Module 15 - Sensory pathways II (hearing, chemical senses, and touch)

Question 1

Olfaction: How does its reception work and where is information passed through to?

Answer 1

Receptors are in the neuron’s dendrites and are exposed to the outside world (in the air space in the nasal cavity)

Neurons that express the same receptor type will converge onto the same place in the brain – a glomerulus

Question 2

Typical pathway of olfaction in humans

Answer 2

Olfactory receptor cells - olfactory bulb -

Question 3

Olfaction and visual: the GPCRs and the ligands

Answer 3

Olfaction:
GPCR - odorant receptors
Ligand - odorants

Visual:
GPCR - rhodopsin
Ligand - light

Question 4

The typical olfaction process

Answer 4

1) Olfactory GPCR binds an odorant which causes G protein activation - its alpha subunit then exchanges GDP for GTP, activating adenylate cyclate

2) When Adenylate cyclate is activated, it breaks down ATP into cyclic AMP.

3) Cyclic AMP then opens cyclic nuc-gated channels, which are permeable to calcium and sodium, causing depolarisation of the olfactory receptor neuron, causing AP firing.

4) Calcium then goes on to activate calmodulin, phosphodiesterases and cam kinase as a complex feedback mechanism.

Question 5

Visual information pathway vs olfactory information pathway

Answer 5

Olfactory - causes depolarisation of the ORN
Vision - causes hyperpolarisation of rod or cone photoreceptor

Question 6

Insect vs human olfactory pathway

Answer 6

Insect olfactory receptors are NOT GPCR’s but instead are ion channels, which are directly opened upon binding of an odorant

Question 7

Combinatorial coding

Answer 7

One odorant may activate several neurons and each with varying affinity/intensity and this may lead to combinatorial coding which allows for the high specificity and identification present in olfaction

Question 8

The olfaction pathway

Answer 8

Olfactory receptor neurons (ORN) -> apical dendrite of mitral cell -> secondary dendrite of mitral cell -> brain

Periglomelular/granule cells connect the synapse of ORNs and mitral cells / connect mitral cells laterally and act as inhibitory cells that allow for processing

Question 9

Only key difference between vision and olfaction

Answer 9

Stimuli and information move in the same direction with olfaction but not with vision

Question 10

The five basic tastes, what they indicate, and what type of receptors are used for them

Answer 10

Bitter - avoid poisons - GPCR
Sweet - sugar & carbohydrate - GPCR
Umami - l-amino acids (monosodium glutamate) - GPCR
Salty - Na⁺ - Epithelial Na⁺ channels (not in humans)
Sour - acids/H⁺ - Trp
Fat – the sixth taste? - ?????

Question 11

The three types of gustatory receptors

Answer 11

Circumvallate - Back of tongue
Foliate - Sides of tongue
Fungiform - front of tongue

Question 12

Pheromones

Answer 12

Vomeronasal organ (Jacobson’s organ)

Used to detect prey and pheromones etc - not found in humans (so far)

Question 13

Hearing: how is sound detected?

Answer 13

Detecting variations in air pressure - humans can detect 20-20,000Hz

Lower frequency (long wavelength) - lower intensity/deeper
Lower amplitude - lower intensity/deeper

• Deeper sounds travel further
• Insects can only hear about 6 inches around them because they detect the speed of particles instead of variations in air pressure

Question 14

Sound detecting system: the components

Answer 14

Auditory system:
External ear - Ear (sound travels through into the ear canal)
Middle ear - Bones (Malleus, incus, stapes)
Inner ear - Cochlea (used to convert air pressure into electrical signals)

Vestibular system:
Semicircular canals - detect head rotation (can have a lag - seasickness/dizziness caused by being stationary but still being told you’re moving)
Otolith organs - use CaCO₃ crystals to detect force of gravity and acceleration (crystals move and activate the mechanical force of hair cells)

Question 14

Cochlea: what is it composed of, how do the membranes cause sound detection, and what do the hair cells do?

Answer 14

Three fluid-filled chambers (scala vestibuli, media, tympani) and the Organ of Corti, which consists of hair cells, support cells and the basilar membrane

Basilar membrane moves due to the movement of fluid, which in turn causes relative movement of the hair cells and the overlaying tectorial membrane. The apical tips of the hair cells are embedded in the tectorial membrane, and are sheared when the basilar membrane vibrates.

