Transcript Characteristic for receptor cells

Sensations & Perceptions
• sensation - is an awareness of sensory
stimuli in brain
• perception - meaningful interpretation
or conscious understanding of sensory
data
5 component of sensation
1- Sensory Receptors
– Structures that detect changes in external &
internal environment.
– Modified neurons or epithelial cells that have
evolved to respond to stimuli (eye, ear, nose).
2- Reception
- Ability of receptor to absorb energy of a
stimulus.
• 3. Transduction
– conversion of stimulus energy into membrane
potential*, i.e.,a Receptor Potential... RP (or generator
potential... GP --> fires an AP)...
– sort of like an EPSP or IPSP...a change in
permeability of a post-synaptic membran
– often graded = proportional to strength of stimulus
• may be amplified and/or may be summed.
• may be strong enough (reaches threshold) to generate action
potentials.
• 4- Transmission
– Receptor potentials transmitted via AP's to CNS
• 5- Integration
– Processing of frequency of receptor potentials.
sensory information is coded as frequency of
Action potentials
Characteristic for receptor cells
• Highly selective for specific stimuli, or
modalities.
• Have the ability to greatly amplify a signal.
The sequence of events in a receptor cell
from stimulus to generation of AP in a
sensory axon is :
sequence from stimulus to AP
•
(1) Stimulus energy of the correct modality produces an alteration in
receptor protein [in a membrane].
•
(2) The receptor may be part of an ion channel or a receptor protein, usually
located in a membrane.
•
(3) It may modulate the activity of ion channels indirectly through an enzyme
cascade, which amplifies the cellular signal.
•
(4) the absorption of energy by the receptor protein eventually leads to a
change in conductivity of the ion channels that carry the receptor current.
•
(5) The receptor current changes.
•
(6) The change in membrane potential, caused by an altered ion distribution
across the receptor cell membrane, is called the receptor potential.
Sensory Adaptation
• an attenuation of the stimulus...
• A decrease in responsiveness by
receptors due to continual stimulation.
• A uniformly maintained stimulus of
constant intensity is perceived as
progressively weaker with time,
• while a variable intensity stimulus of
shorter durations is perceived as a
progressively stronger odor over time
Sense of Vision
• Amount of light entering eye controlled by iris.
– Sphincter muscle that lies between cornea and lens.
– Light passes through pupil, zone in iris.
– Enlarges in dim light to allow more light to enter eye.
• Lenses limited by chromatic aberration.
– a) Short wavelengths refracted or bent more than longer
wavelengths
– b) Short wavelengths focus at different point than long
wavelengths.
– c) Vertebrate eye thus filters out short-wavelength ultraviolet light
– d) Insects do not focus light and can perceive ultraviolet light
• Photoreceptors located on retina.
• retina contains rods and cones
– a) Rods used for black-and-white vision when
illumination is dim.
– b) Cones are used for color vision, are shorter than
rods.
– c) Humans have 100 million rods and 3 million cones
in each retina.
– d) Most cones found in fovea.
(1) Location where eye forms its sharpest
image
(2) Almost no rods found here
Cellular structure of rods and cones
very similar
a) Inner segment
(1) Rich in mitochondria
(2) Contains numerous vesicles filled
with neurotransmitter molecule
b) Outer segment: connected to inner segment by
narrow stalk
(1) Packed with hundreds of flattened
disks, stacked on one another
(2) Light-capturing photopigment
molecules on membranes of these disks
• Rhodopsin is rod cell photopigment
a) Opsin protein coupled to molecule
of cis-retinal.
b) Cis-retinal produced from
carotene.
a) Three kinds of cones, each has cisretinal plus opsin with slightly different
amino acid sequence
b) Sequence shifts absorption maximum
from 500 nanometers of rhodopsin
(1) 455 nm is blue-absorbing.
(2) 530 nm is green-absorbing.
(3) 625 nm is red absorbing.
