NEUR30002 Lecture Notes - Lecture 7: Taste, Ribbon Synapse, Neural Tube
13 Aug 2018
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L7
Special sensory transduction mechanisms
What are the
special senses?
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- taste is a sense since it is responding to chemicals within the mouth
- this is similar to olfaction which is responding to chemicals in the nose
- smell and tast gives flavour which is not present in sensory in transduction
- flavour is a contruct in the brain
- it involves appearance and the texture as well as the smell
- flavour even comes from sound
- flavour involves vision because the TV shows cooking
- the ones that are about cooking are all about seeing rather than any smelling or
eating
- you can still look at it and say you want to eat it even if there isnt any smell or taste
Vision
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- the eye which operates as a camera where at the front is a lens and then in the back
is photo detectors
- within the eye the retina is a multi layer strucutre
- in vertebra its remarkebly similar
- on the inner side is ganglion cells, going back up you get layers of bipolar cells and
anachrome cells
- in the photo receptor layer is the detectors which have a class called horizontal cells
which run across the retina rather than through
- the vast majority of the retina, for the light to pass they need to get through the
other layers which may create distortions
- in humans and other primates we have a place called a fovea where the other layers
are pushed apart so that the light falls directly onto the photoreceptors
- as a result this is the area of the higher visual acuity
- this is what you use to focus on what you are reading at writing
- this is why in low light you cant see terribly well since that region doesnt respond in
low light
- in the fovea they are high intensity photoreceptor
- you have rods and cones
- cones are found in the outer part but are in higher number in the fovea
- other animals dont use such high pricision vision
- prey animals sheep cows and horses have a visiual streak which is a horizontal line
of high density detectors which look and focus on the horizon for danger
- the retina is important
Photo receptors
are
hyperpolarized by
light
-
- a key difference between mechano and visual sensitive
- photoreceptors are hyperpolarised by the presence by the stimulus meaning they go
away from RMP
- in rods they have 2 charactersitics of interests
- rods are dark adapative and so can detect a single quantam of light
- one light particle is enough for a fully dark adaptive rod to pick up
- normally the rod is not that dark adaptive
- the other feature is that the duration of the response is really long
- these are 300msecond up to 600 milliseconds long
- they give a response a very long time after the stimulus
- it takes about 50 milli second for a respond even with cones and rods
- you are running 200-500 milliseconds behind in terms of your visual world
- the people who are faster than that respond earlier to stimuli and so these people
are better at sports
How it happens?
-
- this shows what is going on in the dark
- in the dark a rod or cone which have the same biochemically process the difference
is in the opsin protein
- you normally have a resting flux of Na going into the cell which keeps it effectively
depolarised
- there is continuous efflux of K and so that whole thing is generating a current which
keeps the cell in the RM state
- when light falls on the retina it activates opsin which interact and activates an
enzyme that breaks down cyclic GMP
- the Na channel which were open are cyclic GMP dependent and so Na stops coming
into the cell and the cell hyper polarised
- it happens very quickly
-
How it happens 2
-
- light interacts with opsin which interacts and breaks downd cyclic GMP and low
cGMP closes the Na channels which are dependent
- rods dont care which wavelength of light they are dealing with
- Cones prefferentially chage with the wavelength of the light
- meaning blue opsins respond to shorter wavelength
- this allows colour vision
- there are some that dont have complete colour vision becasue they dont express
differential opsin and so they become red green colour blind
- in rarer cases people become yellow blue colour blind
- essentially they see in grey scale
- even though this is second messenger this is a very fast process
- it is still slow in the outside world
- this is why you are running a bit behind
What happens
next
-
- you go up and current is recorded as larger as you go up, this is because of
convention
- this is important to be aware of these conventions
- current inward is seen as going down and outward current is seen as going up
- outward current is hyperpolarising
- positive charge is going out of the cell in hyper polarising
- current responds to a light stimulus
- the resting potential for the rods are significantly more negative, its just part of the
process
- the outward current is the same as each one
- both cones and rods dark adapt given enough time
Document Summary
What happens next light interacts with opsin which interacts and breaks downd cyclic gmp and low cgmp closes the na channels which are dependent rods dont care which wavelength of light they are dealing with. Cones prefferentially chage with the wavelength of the light. To be perceived signals must reach neurons that generate action potentials there are two basic sorts that we can deal with. When light falls on them, they turn off the ganglion cell. When light falls on their receptors the ones that contact the on center will release glutamate in a normal way that glutamate depolarises and in turn leads to depolarisation of ganglion cells ef. Why is this set up this way. To be perceived signal must reach neurons that generate action potentials. When the absesnce of ap in the ganglion cell there are no signal to the brain and you.