There are 3 rows of outer hair cells and one row of inner hair cells in the cochlea. Outer hair cells don’t send signals to the brain, only the inner hair cells do. Instead, the outer hair cells actively amplify minute vibration of the basilar membrane, which in turn increases the responses of the inner hair cells to quiet sounds. The outer hair cells can change their shape due to the molecule called prestin.

Question 15

How is an electric potential activated in hearing?

Answer 15

1) Stereocilia on the apical side of the hair cells are arranged like a staircase, connected by the tip links

2) Mechanical force will pull on the channel (we don’t know what it is) and physically opens it, allowing K⁺ in and leading to depolarization (This mechanotransduction (direct form of transduction) is very fast - K⁺ goes in because K concentration is high in the extracellular fluid)

3) Hair cells will release excitatory glutamate onto the terminals of spiral ganglion neurons, which transmit signals to the brain.

Question 16

How do different frequencies of sound get heard in the cochlea

Answer 16

Hair cells are tuned to progressively lower frequencies from the base to the apex - the basilar membrane is wider and less stiff at the apex, thus resonates at lower frequencies (quieter sounds)

Meanwhile, hair cells further away from the base of the apex are tuned to higher frequencies - only higher frequency sounds (louder) can keep moving that fat

Question 17

Interaural time difference

Answer 17

The position of a sound source is detected by comparing the time of the sound arrival to both ears

Question 18

Mechanosensation: the different types of touch receptors

Answer 18

SAI and SAII:
Low-threshold mechanoreceptors in the skin that respond to steady pressure - Merkel cells and Ruffini endings that sense skin indentations and are responsible for fie discrimination of textures

RAI and RAII:
Rapidly adapting mechanoreceptors that sense vibrations and have wide myelinated nerve fibres for fast conduction

Question 19

What is piezo protein?

Answer 19

The mechanoreceptor in Merkel cells

Question 20

Vestibular cortex: what type of information does it deal with and where is it located?

Answer 20

Auditory information referring to gravity, acceleration, and head rotation

Parietal lobe - connected to the somatosensory cortex, in front of the gustatory cortex and above both the olfactory (to its upper right) and auditory (to its upper left) cortexes

Question 21

Visual cortex: what type of information does it deal with and where is it located?

Answer 21

Visual information

Occipital lobe - at the back of the head, far from every other cortex

Question 22

Auditory cortex: what type of information does it deal with and where is it located?

Answer 22

Sound information based on differences in air pressure

Temporal lobe - below both the gustatory (directly) and vestibular (to its bottom right) cortex and to the right of the olfactory cortex

Question 23

Gustatory cortex: what type of information does it deal with and where is it located?

Answer 23

Taste information

Insular/frontal lobes - below/to the right of the vestibular cortex and above the auditory cortex

Question 24

Olfactory cortex: what type of information does it deal with and where is it located?

Answer 24

Smell information

Temporal lobe - below the vestibular (to its bottom left) cortex and to the left of the auditory cortex

Question 25

Somatosensory cortex: what type of information does it deal with and where is it located?

Answer 25

Touch information

Parietal lobe - At the top of the head, connected to the vestibular cortex

Question 25

Vision: sensory structures, ligand, receptor molecules, carrier of transduction current, cortical area?

Answer 25

Photoreceptors - rods and cones

Light

Retinal + opsins (GPCR)

Na⁺ (hyperpolarisation)

V1 cortex (occipital lobe)

Question 26

Hearing: sensory structures, ligand, receptor molecules, carrier of transduction current, cortical area?

Answer 26

Hair cells

Air pressure

N/A

K⁺ (depolarisation)

Auditory cortex (temporal lobe)

Question 27

Olfaction: sensory structures, ligand, receptor molecules, carrier of transduction current, cortical area?

Answer 27

Olfactory receptor neurones (ORNs)

Odorants

GPCRs

Na⁺/Ca²⁺ (depolarisation)

Olfactory cortex (temporal lobe)

Question 28

Gustation: sensory structures, ligand, receptor molecules, carrier of transduction current, cortical area?

Answer 28

Taste receptor cells (neuroepithelial cells)

Light

GPCRs/TRP (ion) channels/Epithelial sodium channels (ENaC)

Various (depolarisation)

Gustatory cortex (Insular/frontal lobes)

Question 29

Touch: sensory structures, ligand, receptor molecules, carrier of transduction current, cortical area?

Answer 29

Merkel cells, Ruffini endings, Meissner’s and Pacinian corpuscles

Touch/vibrations

N/A / Piezo

Most likely Na⁺/Ca²⁺ (depolarisation)

Somatosensory cortex (parietal lobe)