Permeable Membranes Wk 7

Front 1

In order to interpret an incoming signal, sensory receptor must be able to encode what information about the stimulus into APs:

Back 1

MILD
• stimulus MODALITY
• stimulus LOCATION
• stimulus INTENSITY
• stimulus DURATION

Front 2

Theory of labeled lines?

Back 2

informs on modality and location

• Each receptor cell connects to a specific sensory area of the cortex through one

afferent neuron pathway

• Cortex interprets any signal coming from that receptor cell as having been caused by a particular type of stimulus in that particular

location

Front 3

Sensory units:

Back 3

multiple sensory receptors that form synapses w/ a single afferent neuron.

In general, all of the sensory receptors associated w/ a single afferent neuron are of the same type, thus the theory of labeled- line perception can account for our ability to distinguish among different stimulus modalities.

Front 4

The fundamental assumption of the labeled-line theory

Back 4

THERE IS A DISCRETE PATHWAY FROM A SENSORY CELL TO THE INTEGRATING CENTER.

Front 5

A receptor sensitive to more than one sensory modality likely encodes info ...

Back 5

in the temporal pattern of its APs, so bursts of APs could convey a diff message than a continuous series

Front 6

How would relative firing patterns of several ADJACENT sensory cells carry info regarding stimulus modality?

Back 6

If first receptor very sensitive to stimulus A but less sensitive to stimulus B, while another receptor has the opp pattern, by comparing the relative intensity of the signal coming from the 2 receptors, an afferent neuron could code info regarding stimulus modality.

Front 7

receptive field:

Back 7

corresponds to the region of the skin that causes a response in that particular afferent neuron.

"the region in sensory space (e.g., the

body surface, or a specialized structure such as the retina) within which a specific stimulus elicits the greatest action potential response"

Front 8

How does variation in size of a receptive field change affect detection of stimuli?

Back 8

- Large fields detect stimuli across larger area,

- small receptive fields provide MORE PRECISE localization of the stimulus = greater acuity.

Front 9

Info from a single afferent neuron only signals whether stimulus occurred w/i receptive field, how do animals make this more precise?

Back 9

having afferent neurons w/ OVERLAPPING RECEPTIVE FIELDS.

Stimulus causing neuron A and B to respond must have happened where their fields overlap

Front 10

Population coding:

Back 10

where information about the stimulus is encoded in the pattern of firing of multiple neurons.

Front 11

Lateral inhibition:

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Neurons on either side of highly stimulated neuron don't fire, they are inhibited. This increases the contrast b/w the signals from neurons at the center of the stimulus + neurons on the edge, allowing finer discrimination

Front 12

Lateral inhibition example:

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Weak touch across receptive fields of neurons A,B,C would cause each neuron to release small amount of NT onto its 2nd-order neuron, so all are stimulated.

- a Strong prick in receptive field of neuron B releases lots of NT onto its 2nd order neuron B2. The skin bends slightly due to prick, weakly stimulating fields for A+C.

Lateral interneurons that form synapses b/w the axon terminals of B and A+C receive the strong NT response B. These interneurons release an inhibitory NT that prevents release of NT from A+C, no response from them.

Front 13

Diagram of receptor location by lateral inhibition

Back 13

Front 14

How do APs code INTENSITY?

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thru changes in FREQUENCY: strong stimuli trigger high-frequency APs

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dynamic range:

Back 15

relatively limited range of intensities that sensory receptor cells can encode stimuli over

Front 16

threshold of detection:

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weakest stimulus that produces a response in a receptor 50% of the time

Front 17

At the top of the dynamic range, the receptor cell is:

Back 17

is saturated + can't increase its response even if the signal strength increases.

if all available ion channels have opened or closed. The receptor will also reach the top of its dynamic range if the membrane potential reaches the equilibrium potential for the particular ion involved in the receptor or generator potential (because no net ion movement will occur beyond this point). The max rate of release of NT from the receptor cell, or the maximum frequency of APs in the afferent neuron can also set the top of the dynamic range.

Front 18

How can a receptor reach the top of its dynamic range?

Back 18

- if all of the available receptor proteins become saturated.

- if all available ion channels have opened or closed.

- if the membrane potential reaches the equilibrium potential for the particular ion involved in the receptor or generator potential (because no net ion movement will occur beyond this point).

Front 19

What can set the top of the dynamic range?

Back 19

- Any of the steps in sensory transduction:

- The max rate of release of NT from the receptor cell

- the maximum frequency of APs in the afferent neuron

Front 20

Dynamic range graph

Back 20

Response of receptor increases with increasing intensity

Front 21

Different dynamic ranges graph:

Back 21

Rcpt A - large dynamic range + can detect both very weak + very strong stimuli.
Rcpt B only detect very weak stimuli, + becomes saturated at moderate stimulus levels

Front 22

With a large range of stimulus intensities + the range of AP frequencies is limited, receptor A has ___ power for discriminating differences in intensity

Back 22

relatively low power to discriminate differences in intensity.

