Transcript 4-22-05

4-22-05
Neurotransmitters
The Nature Of Nerve Signals
1. Chemical or electrical communication between cells occurs at synapse
2. The same neurotransmitter can produce different effects on different types
of cells
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Chemical or electrical
communication between cells
occurs at synapses
• Electrical Synapses.
– Action potentials travels directly from the presynaptic
to the postsynaptic cells via gap junctions.
– Invertebrate giant axons
– Present in vertebrate brain in stereotype behavior like
a fish flapping its tail to escape a predator.
– Are fast connections.
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• Chemical Synapses.
– More common than electrical synapses.
– Postsynaptic chemically-gated channels exist for ions
such as Na+, K+, and Cl-.
• Depending on which gates open the postsynaptic neuron
can depolarize or hyperpolarize.
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Acetylcholine used as neuromuscular junction
Action potential causes
presynaptic membrane to
release a neurotransmitter
into synaptic cleft
Fig. 48.12
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Neural integration occurs at the
cellular level
• Excitatory postsynaptic potentials (EPSP)
depolarize the postsynaptic neuron.
– The binding of neurotransmitter to postsynaptic
receptors open gated channels that allow Na+ to
diffuse into and K+ to diffuse out of the cell.
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• Inhibitory postsynaptic potential (IPSP)
hyperpolarize the postsynaptic neuron.
– The binding of neurotransmitter to postsynaptic
receptors open gated channels that allow K+ to diffuse
out of the cell and/or Cl- to diffuse into the cell
making the inside more negative relative to the
outside. No sodium movement.
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• Summation: graded potentials (EPSPs and
IPSPs) are summed to either depolarize or
hyperpolarize a postsynaptic neuron.
Fig. 48.14
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The same neurotransmitter can
produce different effects on
different types of cells
• Acetylcholine (ACh).
– Excitatory to skeletal muscle (opens Na
channels).
– Inhibitory to cardiac muscle (inhibits adenyl
cyclase and opens K channels which
hyperpolarize the membrane).
– Secreted by the CNS, PNS, and at vertebrate
neuromuscular junctions.
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• Biogenic Amines.
– Epinephrine and norepinephrine.
• Can have excitatory or inhibitory effects.
• Secreted by the CNS and peripheral nervous
system (PNS).
• Secreted by the adrenal glands (increase heart rate).
• Epinephrine also called adrenalin
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• Dopamine
– Generally excitatory; may be inhibitory at
some sites.
• Widespread in the brain.
• Affects sleep, mood, attention, and learning.
– Secreted by the CNS and PNS.
– A lack of dopamine in the brain is associated
with Parkinson’s disease.
– Excessive dopamine is linked to schizophrenia.
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• Serotonin.
– Generally inhibitory.
• Widespread in the brain.
• Affects sleep, mood, attention, and learning
– Secreted by the CNS.
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• Amino Acids
– Gamma aminobutyric acid (GABA).
• Inhibitory –opens chloride channel--IPSPs.
• Secreted by the CNS and at invertebrate
neuromuscular junctions.
• Glycine.
.
• Secreted by the CNS.
• Inhibitory
• Glutamate.
– Excitatory.
– Secreted by the CNS and at invertebrate
neuromuscular junctions.
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• Aspartate.
– Excitatory.
– Secreted by the CNS.
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• Neuropeptides.
– Substance P.
• Excitatory.
• Secreted by the CNS and PNS.
• Mediates our perception of pain.
• Met-enkephalin (an endorphin).
– Generally inhibitory.
– Secreted by the CNS.
– Decrease our perception of pain. Natural
“high”. Long distance runners “feel good”
after a long run associated with increased
endorphins. Morphine binds to same receptor.
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• Gasses that act as local regulators.
– Nitric oxide (NO2).
– Carbon monoxide.
– Persons suffering angina (chest pain from lack
of adequate delivery of oxygen to the heart)
take nitroglycrine. An enzyme breaks it down
into nitric oxide which relaxes cornary arteries
providing almost instant relief.
– Viagra slows the degradation of nitric oxide in
the erectile tissue veins that causes their
relaxation.
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CHAPTER 49
SENSORY AND MOTOR SYSTEMS
Sensing, Acting, and Brains
1. The brain’s processing of sensory input and motor output is cyclical rather
than linear meaning we are continually analyzing sensory input as we sense
the effect of our motor output.
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The brain’s processing of sensory
input and motor output is cyclical
rather than linear
• The way it ISN’T: sensing  brain analysis  action.
• The way it is: sensing, analysis, and action are ongoing
and overlapping processes.
• Sensations begin as different forms of energy that are
detected by sensory receptors.
– This energy is converted to action potentials that
travel to appropriate regions of the brain.
• The limbic region plays a major role in determining the
importance of a particular sensory input (whether it gets
filtered out).
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Introduction To Sensory Reception
1. Sensory receptors transduce stimulus energy and transmit signals to the
nervous system
2. Sensory receptors are categorized by the type of energy they transduce
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• Definitions:
• Sensations are action potentials that reach the
brain via sensory neurons.
• Perception is the awareness and interpretation
of the sensation.
• Next slide has example of taste reception.
