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
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.
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
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.
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.
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 cell’s 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).
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 cell’s 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/ear/research/ashmorelab)
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.
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 the basis of a
simple, non-invasive, test for hearing defects in newborn babies.
An example of multifrequency spontaneous
otoacoustic emissions recorded from a 48year-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.
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.
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 against overstimulation.
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.
Auditory pathways:
Auditory pathways
- Cochlea
Left
Auditory
cortex
Right
Auditory
cortex
Cochlear nuclei (brainstem)
-
Superior olivary nuclei
(brain-stem)
-
Inferior colliculus
(brain-stem)
-
Medial geniculate
nuclei (thalamus)
-
Auditory cortex
Medial geniculate nucleus
Cochlea
Inferior colliculus
Auditory
nerve fiber
-
Ipsilateral
Cochlear
nucleus
Superior
Olivary
nucleus
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 the’Chopper’
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.
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 .
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 .
Tonotopic organisation
The basilar membrane in the cochlea is tonotopically organized. The tonotopic organization is retained at all levels of the central
auditory system.
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 green is a region
important in comprehending speech. It is just
posterior to the primary auditory cortex.
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.
Vision
Kanizsa triangle
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 known receptors in living organisms. There is a
narrow band of wavelengths with medium energies that is called light.
Submodalities of vision
Sensing changes in illumination that vary in time and space is called vision. Vision has large
number of submodalities.
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 eye’s visual axis
what minimizes the aberations
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 rat’s 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.
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
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).
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
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.
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.
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.
„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; offcenter 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.
Circuits generating the responses of the ganglion cells
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.
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 oncenter bipolar cell to become hyperpolarized, the opposite effect of
illumination in the receptive field center.
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.
Cone and rods circuits
Day: Rods are
saturated.
Information from
cones is passed to
‘On’ i ‘Off’ bipolar
cells and from
there to ganglion
cells of the X type
near the fovea.
Covergence:
in a cat: 36:9:1,
in primates) 1:1:1.
Sharp and color
vision.
Night: Rods are
operating. Cones
are inactive but
their responses
may be recorded
due to electrical
synapses from
rods (convergence
50:1). In complete
darkness the cones
are completely
inactive and the
rods transmit
signals through
bipolar cells
(convergence
1500:100:1). Loss
of visual acuity
and color vision
but increase in
sensitivity.