BIOL21341 Sensory Systems

Define sensory systems

Sensory systems provide an interface between the world and our brains

What percentage of the brain is devoted to sensory systems?

Around 50%

What is interoception?

The sense of the internal state of the body

Give an example of interception

Levels of CO2 in the blood

What is exteroception?

The sense of the external environment of the body

Give two examples of exteroception

Vision and hearing

What is the role of the vagus nerve?

The vagus nerve innervates many organs and signals changes in the internal environment to the brain

Define proprioception

The ability to tell where our body is in space and to sense the relative locations of different body parts.

Define sensory receptors

Specialised excitable cells sensitive to a form of physical energy (modality)

Define modality

A physical energy that is detected by a specific receptor

What receptor type responds to mechanical energy?

Mechanoreceptors

What receptor type responds to thermal energy?

Thermoreceptors

What receptor type responds to chemical energy?

Chemoreceptor

What receptor type responds to electromagnetic energy?

Photoreceptors

Light is an example of what type of modality?

Electromagnetic

What receptors type are pain receptors?

Chemoreceptors

Give two examples of mechanoreceptors

Pacinian corpuscle and hair cells

Define sensory transduction

The conversion of physical or chemical stimulus into a receptor potential

How does sensory transduction differ from synaptic transmission?

Synaptic transmission release neurotransmitters that trigger ion gates to open, but the physical energy causes ion gates to open in sensory transduction

What are the two way receptor potentials can be converted into action potentials?

1. Conversion can occur in the same neurone

2. First cell produces receptor potential, then generates a action potential in a separate afferent neurone

Define an afferent neuron

A neurone in the peripheral nervous system that conducts action potentials to the CNS

How would a large stimulus be translated into an action potential?

The frequency of action potential firing would increase

How would a small stimulus be translated into an action potential?

The frequency of action potential firing would decrease

Define sensory coding

Representation of a stimulus in terms of an action potential

What is modulated to translate a strong/small stimulus?

Frequency

What type of ion gate open in response to hair cell vibrations in the cochlea?

Potassium ion gates

What are the different areas in the cochlea responsive to?

Different frequencies

What two components make a sensory unit?

Receptor + Afferent

What is a topographic (biology) map?

The ordered projection of a sensory surface to one or more structures of the central nervous system.

What is stereognosis?

a sense that allows a person to recognize the size, shape, and texture of an object

Why do adult sea squirts digest 1/2 of their nervous system?

As lava they are motile, but in the adult form they have little need for 100s of neurones as they don't move.

What is the sense of taste called?

Gustation

What is the main role of gustation?

Allows us to identify foods that could be dangerous

What are the 5 dimensions of taste?

Sweet, sour, sour, bitter, salty and umami

What molecule induces the taste of umami?

Glutamate

When was umami accepted as the fifth taste by the scientific community?

1990

Sensory cells connect directly the brain? True or False (Explain what they connect to)

False - they release neurotransmitters detected by the cranial nerves

What are the two taste cells? (The ones that detect ligands)

Type II and III

What are type I taste buds described as? (Explain why)

"Glial-like" they have a supporting role in the taste bud

At resting state what two mechanisms contribute to the resting membrane potential?

1. K+ leakage channels = negative charge in cell

2. Sodium/potassium exchanger

What two tastes are detected by type III cells?

Salty and sour

What 3 tastes are detected by type II cells?

Bitter, sweet and umami

What protein type is used in type II taste cells?

G-protein coupled receptors

What shape change occurs in the G-protein when the tastant ligand binds to the extracellular domain?

The alpha subunit separates from the beta-gamma unit

Give 3 examples of second messengers that can be activated when type II g-protein are separated

1. cAMP

2. cGMP

3. IP3

What is the G-protein called in type II taste cells?

Gustducin

How is phospholipase-C (PLC) activated in type II taste cells?

The beta-gamma subunit activates it

What reaction does phospholipase-C (PLC) catalyse in type II taste cells?

The separation of PIP2 into IP3

What is IP3?

Inositol triphosphate

What is DAG?

Diacylglycerol

What is the role of IP3 in type II taste cells?

IP3 interacts with receptors on the endoplasmic reticulum - releasing calcium ions

What channel does calcium released from the endoplasmic reticulum activate?

TRPM5

What ion (and where is it from) activates TRPM5 channels?

Calcium ions from the endoplasmic reticulum of type II taste cells

What neurotransmitter is released from type II taste cells?

ATP

'Nice' type II taste cells can detect which two tastants?

Sweet and umami

What GPCR do sweet ligands bind to in type II taste cells?

T1R2:T1R3

What GPCR do umami ligands bind to in type II taste cells?