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Sensory receptors transduce stimulus energy and
transmit signals to the nervous system. Action
potential in most case not generated directly.
G-protein >Adenylyl cyclase
C-AMP> Protein kinase
phosphorylates some protein?? that
prevents K+ channel from opening.
Action potentials
• Sensory Transduction.
– The conversion of stimulus energy into a change in
membrane potential.
– Receptor potential: a sensory receptor’s version of
a graded potential (change in voltage across the
membrane determined by the strength of stimulus)
Change affects the rate of firing of postsynaptic
neuron.
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• Amplification.
– The strengthening of stimulus energy so that it can
be detected by the nervous system.
• May be a part of, or occur apart from, sensory
transduction.
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• Transmission.
– The conduction of sensory impulses to the CNS.
– Some sensory receptors must transmit chemical
signals to sensory neurons.
• The strength of the stimulus and receptor potential affects
the amount of neurotransmitter released by the sensory
receptor.
– Some sensory receptors are sensory neurons.
• The intensity of the receptor potential affects the
frequency of action potentials.
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• Integration.
– The processing of sensory information.
• Begins at the sensory receptor.
– For example, sensory adaptation is a decrease in responsiveness
to continued stimulation.
– For example, the sensitivity of a receptor to a stimulus will vary
with environmental conditions.
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Sensory receptors are categorized
by the type of energy they
transduce
Fig. 49.3
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• Mechanoreceptors respond to mechanical
energy.
– For example, muscle spindles is an interoreceptor
that responds to the stretching of skeletal muscle.
– For example, hair cells detect motion.
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• Pain receptors = nocioceptors.
– Different types of pain receptors respond to
different types of pain.
– Prostaglandins increase pain by decreasing a pain
receptors threshold.
• Anti-inflammatories work by inhibiting prostaglandin
synthesis.
• Thermoreceptors respond to heat or cold.
– Respond to both surface and body core temperature.
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• Chemoreceptors respond to chemical stimuli.
– General chemoreceptors transmit information about
total solute concentration.
– Specific chemoreceptors respond to specific types
of molecules (olfaction).
– Internal chemoreceptors respond to glucose, O2,
CO2, amino acids, etc.
– External chemoreceptors are gustatory receptors
(taste) and olfactory receptors (smell).
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• Electromagnetic receptors respond to
electromagnetic energy (migrating animals—
birds and whales).
– Photoreceptors respond to the radiation we know
as visible light.
– Electroreceptors: some fish use electric currents
to locate objects. If something enters their electric
field they sense it. Fish in murky waters in the
tropics. Also sharks and duck billed platypus (bill).
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Photoreceptors And Vision
1. A diversity of photoreceptors has evolved among invertebrates
2. Vertebrates have single-lens eyes
3. The light-absorbing pigment rhodopsin triggers a signal-transduction
pathway
4. The retina assists the cerebral cortex in processing visual information
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• Most, if not all, animal photoreceptors may be
homologous (common origin)._
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A diversity of photoreceptors has
evolved among invertebrates
• Eye cups are among the simplest
photoreceptors
– Detect light intensity and direction — no image
formation.
– The movement
of a planarian is
integrated with
photoreception.
Turns away from light
Fig. 49.7
• Image-forming eyes.
– Compound eyes of insects and crustaceans.
• Each eye consists many individual
units called of ommatidia,
each with its own
light-focusing lens.
• This type of eye
is very good at
detecting
movement.
Neurons directly exit the back of eye
Fig. 49.8
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• Single-lens eyes of invertebrates such as jellies,
polychaetes, spiders, and mollusks.
– The eye of an octopus works much like a
camera and is similar to the vertebrate eye.
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Vertebrates have single-lens eyes
• Is structurally analogous to the invertebrate
single-lens eye.
Fig. 49.9
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• Sclera: a tough white layer of connective tissue
that covers all of the eyeball except the cornea.
– Conjunctiva: external cover of the sclera —
keeps the eye moist.
• Cornea: transparent covering of the front of the
eye.
– Allows for the passage of light into the eye
and functions as a fixed lens (refracts light).
– Choroid: thin, pigmented layer lining the interior
surface of the sclera. Prevents light rays from
scattering and distorting the image.
– Anteriorly it forms the iris.
• The iris regulates the size of the pupil.
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• Retina: lines the interior surface of the choroid.
– Contains photoreceptors.
• Except at the optic disk (where the optic nerve
attaches).
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• The lens and ciliary body divide the eye into
two cavities.
– The anterior cavity is filled with aqueous
humor produced by the ciliary body.
• Glaucoma results when the duct that drain
aqueous humor are blocked.
– The posterior cavity is filled with vitreous
humor.
– The lens, the aqueous humor, and the
vitreous humor all play a role in focusing
light onto the retina.
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• Accommodation is the focusing of light on the
retina.
– In squid, octopuses, and many fish this is
accomplished by moving the lens forward
and backward. The cephalopod with a shell,
Nautilus has an eye without a lens (pin hole
camera).
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– In mammals
accommodation
is accomplished
by changing the
shape of the lens.
• The lens is
flattened for
focusing on
distant objects.
• The lens is
rounded for
focusing on near
objects.
Fig. 49.10
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