Transcript book ppt - Castle High School

35
Sensors
Concept 35.1 Sensory Systems Convert Stimuli into Action
Potentials
Sensory receptor cells, or sensors or receptors,
transduce physical and chemical stimuli into a
change in membrane potential.
The change in membrane potential may generate
an action potential that conveys the sensory
information to the CNS for processing.
Sensory transduction—begins with a receptor
protein that can detect a specific stimulus.
The receptor protein opens or closes ion channels
in the membrane, changing the resting potential.
Concept 35.1 Sensory Systems Convert Stimuli into Action
Potentials
Receptor potentials—graded membrane
potentials that travel a short distance.
Receptor potentials can generate action
potentials in two ways:
• Can generate action potentials in the
receptor cell
• Can trigger release of neurotransmitter so
that a postsynaptic neuron generates an
action potential
Concept 35.1 Sensory Systems Convert Stimuli into Action
Potentials
Stretch receptors in crayfish cause receptor
potentials when the attached muscle is
stretched.
Receptor potentials spread to the base of
the axon and generate action potentials.
The rate of firing depends on the magnitude
of the receptor potential, which depends
on the amount of stretching.
Figure 35.1 Stimulating a Sensory Cell Produces a Receptor Potential
Concept 35.1 Sensory Systems Convert Stimuli into Action
Potentials
Different sensory receptors respond to
particular stimuli:
• Mechanoreceptors detect physical forces
such as pressure (touch) and variations in
pressure (sound waves).
• Thermoreceptors respond to temperature.
• Electrosensors are sensitive to changes in
membrane potential.
Concept 35.1 Sensory Systems Convert Stimuli into Action
Potentials
• Chemoreceptors respond to the presence
or absence of certain chemicals.
• Photoreceptors detect light.
Some sensory receptor cells are organized
with other cells in sensory organs, such as
eyes and ears.
Sensory systems include sensory cells,
associated structures, and neural networks
that process the information.
Figure 35.2 Sensory Receptor Proteins Respond to Stimuli by Opening or Closing Ion Channels
Concept 35.1 Sensory Systems Convert Stimuli into Action
Potentials
Sensation depends on which part of the
CNS receives the sensory messages.
Intensity of sensation is coded as the
frequency of action potentials.
Some sensory cells transmit information to
the brain about internal conditions, without
conscious sensation.
Concept 35.1 Sensory Systems Convert Stimuli into Action
Potentials
Adaptation—diminishing response to
repeated stimulation.
Enables animals to ignore background
conditions but remain sensitive to
changing or new stimuli.
Some sensory cells don’t adapt (e.g.,
mechanoreceptors for balance).
Concept 35.2 Chemoreceptors Detect Specific Molecules or Ions
Chemoreceptors—receptor proteins that
bind to various molecules, responsible for
taste and smell.
Also monitor internal environment, such as
CO2 levels in blood.
Olfaction—sense of smell; depends on
chemoreceptive neurons embedded in
epithelial tissue at top of nasal cavity (in
vertebrates).
Figure 35.3 Olfactory Receptors Communicate Directly with the Brain (Part 1)
Figure 35.3 Olfactory Receptors Communicate Directly with the Brain (Part 2)
Concept 35.2 Chemoreceptors Detect Specific Molecules or Ions
Axons from olfactory sensors extend to the
olfactory bulb in the brain—dendrites end
in olfactory hairs on the nasal epithelium.
Odorant—a molecule that activates an
olfactory receptor protein
Odorants bind to receptor proteins on the
olfactory cilia.
Olfactory receptor proteins are specific for
particular odorants.
Concept 35.2 Chemoreceptors Detect Specific Molecules or Ions
When an odorant binds to a receptor
protein, it activates a G protein, which
activates a second messenger (cAMP).
The second messenger causes an influx of
Na+ and depolarizes the olfactory neuron.
Many more odorants can be discriminated
than there are olfactory receptors.
In the olfactory bulb, axons from neurons
with the same receptors converge on
glomeruli.
