efferent
• Interneurons
• Carry information between other neurons

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Glial Cells

• Cells that insulate and support neurons
• Create the myelin sheath
• Remove waste products
• Provide nourishment
• Prevent harmful substances from entering the brain

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Action Potential Properties

• All-or-None Response When the depolarizing
  current exceeds the threshold, a neuron will
  fire. If the depolarizing current fails to
  exceed the threshold, a neuron will not fire.
• Intensity of an action potential remains the same
  throughout the length of the axon.

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Action Potential

• A neural impulse. A brief electrical charge that
  travels down an axon and is generated by the
  movement of positively charged atoms in and out
  of channels in the axons membrane.

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The Neural Impulse

• Ions
• Charged molecules
• Resting Potential
• When more negative ions are inside the neuron
  than outside
• Charge is approximately -70mV
• Neuron is not transmitting information

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Depolarization Hyperpolarization

• Depolarization Depolarization occurs when
  positive ions enter the neuron, making it more
  prone to firing an action potential.
  Hyperpolarization occurs when negative ions enter
  the neuron, making it less prone to firing an
  action potential.

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Threshold

• Threshold Each neuron receives depolarizing and
  hyperpolarizing currents from many neurons. When
  the depolarizing current (positive ions) minus
  the hyperpolarizing current (negative ions)
  exceed minimum intensity (threshold) the neuron
  fires an action potential.

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Refractory Period Pumps

• Refractory Period After a neuron fires an action
  potential it pauses for a short period to
  recharge itself to fire again.
• Sodium-Potassium Pumps Sodium-potassium pumps
  pump positive ions out from the inside of the
  neuron, making them ready for another action
  potential.

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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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The Synapse

• Synaptic space (synaptic cleft)
• Tiny gap between neurons
• Terminal button
• Enlarged area at the end of an axon
• The synapse
• Composed of the terminal button of one neuron,
  the synaptic space, and the dendrites or cell
  body of the receiving neuron

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Transmission Between Neurons

• Synaptic vesicles
• Sacs in terminal button that release chemicals
  into synaptic space
• Neurotransmitters
• Chemicals released by synaptic vesicles
• Receptor sites
• Location on receptor neuron for specific
  neurotransmitter

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Neurotransmitters

• Neurotransmitters (chemicals) released from the
  sending neuron travel across the synapse and bind
  to receptor sites on the receiving neuron,
  thereby influencing it to generate an action
  potential.

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Lock Key Mechanism

• Neurotransmitters bind to the receptors of the
  receiving neuron in a key-lock mechanism.

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Neurotransmitters
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Agonists
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Antagonists
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Some Well-Known Neurotransmitters

• Acetylcholine (ACh)
• Released at the neuromuscular junction
• Plays an important role in arousal and attention
• Loss of ACh producing cells is linked to
  Alzheimers Disease
• Dopamine
• Affects neurons associated with voluntary
  movement
• Plays a role in learning, memory, and emotions
• Loss of dopamine-producing cells causes symptoms
  of Parkinsons Disease

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Some Well-Known Neurotransmitters

• Serotonin
• Found throughout the brain
• Appears to sets an emotional tone
• Low serotonin levels are implicated in depression
• Endorphins
• Reduce pain by inhibiting or turning down
  neurons that transmit pain information

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Reuptake

• Neurotransmitters in the synapse are reabsorbed
  into the sending neurons through the process of
  reuptake. This process applies the brakes on
  neurotransmitter action.

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Psychopharmacology

• Most psychoactive drugs (and toxins) work by
  blocking or enhancing synaptic transmission
• Botulism
• Blocks release of ACh at the neuromuscular
  junction, causing paralysis
• Botox is botulism toxin used to prevent facial
  muscles from making wrinkles

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Psychopharmacology

• Curare
• Can stun or kill prey quickly
• Blocks ACh receptors causing paralysis
• Antipsychotic medications
• Block dopamine receptors
• Reduces schizophrenic hallucinations
• Caffeine
• Increases the release of excitatory
  neurotransmitters by blocking the inhibitory
  neurotransmitter adenosine

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Psychopharmacology

• Cocaine
• Prevents reabsorption of dopamine
• Leads to heightened arousal of entire nervous
  system