LECTURE 7: SENSING THE ENVIRONMENT Flashcards
provide the only channels of communication
from the external world to the nervous system
sensory organs
gathered from the environment and from within the body and processed by the nervous system
Sensory input
Basic Concepts in Sensory Physiology: Information is conveyed in sensory nerves as ______________.
action potential
-cells specialized to convert stimulus energy into neural signals
-housed or clustered together in sense organs (where sensory reception begins
Sensory receptors
Basic Concepts in Sensory Physiology: Sensory organs serve as ________ where external signals generate receptor potentials
transducers
organs containing
cells (receptor cells) that are specialized to
respond to particular kinds of stimuli
Sensory reception
Basic Concepts in Sensory Physiology: Action potentials in all sensory nerves are of the __________ and __________-
same nature and magnitude
Basic Concepts in Sensory Physiology: The magnitude of the action potential is unaffected by _______________
stimulus intensity
Basic Concepts in Sensory Physiology: Stimulus intensity is coded by _______________ of the action potential
frequency modulation
positioned in many
locations (surface of inside)constitute the
first step in gathering sensory information
Sensory organs
carry information
from periphery to CNS
Afferent neurons
Receptor class that is sensitive to mechanical energy
mechanoreceptor
Receptor class that is sensitive to chemical energy
chemoreceptor
carry information
away from the CN
Efferent neuron
Receptor class that is sensitive to electrical energy
electroreceptor
Receptor class that is sensitive to thermal energy
thermoreceptor
Receptor class that is sensitive to light energy
photoreceptor
features that characterize stimuli within a
particular modality
Qualities
ability to modify the conformation of a
receptor molecule
Stimulus
respond to signals from
within the body and communicate this information to
the brain by pathways that typically are not brought into
consciousness
Interoceptive receptors
Properties of Receptor Cells
- Highly selective for a specific kind
of energy (membranes respond
differentially to different types of
energy) - Exquisitely sensitive to their
selected stimuli because they can
amplify the signal that is being
received
form of energy to
which a sensory receptor is most sensitive
Sensory modality
Molecules of _____________ (cell membrane of photoreceptor cell ) absorb photons, capturing their energy.
rhodopsin
Properties of Receptor Cells: form of energy to
which a sensory receptor is most sensitive
Sensory modality
Properties of Receptor Cells: Receptor cells _________ the sensory input
(change the stimulus energy into the
energy of a nerve impulse)
transduce
Properties of Receptor Cell: transduction depends on a_____________ in particular receptor molecules
(proteins)
conformational change
produces a transient structural change that
activates a cascade of associated molecules
Photon
Sensory transduction processes:
- Absorption of stimulus energy
- Transduction proper
- Amplification of output energy
- Integration and Conduction of output potential
The basic events in a receptor cell:
o detection
o amplification
o encoding of the sensory stimulus
Basic events in a receptor cell
- Detection
- Amplification
- Encoding
- threshold of detection - smallest amount of stimulus energy that will produce a response in a receptor 50% of the time
- time constant of sensory reception -receptors must be able to respond quickly and repeatedly (for a sensory system to convey accurate information about rapidly changing stimuli)
*receptors must be interconnected - allows the
population of receptors to extract information
about very rapid events on the basis of their collective
activity
Detection
smallest amount of stimulus
energy that will produce a response in a receptor 50% of
the time
threshold of detection
receptors must be able to respond quickly and repeatedly (for a sensory system to convey accurate information
about rapidly changing stimuli)
time constant of sensory reception
allows the population of receptors to extract information about very rapid events on the basis of their collective activity
receptors must be interconnected
*sensory signals occurs within the receptor cells
* mediated by different intracellular mechanisms
* occurs at the same time that noise is suppressed, so the signal-to-noise ratio improves in the process.
Amplification
_______ is captured by a visual pigment molecule
photon
Capturing of photon by visual pigment molecule activates __________
transducin (GTP-binding protein, or G
protein)
activates a phosphodiesterase to
hydrolyze cyclic guanosine monophosphate A
(cGMP)
Transducin
- sensory information into a neuronal signal to be transmitted to the brain depends on changes in the conductance through membrane ion channels
*channel conductance changes - shift the probability that the neuron will produce an AP
Encoding
Characteristics of Receptor Potential
- An adequate stimulus elicits a graded RP, the amplitude of which is a function of stimulus intensity.
- The frequency of resultant AP in a receptor is a coded representation of the intensity of the adequate stimulus.
Types of Sensory receptors
- Phasic receptor
- Tonic receptors
The graded depolarization of a receptor in response to stimulation
Receptor potential
-produces APs during only part of the
stimulation ( onset or at the
offset of the stimulus -cannot
by itself convey information
about the duration of the
stimulus)
-adapt more rapidly and cease impulse discharge even during a prolonged stimulus.
Phasic receptor
-continue to fire APs throughout the
stimulation (can directly
convey information about the
duration of the stimulus
-adapt relatively slowly and continue to discharge impulses throughout duration of prolonged stimulus.