Concept 35.2 Chemoreceptors Detect Specific Molecules or Ions
Pheromones—chemical signals used by
insects to attract mates.
Example: Female silkworm moth releases
bombykol. Male has receptors for
bombykol on the antennae.
One molecule of bombykol is enough to
generate action potentials.
Figure 35.4 Some Scents Travel Great Distances (Part 1)
Figure 35.4 Some Scents Travel Great Distances (Part 2)
Concept 35.2 Chemoreceptors Detect Specific Molecules or Ions
Vomeronasal organ (VNO) is found in
many vertebrates—specialized for
pheromones
It is a paired tubular structure embedded in
the nasal epithelium.
When animal sniffs, the VNO draws a
sample of fluid over chemoreceptors in
walls.
Information goes to an accessory olfactory
bulb and on to other brain regions.
Concept 35.2 Chemoreceptors Detect Specific Molecules or Ions
Gustation is the sense of taste.
Taste buds—clusters of chemoreceptors.
Some fish have taste buds on the skin; the
duck-billed platypus has taste buds on its
bill.
Human taste buds are embedded in the
tongue epithelium, on papillae. The
sensory cells generate action potentials
when they detect certain chemicals.
Figure 35.5 Taste Buds Are Clusters of Sensory Cells (Part 1)
Figure 35.5 Taste Buds Are Clusters of Sensory Cells (Part 2)
Concept 35.2 Chemoreceptors Detect Specific Molecules or Ions
Humans taste salty, sour, sweet, bitter, and
umami—a savory, meaty taste.
“Salty” receptors respond to Na+
depolarizing the cell.
“Sour” receptors detect acidity as H+, and
“sweet” receptors bind different sugars.
Umami receptors detect the presence of
amino acids, as in MSG.
Bitterness is more complicated and involves
at least 30 different receptors.
Concept 35.3 Mechanoreceptors Detect Physical Forces
Mechanoreceptors are cells that detect
physical forces.
Distortion of their membrane causes ion
channels to open and a receptor potential
to occur.
This may lead to the release of a
neurotransmitter.
Concept 35.3 Mechanoreceptors Detect Physical Forces
The skin has diverse mechanoreceptors:
• Free nerve endings detect heat, cold, and
pain.
• Merkel’s discs: Adapt slowly, give
continuous information.
• Meissner’s corpuscles: Adapt quickly, give
information about change.
Concept 35.3 Mechanoreceptors Detect Physical Forces
• Ruffini endings: Deep, adapt slowly, react
to vibrating stimuli of low frequencies.
• Pacinian corpuscles: Deep, adapt rapidly,
react to vibrating stimuli at high
frequencies.
Figure 35.6 The Skin Feels Many Sensations
Concept 35.3 Mechanoreceptors Detect Physical Forces
Muscle spindles: Mechanoreceptors in
muscle cells, called stretch receptors.
When muscle is stretched, action potentials
are generated in neurons.
CNS adjusts strength of contraction to
match load on muscle.
Concept 35.3 Mechanoreceptors Detect Physical Forces
Golgi tendon organ: Another
mechanoreceptor, in tendons and
ligaments.
Provides information about the force
generated by muscle; prevents muscle
tearing.
Figure 35.7 Stretch Receptors (Part 1)
Figure 35.7 Stretch Receptors (Part 2)
Concept 35.3 Mechanoreceptors Detect Physical Forces
Hair cells—mechanoreceptors in organs of
hearing and equilibrium.
Hair cells have projections called
stereocilia that bend in response to
pressure.
Bending of stereocilia can depolarize or
hyperpolarize the membrane.
Figure 35.8 Hair Cells Have Mechanosensors on Their Stereocilia (Part 1)
Figure 35.8 Hair Cells Have Mechanosensors on Their Stereocilia (Part 2)
Concept 35.3 Mechanoreceptors Detect Physical Forces
Auditory systems use hair cells to convert
pressure waves to receptor potentials.
Outer ear:
Pinnae collect sound waves and direct them
to the auditory canal.
The tympanic membrane covers the end of
the auditory canal and vibrates in
response to pressure waves.
