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Title: Chapter 3 Synapses


1
Chapter 3Synapses
2
The Concept of the Synapse
  • Synapse gap between one neurons presynaptic
    terminal and another receiving neuron
  • Discovered by Santiago Ramon y Cajal in the late
    1800s

3
The Concept of the Synapse
  • In 1906, Charles Scott Sherrington
  • coined the term synapse and proposed that neurons
    communicate by transmitting chemicals at synapses
  • conducted research on synaptic communication by
    studying reflexes (automatic muscular responses
    to stimuli).

4
Reflex arc the complete circuit from sensory
neuron to muscle response
Fig. 3-1, p. 52
5
Sherrington deduced three properties of reflexes
  1. Reflexes are slower than conduction along a
    single axon.

6
Sherrington deduced three properties of reflexes
  • 2. Several weak stimuli presented at slightly
    different times or slightly different locations
    produces a stronger reflex than a single stimulus
    does.

7
Sherrington deduced three properties of reflexes
  • 3. As one set of muscles relaxes, another set
    becomes excited.

8
John Eccles (1964)
  • Postsynaptic neuron receives the message
  • Presynaptic neuron delivers the synaptic
    potential

9
  • Spatial summation synaptic input from several
    locations can have a cumulative effect and
    trigger a nerve impulse
  • Temporal summation repeated stimuli can have a
    cumulative effect and can produce a nerve impulse
    when a single stimuli is too weak.

Fig. 3-4, p. 54
10
Graded potentials
  • either depolarize (excite) or hyperpolarize
    (inhibit) the postsynaptic neuron
  • decay over time and space

11
The Concept of the Synapse
  • Excitatory postsynaptic potential (EPSP)
    depolarizes neurons membrane
  • is a graded (excitatory) potential that decays
    over time and space.
  • The cumulative effect of EPSPs are the basis for
    temporal and spatial summation.
  • Inhibitory postsynaptic potential (IPSP)
    hyperpolarizes membrane.
  • Serves as an active brake, that suppresses
    excitation.

12
The Concept of the Synapse
  • The spontaneous firing rate refers to the
    periodic production of action potentials despite
    synaptic input.
  • EPSPs increase the nerve cells spontaneous
    firing rate.
  • IPSPs decrease the nerve cells spontaneous
    firing rate.

13
Chemical Events at the Synapse
  • Otto Loewi (1920) Discovered that transmission
    of a message across the synapse occurs by
    chemical means.

14
Neuroanatomy Handout 2 The Synapse and
Neurotransmitters
  • Synaptic cleft (E)
  • Axon membrane (A)
  • Presynaptic membrane (A1)
  • Synaptic vesicles (B) tiny spherical packets
    located in the presynaptic terminal where
    neurotransmitters are held for release
  • Neurotransmitters (C) chemicals that travel
    across the synapse and allow communication
    between neurons
  • Neurotransmitter fragments (C1)

15
Neurotransmitters
  • Approx. 100 different kinds
  • Neurons synthesize neurotransmitters and other
    chemicals from substances provided by the diet.
  • Acetylcholine is synthesized from choline found
    in milk, eggs, and nuts.
  • Serotonin is synthesized from tryptophan found in
    turkey and soy.

16
Major sequence of events for neurotransmitters
  1. The neuron synthesizes chemicals that serve as
    neurotransmitters.
  2. Neurons store neurotransmitters in axon terminals
    or transport them there (transportation from cell
    body can take hours or days).
  3. An action potential triggers the release of
    neurotransmitters into the synaptic cleft.

17
Major sequence of events for neurotransmitters
  • Exocytosis (D) refers to the excretion of the
    neurotransmitter from the presynaptic terminal
    into the synaptic cleft.
  • Triggered by an action potential arriving from
    the axon.

18
Major sequence of events for neurotransmitters
  1. The neurotransmitters travel across the cleft and
    attach to the postsynaptic membrane (F) at the
    postsynaptic receptor sites (G).
  2. The neurotransmitters separate from the
    receptors.
  3. The neurotransmitters are taken back into the
    presynaptic neuron, diffuse away, or are
    inactivated by chemicals.

19
Major sequence of events for neurotransmitters
  • Reuptake (endocytosis) (H) refers to the
    presynaptic neuron taking up most of the
    neurotransmitter molecules intact and reusing
    them.

20
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.

21
Pituitary Gland
  • Attached to the hypothalamus and consisting of
    two distinct glands that each release a different
    set of hormones
  • Anterior pituitary- composed of glandular tissue
    and synthesizes six hormones.
  • Posterior pituitary- composed of neural tissue
    and can be considered an extension of the
    hypothalamus

22
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.

