Sensory systems: Transduction

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Sensory systems Transduction

• Sensory cells are either
• 1. epithelial cells that are induced to
  specialize in performing some type of sensory
  transduction or
• 2. neurons that grow into the area where the
  stimuli can be detected.

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There are a variety of ways to categorize senses
the visceral afferents typically do not provide a
conscious sensation and yet provide information
for reflex responses.
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Some basic concepts

• NS has to answer 3 basic questions about a
  stimulus What modality? Where? How much?
• What modality? Receptors are modality-specific
• Where? Brain is organized as a map of the
  locations of individual receptive fields this
  is called somatotopic organization.
• How much? Stimulus intensity is encoded in the
  frequency of action potentials in afferent
  neurons

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Visual system Phototransduction

• Vertebrate photoreceptors rods and cones

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Cell types in the primate retina R, rods C,
conesH, horizontalA, AmacrineFMB, IMB, IDB,
RB all are kinds of bipolar cellsMG, midget
ganglion cellP, parasol cell
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Review of anatomy you may ignore the names of
the layers
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Why upside down? During development, the eye
forms as an outgrowth of the brain. The retina
(located at the back of the eyeball) is designed
so that light must pass through all the layers of
neurons before reaching photoreceptors and
finally being absorbed by the pigment epithelium.
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Anatomy of rods and cones
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Cones have folds and rods have free-floating
disks that hold the photoreceptor pigment. The
receptor cell membranes have the highest
proportion of protein to lipid of any membranes
analyzed
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Membrane responses to light Cation channels
(permeable to Na, K and Ca) are closed in
response to light, which causes membrane
hyperpolarization
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An individual cells responses to light are
graded the more light, the greater the response
of the cell, up to a limit, at which the response
capability of the cell is saturated.
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What is the link between the presence of light
and the cells response?

• The signal must travel
• 1. from the altered receptor molecule
  (rhodopsin-retinal, etc.) which captures energy
  from the photon,
• 2. through second messengers in the cytoplasm
• 3. to the outer membrane, to alter the
  open/closed state of the channels.

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The chromophore retinal (retinene) is a
derivative of Vitamin A. It is bound to the
visual systems 7 transmembrane helix receptors,
rhodopsin and the cone pigments
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The mysterious enzymes will be described later
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The cone pigments have different peak
absorbancies, with Rhodopsin in the middle, at
496nm
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Genes for the cone pigments are called S, M and L
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Molecular basis of trichromatic vision The G
Protein-coupled 7 transmembrane helix receptor
proteins have distinct sequences

• Three cone pigments must all be present to give
  normal color discrimination. If one pigment is
  defective or absent (in dichromats) it is most
  commonly a problem of red- green discrimination.
  Both of these genes are on the X chromosome, and
  the genes are very similar (L vs M). This
  explains why distinctions between red and green
  are easily lost, especially in males, whereas the
  pigment for blue is different. All three cone
  pigments are equally different from rhodopsin,
  the rod pigment.

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Hyperpolarization of rod by light the cation
channel is gated internally by cyclic GMP, which
must bind to open the channel.
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Response to light rhodopsin to transducin

• Cation channels are open as long as cyclic GMP is
  bound to them.
• Dark current (Na through cation channels) is
  turned off when cyclic GMP is converted to 5GMP
  by phosphodiesterase, which is activated by the G
  protein Transducin.

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Another view of the messages that regulate
membrane channels in the light transduction
process
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Events from previous slide

• 1. In the dark, guanyl cyclase is active,
  generating cyclic GMP. In the presence of bound
  cyclic GMP, the cation channels are open,
  admitting both Na and, to a lesser extent, Ca.
• 2. Photon changes the conformation of the
  receptor.
• 3. G protein (transducin) subunit Ga, activates
  phosphodiesterase, which catalyzes the
  degradation of cyclic GMP to 5GMP. As the level
  of cyclic GMP falls, channels close.
• 4. Recovery in the dark involves the ß? subunit
  and a neat molecule called arrestin, which binds
  to phosphorylated rhodopsin and allows the
  receptor to recover by competing with the site
  required for activation of more G proteins
  (transducin). The details of adjustment of the
  sensitivity of the system (adaptation) that
  include arrestin are more than you need to focus
  on.

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What is the effect on synaptic communication if
the photoreceptor cells hyperpolarize in light?

• Hyperpolarization alters the constitutive
  release of neurotransmitter, which leaks, more or
  less, from the receptor, depending on whether the
  cell is receiving a lot or a little light.
• Turning off a signal is as good as turning it on,
  to indicate a change to the nervous system, as
  indicated below.
• The transmitter released by the photoreceptors in
  the dark is referred to as an inhibitory
  transmitter it is glutamate.

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Glutamate is the receptor that the photoreceptors
release in the dark
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When light turns off the release of glutamate,
the next cells in the circuit, the bipolar cells,
are less hyperpolarized, i.e., relatively
depolarized, and they release transmitter that
excites the ganglion cells
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The ganglion cells are constitutively active,
firing action potentials in the dark the level
of action potential generation increases in the
light. The ganglion cell axons form the optic
nerve and their action potentials relay visual
information to higher levels of the brain. Note
that each cell type has a graded potential the
receptor potential, the synaptic potential of
bipolar cells, and the synaptic potential on
which action potentials are superimposed (a
recording like this would be made in the soma).
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Conclusions Visual transduction

• Receptor cells, rods and cones, possess visual
  pigments that are 7 transmembrane receptors that
  are distorted by reception of light energy
  (specifically when 11-cis retinal is converted to
  the all trans form). The activation of the
  associated G protein leads to changes in the
  concentration of cyclic GMP, the ligand for the
  cation channels that are open in the dark. The
  phosphodiesterase that is activated by the
  subunit Ga breaks down the cyclic GMP and so
  channels that lose their ligand will close. The
  ß? subunit is involved in the recovery process.
  The (inhibitory) signal relayed to the
  postsynaptic cell by the receptor is off in the
  light and on in the dark. Rebound from
  inhibition allows the bipolar cells to release
  transmitter, which excites the ganglion cells,
  the first cells in the pathway to generate action
  potentials.