Transcript Sensory Systems

David Sadava H. Craig Heller Gordon H. Orians
William K. Purves David M. Hillis
Biologia.blu
C – Il corpo umano
Sensory Systems
Sensory Systems
• How do sensory cells convert stimuli
into action potentials?
• How do sensory systems detect
chemical stimuli?
• How do sensory systems detect
mechanical forces?
• How do sensory systems detect
light?
Sensory Systems - How do sensory cells convert stimuli into action potentials?
Sensory cells transduce physical and
chemical stimuli into neuronal signals.
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.
Sensory Systems - How do sensory cells convert stimuli into action potentials?
Sensory receptor proteins can be:
• ionotropic - ion channels themselves or
directly affect ion channels
(mechanoreceptors, thermoreceptors,
electrosensors);
• metabotropic - affect ion channels
through G proteins and second
messengers (chemoreceptors,
photoreceptors).
Sensory Systems - How do sensory cells convert stimuli into action potentials?
Sensory receptor cells transduce
energy of a stimulus into a change in
membrane potential.
Receptor potential generates action
potentials in the receptor cell, or
causes release of a neurotransmitter.
Sensory Systems - How do sensory cells convert stimuli into action potentials?
Stimulating a sensory cell
produces a receptor potential
Sensory Systems - How do sensory cells 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,
but there is no conscious sensation.
Sensory Systems - How do sensory cells convert stimuli into action potentials?
Some sensory receptor cells are
organized with other cells in sensory
organs.
Sensory systems include sensory
cells, associated structures, and
neuronal networks that process the
information.
Sensory Systems - How do sensory cells 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).
Sensory Systems - How do sensory systems detect chemical stimuli?
Chemoreceptors: receptor proteins
that bind various ligands; responsible
for taste and smell.
Also monitor internal environment, such
as CO2 levels in blood.
Sensory Systems - How do sensory systems detect chemical stimuli?
Olfaction is the sense of smell.
Olfactory sensors are embedded in epithelial
tissue at top of nasal cavity (in vertebrates).
Axons extend to the olfactory bulb in the brain,
dendrites end in olfactory hairs on the nasal
epithelium.
Odorant: a molecule that binds to a receptor
protein on the olfactory cilia.
Olfactory receptor proteins are specific for
particular odorants.
Sensory Systems - How do sensory systems detect chemical stimuli?
Olfactory receptors communicate
directly with the brain
Sensory Systems - How do sensory systems detect chemical stimuli?
Gustation is the sense of taste.
Taste buds: clusters of chemoreceptors
Human taste buds are embedded in the
tongue epithelium, on the papillae.
Some fish have taste buds on the skin;
duck-billed platypus has taste buds on
the bill.
Sensory Systems - How do sensory systems detect chemical stimuli?
Taste buds are clusters of sensory cells
Sensory Systems - How do sensory systems detect chemical stimuli?
Tongue epithelium is shed and replaced at a
rapid rate. Taste bud cells last about 10 days.
Neurons form new synapses with new taste bud
cells as they are formed.
Humans taste salty, sour, sweet, bitter, and
umami: savory, meaty taste from receptors for
amino acids.
Full complexity of taste involves both gustatory
and olfactory receptors.
Sensory Systems - How do sensory system detect mechanical forces?
Mechanoreceptors: sensory cells that respond to
mechanical forces.
Physical distortion of the plasma membrane causes
ion channels to open, this changes membrane
potential and leads to action potentials.
The skin has diverse mechanoreceptors.
Merkel’s discs: adapt slowly; continuous
information.
Meissner’s corpuscles: adapt quickly: information
about change.
Sensory Systems - How do sensory system detect mechanical forces?
The skin feels many sensations
Sensory Systems - How do sensory system detect mechanical forces?
Ruffini endings: deeper, adapt slowly,
react to vibrating stimuli of low
frequencies.
Pacinian corpuscles: deep, adapt
rapidly, react to vibrating stimuli at high
frequencies.
Sensory Systems - How do sensory system detect mechanical forces?
Mechanoreceptor cells constantly provide
information on position of limbs and stresses on
muscles and joints. Essential for maintaining
posture and coordinating movement.
