Widzenie

1

The travelling wave theory - Von Bekesy (1928).
Nobel 1961
The sound pressure applied to the oval window is
transmitted as a travelling wave along the basila
membrane. The peak diplacements for high
frequencies are toward the base, and for low
frequencies are toward the apex.
Georg von Békésy (1899 1972)
Envelopes induced by sound at 3 different
frequencies
2
Problem envelopes of the travelling waves are
wide while we are hearing pure tones
There must be additional mechanism for tunning of
the auditory system to the sound frequency.
Proof movements of the basilar membrane
Effect of cochlear amplifier. C) The peak due to
cochlear amplifier. D) Amplitude of the passive
movement of basilar membrane in the absence of
the cochlear amplifier.
3
The Organ of Corti
The organ of Corti is the receptor organ of the
inner ear, containing the hair cells and a
variety of supporting cells.
Transsection through cochlea showing the organ of
Corti
4
Two types of hair cells
Scanning electron micrographs of the organ of
Corti after removal of the tectorial membrane.
Inner hair cells are arranged in the single row.
Outer hair cells are arranged in the three rows
and the stereocilia of each cell are arranged in
a V configuration.
5
Organization and properties of the inner and
outer hair cells
A. Innervation pattern 20000 nerve fibers
connect to the 3500 IHC, while 1000 nerve fibers
connect to the 20000 OHC. The IHC are the main
sites of auditory transduction. B, C Response
properties stimulus oscillations (s) trigger
similar oscillations in the membrane potential.
Each cell has the best frequency for which, there
is a peak in the tuning curve.
6
Functional organization of the inner and outer
hair cells
In both types of cells the initial depolarization
is due to influx of K. This leads to activation
of a voltage-gated Ca2 channels. Influx of Ca2
provides for modulation of Ca2 - sensitive K
channels. The interplay of K and Ca2
conductances produces an oscillating potential
which generates an electrical resonance. It
increases the response at the cells best
frequency and sharpens the tuning curve within
the cell. It also provides the means for the
outer hair cells to produce mechanical output
through voltage-mechanical converter (V-M).
7
Rock around the clock Hair Cell
An outer hair cell is being stimulated
electrically by a patch pipette which enters from
the lower left. The cells potential is changed
by by plugging Walkman into the input socket of
the electrophysiology amplifier. The cell changes
its length but its volume stays constant. The
motor is a transmembrane protein that
mechanically contracts and elongates leading to
electromotility. The molecule, discovered in 2000
is called prestin. (from http//www.ucl.ac.uk/e
ar/research/ashmorelab)
8
The cochlear amplifier
Shape changes of the outer hair cells due to
rapidly oscillating membranne potential
contribute to movement of the tectorial and
basilar membranes. Inner hair cells are
stimulated by the relative movements between
these membranes. It is presumed that this
mechanism contributes to the active tunning of
hair cells responses.
9
Otoacoustic emission
An otoacoustic emission (OAE) is a sound which is
generated from within the inner ear. There are
two types of otoacoustic emissions spontaneous
otoacoustic emissions, which can occur without
external stimulation, and evoked otoacoustic
emissions, which require an evoking stimulus.
Most probably, otoacoustic emissions are produced
by the cochlear outer hair cells as they expand
and contract. Otoacoustic emissions are
clinically are the basis of a simple,
non-invasive, test for hearing defects in newborn
babies.
An example of multifrequency spontaneous
otoacoustic emissions recorded from a 48-year-old
woman with normal hearing. The black spikes
represent the response above the noise floor.
An example of evoked otoacoustic emissions and
their spectra. Evoked otoacoustical emissions are
evidence for a cochlear amplifier.
10
Mechanism of frequency tunning 2 dependence on
the location
Many properties of IHC and OHC vary with the
position along the cochlea. These differences are
likely to be correlated with the differing
frequencies that are processed along the cochlea,
but the significance of these changes is still
not understood.
11
Efferent fibers
In addition to afferent fibers, the auditory
nerve also contains efferent fibers, which arises
from cells in the brain-stem. Efferent fibers
inhibit mainly outer hair cells by
hyperpolarizing the hair cells membrane. It
reduces the motor output of the outer hair cells
and reduces the movement of the tectorial and
basilar membranes and the sensory response of the
inner hair cells. Its role is assumed to be a
protection agains overstimulation.
12
Tunning curves
Tuning curves for cochlear hair cells. To
construct a curve, the experimenter presents
sound at each frequency at increasing amplitudes
until the cell produces a criterion response,
here 1 mV. The curve thus reflects the threshold
of the cell for stimulation at a range of
frequencies. Each cell is most sensitive to a
specific frequency, its characteristic (or best)
frequency. The threshold rises briskly
(sensitivity falls abruptly) as the stimulus
frequency is raised or lowered.
13
Auditory pathways

