Sensory Systems

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Sensory Systems
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Sensory Systems

• Sensory Cells Transduction of Stimuli into
  signals for nervous system. Modified neurons
• 1. Chemoreceptors Responding to Specific
  Molecules
• 2. Mechanoreceptors Detecting Stimuli that
  Distort Membranes
• 3. Photoreceptors and Visual Systems Responding
  to Light

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Sensory Cells and Transduction of Stimuli

• Most sensory cells have
• membrane receptor proteins that detect a stimulus
  and respond by altering the flow of ions across
  the plasma membrane.
• The resulting change in membrane potential causes
  the sensory cell to fire action potentials or to
  change its secretion of a neurotransmitter onto
  an associated neuron that fires action
  potentials.
• The intensity of the stimulus is encoded in the
  frequency of the action potentials produced.

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Figure 45.1 Sensory Cell Membrane Receptor
Proteins Respond to Stimuli
1. Mechanoreceptor
2. Chemoreceptor
3. Photoreceptor
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Sensory Cells and Transduction of Stimuli

• Although they are simply depolarization events,
  sensory data are interpreted in different ways
  according to the different places in the CNS
  where messages from different kinds of sensory
  cells arrive.
• e.g. a small patch of skin
• various sensory cells for heat, pressure,
  movement, and tissue damage (pain).
• Interpretation of a stimulus Which sensation??
• which cells of the central nervous system
  receive the signal?

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Sensory Cells and Transduction of Stimuli

• Some information is sensed without our being
  conscious of it.
• levels of CO2, blood sugar, and O2. important
  for the maintenance of homeostasis.
• Sensory cells and other types of cells form
  sensory organs, such as eyes, ears, and noses.
• Sensory systems
• the sensory cells the associated structures
  neuronal networks that process the
  information.

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Sensory Cells and Transduction of Stimuli

• Sensory cells transduce the energy from a
  stimulus into action potentials.
• The first step is activation of a receptor
  protein in the plasma membrane of a sensory cell
  by a stimulus.
• The activated protein opens or closes ion
  channels.

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Sensory Cells and Transduction of Stimuli

• In ionotropic sensory detection, the receptor
  protein itself is part of the ion channel and, by
  changing its conformation, opens or closes the
  channel pore.
• In metabotropic sensory detection, the receptor
  protein is linked to a G protein that activates a
  cascade of intracellular events that eventually
  open or close ion channels.

• The affected receptor ? action potential ?
  nervous system.
• stimulus ? change in the resting membrane
  potential of a sensory cell receptor potential

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Figure 45.2 Stimulating a Sensory Cell Produces
a Receptor Potential
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Sensory Cells and Transduction of Stimuli

• Primary sensory cells generate action potentials
  directly. An example is the crayfish stretch
  receptor.
• Secondary sensory cells generate action
  potentials indirectly by inducing the release of
  neurotransmitter.

• Some sensory cells respond less when stimulation
  is repeated, a phenomenon called adaptation.
  NOT Darwins

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ChemoreceptorsResponding to Specific Molecules

• Chemoreceptors
• detect chemical stimuli.
• Chemoreceptors are responsible for smell and
  taste, and for monitoring internal environmental
  factors such as CO2 and O2 in the blood.
• Corals, for example, can detect protein or even a
  single type of amino acid, causing them to extend
  tentacles in search of food.

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Figure 45.3 Some Scents Travel Great Distances
(Part 1)

• Arthropods use chemical signals called pheromones
  to attract mates.
• Female silkworm moths release a pheromone from
  glands at the tip of the abdomen, and males have
  receptors for bombykol on their antennae.
• A single molecule of the pheromone can stimulate
  a perceivable action potential. 200 hairs or more
  per second are activated, the male flies upwind
  in search of the female.

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ChemoreceptorsResponding to Specific Molecules

• Chemoreceptor Olfaction.
• In vertebrates, olfactory sensors are neurons
  embedded in a layer of epithelial cells at the
  top of the nasal cavity.
• The axons of these sensors project to the
  olfactory bulb of the brain.
• The dendrites end in olfactory hairs at the
  surface of the nasal epithelium.
• Molecules from the environment diffuse through
  nasal mucus to reach the surface of the olfactory
  hairs.

