Download Biophysics of sensory receptors

Theories about sensing
Aristotle (384-322 BC)
cardiocentric sensing.
Biophysics of sensory
receptors
Galenus (129-200 AD) raised
doubts about cardiocentric
sensing.
Cardiocentric sensing
(Medieval reconstruction)
Today:
stimulus sensory receptors receptor potential neuron/nerve action potential central nervous system signal processing sensation
fMRI recording during
sensomotoric function
Sensory homuncle
Sensory receptors
Steps of sensing
OLFACTION
VISION
HEARING
TASTE
TOUCH
Case of hearing
Spectrum
Duration
Direction
Physically
measured sound
properties
Neural
conductance
Sensory receptor
(ear)
Brain
(auditory center)
Loudness
Psychologically
perceived sound
effects
Pitch
Tone
Perceived duration
Perceived direction
Sound recognition
Sound sensation
Intensity
Frequency
Meissner
body
Free axon
Rod
Cone
Sensory receptor: Specialized sensory cell, which responds to a given stimulus (e.g., light, sound,
chemicals) and relays the information to the central nervous system.
Cell surface receptor (different meaning!): Proteins which specifically bind hormones,
neurotransmitters and other molecules, and thus iniate specific cellular reactions.
Five senses?
Steps of signal transduction
Most important sensory modalities (First 11: perceived modalities)
Modality
Receptor
(physical-chemical
effects)
1
Vision
Rods and cones
Eye
2
Hearing
Hair cells
Ear (organ of Corti)
3
Olfaction (smelling)
Olfactory neuron
mucus membrane
4
Taste
Taste receptor cells
Taste buds
5
Angular acceleration
Hair cells
Ear (semicircular canals)
6
Linear acceleration
Hair cells
Ear (utricle and saccule)
7
Touch, pressure
Nerve endings
Multiple types
8
Heat
Nerve endings
Multiple types
9
Pain
Nerve endings
Multiple types
10 Cold
Free nerve endings
...
11 Joint position and motion
Nerve endings
Multiple types
12 Muscle length
Nerve endings
Muscle spindle
13 Muscle stress
Nerve endings
Golgi’s tendon organ
14 Arterial presure
Nerve endings
Sinus caroticus stretch receptors
15 Central venous pressure
Nerve endings
Venous, atrial stretch receptors
16 Lung stress
Nerve endings
Pulmonary stretch receptors
17 etc...
etc...
etc...
Neuron
Uaction
sensory
areas
hearing and
vision
association
association
areas
ASSOCIATION
oten
hearing
Uthres
ion channel
Uact
NERVE
Sensitivity
tials
ten
n po
Uact
o
acti
nerve
from receptor
to
brain
activation of sensory
areas
SENSORY CENTER
STIMULUS
RECEPTOR
perception
interpretation
positive ions
tial
tor p
ACTION POTENTIAL
Central
nervous
system
Step
speech
recognition
p
rece
RECEPTOR POTENTIAL
Uthres
thres
Ugen
thres
Receptor
From stimulus to sensation
Measured signal
STIMULUS
Environment
Organ
eV - size stimulus is sufficient for
evoking action potential:
• sound receptors: thermal motion of
the molecules of air
• light receptors: 1-2 photons
1. Modality
What is coded by the action potential?
Adequate stimulus
Type of energy for which the receptor is most sensitive (e.g.,
light for the eye).
• modality (type)
• intensity (strength)
Action potentials are identical in all nerves. How do we know,
for example, whether an action potential codes for touch and
not cold?
• duration
• localization
Principle of specific sensory energies
of the stimulus
Sensation is determined by the stimulated cortical region!
2. Intensity
3. Duration, adaptation
Which parameters carry information about stimulus strength?
• frequency of action potentials
• number of activated receptor cells
Weber-Fechner
psychophysical law
Stevens’ law
= const lg
0
= const 0 n
= sensation strength
=background intensity
0=absolute threshold
intensity
n=constant specific for the
type of sensation
n<1: compressive function
(hearing, vision)
Stimulus
Adaptation. During constant stimulus the
frequency of action potentials gradually
decreases.
Rapidly adapting (phasic) receptors
E.g., pressure, smell, heat
Slowly and partially adapting (tonic) receptors
E.g., cold, pain (dental pain)
n>1: expansive function
(pressure, taste)
Weber (1795-1878)
Fechner (1801-1887)
Stevens (1906-1973)
Action
potentials
Receptor
potential
No adaptation
Action
potentials
Slow adaptation
Receptor
potential
Action
potentials
Receptor
potential
Fast adaptation
Seconds
4. Localization, receptor fields
Branched nerve endings define receptor fields (convergence). Such can be found in
the skin (touch) and in the peripheral retina (rods).
Biophysics of the
eye
Receptor fields with overlap
Optical illusions - intensity
Optical illusions – space
Optical illusions – motion
Optical illusions – direction
Ooptical illusions point out the remarkable and unusual
processing power of the visual system.
