BIO271 - Lecture 3 - Sensory physiology and chemoreception

where does sensation begin?

-begins in sensory receptor cells

what is the important feature of any stimulus?

-it is its ability to modify the conformation (change) of a receptor molecule

what is facilitated by sensory receptors?

-signal amplification

what is the process of sensation?

1. stimuli

2. goes to selective receptor structures

3. then receptor cell

4. then intracellular amplification of signal occurs

5. which is sent as a neuronal signal to the CNS

what did neurotransmitters evolve from?

-they evolved from ancient signaling molecules

what do many organisms use?

-they use electrical signals, which are really efficient and instantaneous

what do action potentials in non-metazoans involve?

-they involve Ca+

what do animals uniquely have?

-they have unique families of voltage-gated Na+ channels (which evolved much later compared to Ca2+)

how does action potential in paramecium occur?

1. Paramecium cells swim by beating their cilia

2. Ca2+ entering the cilium through voltage-gated Ca2+ channels generates AP

why are unique family of voltage-gated Na+ channels in animals important?

• because they are one of the key evolutionary innovations

• and all animals have at least one gene that codes for voltage-gated Na+ channels

what are sensory receptors?

-they range from single cells to complex sense organs

what are the 6 types of sensory receptors?

1. chemoreceptors

2. mechanoreceptors

3. photoreceptors

4. electroreceptors

5. magnetoreceptors

6. thermoreceptors

what was the current theory made in 2016 about sensory receptors?

-it was that ancient sensory cell types have been reused multiple times in the evolution of complex sense organs

how do create a new cell type?

-by changing the population of transcription factors (TFs)

what can changing TFs lead to?

1. cell type diversification

2. cell type fusion

what is cell diversification?

-the developmental process where undifferentiated stem cells specialize into various distinct cell types with unique structures and functions

(ex. primary mechano-sensory cell that specializes into a hair cell and sensory neuron)

what is cell type fusion?

-a biological process where 2 or more distinct cells combine their plasma membranes and cytoplasm to become a single hybrid cell, often merging their nuclei

(ex. primary mechano-sensory cell + epidermal cell = merkel cell)

what are the 2 steps of sensory reception?

1. reception of the signal

2. transduction of the signal

what is reception of the signal?

• where the stimulus as a physical energy → (binds) to sensory receptor

what is transduction of the signal?

• the process by which receptor cells change stimulus energy into an electrical signal

what does the receptor act as in sensory reception?

-it acts as a transducer

what are sensory receptors?

-they all absorb energy of incoming stimuli and transduce into changes in membrane potential

what is the process of sensory reception?

1. reception → stimulus detected

2. transduction → opening or closing of an ion channel

3. amplification → graded potentials, ΔVm (critical for generating an action potential)

4. transmission → signal sent to integrating center

5. perception → behavioural response

what is the stimulus for chemoreceptors?

-it is a chemical stimulus

what is the stimulus for mechanoreceptors?

-it is a pressure stimulus

what is the stimulus for photoreceptors?

-it is a light stimulus

what are the 2 types of sensory receptors?

1. “sensory neuron” type

2. “epithelial sensory receptor cell” type

what is the “sensory neuron” type?

-sensory neuron where the action potential occurs inside the neuron itself

what is the “epithelial sensory receptor cell” type?

-epithelial sensory cell coupled to a specialize sensory neuron to generate an action potential

what are the 2 types of graded potentials?

1. generator potential

2. receptor potential

generator potential

-sensory receptor is the primary afferent neuron

-△Vm spreads along membrane to trigger zone

receptor potential

-sensory receptor is separate from the primary afferent neuron

-△Vm triggers release of neurotransmitter

-leads to postsynaptic graded potential

where is the trigger zone located?

-it is located analogous (where) to the axon hillock

where are APs generated?

-at the trigger zone, which will initiate AP or not

stimulus modality

-based on type of stimuli, the receptors can detect

-the specific type or category of a stimulus (ex. light, soun) that is detected by sensory receptors

chemoreceptors

-chemicals (ex. smell, taste)

mecanoreceptors

-pressure and movement, touch, balance, bp

photoreceptors

-light (ex. vision)

electroreceptors

-electrical fields

magnetoreceptors

-magnetic fields

thermoreceptors

-temperature

how were sensory receptors usually classified in the past?

