Chapter 3 Synapses

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Chapter 3Synapses
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The Concept of the Synapse

• Neurons communicate by transmitting chemicals at
  junctions called synapses
• In 1906, Charles Scott Sherrington coined the
  term synapse to describe the specialized gap that
  existed between neurons.

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The Concept of the Synapse

• Sherrington observed that repeated stimuli over a
  short period of time produced a stronger
  response.
• Led to the idea of temporal summation or that
  repeated stimuli can have a cumulative effect and
  can produce a nerve impulse when a single stimuli
  is too weak.

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Fig. 3-3, p. 54
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The Concept of the Synapse

• Sherrington also noticed that several small
  stimuli on a similar location produced a reflex
  when a single stimuli did not.
• This led to the idea of spatial summation or that
  synaptic input from several locations can have a
  cumulative effect and trigger a nerve impulse.

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Fig. 3-4, p. 54
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The Concept of the Synapse

• Excitatory postsynaptic potential (EPSP) is a
  graded potential that decays over time and space.
• Graded potentials are different than action
  potentials!!
• The cumulative effect of EPSPs are the basis for
  temporal and spatial summation.

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The Concept of the Synapse

• Sherrington also noticed that during the reflex
  that occurred, the foot of a dog that was pinched
  retracted while the other three feet were
  extended.
• He suggested that an interneuron in the spinal
  cord sent an excitatory message to the flexor
  muscles of one leg and an inhibitory message was
  sent to the other three legs.

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Fig. 3-5, p. 55
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The Concept of the Synapse

• This led to the idea of inhibitory postsynaptic
  potential or the temporary hyperpolarization of a
  membrane.
• An IPSP occurs when synaptic input selectively
  opens the gates for positively charged potassium
  ions to leave the cell or for negatively charged
  chloride ions to enter the cells.
• Serves as an active brake, that suppresses
  excitation.

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The Concept of the Synapse

• Neurons can have thousands of synapses.
• Both temporal and spatial summation can occur
  within a neuron.
• The likelihood of an action potential depends
  upon the ratio of IPSPs to EPSPs at a given
  moment.

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Chemical Events at the Synapse

• Transmission of a message across the synapse
  occurs by chemical means.
• Neurotransmitters are chemicals that travel
  across the synapse and allow communication
  between neurons.

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Chemical Events at the Synapse

• The major sequence of events that allow
  communication between neurons across the synapse
  are as follows
• The neuron synthesizes chemicals that serve as
  neurotransmitters.
• Neurons store neurotransmitters in axon terminals
  or transport them there.
• An action potential triggers the release of
  neurotransmitters into the synaptic cleft.

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Chemical Events at the Synapse (cont.)

• The neurotransmitters travel across the cleft and
  attach to receptors on the postsynaptic neuron.
• The neurotransmitters separate from the
  receptors.
• The neurotransmitters are taken back into the
  presynaptic neuron (reuptake), diffuse away, or
  are inactivated by chemicals.
• The postsynaptic cell may send negative feedback
  to slow the release of further neurotransmitters.

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Fig. 3-8, p. 59
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Chemical Events at the Synapse

• Major categories of neurotransmitters include the
  following
• Amino acids glutamate, GABA,
• Acetylcholine
• Monoamines serotonin, dopamine,
  norephinephrine, epinephrine
• Purines --adenosine
• Gases nitric oxide (not laughing gas!)

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Chemical Events at the Synapse

• Neurons synthesize neurotransmitters and other
  chemicals from substances provided by the diet.
• Smaller neurotransmitters are synthesized in the
  presynaptic terminal and held there for release.
• Example acetylcholine
• Larger neurotransmitters are synthesized in the
  cell body and transported down the axon.
• Example peptides

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Chemical Events at the Synapse

• Vesicles are tiny spherical packets located in
  the presynaptic terminal where neurotransmitters
  are held for release.
• Exocytosis refers to the excretion of the
  neurotransmitter from the presynaptic terminal
  into the synaptic cleft.
• Triggered by an action potential arriving fro the
  axon.

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Fig. 3-10, p. 61
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Chemical Events at the Synapse

• Transmission across the synaptic cleft by a
  neurotransmitter takes fewer than 10
  microseconds.
• Most individual neurons release at least two or
  more different kinds of neurotransmitters.
• A neuron may respond to more types of
  neurotransmitters than it releases.

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Chemical Events at the Synapse

• An ionotropic effect refers to when a
  neurotransmitter attaches to receptors and
  immediately opens ion channels.
• Ionotropic effects occur very quickly and are
  very short lasting.
• Most of the brains excitatory ionotropic
  synapses use glutamate or acetylcholine as a
  neurotransmitter.

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Chemical Events at the Synapse

• Metabotropic effects refer to when a
  neurotransmitter attaches to a receptor and
  initiates a sequence of metabolic reactions that
  are slower and longer lasting.
• Metabotropic events include such behaviors as
  hunger, fear, thirst, or anger.

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Chemical Events at the Synapse

• Metabotropic effects utilize a number of
  different neurotransmitters and are often called
  neuromodulators because they do not directly
  excite or inhibit the postsynaptic cell.
• Instead, neuromodulators
• increase or decrease the release of other
  neurotransmitters
• alter the response of postsynaptic cells to
  various inputs.

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Chemical Events at the Synapse

• A hormone is a chemical secreted by a gland or
  other cells that is transported to other organs
  by the blood where it alters activity.
• Endocrine glands are responsible for the
  production of hormones.
• Hormones are important for triggering
  long-lasting changes in multiple parts of the
  body.