Tonic receptors
Steps link the onset of a stimulus to the production of APs in a
sensory pathway
*receptor cells (RC) generate and carry APs into the CNS
* RC synaptically modulate APs in other neurons that carry the
signal into the CNS
The decrease in the response of a receptor to a steadily maintained stimulus over time
Receptor Adaptation
Stages from stimulus to output where
adaptation can take place:
- Mechanical properties of the receptor cell -
act as a filter that preferentially passes
transient, rather than sustained, stimuli. - The transducer molecules “run down” during
a constant stimulus. - Enzyme cascade activated by a transducer
molecule may be inhibited by the accumulation
of a product or an intermediate substance. - Electrical properties of the receptor cell may
change in the course of sustained stimulation. - Membrane of the spike-initiating zone
may become less excitable during
sustained stimuli. - Sensory adaptation can also take place
in higher-order cells in the CNS
o pressure and vibration receptor found in the skin, muscles, mesentery, tendons, and joints of mammals
o region of receptor membrane -sensitive to mechanical stimuli (surrounded by concentric
lamellae of connective tissue)
o something presses on the corpuscle (deforming it) -disturbance is transmitted
mechanically through the layers to the sensitive membrane of the receptor neuron
Pacinian corpuscle
adapts
quickly to constant
stretch, producing only a
short train of impulses
Phasic receptor
fires
steadily during
maintained stretch but
frequency decreases
Tonic receptor
transducing photons of light into
electrical signals that can be interpreted by the nervous
system, and photoreceptive organs (eyes)
Photoreception
*Adaptation in the Pacinian corpuscle depends on the ______________________________________-
mechanical properties of accessory structures
based upon a very highly conserved
set of protein molecules that provide an optical pathway
leading light to the photo-receptive surface and that
capture photons within the photoreceptors
visual transduction
o protein visual pigment molecules
o each molecule - seven transmembrane domains
o coupled to photopigment molecules, which are structurally altered by the absorption of photons and which in turn modify the properties of the opsin protein
opsins
-few receptors in an
open cup of screening pigment
cells
- surrounding distribution of
light and dark, but they do not
provide enough information to
allow detection of either predators
or prey
eyespots
o spherical lens in aquatic animals
o provides the high refractive
power needed to focus images
under water (spherical aberration)
o lens is not homogeneous (dense
with a high refractive index in the
center and has a gradient of
decreasing density and refractive
index toward the periphery.
single-chambered eye
reducing the size of
the aperture to produce a pinhole
eye (B)
simplest eyes
three lenses in series
that together correct for
spherical aberration (D)
eye with multiple lenses
o combines small aperture
with a refractile lens
o very high quality image
that is focused on the layer
of photoreceptors in the
retina
vertebrate eye (E)
Main parts of the vertebrate eye
-sclera
-choroid
-retina
-lens
-iris
-optic disk
white outer layer, including cornea
sclera
pigmented layer
choroid
regulates the size of the pupil
iris
2 cavities of the eye
-Anterior cavity
-Posterior cavity
focuses light on the retina
lens
a blind spot in the retina where the
optic nerve attaches to the eye
optic disk
contains photoreceptors
retina
filled with watery aqueous humor
anterior cavity
filled with jellylike vitreous humor
posterior cavity
The ___________ produces the aqueous humor
ciliary body
In mammals
accommodation is
accomplished by
_______________________
changing the shape of the lens
____________________ is the
focusing of light in the
retina.
Accommodation
two types of photoreceptors
- Rods
- Cones
light-sensitive but don’t distinguish colors
Rods
distinguish colors but are not as sensitive to
light
Cones
In humans, cones are concentrated in the ______
fovea
What is Fovea?
the center of the visual field
is the visual pigment of rods.
Rhodopsin (retinal + opsin)
rods are more concentrated around the ________________
periphery of the retina
The absorption of light by rhodopsin initiates a __________________________-
signal-transduction pathway
Visual processing begins with
rods and cones synapsing with
_______________
bipolar cells
Bipolar cells synapse with
________________
ganglion cells
Visual processing in the retina
also involves ___________ and ___________
horizontal cells
and amacrine cells
Vertical pathway of Visual processing
photoreceptors → bipolar cells → ganglion
cells axons
Lateral pathways of visual processing
Photoreceptors → horizontal cells → other
photoreceptors
result to more distance photoreceptors and bipolar
cells are inhibited → sharpens edges and
enhances contrast in the image.
Lateral Inhibition
Pathway that is a results in lateral inhibition, this time of the ganglion cells
Photoreceptors → bipolar cells → amacrine
cells → ganglion cells
Stages of the vertebrate eye
- Initial stage
- Further bent or refracted as they pass through the lens and finally form an inverted image on the rear internal surface of the eye (retina)
incident light rays are bent as they pass
through the clear outer surface of the eye (cornea)
Initial stage
incident light is refracted by the cornea and the lens and is focused
on the _________________
photosensitive retina
image focused on the retina is inverted by the lens. The lens is held in place by the ______________
zonularfibers
image is focused - changing the curvature and thickness of the lens –changes the distance at which an image passed through the
lens comes into focus (_________________)
focal length of the lens
When ciliary muscle fibers contract, tension on the zonular fibers is __________, and the elastic properties of the lens cause it to become
more ______________, shortening the focal length.
-reduced
-rounded
modification of the tension
exerted on the perimeter of the lens is caused by
shape of the lens is changed
lens is held in place within the eye by the radially oriented fibers
of the _________
zonula