Figure 35.9 Structures of the Human Ear (Part 1)
Concept 35.3 Mechanoreceptors Detect Physical Forces
Middle ear—air filled cavity:
Open to the throat via the eustachian tube.
Eustachian tubes equilibrate air pressure
between the middle ear and the outside.
Ossicles—malleus, incus, stapes—
transmit vibrations of tympanic membrane
to the oval window.
Figure 35.9 Structures of the Human Ear (Part 2)
Concept 35.3 Mechanoreceptors Detect Physical Forces
Inner ear has two sets of canals—the
vestibular system for balance and the
cochlea for hearing.
The cochlea is a tapered and coiled
chamber composed of three parallel
canals separated by Reissner’s
membrane and the basilar membrane.
Figure 35.9 Structures of the Human Ear (Part 3)
Concept 35.3 Mechanoreceptors Detect Physical Forces
The organ of Corti sits on the basilar
membrane—transduces pressure waves
into action potentials.
Contains hair cells with stereocilia—tips are
embedded in the tectorial membrane.
Hair cells bend and create a graded
potential that can alter neurotransmitter
release.
Concept 35.3 Mechanoreceptors Detect Physical Forces
Upper and lower canals of the cochlea are
joined at distal end.
The round window is a flexible membrane
at the end of the canal.
Traveling pressure waves of different
frequencies will produce flexion of the
basilar membrane.
Concept 35.3 Mechanoreceptors Detect Physical Forces
Different pitches, or frequency of vibration,
flex the basilar membrane at different
locations.
Action potentials stimulated by
mechanoreceptors at different positions
along the organ of Corti are transmitted to
regions of the auditory cortex via the
auditory nerve.
Figure 35.10 Sensing Pressure Waves in the Inner Ear
Concept 35.3 Mechanoreceptors Detect Physical Forces
Conduction deafness: Loss of function of
tympanic membrane or ossicles.
Nerve deafness: Damage to inner ear or
auditory nerve pathways.
Hair cells in the organ of Corti can be
damaged by loud sounds. This damage is
cumulative and irreversible.
Concept 35.3 Mechanoreceptors Detect Physical Forces
The vestibular system in the mammalian
inner ear has three semicircular canals
at angles to each other, and two
chambers—the saccule and the utricle.
Hair cells sense position and orientation of
head by shifting of endolymph.
Cupulae in canals contain hair cell
stereocilia—otoliths in membrane exert
pressure and bend stereocilia.
Figure 35.11 Organs of Equilibrium (Part 1)
Figure 35.11 Organs of Equilibrium (Part 2)
Figure 35.11 Organs of Equilibrium (Part 3)
Concept 35.4 Photoreceptors Detect Light
Photosensitivity—sensitivity to light
A range of animal species from simple to
complex can sense and respond to light.
All use same pigments—rhodopsins.
Concept 35.4 Photoreceptors Detect Light
Rhodopsin molecule consists of opsin (a
protein) and a light-absorbing group, 11cis-retinal.
Rhodopsin molecule sits in plasma
membrane of a photoreceptor cell.
11-cis-retinal absorbs photons of light and
changes to the isomer all-trans-retinal—
changes the conformation of opsin.
Concept 35.4 Photoreceptors Detect Light
In vertebrate eyes, the retinal and opsin
eventually separate, called bleaching.
A series of enzymatic reactions is required
to return all-trans-retinal back to 11-cisretinal, which recombines with opsin to
become photosensitive rhodopsin again.
Figure 35.12 Light Changes the Conformation of Rhodopsin
Concept 35.4 Photoreceptors Detect Light
Rod cells are modified neurons with:
• An outer segment with discs of plasma
membrane containing rhodopsin to capture
photons
• An inner segment that contains the
nucleus and organelles
• A synaptic terminal where the rod cell
communicates with other neurons
Figure 35.13 A Rod Cell Responds to Light (Part 1)
Figure 35.13 A Rod Cell Responds to Light (Part 2)
Concept 35.4 Photoreceptors Detect Light
Stimulation of rod cells by light makes the
membrane potential more negative
(hyperpolarized)—the opposite of other
sensory cells responding to their stimuli.
