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Biophysics of
excitable
cells
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Axons are specialized for the conduction of an electrical
impulse called an action potential
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Specialized regions of neurons carry out different
functions
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Experimental techniques are conceptually simple
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Cell-Semiconductor-Hybrids:
Neuron on the Chip
More: Fromherz
MPI Martinsried
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Cell-Semiconductor-Hybrids: Neuron on the Chip
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Synapses are specialized sites where neurons
communicate with other cells
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Multiple exitatory and inhibitory synaptic contacts allow
complex neuronal interconnects
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Neurons are organized into circuits
The knee-jerk reflex arc in the human.
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A schematic of the vertebrate nervous system
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Membrane depolarizations spread passively only
short distances
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The electrical activity of neurons results from the opening
and closing of specific ion-channels proteins in the neuron
plasma membrane
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Voltage-gated cation channels generate action potentials
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The structure and function of the voltage-gated Na+ channel
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Action potentials are propagated unidirectionally
without diminution
Movements of only
a few Na+ and K+
ions generate the
action potential
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Myelination increases the velocity of impulse conduction
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Formation and structure of a myelin sheath in the peripheral
nervous system
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Each region of myelin formed by an individual glial cell is
separated from the next region by an unmyelinated area
called the node of Ranvier
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Action potentials travel rapidly from one node to the next
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Patch clamps permit measurement of ion movements
through single channels
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Different patch clamping configurations
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Current flux through individual voltage-gated channels
determined by patch clamping of muscle cells
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The oocyte expression assay can be used to determine if a
protein is an ion channel
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Voltage-gated K+ channels have four subunits each
containing six transmembrane  helices
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All five subunits in the nicotinic acetylcholine receptor
contribute to the ion channel
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P segments form the ion-selectivity filter
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All pore-forming ion channels are similar in structure
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Acetylcholine and other transmitters can activate multiple
receptors
Acetylcholine is released
by motor neurons at
neuromuscular junctions
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Neurotransmitters are small molecules that transmit
impulses at chemical synapses
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Influx of Ca2+ triggers release of neurotransmitters
Synaptic vesicles can be
filled, exocytosed, and
recycled within a minute
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Synaptic-vesicle and plasma-membrane proteins important
for vesicle docking and fusion
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Chemical synapses can be excitatory or inhibitory
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Ligand-gated receptor ion channels function at fast synapses
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G protein-coupled receptors function at slow synapses
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Transmitter-mediated signaling is terminated by
several mechanisms
• Following release of a neurotransmitter or neuropeptide, it must be removed or
destroyed to prevent continued stimulation of the post-synaptic cell
• To end the signaling, the transmitter may
– diffuse away from the synaptic cleft
– be taken up by the pre-synaptic neuron
– be enzymatically degraded
• Signaling by acetylcholine and neuropeptides is terminated by enzymatic
degradation
• Signaling by most classic neurotransmitters is terminated by uptake
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Impulses transmitted across chemical synapses can be
amplified and computed
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Opening of acetylcholine-gated cation channels leads to
muscle contraction
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Cardiac muscarinic acetylcholine receptors activate a G
protein that opens an ion channel
Catecholamine receptors also induce changes in second-messenger levels
that affect ion-channel activity
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A serotonin
receptor indirectly
modulates K+
channel function
by activating
adenylate cyclase
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Membrane disks in
the outer segments
of rod cells contain
rhodopsin, a lightsensitive protein
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Absorption of a photon triggers isomerization of retinal
and activation of opsin
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Cyclic GMP is a key transducing molecule in rod cells
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A thousand different G protein-coupled receptors detect odors
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Impulse transmission across electric synapses is
nearly instantaneous
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Comparison of action potential transmission across electric
and chemical synapses
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Learning and memory
• Learning is the process by which animals modify their behavior as a result of
experience or acquisition of information about the environment
• Memory is the process by which this information is stored and retrieved
– Long term memory involves the formation or elimination of certain synapses
– Short-term memory involves changes in the release and function of
neurotransmitters at specific synapses
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Study of the gill withdrawal reflex of
Aplysia has provided insight into shortterm learning processes
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This simple behavior
exhibits the most
elementary forms of
learning familiar
in vertebrates:
habituation,
sensitization, and
classical conditioning
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Facilitator neurons mediate sensitization of Aplysia
withdrawal reflex
Individuals were restrained in small
aquariums in a manner that the gill
was exposed. A tactile stimulus was
administered to the siphon and elicited
the gill and siphon withdrawal reflex.
A photocell was placed under the gill
to record amplitude and duration of
the response elicited by the stimulus.
Habituation was observed when the
stimulus was delivered repeatedly to
the siphon. Stimulus every 90 seconds
resulted in a rapidly declined
response. By delivering an electric
shock to the tail the response was
rapidly restored, dishabituation
occurred. Sensitization was observed
when a strong stimulus was
administered to the tail, this enhanced
a completely rested reflex in Aplysia
californica.
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Coincidence detectors participate
in classical conditioning and
sensitization
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