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Figure 45.4 Olfactory Receptors Communicate
Directly with the Brain (Part 1)
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Figure 45.4 Olfactory Receptors Communicate
Directly with the Brain (Part 2)
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ChemoreceptorsResponding to Specific Molecules

• Odorants are chemicals that bind to olfactory
  receptor proteins.
• Each olfactory receptor protein binds particular
  odorant molecules, which activates a G protein.
• The G protein then activates an enzyme that
  increases levels of a second messenger, such as
  cAMP.
• The second messenger binds to sodium channels in
  the plasma membrane and opens them. The influx of
  Na depolarizes the membrane and an action
  potential is fired.

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ChemoreceptorsResponding to Specific Molecules

• The number of odorant molecules greatly exceeds
  the number of different receptor proteins.
• Each odorant may bind to one or more specific
  receptor proteins.
• A specific odorant is distinguished according to
  the different and unique combination of cells it
  activates.
• The strength of the odor depends on the number of
  odorant molecules detected.
• More odorant molecules produce more action
  potentials per unit of time and are perceived as
  stronger odors.

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ChemoreceptorsResponding to Specific Molecules

• Vomeronasal organ (VNO) is
• a small, paired tubular structure embedded in the
  nasal epithelium.
• The VNO has a pore opening into the nasal cavity
  when an animal sniffs it draws a sample of nasal
  fluid over the chemoreceptors of the VNO.
• The information from the VNO chemoreceptors goes
  to an accessory olfactory bulb in the brain.
• From the olfactory bulb, information is routed to
  regions of the brain involved in sexual and other
  instinctive behaviors.

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ChemoreceptorsResponding to Specific Molecules

• Experiments with mice have confirmed that the VNO
  detects pheromones.
• In snakes, the VNO opens into the mouth cavity.
  The snakes forked tongue fits into the VNO and
  molecules collected from the air contact the
  chemoreceptors in the VNO.
• The snake uses its tongue to smell its
  environment.

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ChemoreceptorsResponding to Specific Molecules

• Gustation, the sense of taste, depends on
  clusters of sensory cells called taste buds.
• Humans have 10,000 taste buds embedded in the
  epithelium of the tongue.
• Many are in raised papillae, the small bumps on
  human tongues.
• The outer surface of each bud has a pore that
  exposes the tips of sensory cells. Microvilli
  increase the surface area of the cells.
• The sensory cells form synapses with dendrites of
  sensory neurons.

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Figure 45.5 Taste Buds Are Clusters of Sensory
Cells
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ChemoreceptorsResponding to Specific Molecules

• Receptor proteins in the microvilli bind specific
  molecules. This causes the release of
  neurotransmitters to the dendrites of associated
  sensory neurons.
• Taste buds are replaced every few days, but the
  associated neurons live on.
• Taste buds can distinguish sweet, salty, sour,
  and bitter tastes.
• Recently the savory meaty taste umami has been
  added to the list of distinguishable tastes.

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MechanoreceptorsDetecting Stimuli that Distort
Membranes

• Mechanoreceptors
• sensitive to mechanical forces
• skin sensations and sensing blood pressure.
• Physical distortion of a mechanoreceptors plasma
  membrane causes ion channels to open, which leads
  to the generation of action potentials.
• The rate of the action potentials is related to
  the strength of the stimulus.

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Figure 45.6 The Skin Feels Many Sensations
Skin - diverse mechanoreceptors
Non-hairy skin
Low frequency
Higher frequency
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MechanoreceptorsDetecting Stimuli that Distort
Membranes

• Density of tactile mechanoreceptors influences
  how finely stimulation can be resolved.
• On the back, two stimuli must be fairly far apart
  before they can be resolved.
• On fingertips, finer spatial discrimination is
  possible because mechanoreceptors are much more
  dense.

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MechanoreceptorsDetecting Stimuli that Distort
Membranes

• Stretch receptors provide an animal with
  information about the position of its limbs and
  the stresses on its muscles and joints. They feed
  information continuously to the CNS.
• Stretch receptors embedded in connective tissues
  in skeletal muscle are called muscle spindles.
• They are modified muscle fibers that are
  innervated in the center with extensions of
  sensory neurons.
• The CNS uses information from muscle spindles to
  maintain muscle tone.

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Figure 45.7 Stretch Receptors Are Activated when
Limbs Are Stretched (Part 1)
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MechanoreceptorsDetecting Stimuli that Distort
Membranes

• The Golgi tendon organ is a stretch receptor
  found in tendons and ligaments.
• When a muscle contraction becomes too forceful,
  the Golgi tendon organ sends signals to the CNS
  that inhibits motor neurons and the muscle
  relaxes.
• This prevents muscle damage by limiting the force
  of contracting muscles when excessive force could
  injure connective tissue.