-they were classified based on stimulus location (telereceptors, exteroreceptors, interoreceptors, proprioceptors)

-but this tells us little to nothing about how the receptors work

-more of a general classification

how are sensory receptors classified now?

-they are classified based on the type of stimuli, the receptor can detect (stimulus modality)

adequate stimulus

-preferred stimulus modality (most sensitive)

polymodal receptors

-sensitive to more than one stimulus modality

ampullae of lorenzini

-sense organ sensitive to electric fields, temperature and touch stimuli

what can many receptors be excited by?

-many receptors can be excites by other stimuli, if sufficiently strong

(ex. pressure on eyelid → perceive light)

what is coding of sensory information?

-any type of stimulus is converted by sensory receptors to action potentials (all AP are the same) in an afferent neuron

How can an organism differentiate among stimuli, or detect the strength of a signal?

-it can do this through:

• stimulus modality

• stimulus location

• stimulus intensity

• stimulus duration

How the CNS differentiate between APs from different stimuli?

-it can do this through the theory of labeled lines:

• receptor location encodes both stimulus modality and location

• integrating center interprets modality and location

what is the theory of labeled lines?

-by Johannes Muller

-discrete pathway from the sensory cell to the integrating center

-sensory neuronal receptive fields are organized in cortical sensory areas to form topographical maps.

what types of receptors are exceptions to labeled lines?

-polymodal receptors are because they encode modality via temporal patterns of APs

what are topographical maps?

-primary somatosensory cortex: (somatosensory homunculus)

-the larger areas are coded since they are more sensitive

-sensory neuronal receptive fields are organized in cortical sensory areas to form these

receptive field

-the region that causes a response in a particular afferent neuron (can be many sensory receptors present)

-region of the sensory surface that generates a response when stimulated

-each sensory receptor is most sensitive to stimulation of a specific area, which defines this for the receptor

-the brain uses timing difference of receptor activation receptors

how is the location of a stimulus coded?

-it is coded according to which group of neurons is active

what is the exceptions for receptive fields?

-the exception is auditory information

what has no receptive fields?

-neurons do not

population encoding

-overlapping receptive fields allows for improved ability to localize a stimulus, through this phenomenon

lateral inhibition

-signals from neurons at the center of the stimulated area inhibit neurons on the periphery

-enhances contrast between activated receptive fields and inactive neighbors and makes a stimulus easier to perceive; involves pre-synaptic inhibition

(additional cells will be recruited)

How then does an afferent neuron distinguish between a strong stimulus at edge of field and a weak stimulus at center?

-by using more than one sensory receptor cell and lateral inhibition

what does a smaller receptive field entail?

• smaller receptive field → more precise location of the stimulus → greater acuity

where is maximal sensitivity located?

-at the center of the receptive field because of higher density of receptors

what is the process of lateral inhibition?

1. response proportional to stimulus strength

2. lateral inhibitory neurons inhibit pathways A and C (they secrete GABA onto A and C, so it’s just B that’s detected)

3. lateral inhibition enhances contrast

GABA

-inhibitory neurotransmitter

dynamic range

-range of stimulus intensities over which a receptor exhibits an increased response

threshold of detection

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

saturation

-top of the dynamic range (maximal response)

stimulus intensity

-sensory neurons code stimulus intensity by changes in action potential frequency

(ex. strong stimuli → high frequency)

what is the limiting factor that affects the highest AP?

-the absolute refractory period

what is a large dynamic range? (trade-off between dynamic range and discrimination)

-large change in stimulus causes a small change in AP frequency

-poor sensory discrimination

-less sensitive to change in stimulus intensity

what is a narrow dynamic range? (trade-off between dynamic range and discrimination)

-small change in stimulus causes a large change in AP frequency

-good sensory discrimination

-more sensitive to change in stimulus intensity

range fractionation

-groups of receptors work together to increase dynamic range without decreasing sensory discrimination

what is encoding logarithmically?