The dark current is a flow of Na+ ions that
continually enters the rod cell in the dark.
Rod cell is depolarized and releases
neurotransmitter continually.
Hyperpolarizing effect of light decreases
neurotransmitter release.
Concept 35.4 Photoreceptors Detect Light
When rhodopsin absorbs a photon of light, a
cascade of events begins, starting with the
activation of a G protein, transducin.
Transducin activates PDE which converts
cGMP to GMP—the Na+ channels close,
and the membrane is hyperpolarized.
Figure 35.14 Light Absorption Closes Sodium Channels
Concept 35.4 Photoreceptors Detect Light
Rhodopsin in a variety of visual systems:
Flatworms—photoreceptor cells in paired
eye cups.
Arthropods—compound eyes. Each eye
consists of units called ommatidia.
Each ommatidium has a lens to focus light
onto photoreceptor cells.
Figure 35.15 Ommatidia: The Functional Units of Insect Eyes (Part 1)
Figure 35.15 Ommatidia: The Functional Units of Insect Eyes (Part 2)
Concept 35.4 Photoreceptors Detect Light
Vertebrates have image-forming eyes—
bounded by sclera, connective tissue that
becomes transparent cornea on front of
eye.
Iris (pigmented)—controls amount of light
reaching photoreceptors; opening—pupil.
Lens—crystalline protein, focuses image,
allows accommodation, can change
shape.
Retina—photosensitive layer, back of eye.
Figure 35.16 The Human Eye (Part 1)
Concept 35.4 Photoreceptors Detect Light
The retina has five layers of neurons
including photoreceptors (rods and cones)
at the back.
Photoreceptors send information to bipolar
cells, which send information to the
ganglion cell layer.
Axons from ganglion cells conduct
information to the brain.
Figure 35.16 The Human Eye (Part 2)
Concept 35.4 Photoreceptors Detect Light
Two other cell types communicate laterally
across the retina:
Horizontal cells form synapses with bipolar
cells and photoreceptors.
Amacrine cells form local synapses with
bipolar cells and ganglion cells.
Ultimately, all information converges on
ganglion cells.
Concept 35.4 Photoreceptors Detect Light
A receptive field—a group of
photoreceptors that receive information
from a small area of the visual field and
activate one ganglion cell.
The receptive field of a ganglion cell results
from a pattern of synapses between
photoreceptors, bipolar cells and lateral
connections.
Concept 35.4 Photoreceptors Detect Light
Receptive fields have two concentric
regions, a center and a surround.
A field can be either on- or off-center.
Light falling on an on-center receptive field
excites the ganglion cell, while light falling
on an off-center receptive field inhibits the
ganglion cell.
The surround area has the opposite effect
so ganglion cell activity depends on which
part of the field is stimulated.
Concept 35.4 Photoreceptors Detect Light
Neurons of the visual cortex, like retinal
ganglion cells, have receptive fields.
Cortical neurons are stimulated by bars of
light in a particular orientation,
corresponding to rows of circular receptive
fields of ganglion cells.
The brain assembles a mental image of the
world by analyzing the edges in patterns of
light and dark.
Concept 35.4 Photoreceptors Detect Light
Vertebrate photoreceptors consist of rod
cells and cone cells.
Rod cells are responsible for night vision;
cone cells are responsible for color vision.
Fovea—area where cone cell density is
highest.
Figure 35.17 Rods and Cones (Part 1)
Concept 35.4 Photoreceptors Detect Light
Humans have three types of cone cells with
slightly different opsin molecules—they
absorb different wavelengths of light.
This allows the brain to interpret input from
the different cones as a full range of color.
Color blindness is the loss of function of a
type of cone cell—the result of a
nonfunctional gene.
Figure 35.17 Rods and Cones (Part 2)
Answer to Opening Question
All of these animals make use of other senses
besides vision to perceive their surroundings in
the dark.
Information is also conveyed through tactile
stimuli, olfaction, heat-detection, and auditory
input.