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Chemical Events at the Synapse

• Neurotransmitters released into the synapse do
  not remain and are subject to either inactivation
  or reuptake.
• Reuptake refers to when the presynaptic neuron
  takes up most of the neurotransmitter molecules
  intact and reuses it.
• Transporters are special membrane proteins that
  facilitate reuptake.
• Example Serotonin is taken back up into the
  presynaptic terminal.

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Chemical Events at the Synapse

• Examples of inactivation and reuptake include
• Acetylcholine is broken down by
  acetylcholinesterase into acetate and choline.
• Some serotonin and catecholamine molecules are
  converted into inactive chemicals
• COMT (catechol-o-methyltranferase)and MAO
  (monoamine oxidase) are enzymes that convert
  catecholamine transmitters into inactive
  chemicals.

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Chemical Events at the Synapse

• Research has begun to investigate the role of
  events at the synapse and their effects on
  personality.
• Research suggests that some dopamine receptors
  may be related to pleasure-seeking and
  thrill-seeking behaviors.

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Drugs and the Synapse

• The study of the influence of various kinds of
  drugs has provided us with knowledge about many
  aspects of neural communication at the synaptic
  level.
• Drugs either facilitate or inhibit activity at
  the synapse.
• Antagonistic drugs block the effects of
  neurotransmitters (e.g., novacaine, caffeine).
• Agonist drugs mimic or increase the effects of
  neurotransmitters (e.g., receptors in the brain
  respond to heroin, LSD and cocaine)
• Drugs alter various stages of synaptic processing.

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Drugs and the Synapse

• Drugs work by doing one or more of the following
  to neurotransmitters
• Increasing the synthesis.
• Causing vesicles to leak.
• Increasing release.
• Decreasing reuptake.
• Blocking the breakdown into inactive chemical.
• Directly stimulating or blocking postsynaptic
  receptors.

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Drugs and the Synapse

• A drug has an affinity for a particular type of
  receptor if it binds to that receptor.
• Can vary from strong to weak.
• The efficacy of the drug is its tendency to
  activate the receptor .
• Drugs can have a high affinity but low efficacy.

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Drugs and the Synapse

• Almost all abused drugs stimulate dopamine
  release in the nucleus accumbens
• small subcortical area rich in dopamine receptors
• an area responsible for feelings of pleasure
• Sustained bursts of dopamine in the nucleus
  accumbens inhibit cells that release the
  inhibitory neurotransmitter GABA

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Fig. 3-18, p. 72
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Drugs and the Synapse

• Drugs are categorized according to their
  predominant action or effect upon behavior
• Stimulant drugs increase excitement, alertness,
  motor activity and elevate mood.
• Examples amphetamines, cocaine, methylphenidate
  (Ritalin), MDMA (Ecstasy), nicotine
• Stimulant drugs directly stimulate dopamine
  receptor types D2, D3, and D4.

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Fig. 3-19, p. 72
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Drugs and the Synapse

• Amphetamine stimulate dopamine synapses by
  increasing the release of dopamine from the
  presynaptic terminal.
• Cocaine blocks the reuptake of dopamine,
  norepinephrine, and serotonin.
• Methylphenidate (Ritalin) also blocks the
  reuptake of dopamine but in a more gradual and
  more controlled rate.

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Drugs and the Synapse

• Ecstasy increases the release of dopamine at low
  doses that account for its stimulant properties.
• Ecstasy increases the release of serotonin at
  higher doses accounting for its hallucinogenic
  properties.
• Research indicates ecstasy use may contribute to
  higher incidences of anxiety and depression as
  well as memory loss and other cognitive deficits.

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Fig. 3-20, p. 73
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Drugs and the Synapse

• Nicotine stimulates one type of acetylcholine
  receptor known as the nicotinic receptor.
• Nicotinic receptors are found in the central
  nervous system, the nerve-muscle junction of
  skeletal muscles and in the nucleus accumbens
  (facilitate dopamine release).

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Drugs and the Synapse

• Opiate drugs are those that are derived from (or
  similar to those derived from) the opium poppy.
• Opiates decrease sensitivity to pain and increase
  relaxation.
• Examples morphine, heroin, methadone.

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Drugs and the Synapse

• The brain produces peptides called endorphins.
• Endorphin synapses may contribute to certain
  kinds of reinforcement by inhibiting the release
  of GABA indirectly.
• Inhibiting GABA indirectly releases dopamine.
• Endorphins attach to the same receptors to which
  opiates attach.

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Drugs and the Synapse

• Opiates also block the locus coeruleus.
• involved in our response to arousing stimuli by
  release of norepinephrine
• also involved in memory storage.

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Drugs and the Synapse

• Tetrahydocannabinol (THC) is the active
  ingredient in marijuana.
• THC attaches to cannabinoid receptors throughout
  the brain but especially the cerebral cortex,
  cerebellum, basal ganglia, and hippocampus.
• Anandamide and 2-AG are the endogenous chemicals
  that attach to these receptors.

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Drugs and the Synapse

• The location of the receptors in the brain may
  account for the subjective effects of loss of
  time, an intensification of sensory experience,
  and also memory impairment.
• The cannabinoid receptors are located on the
  presynaptic neuron and inhibit the release of
  glutamate and GABA.

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Drugs and the Synapse

• Hallucinogenic drugs cause distorted perception.
• Many hallucinogenic drugs resemble serotonin in
  their molecular shape.
• Hallucinogenic drugs stimulate serotonin type 2A
  receptors (5-HT2A) at inappropriate times or for
  longer duration than usual thus causing their
  subjective effect.