Transcript 5. Nervous System Physiology 3

ANIMAL PHYSIOLOGY AND
ENDOCRINOLOGY
Nervous System Physiology:
Neurotransmitters and synaptic
transmission
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NEUROTRANSMITTERS
Neurotransmitter is a chemical:
I.
it must be released from presynaptic terminal,
ii. must elicit the normal depolarization,
iii. The effect of the substance must be blocked by the
same agents that block synaptic transmission.
Neuromodulators alter or modify the functioning of
synapses.
Agonists are the chemicals that have the same effect as a
neurotransmitter.
Anatagonists are the chemicals that reduce or prevent the
synaptic transmission.
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Table 4-1.
Selected Neurortransmitters and Neuromodulators
Compound
Site of Action of Neurons
Acetylcholine
Neuromuscular Junction, autonomic endings,
autonomic ganglia, sweat glands, brain, retina, GI
tract
Biogenic amines
Epinephrine
Norephinephrine
Dopamine
Serotonin
Histamine
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Brain, spinal cord
Sympathetic endings, brain, spinal cord, GI tract
Brain, Sympathetic ganglia, retina
Brain, spinal cord, retina, GI tract
Brain, GI tract
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Compound
Site of Action of Neurons
Amino acids
GABA
Glutamate
Aspartate
Glycine
Brain, retina
Brain
Spinal cord, brain?
Spinal cord, brain, retina
Prines/Purine
nucleotides
Adenosine
ATP
Gas
Nitric oxide
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Brain
Autonomic ganglia, brain
Brain, spinal cord, GI tract
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PRINCIPAL NEUROTRASMITTER SYSTEMS
Following is a summarized overview of principal
neurotransmitters and their receptors.
ACETYLCHOLINE
Acetylcholine is the principal neurotransmitter at
neuromuscular junctions. The neurons that
synthesize and release acetylcholine are called
“cholinergic neurons”.
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Active uptake
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ECITATORY AMINO ACIDS
• Glutamate and aspartate are excitatory amino
acids. These depolarizes many mammalian
neurons.
• Glutamate is calculated as responsible for 75 %
excitatory transmission in brain and spinal cord.
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INHIBITROY AMINO ACIDS
• Gamaamino Butyric Acid (GABA) and Glycine
are inhibitory amino acid that act as
neurotransmitters.
• GABA is a major inhibitory neurotransmitter in
the brain, where it is transmitter for 20 %
synapses. It is also fund in retina.
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Figure 45-1 Structure of a
large neuron in the brain,
showing its important
functional parts.
(Redrawn from Guyton
AC: Basic Neuroscience:
Anatomy and Physiology.
Philadelphia: WB
Saunders Co, 1987.)
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SYNAPTIC TRANSMISSION
Synaptic transmission occur via two systems:
– Electrical Synapses
Electrical synapses are very closely connected
channels between two neurons. It allows transmission
of nerve impulse directly from one neuron to the
other.
– Chemical Synapses
In chemical synapse, chemicals (neurotransmitters)
are released at synapses and attach at other neuron’s
receptors to transmit nerve impulse.
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ELECTRICAL SYNAPSES
• Electrical synapses transfer impulse by direct ionic
coupling.
• At electrical synapse, the plasma membranes of preand post-synaptic cells are in close apposition and
communication between cells takes place by way of
protein channels called GAP JUCTIONS.
• Ions can flow from one cell to the other by way of these
gap junction to produce an almost equal though
somewhat attenuated signal in the postsynaptic cell.
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ELECTRICAL SYNAPSES
An Electron
Micrograph
showing
CONNEXONS
– channels in
the membrane
for Electrical
Synapses.
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In Electrical
Synapses action
potential directly
transmit from
presynaptic
membrane to the
postsynaptic
membrane.
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Electrical synapses were first discovered in 1959
in study on crayfish.
Electrical transmission of nerve impulse is found
at many locations in various animals, e.g.,
vertebrate retina, some locations of vertebrate
nervous system, smooth muscle and cardiac
muscle fibers and sensory neurons.
Electrical transmission is possible in both
directions at gap junctions but at some places it
is in only one direction, such junctions are called
rectifying.
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Electrical transmission is quite faster than
that of Chemical transmission due to the
long pathway of chemical synapses, i.e.
release of neurotransmitter – attachment
of it to receptors at postsynaptic
membrane – opening of ion channels in
post synaptic membrane – generation of
impulse.
At some locations both chemical and
electrical transmission occur. It was first
discovered in birds.
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CHEMICAL SYNAPSES
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Neuron –
Neuron
Synapse
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Fast Chemical Synaptic
Transmission
-Neurotransmitter attaches to a
receptor
- Receptor is an ion channel, which
opens in response
- Ions flow inside.
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Slow Chemical Transmission
When the receptor is activated by
the neurotransmitter, its G-protein
site becomes receptive to the Gprotein. That protein moves into
this interaction, which yields three
major changes:
(1) The GDP molecule is released
and a GTP replaces it.
(2) The alpha portion of the Gprotein dissociates from the beta
moiety, and moves to connection
with the gate molecule.
(3) This causes the gate to open
and an ionic current occurs.
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