• Auditory pathways
• - Cochlea
• Cochlear nuclei (brain-stem)
• Superior olivary nuclei (brain-stem)
• Inferior colliculus (brain-stem)
• Medial geniculate nuclei (thalamus)
• Auditory cortex

Left Auditory cortex
Right Auditory cortex
Medial geniculate nucleus
Cochlea
Inferior colliculus
Auditory nerve fiber
Superior Olivary nucleus
Ipsilateral Cochlear nucleus
14
Types of cells in the cochlear nuclei
Auditory nerve fibers terminate in the cochlear
nuclei (CN) on different types of cells with
different response properties. Responses to a
tone burst of 50 ms are shown. The Primary-like
preserve the envelope of the input signal, the
Pauser and theChopper provide for
differentation between onset and ensuing phases
of the tone, the On cells signal the onset or
timing of a sound. Each cell type represents an
abstraction of one particular feature of the
input. Different functional properties are
processed and transmitted in parallel pathways.
In humans, the receptor potentials of certain
hair cells and the action potentials of their
associated auditory nerve fiber can follow
stimuli of up to about 3 kHz in a one-to-one
fashion.
15
Sound localization in medial superior olive nuclei
Diagram illustrating how the MSO computes the
location of a sound by interaural time
differences. A given MSO neuron responds most
strongly when the two inputs arrive
simultaneously, as occurs when the contralateral
and ipsilateral inputs precisely compensate (via
their different lengths) for differences in the
time of arrival of a sound at the two ears. The
systematic (and inverse) variation in the delay
lengths of the two inputs creates a map of sound
location In this model, E would be most
sensitive to sounds located to the left, and A to
sounds from the right C would respond best to
sounds coming from directly in front of the
listener. Psychophysical experiments show that
humans can actually detect interaural time
differences as small as 10 microseconds This
sensitivity translates into an accuracy for sound
localization of about 1. Interaural time
differences are used to localize the source for
frequencies below 3 kHz .
16
Sound localization in lateral superior olive
nuclei
Lateral superior olive neurons encode sound
location through interaural intensity
differences. LSO neurons receive direct
excitation from the ipsilateral cochlear nucleus
input from the contralateral cochlear nucleus is
relayed via inhibitory interneurons in the MNTB
(medial nucleus of the trapezoid body). This
excitatory/inhibitory interaction results in a
net excitation of the LSO on the same side of the
body as the sound source. In contrast, sounds
arising from in front of the listener, will
silence the LSO output. Interaural intensity
differences are used to localize the source for
frequencies above 2 kHz .
17
Tonotopic organisation
The basilar membrane in the cochlea is
tonotopically organized. The tonotopic
organization is retained at all levels of the
central auditory system.
18
The auditory cortex
Diagram showing the brain in left lateral view,
including the depths of the lateral sulcus, where
part of the auditory cortex occupying the
superior temporal gyrus normally lies hidden. The
primary auditory cortex (A1) is shown in blue
the surrounding belt areas of the auditory cortex
are in red. The primary auditory cortex has a
tonotopic organization, as shown in the blowup
diagram of a segment of A1. The Wernicke's area
shown in gren is a region important in
comprehending speech. It is just posterior to the
primary auditory cortex.
19
Noise and music
fMRI activation during listening to noise (left)
and music (right). Moderate activity level is
present in the auditory areas during noise
listening. These areas become more active during
listening to the music. Besides, new areas are
activated.
20
Vision
Kanizsa triangle
21
The electromagnetic spectrum
Electromagnetic waves with high frequencies has
high energies that disrupt moelcular bonds. Waves
with low frequencies have lower energies for
which there are few know receptors in living
organisms. There is a narrow band of wavelengths
with medium energies that is called light.
22
Submodalities of vision
Sensing changes in illumination that vary in time
and space is called vision. Vision has large
number of submodalities.
23
The eye
The eye is designed to focus the visual image on
the retina with minimal optical distortion. Lens
change shape to focus light from different
distances on the retina where photoreceptors are
located. In one region of the retina, the fovea,
the cell bodies of the proximal retinal neurons
are shifted to the side, enabling the
photoreceptors there to receive the visual image
in its least distorted form. Humans constantly
move their eyes so that scenes of interest are
projected onto the fovea.