Muscle spindles are mechanoreceptors in muscle
cells: stretch receptors. When muscle is
stretched, action potentials are generated in
neuron. CNS adjusts strength of contraction to
match load on muscle.
Golgi tendon organ: in tendons and ligaments.
Provides information about the force generated
by muscle; prevents muscle tearing.
Sensory Systems - How do sensory system detect mechanical forces?
Stretch receptors
Sensory Systems - How do sensory system detect mechanical forces?
Auditory systems use mechanoreceptors
to convert pressure waves to receptor
potentials.
Human ears: 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.
Sensory Systems - How do sensory system detect mechanical forces?
Structures of the human ear
Sensory Systems - How do sensory system detect mechanical forces?
Middle ear: air filled cavity, open to the throat via the
eustachian tube, it equilibrates air pressure between
middle ear and the outside.
Ossicles: malleus, incus, stapes: transmit vibrations
of tympanic membrane to the oval window.
The ossicles translate vibration of tympanic
membrane to smaller movement but greater force at
the smaller oval window.
Movement of oval window is translated into pressure
changes in the fluid-filled inner ear.
Sensory Systems - How do sensory system detect mechanical forces?
Inner ear includes the cochlea (tapered and coiled
chamber composed of three parallel canals
separated by two membranes), Reissner’s
membrane, and basilar membrane.
Organ of Corti sits on the basilar membrane:
transduces pressure waves into action potentials.
Hair cells: mechanoreceptors in organs of hearing
and balance. They have microvilli called stereocilia.
Bending of stereocilia open or close ion channels.
When the plasma membrane is depolarized,
neurotransmitters are released.
Sensory Systems - How do sensory system detect mechanical forces?
Hair cells have mechanosensors on their stereocilia
Sensory Systems - How do sensory system detect mechanical forces?
Mammalian inner ear has three
semicircular canals at angles to each
other; sense position and orientation of
head.
The vestibular apparatus has two
chambers that sense position of head
and acceleration.
Sensory Systems - How do sensory system detect mechanical forces?
Organs of equilibrium (part 1)
Sensory Systems - How do sensory system detect mechanical forces?
Organs of equilibrium (part 2)
Sensory Systems - How do sensory system detect light?
Photosensitivity: sensitivity to light.
A range of animal species from simple to
complex can sense and respond to
light.
All use the same pigments: rhodopsins.
Rhodopsin: opsin (a protein) and a lightabsorbing group; it sits in plasma
membrane of a photoreceptor cell.
Sensory Systems - How do sensory system detect light?
In vertebrate eyes, the retinal and opsin
separate, this is called bleaching.
Rod cell: one type of vertebrate
photoreceptor, which release
neurotransmitters from base of cell that
synapses with a neuron.
Outer segment is a stack of plasma
membrane discs packed with rhodopsin.
Sensory Systems - How do sensory system detect light?
Eye anatomy
Sensory Systems - How do sensory system detect light?
Sclera: tough connective tissue, becomes
transparent cornea on front.
Iris (pigmented): controls amount of light
reaching photoreceptors; opening is the
pupil.
Lens: crystalline protein; focuses image –
allows accommodation; changes shape.
Retina: photoreceptor layer.
Sensory Systems - How do sensory system detect light?
Staying in focus
Sensory Systems - How do sensory system detect light?
Vertebrate photoreceptors: rod cells and
cone cells.
Fovea: area where cone cell density is
highest
Cone cells have low sensitivity to light;
rod cells are responsible for night
vision.
Sensory Systems - How do sensory system detect light?
Rods and cones
Sensory Systems - How do sensory system detect light?
The retina has five layers of cells.
Light must pass through all layers before it is
captured by rhodopsin.
Light not captured is absorbed by a layer of
pigmented epithelial cells.
Layer at front consists of ganglion cells—their
axons form the optic nerve.
Photoreceptor cells are connected to ganglion cells
via bipolar cells.
Sensory Systems - How do sensory system detect light?
The retina (part 1)
Sensory Systems - How do sensory system detect light?
The retina (part 2)