Neurotransmitters

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Neuroanatomy
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• Neuroanatomy refers to the study of the parts and
  function of neurons.
• Neurons are individual nerve cells.
• The entirety of the human bodys neurons make up
  the nervous system, from the brain to the tips of
  the toes.

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The Basic Parts of a Neuron
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• A. Dendrites
• Thin, branching fibers lined with receptors at
  which the dendrite receives information from
  other neurons. The greater the surface area, the
  greater the amount of information. Some dendrites
  are covered with spines which greatly increase
  its surface area.

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• B. Cell Body/Soma
• Contains the (C) nucleus and other parts of the
  cell needed to sustain life

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• D. Axon
• Wire-like structure ending in the terminal
  buttons that extends from the cell body

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• E. Myelin Sheath
• An insulating, fatty covering around the axon
  that speeds neural transmissions. Axons that are
  myelinated appear white. Known as white matter.

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F. Schwann Cells

• Provide for the growth of the myelin sheath.

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• G. Nodes of Ranvier
• Regularly spaced gaps in the myelin sheath around
  an axon or nerve fiber. This is where
  depolarization takes place.

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• H. Terminal Buttons
• The branched end of the axon that contains
  neurotransmitters

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Neural Transmission
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• Synapse
• The space between the terminal buttons on one
  neuron and dendrites of the next neuron

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• Neurotransmitters

• Chemicals contained in the terminal buttons
  that enable neurons to communicate.
• Neurotransmitters fit into receptor sites on the
  dendrites of neurons like a key fits into a lock.

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Neurotransmitters
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• At the terminal buttons, neurotransmitters are
  released into the synapse and passed along to the
  dendrites of the next neuron.

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• If enough neurotransmitters have been sent, the
  next neuron will fire. If not, the message ends.
  This is called the all-or-nothing principle.

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• After a neuron fires its message, there is a
  brief period of time before it can fire again.
  This is called a neurons refractory period.
• During the refractory period, excess
  neurotransmitters are reabsorbed by the sending
  neuron, called re-uptake, as well as the cell
  becoming polarized once again.

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• In its resting state (resting potential or
  polarization), a neuron has a negative charge
  because mostly negative ions are within the cell.
  Surrounding the cell are positively charged
  ions. The ions cannot mix because it its resting
  stage, the cell membrane is semi-impermeable.

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• A neuron has a pre-set level of stimulation that
  needs to be met or exceeded in order for it to
  pass the received impulses on to the next neuron.
  This is called a neurons threshold.

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• If the threshold has been met or exceeded, a
  chain reaction begins.
• With threshold being met, the cell becomes
  depolarized and allows positively charged ions
  into the axon at the nodes of ranvier. This mix
  of positive and negative ions causes an
  electrical charge to form (an action potential).
  At 120 meters per second, the action potential
  travels to the terminal buttons.

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Axon inside and out
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Resting Potential

• The state of a neuron when it is at rest and
  capable of generating an action potential
• The neuron is set and ready to fire

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Action Potential

• A brief electrical charge that travels down the
  axon of the neuron.
• A neural impulse
• Considered an on condition of the neuron

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Refractory Period

• The recharging phase when a neuron, after
  firing, cannot generate another action potential
• Once the refractory period is complete the neuron
  can fire again

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Neuron firing like a Toilet

1. Like a Neuron, a toilet has an action potential.
   When you flush, an impulse is sent down the
   sewer pipe

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Neuron firing like a Toilet

• 2. Like a neuron, a toilet has a refractory
  period. There is a short delay after flushing
  when the toilet cannot be flushed again because
  the tank is being refilled

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Neuron firing like a Toilet

1. Like a Neuron, a toilet has a resting potential.
   The toilet is charged when there is water in
   the tank and it is capable of being flushed again
2. Like a Neuron, a toilet operates on the
   all-or-none principle it always flushes with
   the same intensity, no matter how much force you
   apply to the handle

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All-or-None Principle

• The principle that if a neuron fires it will
  always fire at the same intensity
• All action potentials are of the same strength.
• A neuron does NOT fire at 30, 45 or 90 but at
  100 each time it fires.