A relatively large change in stimulus intensity causes only small change in response of receptor A (see slope), while a small change in intensity causes large change in response of receptor B.

Front 23

With a small range of intensities, B has ___ power for discriminating intensities

Back 23

Higher, B is sensitive to only a small portion of the possible range of stimulus intensities, but it has the ability to provide very fine discrimination w/i that range (as a small change in intensity causes large change in response)

Front 24

Range fractionation strategy:

Back 24

Use behaviours of populations of receptors to improve sensory discrimination: groups of receptors, each sensitive to a different range of stimulus intensities, can work together to provide fine discrimination across a wider range of intensities

Front 25

individual receptor cells are sensitive to only a small portion of possible range of intensities, so how does range fractionation work?

Back 25

Multiple receptors cover different parts of the range.

This system does intensity thru behaviour of populations of sensory receptors.

Front 26

Range fractionation graph:

Back 26

rcptrs work together to provide fine discrim across wide range of intensities

Front 27

upper limit of the frequency of APs is set by

Back 27

by refractory periods of the v-gated channels involved in the AP.

Front 28

How can a sensory receptor code for such a wide range of stimulus intensities (1-1.4 million) with such a small range of AP frequencies (1/sec to 1000/sec)?

Back 28

- Range fractionation can extend the dynamic range

- BUT LOG ENCODING!! many receptors use this strategy

Front 29

Log encoding:

Back 29

the response increases linearly w/ the log of the stimulus intensity
Curve represents compromise b/w broad dynamic range + fine discrimination b/w similar stimulus intensities
allows receptor to have a constant response to a given % change in stimulus intensity

Front 30

Log encoding graph:

Back 30

fine discrimination at low intensities + coarser discrimination at high intensities (limited change in response when large change in intensity)

Front 31

Weber-Fenchner relationship

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= log relationship b/w actual and perceived stimulus intensity

Sensations that obey the Weber-Fenchner relationship: brightness, loudness, and weight.

Day/night candle, book on sofa u carry

Front 32

Two functional classes of sensory receptors that code for stimulus DURATION:

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• tonic receptor
• phasic receptor

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• tonic receptor

Back 33

fire APs as long as stimulus continues, so can convey info about HOW LONG stimulus lasts. Most do not fire APs at same frequency throughout prolonged stimulus

Front 34

tonic receptor graph

Back 34

are depolarized throughout the duration of a stimulus, can show adaptation

Front 35

receptor adaptation

Back 35

Typically AP frequency often declines if stimulus intensity is maintained at a constant level

Front 36

phasic receptor

Back 36

Some receptors that adapt so quickly that they produce APs only when stimulus begins

-code changes in the stimulus but do not explicitly encode stimulus duration

Front 37

phasic receptor graph

Back 37

depolarize only at beginning, adapt rapidly

Front 38

olfaction

Back 38

= sense of smell, the detection of chemicals carried in the air. provides the ability to sense chemicals whose source is locate away some distance

Front 39

gustation

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= sense of taste, detect dissolved chemicals emitted from ingested food

Front 40

How are olfaction + gustation distinct from one another?

Back 40

based on structural criteria;

- are performed by diff sense organs,

- use diff Signal transduction mechanisms

- separate integrating centers process the incoming info from the senses of taste + smell

Front 41

Odorant receptor proteins:

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GPCRs. Each odorant receptor cell expresses only a single kind of odorant receptor protein.

Front 42

Odorant pathway:

Back 42

Receptor change signals G(olf) to activate, which signals via adenylate cyclase, activating cAMP pathway that eventually -> depolarizing generator potential.
- APs triggered in dendrite of olfactory receptor neuron, these APs travel TOWARD THE CELL BODY of this BIPOLAR neuron, (as opposed to motor neuron). These APs transmitted to axon terminals form synapses w/ neurons of brain olfactory bulb.

Front 43

bipolar olfactory neuron image:

Back 43

Front 44

odorant pathway diagram:

Back 44

-cAMP opens cAMP-gated ion channels, allowing Ca2+/Na+ to enter (generator potential)
-Presence of Ca2+ opens Cl- channels, so they leave, increasing depolarization
- So depolarized that v-gated Na+ channels open, AP OCCURS!

Front 45

Some odorant receptors are coupled to PLC cascade:

Back 45

PLC hydrolyzes PIP2 in the PM, producing IP3 and DAG, resulting in increase in intracellular Ca2+ just like in cAMP pathway

-This causes PM Cl- channels to open, ultimately depolarizing cell even more, triggering APs.

Front 46

How do you account for the fact that the total # of odors that an animal can distinguish is even larger than the amount of genes coding for odorant receptor proteins?

Back 46

1 olfactory neuron -> 1 odorant receptor gene, but each odorant receptor can recognize more than 1 odorant. Thus a given odorant excites multiple olfactory neurons.

combinatorial code: Even if each odorant were coded by a combo of only 3 diff receptors, there would be approximately 1 bill potential combos. The code for each odor actually involves more than 3 receptors!