Sensory Transduction in Photoreceptors
1. Rod or cone contains many Na+ channels
in plasma membrane of outer segment
a) In dark many channels are open
b) Na+ ions continually diffuse into
outer segment, across stalk to inner
segment
c) Small flow in absence of light
called the dark current
d) Causes membrane to be
somewhat depolarized in the dark
2. In the light, Na+ channels in outer
segment close rapidly
a) Reduces dark current
b) Causes photoreceptor to hyperpolarize
c) Receptor respond by hyperpolarizing
rather than depolarizing.
3. Light causes Na+ channels to close
a) Cis-retinal is converted to trans-retinal
when the photopigment absorbs light
b) Isomerization causes retinal to
dissociate from opsin: bleaching reaction
c) Opsin protein changes shape
d) Shape change activates G protein
e) In turn activates hundreds of
phosphodiester molecules
f) This breaks down intracellular
messenger cyclic guanosine monophosphate
(cGMP)
4. Photopigments, G proteins and phosphodiesterase
embedded in outer segment disks
a) cGMP found in cytoplasm between disks and
plasma membrane
b) cGMP serves as link between events in disk
membrane and Na+ channels in plasma membrane
c) cGMP is required to keep channels open
d) When light is absorbed by photopigment, cGMP
is broken down
e) Channels close at rate of 1000 per second
f) Each photopigment coupled to many G proteins
each to many phosphodiesterases
g) Absorption of one photon cascades to block
entry of over a million Na+molecules
h) Photoreceptor thus hyperpolarizes
• In the DARK,
Rhodopsin is inactive (CIS) and cGMP is bound
to rod cell membranes;
Na channel is open; rod cell is DEPOLARIZED
and releases neurotransmitter GLUTAMATE
which excites or inhibits adjacent Bipolar
cells = no sensation of light.
In the LIGHT
Rhodopsin changes conformation (TRANS), which
activated
enzyme
TRANSDUCIN;
transducin
activates a PHOSPHODIESTRASE, converts cyclicGMP to GMP; GMP closes the Na channels, and
HYPERPOLARIZES
rod
cells
(RECEPTOR
POTENTIAL); hyperpolarized rod cell stops releasing
GLUTAMATE, allowing bipolar cells repsond
• Two basic types of
photoreceptor, rods and
cones, exist in the
vertebrate retina.
• The rods are
photoreceptors that
contain the visual
pigment - rhodopsin
and are sensitive to
blue-green light with a
peak sensitivity around
500 nm wavelength of
light.
• Rods are highly sensitive photoreceptors and
are used for vision under dark-dim conditions
at night.
• Cones contain cone opsins as
their visual pigments
• and, depending on the exact
structure of the opsin molecule,
are maximally sensitive to either
long wavelengths of light
[564nm],medium wavelengths of
light [533nm] or short wavelengths
of light [437nm].
• Cones of different wavelength
sensitivity and the consequent
pathways of connectivity to the
brain are the basis of color
perception in our visual image.
• Three different cone mechanisms can be
detected in behavioral, psychophysical and
physiological testing.
• These mechanisms are the basis of so called
trichromatic vision which most humans have.
• Where only one or two visual pigment bearing
types of cone are present the vision is said to be
monochromatic or dichromatic.
• Most mammalian species are dichromatic
containing as well as rods only middle and
short wavelength sensitive cones in their
retinas.
• Primates and humans, birds, reptiles and
fish are trichromatic, tetrachromatic and
some even pentachromatic (the latter
three vertebrate phyla).
Binocular Vision
1- Visual images of vertebrate eyes
a) Eyes on opposite sides of head, each sees object
at different angle
b) Parallax permits sensitive depth perception,
stereoscopic vision
2- Predators have eyes set in front of head to increase
stereoscopic vision
3- Prey have eyes set on sides of head to enlarge total
receptive field.
4- Must learn to perceive distance, not inborn.
Chemical senses
• Distinctions between gustation and olfaction.
– Gustation used to detect and identify foodrelated chemicals in near environment.