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Figure 45.7 Stretch Receptors Are Activated when
Limbs Are Stretched (Part 2)
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MechanoreceptorsDetecting Stimuli that Distort
Membranes

• Hair cells are also mechanoreceptors.
• Each hair cell has a set of stereocilia
  (microvilli).
• When the stereocilia are bent in one direction,
  receptor potential becomes more negative when
  they are bent in the other direction, it becomes
  more positive.
• When the membrane potential becomes more
  positive, the hair cell releases a
  neurotransmitter to the sensory neuron associated
  with it, and the sensory neuron sends action
  potentials to the CNS.

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MechanoreceptorsDetecting Stimuli that Distort
Membranes

• Hair cells are found in the lateral line system
  of fishes, providing information about movement
  through the water and moving objects that cause
  pressure waves in water.
• Vertebrate organs of equilibrium use hair cells
  to detect the position of the body with respect
  to gravity.
• Semicircular canals and the vestibular apparatus
  in the mammalian inner ear use hair cells to
  detect position and orientation of the head, as
  well as acceleration produced by movement.

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Figure 45.8 The Lateral Line System Contains
Mechanoreceptors
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Figure 45.9 Organs in the Inner Ear of Mammals
Provide the Sense of Equilibrium (Part 1)
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Figure 45.9 Organs in the Inner Ear of Mammals
Provide the Sense of Equilibrium (Part 2)
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MechanoreceptorsDetecting Stimuli that Distort
Membranes

• Auditory systems use mechanoreceptors to convert
  pressure waves into action potentials.
• Pinnae collect sound waves and direct them into
  the auditory canal, which leads to the middle
  inner ear.
• The eardrum (tympanic membrane) covers the end of
  the auditory canal and vibrates in response to
  pressure waves. On the other side is the
  fluid-filled middle ear.
• Pressure on both sides of the eardrum
  equilibrates because the Eustachian tube allows
  airflow.

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Figure 45.10 Structures of the Human Ear (Part 1)
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MechanoreceptorsDetecting Stimuli that Distort
Membranes

• Three delicate bones in the middle ear called the
  ear ossicles (the malleus, incus, and stapes)
  transfer the vibrations of the eardrum to the
  oval window.
• Behind the oval window is the fluid-filled inner
  ear. Movements of the oval window result in
  pressure changes in the inner ear.
• The inner ear is a long, tapered, coiled chamber
  called the cochlea, composed of three parallel
  canals separated by two membranes, Reissners
  membrane and the basilar membrane.

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Figure 45.10 Structures of the Human Ear (Part 2)
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MechanoreceptorsDetecting Stimuli that Distort
Membranes

• The organ of Corti rests on the basilar membrane.
• The organ of Corti actually transduces pressure
  waves into action potentials in the auditory
  nerve.
• The organ of Corti contains hair cells whose
  stereocilia are in contact with the tectorial
  membrane.
• When the basilar membrane flexes, the tectorial
  membrane bends the hair cell stereocilia.

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Figure 45.10 Structures of the Human Ear (Part 3)
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MechanoreceptorsDetecting Stimuli that Distort
Membranes

• What causes the basilar membrane to flex?
• The cochlea is filled with fluid and the upper
  and lower canals are connected at the distal end.
  Pressure waves displace the fluid in the upper
  canal of the cochlea.
• Instead of traveling all the way around the
  canals, the waves of fluid cross the basilar
  membrane, causing it to flex.
• High frequency causes the basilar membrane
  nearest the oval window to flex.
• Low frequency causes flexing farther down the
  membrane.

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Figure 45.11 Sensing Pressure Waves in the Inner
Ear (Part 1)
22,000 Hz
3,000 Hz
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MechanoreceptorsDetecting Stimuli that Distort
Membranes

• Deafness has two general causes
• Conduction deafness is loss of function of the
  tympanic membrane or ossicles of the middle ear.
  The ossicles stiffen with age causing loss of
  ability to hear high frequency sound.
• Nerve deafness is caused by inner ear or auditory
  pathway damage, including damage to hair cells.
• Rock music and other loud noises can cause damage
  to hair cells. This damage is cumulative and
  permanent.

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Photoreceptors and Visual Systems Responding to
Light

• Photosensitivity
• the sensitivity to light.
• It ranges from the ability to orient to the sun
  to the ability to see.
• Evolution has conserved molecules used for
  photosensitivity across the entire range of
  animal species. These are a family of pigments
  called rhodopsins.

