Lecture 4, 5

Question 1

How does the olfactory system function?

Answer 1

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

Question 2

What are the transduction mechanisms in receptor cilia for odor?

Answer 2

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

Question 3

What are the 3 steps in transduction in olfactory receptor neurons?

Answer 3

1. Odorants generate slow receptor potential in the cilia
2. The receptor potential propagates along the dendrite and triggers a series of action potentials within the soma of the olfactory cell
3. The action potentials propagate a long the olfactory nerve axon

Slide 3 lecture 4

Question 4

What are the 3 ways we distinguish between odors?

Answer 4

1. Olfactory population coding- each odor is represented by the activity of a large population of neurons
2. Olfactory spatial maps- the neurons responsive to particular odors may be organized into spatial maps
3. 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

Question 5

What are the central olfactory pathways?

Answer 5

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

Question 6

Where do olfactory receptor neurons expressing a particular receptor gene all send their axons?

Answer 6

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

Question 7

What is the zone-to-zone protection in the olfactory system?

Spatial maps

Answer 7

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

Question 8

What is spatial code?

Answer 8

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

Question 9

What is temporal coding in the olfactory system?

Answer 9

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

Question 10

How do you tune the specificity of mitral cells?

Answer 10

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

1. The overall stereochemical structure of the hydrocarbon chain
2. The type and position of the attached functional group

Slide 14 lecture 4

Question 11

What is lateral inhibition?

Answer 11

Using inhibitory interneurons to enhance contrast

Question 12

What are the interactions between mitral cells and granule cells?

Answer 12

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

Question 13

What maintains the segregation of information in the olfactory bulb?

Answer 13

Not the olfactory cortex
Individual cortical neurons are more broadly tuned to different odors than in the olfactory bulb

Slide 17 lecture 4

Question 14

What are anosmias?

Answer 14

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

Question 15

How can odorants elicit physiological responses?

Answer 15

Visceral motor responses to food (salivation and increased gastric motility) or a noxious smell (gagging)

Question 16

How can odorants influence reproductive and endocrine functions?

Answer 16

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

Question 17

What is the fovea?

Answer 17

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

Question 18

How does viewing light work for us?

Answer 18

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

Question 19

How do we convert light into neural signals?

Answer 19

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

Question 20

What is colour blindness?

Answer 20

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

Question 21

What is the peripheral retina?

Answer 21

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

Question 22

What is the central retina?

Answer 22

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

Question 23

What is the blind spot in the eye?

Answer 23

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

Question 24

What is the phototransduction pathway in the eye?

Answer 24

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

Question 25

What happens when light is shined on a photoreceptor (rod and cone)?

Answer 25

Leads to membrane hyperpolarization
Photoreceptors release fewer transmitter molecules in the light than in the dark

Slide 12 lecture 5

Question 26

What is the path for information flow from photoreceptors?

Answer 26

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

Question 27

What are the 2 classes of bipolar cells?

Answer 27

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