• Fovea is characterized by
• - high density of photoreceptors
• lack of blood vessels
• location on the eyes visual axis what minimizes
  the aberations

24
The mistake of evolution?
Hypotheses - protection against the damaging
effects of light - sustaining the photoreceptors
by the retinal pigment epithelium (recycling and
metabolising their products) Side effect - blind
spot
Left schematic diagram of the retina by Santiago
Ramon y Cajal (1900). Right section of rats
retina.
In vertebrates retina the light must pass through
several inner layers of nerve cells and their
processes before it reaches the photoreceptors.
It is typical of vertebrates but rare among
invertebrates.
25
Photoreceptors rods and cones
Distribution of rods and cones in the human retina

• The human retina contains two types of
  photoreceptors, rods and cones. Cones are
  responsible for day vision. Rods mediate night
  vision
• There are 20 times more rods than cones
• Rods are 1000 times more sensitive to light than
  cones.

Rods and cones in electron micorgraph
26
Visual acuity is highest in the fovea and
decreases with distance from the fovea
Special chart prepared to demonstrate how visual
acuity decreases rapidly with target distance
from the fovea. According to Anstis (1974) when
the center of the chart is fixated at
approximately normal reading distance, all the
letters should be equally well readable, since
increasing target distance from the fovea is
compensated by a corresponding increase in letter
size. From Anstis, S. A chart demonstrating
variation in acuity with retinal position, Anstis
S. Vision Research, 14 , 589-592 (1974).
27
Rods and cones
Rods Cones
High sensitivity to light, specialized for night vision Lower sensitivity, specialized for day vision
More photopigment, capture more light Less photopigment
High amplification, single photon detection Lower amplification
Low temporal resolution slow response, long integration time High temporal resolution fast response, short integration time
More sensitive to scattered light Most sensitive to direct axial rays
Rod system Cone system
Low acuity not present in central fovea, highly convergent retinal pathways High acuity concentrated in fovea, dispersed retinal pathways
Achromatic one type of rod pigment Chromatic three types of cones, each with a distinct pigment that is most sensitive to a different part of the visible light spectrum
28
The dark current
In darkness two currents flow in a photoreceptor.
An inward Na current flows through cGMP-gated
channels, while an outward K current flows
through nongated K-selective channels. The
outward current carried by the K channels tends
to hyperpolarize the photoreceptor. The inward
current tends to depolarize the photoreceptor. As
a result, in darkness the photoreceptor's
membrane potential is around -40 mV. The
photoreceptor is able to maintain steady
intracellular concentrations of Na and K in the
face of these large fluxes because its inner
segment has a high density of Na-K pumps, which
pump out Na and pump in K . In darkness the
cytoplasmic concentration of cGMP is high, thus
maintaining the cGMP-gated channels in an open
state and allowing a steady inward current,
called the dark current. When light reduces the
level of cGMP, thus closing cGMP-gated channels,
the inward current that flows through these
channels is reduced and the cell becomes
hyperpolarized to around -70 mV.
29
Three stages of phototransduction
Phototransduction involves the closing of Na
channels in the outer segment of the
photoreceptor membrane. In the absence of light
these channels are kept open by intracellular
cGMP and conduct an inward Na current..

