Lecture 4, 5 Flashcards
How does the olfactory system function?
Abides by the same basic principle that governs other sensory modalities: stimuli at the periphery are transduced and encoded by receptors into electrical signals, which are then relayed to higher-order centers
Olfactory system consists of a layer of olfactory receptor cells, supporting cells, and basal cells
Odorants dissolve in the mucus layer and contract the cilia of the olfactory cells
Axons of the olfactory cells penetrate the bony cribriform plate on way to CNS
What are the transduction mechanisms in receptor cilia for odor?
Odor transduction begins with odorant binding to specific odorant receptor proteins concentrated on the external surface of the olfactory cilia
Each receptor protein has unique structure and bind diff odorants
Slide 2 lecture 4
What are the 3 steps in transduction in olfactory receptor neurons?
- Odorants generate slow receptor potential in the cilia
- The receptor potential propagates along the dendrite and triggers a series of action potentials within the soma of the olfactory cell
- The action potentials propagate a long the olfactory nerve axon
Slide 3 lecture 4
What are the 3 ways we distinguish between odors?
- Olfactory population coding- each odor is represented by the activity of a large population of neurons
- Olfactory spatial maps- the neurons responsive to particular odors may be organized into spatial maps
- Olfactory temporal coding- the timing of action potentials may be an essential code for odor discrimination
Each receptor cell expresses a single olfactory receptor protein (they bind lots of odors, but prefer 1 over the others, the combo of preferences is what allows us to distinguish between smells)
Slide 4-6 lecture 4
What are the central olfactory pathways?
Olfactory receptor neurons sends axons into the 2 olfactory bulbs
The input layer of each bulb contains about 2000 spherical structures called glomeruli
The endings of ~25k primary olfactory axons converge and terminate on the dendrites of about 25-100 second order olfactory neurons within each glomerulus
Slide 7 lecture 4
Where do olfactory receptor neurons expressing a particular receptor gene all send their axons?
Olfactory receptor neurons expressing a particular receptor gene all send their axons to the same glomeruli (very selective)
An array of glomeruli create a map of odor information
What is the zone-to-zone protection in the olfactory system?
Spatial maps
Odorant receptors with highly homologous amino acid sequences tend to be localized in the same zone of the olfactory epithelium
Many bulb neurons may be activated by one odor, but the neurons positions form complex but reproducible spatial patterns
The smell of a specific odor is converted to a specific map within neural space of bulb
Slide 9 lecture 4
What is spatial code?
A code in which information is conveyed by the relative positions of activated neurons
Different odors evoke different patterns of glomerular activation
Slide 10 lecture 4
What is temporal coding in the olfactory system?
Odor info may be encoded by the detailed timing of spikes within cells and between groups of cells as well as by the number, temporal pattern, rhythmicity, and cell to cell synchrony of spikes
Brain analyzes odor not only by keeping track of which olfactory neurons fire but also when they fire
Slide 11-13 lecture 4
How do you tune the specificity of mitral cells?
Odor molecule quality is coded by a combo of activated glomeruli
The molecular receptive range of individual mitral cells consists of a range of odor molecules that share characteristic structural futures
- The overall stereochemical structure of the hydrocarbon chain
- The type and position of the attached functional group
Slide 14 lecture 4
What is lateral inhibition?
Using inhibitory interneurons to enhance contrast
What are the interactions between mitral cells and granule cells?
Mitral cell dendrodendritic reciprocal synapses with granule cells may enhance the contrast between strongly activated and faintly activated glomeruli (lateral inhibition)
This sharpens the tuning specificity of individual mitral cells to odor molecules
Second order mitral cells may therefore be more sharply tuned to specific molecular feature than olfactory sensory neurons are
Slide 16 lecture 4
What maintains the segregation of information in the olfactory bulb?
Not the olfactory cortex
Individual cortical neurons are more broadly tuned to different odors than in the olfactory bulb
Slide 17 lecture 4
What are anosmias?