Front 47

Unlike olfactory, the gustatory system not able to discriminate among 1000s: grouped into 5 classes:

Back 47

• salty
• sweet
• bitter
• sour
• umami

Front 48

What do the different tastes indicate?

Back 48

Sweet, umami, and salty tastes indicate nutritionally important carbs, proteins, and ions,

bitter + sour tastes indicate toxic substances

Front 49

In terrestrial vertebrates, Taste receptor cells found on:

Back 49

• tongue,
• soft palate
• larynx
• esophagus

Front 50

Taste buds:

Back 50

onion-shaped structures that contain multiple taste receptor cells (b/w 50 and 100), with a pore that opens out to surface of the body.

Front 51

Taste bud diagram:

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apical surface of the taste cell is folded into numerous microvilli, which contain the receptors + ion channels that mediate the transduction of the taste signal.

Front 52

tastants,

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= Dissolved chemicals from food,

enter thru this pore + contact taste receptor cell.

Front 53

What chemicals are flavours conveyed by:

Back 53

• SALTY: conveyed by Na+ ions in food
• SOUR: " " H+ ions
• SWEET/UMAMI: sugars + related org molecules, AAs+related mols
• BITTER: wide range of org molecules, including caffeine, nicotine, and quinine.

Front 54

Salty pathway:

Back 54

Salty substance receptor is actually not receptor but an Na+ ion channel, so it enters cell and so depolarizes it, triggering v-gated Ca2+ to release NTs

Front 55

Salty pathway diagram:

Back 55

Front 56

The Na+ channels involved in "salty" are also permeable to H+ ions, what does this indicate?

Back 56

may play role in perception of sour.

Since Na+ and H+ compete for the channel, these channels are probably important for sourness perception only in species with relatively low Na+ levels in their saliva, use these channels to detect sourness. we have high saliva Na+ so can taste sourness thru other mechanisms.

Front 57

Example of a sour transduction mechanism:

(there are many)

Back 57

Apically localized K+ channel is blocked directly by H+ protons.

Blocking these K+ channels leads to depolarization of the taste cells by decreasing K+ permeability and altering the resting membrane potential (Goldman eq).

This depolarization opens v-gated Ca2+, causes NT release.

Front 58

Sour pathway diagram

Back 58

Front 59

Sweet signal:

Back 59

Sugars bind to GPCRs at the apical cell surface, activating G protein gustducin, which signals thru an adenylate cyclase signal transduction pathway

(with cAMP, protein kinase that closes K+ channels, depolarizing for v-gated Ca2+, releasing NT).

Front 60

Another pathway for sweet (artificial)

Back 60

IP3 instead of adenylate cyclase, but same closing of K+ channels, depolarizing etc

Front 61

Sweet pathway diagram

Back 61

Front 62

Sweetness receptors are sensitive to:

Back 62

Sweetness receptors are sensitive to:
• monosaccharides
• polysaccharides
• high potency sweetener
• some amino acids

Front 63

Umami detected by 2 different kinds of receptors:

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• 1 similar to sweet receptors
• 1 similar to glutamate receptors found in brain

-- Umami caused by L-glutamate and other AAs found in foods, as well as the food additive MSG,

Front 64

Umami pathway with glutamate receptors:

Back 64

When glutamate binds to this modified glutamate receptor, conformation changes, activating an associated G protein, which then activates a phosphodiesterase that degrades cAMP into AMP.

->The decreases in cAMP thought to trigger NT release, though precise pathways involved have not yet been ID'd.

Front 65

How is Bitter very complex?

Back 65

more specific than sweet, we have at least 25 genes coding for bitter-taste receptors, and each taste cell that is sensitive to "bitterness" expresses many of these genes.

The way this complex pattern of expression is translated into perception of bitterness is unknown but we know the signal transduction mechanisms

Front 66

Bitter pathway:

Back 66

Activated G protein transducin activates PLC-> split into IP3 -> causes release of Ca2+ from intracellular stores-> NT release

Front 67

Bitter diagram:

Back 67

Front 68

What has to happen for you to be able to take in

sensory information from your surroundings?

Back 68

1. Sensory receptor detects signal

2. Transduce signals into a form that can be interpreted

3. Amplifies signals (maybe)

4. Signal sent to an integrating centre (e.g. brain)

Front 69

adequate stimulus:

Back 69

Most receptors are most sensitive to one specific type of stimulus

Front 70

polymodal receptors

Back 70

Certain receptors respond to several different stimuli with similar sensitivity

Front 71

example of a polymodal receptor

in humans?

Back 71

eg: TRP channels

• TRPV1: heat, acidic pH, and capsaicin

• TRPM8: cold and menthol

Front 72

Olfactory receptor neurons located in:

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olfactory epithelium.

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Olfactory bulb:

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where projection neurons are located.

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• Olfactory tract:

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projects to olfactory cortex via limbic
system.

Front 75

Taste travels pathway:

Back 75

Taste buds -> cranial

nerves -> brain stem - >

thalamus and limbic

system