-encoding a wide range of stimulus intensities using a single receptor cell

• fine discrimination at certain intensities

• coarse discrimination at other intensities

(ex. need much larger change in stimulus to perceive the stimulus)

what is the weber-fechner law?

-resolution of perception diminishes for stimuli of greater magnitude

what are the 2 classes of receptors that encode stimulus duration?

1. tonic receptors

2. phasic receptors

tonic receptors

-produce APs as long as the stimulus continues

-encode duration of stimulus

-can fire at the start of a stimulus

phasic receptors

-produce APs at the beginning and/or end of the stimulus

-encode change in stimulus, but not stimulus duration

receptor adaptation

-AP frequency decreases in response to a sustained stimulus

(ex. forgetting a smell in the house after you have adapted to it for a while)

what are the 2 chemical senses?

1. taste

2. smell

chemoreception

-used by multicellular organisms to sense chemical composition in their external and internal environments

-taste and smell only separated when animals moved to land

what is an example of chemoreception?

-in the sea, all chemicals are dissolved in the same medium (water) there is no need for two separate senses.

-fish and other sea creatures have one general chemical sense

-taste is not just in the mouth. Catfish have chemoreceptors all along their body (a catfish is like a giant tongue), and flies have receptors on their feet so that they can tell immediately upon landing whether an object is good to eat

olfaction

-sense of smell

-the detection of volatile chemicals carried in the air/the distant sense

-chemicals located at some distance from the body

-environmental chemicals from food, predators, potential mates, and particular locations

gustation

-sense of taste

-taste is an immediate sense/ the contact sense

-the detection of dissolved chemicals emitted from ingested food before entering the body

what are the qualities of both olfaction and gustation?

• performed by different sense organs

• use different signal transduction mechanisms

• the incoming/sensory information is processed by separate integrating centers

for many animals, what is the most important sense?

-smell is the most important sense

why is smell such an important sense?

-because of food, predators, potential mates, locations

olfactory mucosa

-is located high inside the nasal cavity and is the site of olfactory transduction

-contains olfactory receptor neurons

olfactory receptor neurons

-have cilia (little hair-like projections) which contain the olfactory receptor proteins

what is the organization of the vertebrate olfactory system?

• odorant-binding proteins → odorant receptors → olfactory receptor cell (olfactory epithelium) → cribriform plate → olfactory bulb → interneuron → CNS

sustentacular cells

-are supporting epithelial cells

-express ACE2 receptors

-physically supports the olfactory sensory neurons

why does COVID19 cause anosmia?

-because of a temporary loss of function of supporting cells (sustentacular cells) in the olfactory epithelium, which causes changes to olfactory sensory neurons, resulting into death of olfactory neurons

(progression of loss of sense of smell)

odorant receptor proteins

-are G protein-coupled receptors that activate adenylate cyclase pathway (metabotropic)

in the signal transduction of olfactory receptor cells, where do APs travel?

-APs travel toward the cell body of the bipolar (different ends) neurons

what additional signal transduction pathways may be involved in odorant detection?

-phospholipase C (PLC)-mediated signal transduction cascade

what is the process of signal transduction pathway in an olfactory receptor cell?

1. binding of odorant to an odorant receptor causes a conformational change

2. the activated G protein G_olf, moves through the membrane and activates adenylate cyclase

3. adenylate cyclase converts ATP into cAMP

4. cAMP opens cAMP-gated ion channels

5. Ca2+ and Na+ enter the cell, causing a generator potential

6. the Ca2+ also opens Ca2+-activated Cl- channels, causing Cl- to leave the cell, increasing the depolarization

7. the generator potential opens voltage-gated Na+ channels, triggering action potentials

odotope

-is a group of odorants that share some chemical feature and cause similar patters of neural firing. Neurons that fire to the same of this are usually located near each other

what does one odorant excite?

-one odorant excites a unique combination of olfactory neurons

what happens if one odor was encoded by 3 different receptors?

-around 1 billion odor combinations could be coded

what are different areas of the mucosa are sensitive to?

-they are sensitive to different types of odorant

how are smells organized?

-smells appear to be organized spatially in the olfactory bulb (similar smells are grouped together)