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Summary Overview neuronal networks and behaviour!
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Sensory systems1. Reception Sensory receptors translate the energy of the stimulus into electrical signals (modality (type), location, intensity and timing)
2. Transport: Axons transport the signal to the series of relay nuclei (parallel processing, topographical representation, cross-over, feedback connections)
3. Processing Interneurons and local circuitry in nuclei process the signal
Visual system Auditory system
Reception
- Modality Photoreceptors (in rods & cones) activated by photons/light Mechanical receptor (hair cells) activated by displacement of stereocilia
types of sensory
receptors Retina Cochlea
• Mechanical (touch and - Inner segment: cell body/ nucleus - The cochlea contains sound detectors: hair cells
proprioception, hearing, - Outer segment: rods and cones - Organ of Corti
balance) o Rods and cones contain these photoreceptors in their o Tectorial basilar membrane vibrates with the frequency of
- physical stretch optic discs which are folded membranes that increase sound
or tension on the surface o Movement of the tectorial basilar membrane bends the
receptor deforms Rods have longer outer segments, more optic stereocilia of the hair cells
the membrane discs and thus more photoreceptors. Rods are Stereocilia are connected to each other via tip links, the
and opens the more sensitive to light. tip links will physically open the ion channels upon fluid
channels o The outer segments lay in pigment epithelium which vibration causing movement
• Chemical (pain, itch, removes disks and regenerates photopigment (takes o Inner and outer hair cells convert the mechanical motion of the
smell, taste) approx. 12 days). This is necessary as discs become tectorial basilar membrane into neural signals
- binding of a desensitized and bleached over time.
chemical to the
receptor (often Distribution of cones and rods
have 7 - Cones mostly at the fovea, non in the periphery
transmembrane - Rods are high in the peripheral part but close to the fovea.
domains and will - The more to the fovea, the more photoreceptors
have a
conformational Differences between rod and cone: light response and connectivity
change upon - Cones are responsible for visual acuity and color: one-to-one
chemical binding) connection to bipolar cells and sharp, short responses
• Photoreceptors (vision: o S cones (small) (blue light) (only 5-10% of the total;
photoreceptors in retina) absent in fovea)
- change in o M cones (medium) (green light)
conformation of a
o L cones (long) (red light)
photosensitive
Ratio of M to L cones differs largely from
protein
individual to individual (no impact on colour
• Thermal (temperature)
perception)
(are less studied)
8% males are colour blind (dichromacy): red-
green colour blindness due to the absence of
, either M or L cones
o Achromatopsia: no cones
o We don’t see absolute colour but we compare out retina
and our visual system and look at the context
- Many rods converge on one bipolar cell (high connectivity), and
have longer responses. They are much better at detecting light
Circuitry: from stimulus Retinal circuitry
to AP - Rods and cones,
- horizontal cells (in-between rods/cones and bipolar cells)
- bipolar cells (switch sign: hyperpolarizing> depolarizing)
- amacrine cells
- ganglion cell (first cell that gives AP)
Extra: Specialized cells Luminance contrast on and off-center ganglion cells
- How does the retina do this?
o Receptive field – part of visual field that one cell monitors
– can be seen as having a center and surround (and is
, thus not only light or no light).
o Different cells with the same receptive field will respond
differently to the center and surround
- How can cells distinguish on-center and off-center?
o On-center and off-center ganglion cells detect contrasts
On center ganglion cells fire AP when there is a
light spot in the center
Off center ganglion cells fire AP when the
surrounding is brighter than the center
When both center and surround are luminated,
both on and off-center ganglion cells fire but at a
lower frequency and similar frequency no
contrast detection
-
- Location Topographical arrangement: Retinotopy in retina Topographical arrangement: Tonotopy in cochlea
What is the position of - Each point of the visual space will be perceived by a certain part
the stimulus relative to of the retina
the body?
• Topographical Spatial resolution: fovea in retina
arrangement of neuronal
receptive fields On- and off-center
• spatial resolution is
determined by the size of
receptive field and density
of receptors.
- Intensity Phototransduction The cochlea: two compartments with different cation concentrations
How strong is the - Scala media contains endolymph that has high K+ and low Na+
stimulus? Presence of light concentration, this drives K+ into the cells (+80mV)
• Sensory threshold When the light hits the photoreceptors, these cells will respond with - Scala tympani contains perilymph (high Na+, low K+) (0mV)
(determined by the graded changes (more light> higher response) in their potential by
sensitivity of the hyperpolarization The organ of Corti, with the hair cells (-45mV to -60mV) is in the middle of
receptors). these fluids. The difference in the potential between the cell and the fluid is
How much stimulation is Principal steps of phototransduction: single photon closes ~ 200 channels around 125 mV
needed to trigger action - Light is absorbed by and activates pigment molecules - Potassium channels in the apical of the cell
potentials? (photoreceptors) (opsin or rhodopsin in rods). The photoreceptor o when open, K+ will go in (as endolymph has high K+)
• Change in energy of the changes its conformation starting an intracellular cascade. o Depolarizationca2+ channels open causing NT release
stimulus - The activated pigment stimulates a G protein (transducin), which - Potassium channels at the base of the cell
- change in membrane in turn activates cGMP phosphodiesterase. This enzyme catalyzes o when open K+ will flow out (as perilymph has low K+)
potential the breakdown of cGMP to 5-GMP. o hyperpolarization
- translation into digital - As the cGMP concentration is lowered, the cGMP-gated channels *K+ can both depolarize and hyperpolarize
code of action potentials close, reducing the inward current and causing the photoreceptor
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