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Photoreceptors and Visual Systems Responding to
Light

• Rhodopsin molecules can absorb photons of light
  and undergo shape changes.
• Rhodopsin molecules consist of a protein called
  opsin and a light-absorbing group,
  11-cis-retinal.
• The retinal group is in the center of the opsin,
  and the entire complex is within the plasma
  membrane of a photoreceptor cell.
• When 11-cis-retinal absorbs a photon, it changes
  to all-trans-retinal, which changes the
  conformation of the opsin. This change signals
  detection of light.

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Figure 45.12 Rhodopsin A Photosensitive Molecule
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Photoreceptors and Visual Systems Responding to
Light

• The all-trans form of retinal and opsin complex
  passes through several intermediate stages.
• One stage, known as photoexcited rhodopsin,
  triggers a cascade that results in alteration of
  membrane potential of a neuron.

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Photoreceptors and Visual Systems Responding to
Light

• A rod cell is a modified neuron. It releases
  neurotransmitters that influences other neurons.
• Rod cells have an outer segment, an inner
  segment, and a synaptic terminal.
• The inner segment has the nucleus and many
  mitochondria.
• The outer segment has a stack of discs of plasma
  membrane densely packed with rhodopsin. The discs
  function to capture photons.

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Figure 45.13 A Rod Cell Responds to Light
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Photoreceptors and Visual Systems Responding to
Light

• When a rod cell is in the dark, it has a
  depolarized resting potential. Na ions can
  continually enter the outer segment.
• When light flashes on the rod cell, the outer
  segment becomes more negative, or hyperpolarized.
• When light is absorbed by rhodopsin, it becomes
  photoexcited and activates a G protein called
  transducin.
• The activated transducin activates a
  phosphodiesterase, which converts cGMP to GMP.
• cGMP keeps sodium channels open in light, GMP
  levels rise and channels close.

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Figure 45.14 Light Absorption Closes Sodium
Channels (Part 1)
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Figure 45.14 Light Absorption Closes Sodium
Channels (Part 2)
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Photoreceptors and Visual Systems Responding to
Light

• The advantage of this system is that it amplifies
  the signal.
• Each single photon can cause activation of
  several hundred transducin molecules, which in
  turn, activate many phosphodiesterase molecules.
• A single photon can close a huge number of sodium
  channels.

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Photoreceptors and Visual Systems Responding to
Light

• Invertebrates have a variety of visual systems.
• Flatworms obtain directional information from
  photoreceptors that are organized into paired eye
  cups, shielded by layers of pigmented cells.
• Because of the shielding, photoreceptors on the
  two sides of the animal are unequally stimulated
  unless the animal is facing directly toward or
  away from the light.

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Photoreceptors and Visual Systems Responding to
Light

• Arthropods have compound eyes consisting of many
  optical units called ommatidia.
• Each ommatidium has a lens that directs light
  onto photoreceptor cells (retinula cells). These
  cells have microvilli with rhodopsin, and their
  axons communicate with the nervous system.
• Each ommatidium gives a slightly different view,
  resulting in broken-up images.
• The number of ommatidia in an eye varies, from a
  few in certain ants to 10,000 in dragonflies.

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Figure 45.15 Ommatidia The Functional Units of
Insect Eyes
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Photoreceptors and Visual Systems Responding to
Light

• Both vertebrates and cephalopod mollusks have
  highly evolved eyes.
• Vertebrate eyes are fluid-filled spheres bound by
  tough connective tissue called sclera.
• A transparent cornea in the front allows light
  passage.
• Inside the cornea is the pigmented iris, which
  controls the amount of light that can enter.
• The pupil is the region where light enters.
• The lens makes fine adjustments in the focus of
  images on the photosensitive retina at the back
  of the eye.

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Figure 45.16 Eyes Like Cameras
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Photoreceptors and Visual Systems Responding to
Light

• The most sensitive area of the retina is the
  fovea.
• The lenses allow the eyes to focus light.
• Fishes, amphibians, and reptiles focus by moving
  the lenses of their eyes closer to or farther
  from their retinas.
• Mammals and birds alter the shape of the lens to
  focus.

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Figure 45.17 Staying in Focus
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Photoreceptors and Visual Systems Responding to
Light

• The shape of the lens changes due to the action
  of two structures.
• Connective tissue surrounding the lens keeps it
  spherical, but suspensory ligaments pull it into
  a flatter shape.
• Ciliary muscles counteract the pull of the
  ligaments and allow the lens to become round.
• The flatter lens is able to focus distant images
  but not nearer ones, which need the light-bending
  properties of the round lens to bring close
  images into focus.
• Lenses become less elastic with age and we lose
  the ability to focus on objects close at hand.