Stimulus: light
Stimulus: light
Electromagnetic wave
Transverse wave
The eye is sensitive to: wavelength and amplitude (~intensity)
The eye is insensitive to: phase and polarization
“Receptor-organ”: eye
Photoreceptors
pigment
epithelium
light
receptors
plasma membrane
rods
sclera
cones
choroid
vitreous humour
discs
membrane
disc
discs
optic nerve
fibers
cytoplasm
retina
cones
ciliary body
plasma
membrane
t
ligh
zonulae ciliares
cilia
(suspensory
ligaments)
membrane disc
mitochondria
lens
eye axis
anterior
chamber
rods
ganglion cells
macula
bipolar
cells
nucleus
fovea
retinal
optic nerve
synapse
cornea
opsin
iris
posterior
chamber
rod
optic disc
(blind spot)
Optics of the eye
cone
rhodopsin molecule
Accomodation and refractive problems
flattened
lens
Total refractive power
of eye: 62 dptr
relaxed ciliary
muscle
bulged
lens
contracted
ciliary muscle
Optical power entering the eye (P)
d
P = J 2
2
J=intensity (W/m2)
d=pupil diameter
stretched
ligaments
2
Pmax dmax =
= 16
Pmin dmin dmax=8 mm
dmin=2 mm
Hypermetropia
Astigmatism
air
cornea
aqueous
humour
lens
vitreous
humour
Refractive power of surfaces (D)
D=
n n'
r
relaxed
ligaments
n-n’=refractive index difference
between refractive media
r=radius of curvature of
refractive surface
Myopia
Correction with cylindric lens
LASIK
(Laser Assisted In Situ
Keratomileusis)
Spatial resolution of the eye
Generation of visual stimulus
visual angle: the smallest angle at which two object points can be
distinguished.
For a healthy eye: 1’ (angular minute, 1/60 degree)
visual _ acuity =
1'
100%
Sensitivity of the human eye
=experimental visual angle
Resolution has wave optics (diffraction) and biological (receptor density) limitations.
Object
Image on receptors
Night vision
Day vision
Astronomical magnitude
Sensation
~2m
Cloudy
night sky
Photoreceptor distribution in retina
Clear
night sky
Quarter Full moon
moon
Twilight
Sunrise
sunset
Cloudy
day sky
Direct sun
exposure
Properties of receptor cells
blind spot
Rods
Rods
blind spot
Cones
fovea
optic
nerve
Cones
Degree (˚)
Rods
Cones
Rod
Cone
Stimulated by very small intensity
(optimally 1 photon!)
Smaller sensitivity, but functions at high
intensities
Saturates at average intensities
No saturation
Found mainly in the peripheral retina
In the fovea, mainly central fovea
Many rods per ganglion (convergence);
greater sensitivity, smaller spatial
resolution
Small convergence;
greater spatial resolution
No color sensitivity
Sensitivity to colors
Photochemical reaction
optical excitation
all-trans-retinal
In human: 3 types of receptors. Each senses different
colors - absorbes at different wavelengths (R=64%,
G=32%, B=2%).
Relative absorption
11-cis-retinal
1 rhodopsin absorbs 1 photon
500 transducin molecules activated
500 phosphodiestherase molecules
activated, and
105 cGMP molecules hydrolyzed
250 Na+-channels closed
Entrance of 106-107 Na+ ions/s inhibited
cell hyperpolarized (1 mV)
transmitter release reduced (glutamate:
inhibitory neurotransmitter).
Color vision
Light reflection from butterfly
retina. The different receptors
reflect dfferent colors.
rod
cones
Wavelength (nm)
X = rR + gG + bB
Additive color mixing
Stimulus: sound
Biophysics of
hearing
Longitudinal
mechanical wave
(pressure wave)
A
Longitudinal wave
Harmonic oscillation:
Transverse wave
y ( t ) = Asin ( ft + )
y=actual pressure; t=time
f=frequency; A=amplitude
=phase shift
Sounds and their spectra
Frequency and intensity of sounds
J Intensity level: n(dB) = 10 lg 1 J2 TYMPANIC MEMBRANE
RUPTURE
PAIN
(Fourier spectrum)
pure
sound
MU
INFRASOUND
ULTRASOUND
SIC
HEARING
Continuous spectra
white
noise
HEARING
Intensity
musical
sound
Discrete spectra
harmonics
Pressure wave therapy
Ultrasound diagnostics,
therapy
DISCOMFORT
fundamental
frequency
SUPERSOUND
HYPERSOUND
SUPERSOUND
Frequency
domain
Tu
rbi
dri ne
ll
Fourier transformation
Inverse Fourier transormation
Time domain
Cardiac sounds
SILENCE
Frequency
drum beat
Wavelength
(in air)
“Receptor-organ”: ear
Outer ear
Middle ear
Inner ear
Physical schematics of the ear
Semicircular
canals (vestibular
organ)
Auricula
The pressure wave
of the fluid generates
surface waves on the
basal membrane
Lever system of ossicles
results transmits waves with
inceased forces
Ossicular
muscles
Outer ear
Oval
window
Middle ear
Axis of rotation
of lever system
Inner ear
The basal membrane is
narrow and taught at the
beginning of the cochlea
The basal membrane is
wide and slack at the
end of the cochlea
Apex
Cochlea extended (35 mm)
Auditory
nerve
External
auditory canal
Eardrum
Apex
cochleae
Auditory ossicles
(malleus, incus, stapes)
Round
window
Cochlea
Eustachian
tube
Auditory nerve
External auditory canal:
resonator with quarter
wavelength.