– Olfaction used not just to detect food, also
detection of enemies and mates,
The gustatory system: Taste bud
 Gustatory receptors
found in clusterstaste buds, receptors
anaxonal but
generate AP's.
 Gustatory receptors
on tongue, throat, and
mouth synapse with
afferent fibers that
travel to brain via 3
cranial nerves
Gustatory transduction
• Two kinds of transduction mechanisms:
– 1) Direct effect of chemical on one or
more ion channels in receptor
– 2) Second messenger pathway that
indirectly changes flow of ions through
channels.
Sour and salty stimuli
Substances that evoke sour or salty taste exert effects by
directly influencing ion channels in membrane of
receptor
• Weak acid vinegar ionize in water to produce protons
(H+) and anions (- ions), in mud puppy, H+ ions block
specific type of K+ channel in receptor
• For salty substances like table salt Na+ and other
cations act as stimuli, Na+ ions of salt enter receptor
Na+ channels directly, depolarizing cell, anion exerts
some other effect, taste of sodium chloride distinct from
sodium bicarbonate
Sweet stimuli
 Produce response via 2nd messenger, but
many kind of receptors, even in single
species
 Binding of sucrose to receptor activates
specific G protein, activates enzyme
adelylyl cyclase, catalyses the conversion
of ATP to cyclic adenosine monophosphate
(cAMP), turns on cAMP dependent protein
kinase A, protein kinase phosphorylates K+
channel in cell membrane, causes closing,
reduction in K+ efflux, depolarization,
possible AP.
• Because variety of receptors, responses
vary, not all sweet substances taste same
• Rate of adaptation to different sweet
stimuli not same, perception of sweet taste
of saccharine declines more than twice as
fast as sweet taste of sugar on successive
samples
Bitter stimuli
•
•
•
1) Direct effect on ion channels- blockage of K+
channels
2) Receptor controlled ion pump- activation of chloride
pump depolarizes
3) Second messenger systems- example two pathway
system, one path releases Ca2+ which directly causes
transmitter release, other closes K+ channel causing
depolarization, also some bitter receptors have
pathway remarkably like rod and cone pathway, G
protein 90% similar to transductin in rods and cones
called gustductin, activates phosphodiesterase (PDE)
that catalyses breakdown of cAMP, rod transductin
itself also found in taste cells
Umami taste
• Amino acids
• Some animals have gustatory receptor
cells that respond to amino acids, to
humans, most amino acids taste bitter,
alanine and serine sweet, a few such as
salts of glutamic acid have unique taste,
umami, neither sour, salty, sweet, or bitter
• In humans, afferent fibers from tip of
tongue travel via facial nerve
(cranial nerve VII), back of tongue
via glossopharyngeal nerve (IX),
pharynx via vagus nerve (X)
• After entering brainstem all
gustatory afferents synapse in
solitary nuclear complex of
medulla.
 In primates two cortical areas
known to be involved in taste
processing, facial part of
somatosensory cortex, and insula
Olfactory transduction
• In land animals, odorant must dissolve in mucus
or fluid covering receptors
• Odorant binding proteins bind with dissolved
odorant, move to receptor cell membrane
• Two transduction pathways in vertebrate
olfactory neurons, each with G protein second
messenger cascade and ligand gated ion
channel, both pathways may be present in one
neuron
•
•
A) G protein linked to
adenylyl cyclase,
promotes synthesis of
cAMP, opens nonspecific cation channel,
depolarizes cell
B) G protein activates
phospholipase C,
stimulates production of
IP3 and DAG, IP3 may
act on Ca2+ channel
The olfactory system
 Olfactory receptors line nasal epithelium, axon
extends through pores in bones of skull, dendrite
has branches, olfactory cilia that contain
receptor proteins for odorants, protected by thin
layer of mucus
• Receptors send axons to olfactory bulb,
in primates lies over nose, in other
vertebrates posterior to nose
• Axons from olfactory bulb enter brain via
olfactory tract (cranial nerve I), project to
primary olfactory cortex or pyriform
cortex on anterioventral of telencephalon,
no passage through thalamus