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Functional Human Physiology for the Exercise and Sport Sciences Synaptic Transmission and Neural Integration

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Title: Functional Human Physiology for the Exercise and Sport Sciences Synaptic Transmission and Neural Integration


1
Functional Human Physiologyfor the Exercise and
Sport Sciences Synaptic Transmission and Neural
Integration
  • Jennifer L. Doherty, MS, ATC
  • Department of Health, Physical Education, and
    Recreation
  • Florida International University

2
Types of synapses
  • Electrical Synapses
  • Nerve impulse transmission directly from one
    neuron to another through membranes connected by
    gap junctions
  • No neurotransmitters
  • Chemical Synapses
  • Nerve impulse is transmitted from one neuron to
    another across a synapse
  • A region of functional contact (not actual
    contact) with another neuron
  • Specialized to release and receive
    neurotransmitters

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4
Chemical Synapses Functional Anatomy
  • Neuron-to-Neuron Synapses
  • Close junction between the axon terminal of one
    neuron and the plasma membrane of another neuron
  • Presynaptic Neuron
  • At a synapse, a neuron that transmits signals to
    a second neuron
  • Communicates with a post-synaptic neuron through
    the axon

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6
Chemical Synapses Functional Anatomy
  • Synaptic Knob (Bouton)
  • The rounded terminal end of the axon on a
    presynaptic neuron
  • Many synaptic knobs of many axons may terminate
    on the cell body or dendrites of postsynaptic
    neurons.
  • Contains many mitochondria
  • Contains Synaptic Vesicles
  • Store neurotransmitters

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Chemical Synapses Functional Anatomy
  • Synaptic Vesicles
  • Release neurotransmitters to diffuse across the
    synaptic cleft
  • Neurotransmitters attach to receptors on the
    post-synaptic neuron cell membrane
  • Synaptic cleft
  • Narrow, fluid-filled space between presynaptic
    and post-synaptic neurons
  • 30 - 50 nm or 1 millionth of an inch wide
  • No direct contact between neurons.

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10
Chemical Synapses Functional Anatomy
  • Post-synaptic neuron
  • At a synapse, the neuron that receives signals
    from another neuron
  • Post-synaptic membrane
  • Contains neurotransmitter receptors
  • Specialized protein receptors that react with (or
    receive) a specified neurotransmitter

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12
Signal Transduction Nerve Impulses (Wave of
Action Potentials)
  • Presynaptic Neuron Signal Transduction
  • A wave of action potentials are propagated along
    the cell membrane of the presynaptic axon
  • The wave of action potentials arrive at the
    synaptic knob.
  • Ca voltage-gated channels in the synaptic knob
    open in response to depolarization of the cell
    membrane
  • Ca ions from the extracellular fluid enter the
    synaptic knob

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14
Signal Transduction Nerve Impulses (Wave of
Action Potentials)
  • Increased amounts of intracellular Ca ions
  • Causes the synaptic vesicles to move toward and
    fuse with the membrane of the synaptic knob
  • The synaptic vesicles release neurotransmitters
    into the synaptic cleft via exocytosis
  • Unless another action potential is present
  • The Ca voltage-gated channels close
  • Ca is actively transported out of the synaptic
    knob.

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16
Signal Transduction Nerve Impulses (Wave of
Action Potentials)
  • Synaptic Cleft
  • Neurotransmitters diffuse across the synaptic
    cleft
  • Transmission of a nerve impulse across the
    synaptic cleft can only occur in one direction
  • Neurotransmitters bind to specific protein
    receptors on the post-synaptic membrane
  • Produces a response on the post-synaptic neuron
  • Neurotransmitters are quickly removed from the
    protein receptors by
  • Degradation by specific enzymes,
  • Diffusing away from the synapse, or
  • Reuptake by neuroglial cells or the presynaptic
    neuron

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18
Signal Transduction Nerve Impulses (Wave of
Action Potentials)
  • Post-Synaptic Signal Transduction
  • Neurotransmitters bind to specific protein
    receptors on the post-synaptic membrane
  • Ion channels on the post-synaptic membrane open
  • Results in a change of the membrane potential of
    the post-synaptic neuron
  • Depolarization occurs
  • Results in a wave of action potentials that is
    propagated along the cell membrane of the
    post-synaptic neuron