23
Table 3-3, p. 76
24

Link to Mouse Party
25
Drugs and the Synapse
  • Drugs either facilitate or inhibit activity at
    the synapse.
  • Antagonistic drugs block the effects of
    neurotransmitters.
  • Agonist drugs mimic or increase the effects of
    neurotransmitters.

26
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.

27
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

28
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.

29
Drugs and the Synapse
  • Amphetamines 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.
  • Often prescribed for people with ADD

30
Drugs and the Synapse
  • MDMA (Ecstasy)
  • increases the release of dopamine at low doses
    that account for its stimulant properties
  • 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.

31
Drugs and the Synapse
  • Nicotine
  • active ingredient in tobacco
  • stimulates an acetylcholine receptor known as the
    nicotinic receptor, found in
  • central nervous system
  • nerve-muscle junction of skeletal muscles
  • nucleus accumbens

32
Drugs and the Synapse
  • Opiate drugs
  • derived from opium poppy
  • decrease sensitivity to pain and increase
    relaxation
  • Examples morphine, heroin, methadone.

33
Drugs and the Synapse
  • Endorphins
  • naturally produced neurotransmitters
  • ease pain
  • inhibit GABA, allowing dopamine to exert its
    effect
  • attach to the same receptors to which opiates
    attach.

34
Drugs and the Synapse
  • Tetrahydocannabinol (THC)
  • active ingredient in marijuana
  • attaches to cannabinoid receptors, especially in
    the cerebral cortex, cerebellum, basal ganglia,
    and hippocampus.
  • Cannabinoids chemicals related to THC, typically
    used medically
  • Anandamide and 2-AG are the endogenous chemicals
    that attach to these receptors.

35
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36
Drugs and the Synapse
  • Hallucinogenic drugs
  • cause distorted perception
  • may resemble serotonin in their molecular shape
  • stimulate serotonin type 2A receptors (5-HT2A) at
    inappropriate times or for longer duration than
    usual thus causing their subjective effect.

37
Alcohol and Alcoholism
  • Alcohol
  • is associated with relaxation
  • in greater amounts impairs judgment and damages
    the liver and other organs
  • dependence (alcoholism) is the habitual use of
    alcohol despite medical or social harm

38
Alcohol and Alcoholism
  • Alcohol has a number of diverse physiological
    effects, including
  • Enhanced response by the GABAA receptor
  • Blockage of glutamate receptors
  • Increased dopamine activity

39
Alcohol and Alcoholism
  • Strong influence of genetics on alcoholism
  • The genetic basis for early-onset alcoholism is
    stronger than for later-onset, especially in men
  • Researchers distinguish between two types of
    alcoholism
  • Type I/Type A
  • Type II/Type B

40
Alcohol and Alcoholism
  • Type I/Type A characteristics include
  • Later onset (usually after 25)
  • Gradual onset
  • Fewer genetic relatives with alcoholism
  • Equal quantity between men and women

41
Alcohol and Alcoholism
  • Type II/Type B characteristics include
  • Earlier onset (usually before 25)
  • More rapid onset
  • More genetic relatives with alcoholism
  • Men outnumber women

42
Alcohol and Alcoholism
  • Genes influence the likelihood of alcoholism in
    many ways, such as
  • being more sensitive and needing more alcohol to
    provide reinforcement
  • being linked with impulsivity
  • influencing responses to stress and
    anxiety-inducing experiences
  • likelihood of prenatal exposure to alcohol

43
Alcohol and Alcoholism
  • Research on sons of alcoholic fathers shows
  • Less average intoxication after one drink
  • Stress decreases more than for the average person
  • Smaller than normal amygdala

44
Addiction
  • Various factors contribute to continued substance
    abuse
  • Tolerance The bodys decrease in response to a
    drug with repeated use
  • Withdrawal Uncomfortable/painful symptoms once
    drug use is discontinued
  • Cravings develop in response to cues
  • Brain reorganization (nucleus accumbens and
    prefrontal cortex)

45
Medications to Combat Alcohol Abuse
  • Revia (naloxone) blocks opiate receptors, thereby
    decreasing the pleasure from alcohol.
  • Antabuse (disulfiram) works by making user sick
  • ?

46
Medications to Combat Alcohol Abuse
  • Most studies suggest that Antabuse has been only
    moderately effective
  • When effective, it supplements the alcoholics
    own commitment to quit
  • Daily routine of pill ingestion may reaffirm
    commitment not to drink
  • Many quit taking the pill and continue to drink

47
Medications to Combat Opiate Abuse
  • Methadone is an opiate similar to heroin and
    morphine but is absorbed and metabolized slowly
  • Perceived to be less harmful than other drugs
  • Assumed to satisfy cravings associated with
    previous drug use
  • Levomethadyl acetate (LAAM) is similar to
    morphine but can be taken three times a week
    rather than daily
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