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Photoreceptors and Visual Systems Responding to
Light

• The retina includes layers of cells that process
  visual information from the photoreceptors and
  produce an output signal that is transmitted via
  the optic nerve.
• Light must pass through all the layers of cells
  before photons are captured by rhodopsin.
• There are two types of vertebrate photoreceptors
  cones and rods.
• Rod cells are more sensitive to light. Cone cells
  respond to different wavelengths of light for
  color vision.
• Cones also provide the sharpest vision. The fovea
  has only cone cells.

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Photoreceptors and Visual Systems Responding to
Light

• Humans have three kinds of cone cells One type
  absorbs violet and blue wavelengths, one absorbs
  green, and one absorbs yellow and red.
• The human fovea has about 160,000 cone cells per
  square millimeter a hawk has 1,000,000.
• Hawks also have two foveas per eye and can see
  both their flight path and the ground below.
• There are no photoreceptors where blood vessels
  and bundles of axons going to the brain pass
  through the back of the eye. This creates a blind
  spot on the retina.

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Figure 45.19 Absorption Spectra of Cone Cells
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Photoreceptors and Visual Systems Responding to
Light

• The human retina is organized into five layers of
  cells.
• Cells at the front of the retina are ganglion
  cells. They fire action potentials and their
  axons form the optic nerves.
• The photoreceptor cells are at the back of the
  retina. Ganglion cells and photoreceptors are
  connected by bipolar cells.
• Photoreceptor cells ? bipolar cells ? ganglion
  cells

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Figure 45.20 The Retina
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Photoreceptors and Visual Systems Responding to
Light

• Horizontal cells connect neighboring pairs of
  photoreceptors and bipolar cells.
• This provides a means for the lateral flow of
  information.
• Amacrine cells connect neighboring pairs of
  bipolar cells and ganglion cells.
• These help make eyes more sensitive to small but
  rapid changes.

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Photoreceptors and Visual Systems Responding to
Light

• Each ganglion cell has a well-defined receptive
  field, which consists of a specific group of
  photoreceptor cells.
• This integrates the light signal into one output.
• The receptive field of a ganglion cell can be
  divided into two concentric areas, called the
  center and the surround.

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Photoreceptors and Visual Systems Responding to
Light

• There are two kinds of receptive fields
  on-center and off-center.
• Ganglia with on-center receptive fields are
  maximally excited by light falling on the center.
• Ganglia with off-center receptive fields are
  maximally stimulated by light falling on the
  surround.
• Center effects are always stronger than surround
  effects.
• The photoreceptors in the center of the receptive
  field of a ganglion cell are connected to that
  ganglion via bipolar cells.

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Figure 45.21 What Does the Eye Tell the Brain?
(Part 1)
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Figure 45.21 What Does the Eye Tell the Brain?
(Part 2)
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Sensory Worlds Beyond Human Experience
Other animals?

• Some species can see infrared and ultraviolet
  light.
• One of the seven photoreceptors in each
  ommatidium of a fruit fly is sensitive to
  ultraviolet light.
• Some flowers have patterns that are invisible to
  humans but can be seen by flies.
• Pit vipers have pit organs, one in front of each
  eye, which can sense and locate infrared
  radiation in total darkness.

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Sensory Worlds Beyond Human Experience
Other animals?

• Elephants can communicate with infrasound, sounds
  below the range of human hearing.
• The advantage of using low frequency sound to
  communicate is that it carries over very long
  distances.

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Sensory Worlds Beyond Human Experience
Other animals?

• Echolocation is sensing the world through
  reflected sound.
• Dolphins, bats, and whales can use noises to
  echolocate.
• They generate sounds at frequencies above human
  hearing.
• These animals use muscles in the middle ear to
  dampen their sensitivity to sound while they are
  emitting sounds in order to protect their
  hearing.
• To hear the returning echoes, they relax the
  muscles.

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Sensory Worlds Beyond Human Experience
Other animals?

• Some fish can sense electric fields.
• Lateral lines of some species, such as catfish,
  contain electroreceptors.
• These enable the fish to detect weak electric
  fields, which helps them locate prey.
• Some fishes, such as electric fish, can use
  electric fields to navigate. Rocks, plants, and
  other structures disrupt their field and are
  interpreted.

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Sensory Systems

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