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Click here to see a neuron in action!
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• Depending on what type of neurotransmitter has
  been released, the next neuron will react
  differently.
• .so, since the entire body is a connection
  of nerves,

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Acetylcholine (ACh)

• Enables muscle action, learning, and memory
• Undersupply, as ACh-producing neurons
  deteriorate, marks Alzheimers disease

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Dopamine

• Reward and Motivation, Motor Control over
  Voluntary Movements
• Excessive dopamine linked to schizophrenia a
  lack of dopamine produces the tremors and lack of
  mobility (like in Parkinsons disease)

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Serotonin

• Affects mood, hunger, sleep, and arousal
• Undersupply is linked to depression Prozac and
  other anti-depressants raise serotonin levels

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Norepinephrine

• Helps to control alertness and arousal
• Undersupply can depress mood

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GABA

• Muscular movement
• Undersupply linked to seizures, tremors, and
  insomnia

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Glutamate

• Involved in memory
• Oversupply can overstimulate the brain, producing
  migraines or seizures

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Endorphins

• Natural opiates that are released in response to
  pain and vigorous exercise

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Epinephrine

• Adrenaline Burst of Energy (small amounts in
  brain)

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Drugs and Chemical Interactions with Neural
Transmission
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• Some drugs that people put into their bodies are
  classified as agonists.
• Agonists may either speed up the neural process,
  cause an over-release or absorption of a
  neurotransmitter, or block the re-uptake process.

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Prozac blocking the re-uptake of Serotonin
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• Some agonists mimic the effects of a naturally
  occurring neurotransmitter

Agonist (like morphine replacing natural
endorphines)
Dendrite of receiving Neuron
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• After a neuron fires, if re-uptake is blocked,
  the lingering neurotransmitters in the synapse
  will continue to be absorbed by the receiving
  neuron until it is gone.
• Therefore, a lingering feeling will occur

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Examples of Agonists

• Cocaine blocks the re-uptake of dopamine
• MDMA (Ecstasy) blocks the reuptake of serotonin
• Repeated use destroys serotonin producing cells

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• Some drugs that people put into their bodies are
  classified as antagonists.
• Antagonists may slow or stop the transmission of
  a neurotransmitter, or they may bind themselves
  to receptors on a neurons dendrite, thus not
  allowing a message to be passed on.

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Examples of Antagonists

• Curare a poison that stops the flow of Ach
  causes paralysis

Antagonist (like curare)
Neurotransmitter (such as Ach)
Dendrite of receiving Neuron
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Types of Neurons

• There are three types of neurons
• Afferent Neurons (Sensory Neurons)
• Interneurons
• Efferent Neurons (Motor Neurons)

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• Afferent Neurons are responsible for taking
  information from the senses TO the brain.

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• Interneurons are located in the spinal cord and
  the brain.
• Interneurons in the brain are responsible for
  sending messages along to the various areas of
  the brain for processing.

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• Efferent Neurons responsible for taking
  information FROM the brain and the spinal cord,
  and back to the rest of the body.

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• The exceptions to the general pathway of neural
  activity are reflexes.

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• Reflexes are controlled by the spinal cord
  without any conscious effort on behalf of the
  brain.

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• Reflexes are primitive responses

• protect our bodies from danger

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Reflex

• We cough, for example, when an irritant enters
  our windpipe and we need to expel it through our
  mouth. We sneeze when we need to clear our nasal
  air passages of irritants and allergens. We blink
  when danger threatens the sensitive tissues of
  the eye and when we need to moisten and clean the
  cornea. (This reflex occurs 900 times an hour!)
  We yawn when nerves in the brain stem find
  there's too much carbon dioxide in the blood.

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Reflex
Spinal Cord
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