19
Signal Transduction Nerve Impulses (Wave of
Action Potentials)
  • Rarely is only one neuron responsible for
    producing an action potential on the cell
    membrane of another neuron
  • Several neurons must produce enough graded
    potentials to reach the threshold for
    generating an action potential
  • Two determinants of signal transduction
  • The type of neurotransmitter, and
  • The receptor proteins
  • Result is either
  • Excitation (depolarization), or
  • Inhibition (hyperpolarization)

20
Excitatory Synapses
  • Bring the membrane potential closer to the
    threshold for generating an action potential
  • Excitatory neurotransmitters depolarize the
    post-synaptic membrane
  • This is called an Excitatory Postsynaptic
    Potential (EPSP)
  • Opens Na voltage-gated channels on the
    post-synaptic membrane allowing Na ions to
    enter the cell
  • Fast response occurs via Ionotropic receptors
  • Channel-linked receptors
  • Slow response occurs via Metabotropic receptors
  • G-protein linked receptors

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22
Inhibitory Synapses
  • Bring the membrane potential away from the
    threshold for generating an action potential
  • Inhibitory neurotransmitters hyperpolarize or
    stabilize the post-synaptic membrane
  • This is called an Inhibitory Postsynaptic
    Potential (IPSP)
  • Opens K voltage-gated channels on the
    post-synaptic membrane allowing K ions to leave
    the cell

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24
Neural Integration
  • The net effect of EPSPs and IPSPs on the
    post-synaptic membrane will determine if the net
    effect is excitatory or inhibitory.
  • If the net effect is more excitatory than
    inhibitory, an action potential will be generated
    on the post-synaptic membrane and impulse
    transduction will occur
  • The opposite is also true, a net inhibitory
    effect will not produce an action potential.

25
Neurotransmitters
  • Structure
  • Chemical compounds
  • Over 30 different types of neurotransmitters
  • Major categories include
  • Choline derivatives
  • Biogenic amines
  • Amino acids
  • Neuropeptides
  • Synthesis
  • Neurotransmitters are synthesized in the
    cytoplasm of the nerve cell body or the synaptic
    knob
  • Neurotransmitters are stored in the synaptic
    vesicles
  • More than one neurotransmitter may be produced by
    a neuron

26
Acetylcholine (ACh)
  • Choline derivative neurotransmitter
  • Found at the neuromuscular junction predominantly
  • Responsible for stimulating muscles to contract
  • Acetylcholinesterase
  • Enzyme that degrades ACh into acetate and choline
  • Acetylcholine Receptors
  • Nicotinic cholinergic receptors
  • Excitatory
  • Found on skeletal muscles (somatic nervous
    system) and in neurons in the autonomic nervous
    system
  • Muscarinic cholinergic receptors
  • Either excitatory or inhibitory
  • Found in the central nervous system

27
Biogenic Amines Neurotransmitters derived from
Amino Acids
  • Catecholamines
  • Includes the neurotransmitters
  • Norepinephrine (NorE)
  • Epinephrine (Epi)
  • Dopamine
  • Important in many of the motor functions of the
    autonomic nervous system
  • Serotonin
  • Found in brainstem
  • Regulates sleep and emotions
  • Histamine
  • Found in hypothalamus
  • Better known for release from tissue cells in
    inflammatory response

28
Biogenic Amines Neurotransmitters derived from
Amino Acids
  • Adrenergic receptors
  • Alpha adrenergic
  • Norepinephrine mostly, but binds to both
  • Beta adrenergic
  • Epinephrine mostly, but binds to both
  • Degradation enzymes
  • Monoamine oxidase (MAO)
  • Catechol-O-methyltransferase (COMT)

29
Amino Acid Neurotransmitters
  • Most abundant class of neurotransmitters
  • Function only in the central nervous system
  • Excitatory neurotransmitters
  • Glutamate
  • Aspartate
  • Inhibitory neurotransmitters
  • Glycine
  • Gamma-aminobutyric acid (GABA)

30
Neuropeptides
  • Short chains of amino acids that function as
    neuromodulators
  • A neuromodulator is a substance that alters the
    response of a neuron to a neurotransmitter, or it
    blocks the release of a certain neurotransmitter.
  • Neuropeptides are more classically known as
    hormones
  • Examples
  • TRH regulates the release of TSH
  • Substance P decreases gastrointestinal motility
  • Vasopressin regulates urine output by the
    kidneys
  • Endogenous opiates analgesic effect, euphoric
    response
  • Enkaphalins
  • Endorphis
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