Chemosensory deficits
May be acquired following chronic sinus infection or inflammation, traumatic head injury, or exposure to toxins
Olfactory loss common consequence of aging- either diminished peripheral sensitivity or altered activity of central structures
Slide 18 lecture 4
How can odorants elicit physiological responses?
Visceral motor responses to food (salivation and increased gastric motility) or a noxious smell (gagging)
How can odorants influence reproductive and endocrine functions?
Phenomenon of synchronized menstrual cycles in women housed in single sex dormitories appears to be mediated by olfaction, infants recognize their mothers within hours after birth by smell
Exposure to androgen and estrogen like compounds at low concentrations can elicit both behavioural responses and different patterns of brain activation in adult female and male subjects
Slide 19 lecture 4
What is the fovea?
Area of retina specialized for high visual acuity in the center of the macula; contains high density of cones
Fovea is why our eyes move around, target what we wanna see at the fovea (for the high visual acuity)
Little divot at fovea for clear image
Slide 2-3 lecture 5
How does viewing light work for us?
There is no such thing as colour in the physical world
Only a spectrum of visible wavelengths of light that are reflected by objects around us
Colours themselves are coloured by the brain
400nm = purple 700nm = red
Slide 4 lecture 5
How do we convert light into neural signals?
Our eyes use rods and cones
Rods- greater number of disks and higher photo pigment concentration 1000 times more sensitive to light than cones
Rods give us no indication of colour
Used for low light levels (twilight vision)
120 million rods
Cones- responsible for our ability to see colour
Contain one of 3 different photopigments
5 million cones, one can detect the tiniest flash of light that reaches only one cone
Slide 5 lecture 5
Slide 8 lecture 5
What is colour blindness?
Missing one cone or another
Or a genetic error (red cone might be subtly changed in that it overlaps with a cone)
Slide 6 lecture 5
What is the peripheral retina?
Much higher ratio of rods to cones higher ratio of photoreceptors to ganglion cells
Peripheral retina much more sensitive to low level lights
Much higher ratio of rods to cones in periphery
Much higher ratio of photoreceptors to ganglion cells
Slide 7-8 lecture 5
What is the central retina?
Cones only
Low ratio of photoreceptors to ganglion cells
Specialized for high resolution vision (since one cell transfers info to one ganglion)
Lateral displacement of cells above photoreceptors at the fovea maximizes visual acuity
No scattered light so no image blur
Slide 7-8 lecture 5
What is the blind spot in the eye?
No photoreceptors where optic nerve leaves eye
We create the world around the black spot in order to ignore it
Brain takes info from around the blind spot and creates imaginary image over it
Slides 8-9 lecture 5
What is the phototransduction pathway in the eye?
Light acts as stimulus
Change in protein confirmation activates the receptor
G protein binds GTP
Decreases second messenger
Decreases Na channel conductance (ion channel closes)
Retina undergoes a conformational change when it absorbs light and activates opsin
Many G proteins are activated by each photopigment molecule
Each PDE enzyme -> many cGMP
In dark Na channel are open, need GMP, light presents itself, phosphodiesterase cleaves GMP, closes sodium channel, less depolarization for channel
Slide 10-11 lecture 5
What happens when light is shined on a photoreceptor (rod and cone)?
Leads to membrane hyperpolarization
Photoreceptors release fewer transmitter molecules in the light than in the dark
Slide 12 lecture 5
What is the path for information flow from photoreceptors?
Photoreceptor -> bipolar cell -> ganglion cell
Only ganglion cells fire APs and are the sole source of output from the retina to the rest of the brain
Contain horizontal and amacrine cells that run horizontally
Interactions with horizontal and amacrine cells that release the inhibitory transmitter GABA influence signalling laterally
Slide 13 lecture 5
What are the 2 classes of bipolar cells?
ON bipolar cells- have G protein couple receptors and hyperpolarize to glutamate released by photoreceptors
OFF bipolar cells- have glutamate gated action channels (AMPA, kainate) and depolarize ti glutamate release
OFF and ON refer to whether cells depolarizes when the light is OFF (more glutamate) or ON (less glutamate)
Slide 14 lecture 5
Slide 1 lecture 6