The smaller surface
area of the oval window,
compared with that of
the eardrum, results in
pressure increase.
Hair cells on the vibrating
basal membrane are bent
and excite auditory nerve
endings
While the oval window
is pressed inside, the
round window is
pressed outside
Pathway of
atmospheric pressure
equilibration
Middle ear: mechanical amplifier
Outer ear: sound collector
Auricula
Sound is steered into the external auditory
canal.
Auditory ossicles (malleus,
incus, stapes)
They amplify eardrum
resonance and transmit it to
the oval window.
External auditory canal
Conducts pressure waves towards the
eardrum. More efficient in certain freqency
range (2000-5000 Hz).
Auditory
ossicles
Oval
window
Eardrum
Amplification:
due to area ratio: 17 due to lever action: 1,3 water
air
Total amplification: 22
(pressure increase)
Eardrum
Brought into resonance by sound waves.
drum
drum
Ultrastructure of the inner ear
Inner ear: sensor
Vestibular organ: semicircular canals
Inner hair cells. Appr. 500
Cochlea: 2.5-pitch, 35-mm-long fluid-filled channel.
It is halved in length partly by an osseous, partly by a membranaeous
wall, the basal membrane.
Sensory organ of sound.
Stapes
Semicircular
canals
Oval window
Cochlea
Eardrum
Basal membrane
Round window
Outer hair cells. Appr. 12-20000
Velocity of wave propagation
Velocity of “surf” wave is smaller
than that of sound (1440 m/s)
Frequency
“Surf” wave
Apex
Oval window
Oval window
Békésy: propagating surface waves on basal membrane
Function of the organ of Corti
Due to the bending of the basal membrane, hair cells become tilted and depolarized.
Ape
Basa
x
2. Tectorial membrane
becomes lifted
l mem
brane
Inner hair cell
Pivot point
3. Shear
force
Outer hair
cells
Oval window
Surface curve of
propagating wave
Relative deflection
Apex
Location of propagating wave at
subseqent times
György Békésy
Nobel-prize 1961
Distance from oval window (mm)
Basal membrane at rest
Pivot point
1. Basal membrane
becomes distorted
The frequency-dependence of the location of propagating wave maxima
provide a rough frequency-discrimination.
endolymph
mechanical
effect, stereocilia
deform
Tight tip
link
Passive mechanism
(basal membrane)
force
cation gates open (tip link
effect), K+ ions enter
support cells
hair cell becomes
depolarized
hair cell
nucl.
Ca2+ channels open,
Ca2+ ions enter
Active mechanism
(outer hair cells)
Base
stereocilia
Oscillation amplitude
(log)
Loose
tip link
Outer hair cells: amplifiers
Basal membrane
Distance from oval membrane (mm)
Outer hair cells
synapse
neurotransmitter is
released into synaptic cleft
efferent
afferent
neurons
afferent neuron
depolarizes, frequency of
action potentials increases
Regenerative amplifier:
positive feedback
(large amplification in narrow frequency range)
Apex
Inner hair cells:
Mechanoelectric transducers
Psychoacoustics: loudness (Fletscher-Munson)
Coding of acoustic information
ethalon sound
loudness Hson (son)
w
surface wave
base
basal membrane
oval wido
Frequency sensing coded
locally.
pain threshold
Intensity level
stimulus
Pressure
Volleyball theory
Intensity
loudness level Hphon (phon)
Location theory
Disco
120 phon
apex
loc
Street
noise
80 phon
oscillatio
n
amplitu
de
Basis:
1. Weak frequency-dependence
of the amplitude maxima of
propagating surface waves.
2. Active amplification.
3. Frequency senitivity of
afferent neurons (threshold
stimulus depends on freqency).
speak
ing ra
t (tim
hair
action
cells
Whisper
30 phon
poten
tials
ltane
o
pote us action
ntials
(so
v
Ste
loudness level
loudness
law
Weber-Fechner
ale (phon)
mic) loudness sc
rith
ga
(lo
d
cte
Constru
a
at
ld
a
nt
e
rim
pe
ex
Intensity
Intensity level
es
ho
ld
of
he
arin
g
frequency f (Hz)
n)
aw
thr
reference intensity, J0
Phon and son scales
’l
ens
nge
e)
simu
Simultaneous appearance
of action potentials in the
auditory nerve may
encode large frequencies.
Loud
speaking
60 phon