T1R1:T1R3

What type of dimer is T1R2:T1R3?

It is a heterodimer - made of two GPCRs

What other receptors (not GPCRs) may umami activate? (2 types, similar names)

mGluR1 and mGluR4

What receptors do bitter tastants bind to in type II taste cells?

T2R receptors

How many T2R genes are used to recognise bitter in humans?

Around 29

What are the two ways organic acids (that can cross the membrane) are detected?

1. H+ ions diffuse directly into the cell

2. Acidification of the cell

What does acidification of type III taste cells cause?

Inhibition of potassium leakage cells = depolarisation

What neurotransmitter is released from type III taste cells?

Serotonin (5HT)

What neurotransmitter does 5HT refer to?

Serotonin

What channel allows sodium ions into type III taste cells?

ENaC (Epithelial sodium channels)

What is the benefit of having so many T2R (bitter) receptors?

GPCRs have a limited range of ligands they can detect. Having a small range would limit the number of compounds that can be detected as bitter - which could be poisonous

There is a way to distinguish between different types of bitter compounds binding to the wide variety of T2R receptors. True or False

False - there is no way to distinguish

"A 1930s experiment separated humans into those that can taste PTC as bitter and those that can't". What is this an example of?

Mendelian inheritance

A study showed that those couldn't taste PTC as bitter had a higher or lower iron deficiency?

Higher iron deficiency in those that couldn't taste PTC as bitter

Why is Isothiocyanate (found in cabbages) important to limit intake?

Isothiocyanate reduces the uptake of iron

Why is Isothiocyanate described as an 'Iron collator'?

Isothiocyanate reduces the uptake of iron

In what gene were polymorphisms detected that affected PTC sensitivity?

TAS2R38

In the PTC sensitivity experiment what genotype has advantage?

Heterozygous advantage

Why is receptor affinity for different tastants under strong selection?

It balances avoiding toxins, but also eating enough to survive

What is the threshold for quinine hydrochloride (a very bitter compound) in browser herbivores?

3.00 mM

What is the threshold for quinine hydrochloride (a very bitter compound) in humans?

0.03 mM

What 3 cranial nerves are sensory for taste?

7, 9 and 10

Where do cranial nerves synapse with neurones from the brain stem?

At the nucleus of the solitary tract

What does the 'Labelled Lines' prediction of taste separation state?

It suggests that each neurone at each stage of the hierarchy encodes a different dimension of taste leaving the brain to merge

Why is the prediction that each receptor cell should detect a single taste dimension wrong?

As we can distinguish between sweet and umami - even-though they are detected by the same receptor

What are satiety signals?

Signal fulness to the brain

Give an example of learned negative in terms of taste preferences (Hint - radiation)

Foxes radiated after consuming sweeten water (which they like), avoided the water.

What is a learned positive in terms of taste preferences?

Associate the taste with a meal of high dietary reward

Where are olfactory receptors located? (What type of epithelium)

Olfactory epithelium

Axons from the olfactory neurones go directly to the brain (olfactory bulb. True of false

True

What is the role of cilia on olfactory neurones?

They detect odorant's in the mucus

What is the role of odourant receptor proteins in mucus?

Odourant receptor proteins bind to the odourant and transport it to the olfactory sensory receptor binding site

What type of enzymes are located in the mucus that clear the mucus of odourants?

Degrading enzymes

What type of receptor are olfactory receptors?

GPCR

Upon binding to the olfactory receptor what G-protein is activated?

G-olf

What occurs after the G-olf G-protein has been activated?

The alpha subunit dissociates and binds to adenylate cyclase

In the olfactory transduction cascade what reaction does adenylate cyclase? ( ____ → ___ )

Generates cAMP from ATP

What is the role of cAMP in the olfactory transduction cascade?

cAMP causes cation channels in the membrane to open

What two cation move into olfactory neurones to trigger depolarisation after cAMP has opened the gates? (full names)

Sodium and calcium

What is the additional action of calcium cations in the olfactory transduction cascade?

Calcium ions opens chloride channel in the membrane, allowing chloride to exit - increasing depolarisation

Calcium also switches off the olfactory transduction cascade by binding to which compound? (Hint - Calcium ___________ )

Calcium calmodulin

Calcium calmodulin has 3 actions. What are they?

1. Closes cation channels 2. Activates phosphodiesterase 3. Inhibits adenylate cyclase

What is the role of phosphodiesterase in terminated the olfactory signalling cascade?

Phosphodiesterase hydrolyses cAMP

What is the effect of calcium calmodulin inhibiting adenylate cyclase?

Adenylate cyclase produces cAMP from ATP - stopping production reduces depolarisation