1. Light is absorbed and activates pigment molecules
   (rhodopsin in rods) in the disc membrane.
2. The activated pigment stimulates a G protein
   (transducin in rods), which in turn activates
   cGMP phosphodiesterase. This enzyme catalyzes the
   breakdown of cGMP to 5c-GMP.
3. As the cGMP concentration is lowered, the
   cGMP-gated channels close, thereby reducing the
   inward current and causing the photoreceptor to
   hyperpolarize.

30
Retinal circuits
The retina has three major functional classes of
neurons. Photoreceptors (rods and cones) lie in
the outer nuclear layer, interneurons (bipolar,
horizontal, and amacrine cells) in the inner
nuclear layer, and ganglion cells in the ganglion
cell layer. Photoreceptors, bipolar cells, and
horizontal cells make synaptic connections with
each other in the outer plexiform layer.
Information flows vertically from photoreceptors
to bipolar cells to ganglion cells, as well as
laterally via horizontal cells in the outer
plexiform layer and amacrine cells in the inner
plexiform layer.
31
On and Off ganglion cells
Ganglion cells have circular receptive fields,
with specialized center (pink) and surround
(gray) regions. On-center cells are excited when
stimulated by light in the center and inhibited
when stimulated in the surround off- center
cells have the opposite responses. A. On-center
cells respond best when the entire central part
of the receptive field is stimulated (3). These
cells also respond well, when only a portion of
the central field is stimulated by a spot of
light (1). Illumination of the surround with a
spot of light (2) or ring of light (4) reduces or
suppresses the cell firing. Diffuse illumination
of the entire receptive field (5) elicits a
relatively weak discharge because the center and
surround oppose each other's effects. B. The
spontaneous firing of off-center cells is
suppressed when the central area of the receptive
field is illuminated (1, 3). Light shone onto the
surround of the receptive field excites the cell
(2, 4). Conclusion retinal ganglion cells
respond optimally to contrast in their receptive
fields.
32
Circuits generating the responses of the ganglion
cells
The horizontal cell receives input from a cone in
the surround of the on-center bipolar cell and
also has a connection with a postsynaptic cone in
the center of the bipolar cell's receptive field.
In the dark, horizontal cells release an
inhibitory transmitter that maintains
postsynaptic cones in the receptive field center
in a slightly hyperpolarized state. Illumination
of cones in the bipolar cell's surround
hyperpolarizes those cones, which in turn
hyperpolarize the postsynaptic horizontal cell.
This hyperpolarization of the horizontal cell
reduces the amount of inhibitory transmitter
released by the horizontal cell onto postsynaptic
cones in the receptive field center, and as a
result these cones become depolarized (the
opposite effect of light absorption by these
cones). This in turn allows the on- center
bipolar cell to become hyperpolarized, the
opposite effect of illumination in the receptive
field center.
Each bipolar cell makes an excitatory connection
with a ganglion cell of the same type. When the
cone is hyperpolarized by light, the on-center
bipolar cell is excited and the off-center
bipolar cell is inhibited. This is because the
two types of bipolar cells have different
postsynaptic receptors to the transmitter
released by the cone. The responses of the
ganglion cells are largely determined by the
inputs from the bipolar cells.
33
Ganglion cell types
Each region of the retina has several
functionally distinct subsets of ganglion cells
that convey, in parallel pathways, signals from
the same photoreceptors. Most ganglion cells in
the primate retina fall into two functional
classes, M (for magni, or large) and P (for
parvi, or small). Each class includes both
on-center and off- center cells.
(P)
(M)
Parallel networks of ganglion cells with
different functional properties are the beginning
of the segregation of information into parallel
processing pathways in the visual system.