Special senses Flashcards
How does perceived sensation occur?
• Perceived sensation occurs only after impulses are interpreted by the brain. Steps involved in sensory perception include:
o Stimulus-> receptor-> action potential propagation to CNS->interpretation
What is sensation?
o Sensation- detection of physical properties of stimuli (colour, brightness, warmth or sweetness)
What is perception?
o Perception- conscious sensory experience (interpretation of stimuli)
What are psychophysics?
• Psychophysics- the study of the relationship between physical stimuli and their perceptions produced in a human observer
What is a psychometric function? Describe what variables are graphed in a psychometric graph.
o Psychometric function- describes the relationship between the parameter of a physical stimulus and response. Sigmoid function.
When graphed:
• X axis- stimulus intensity
• Y axis- probability of detection (%)
o Found by measuring human perception according to stimulus intensity
What is absolute threshold?
Absolute threshold- the minimum stimulation required to detect a stimulus 50% of the time
Can receptors only detect large amounts of stimulus or can they detect small amounts of stimulus? Give examples
• Many receptors can detect the smallest physical unit of stimulus possible
o Vision: photoreceptors-> single photon detection
o Olfaction: chemoreceptors-> single odorant molecule detection
o Audition: hair cell->motion of only a few angstroms for detection
What are environmental stimuli significant in sensory perceptions?
o Electromagnetic and thermal energy
o Mechanical energy and mechanical force
o Chemical agents
What environmental stimuli are included in the electromagnetic and thermal energy category?
Light Infrared radiation Thermal Electric Magnetic
What environmental stimuli are included in the mechanical energy and mechanical force category?
Sound and sonar Touch and vibration Pressure Gravity Inertia
What environmental stimuli are included in the chemical agents category?
Taste
Smell
Humidity
What are sensory receptors? What can they do and which form can they take?
• Sensory receptors- specialized sensory cells that detect changes inside (interoceptors) and outside (exteroceptors) the body
o Receptors can initiate nerve impulses by opening or closing stimulus-gated channels
o Receptor cells can be neurons (somatosensory/visual) or non-neural cells (hearing/gustatory)
Describe the 5 main groups of sensory receptors
- Photoreceptors
- Mechanoreceptors
- Thermoreceptors
- Chemoreceptors
- Nociceptors
What is the energy handled by photoreceptors? Give an example.
Energy handled: Visible light
Example: Vision
What is the energy handled by mechanoreceptors? Give an example.
Energy handled: Mechanical- pressure or physical displacement
Example: Proprioreceptors in joints, receptors for hearing and equilibrium
What is the energy handled by thermoreceptors? Give an example.
Energy handled: Changes in temperature
Example: Heat or cold
What is the energy handled by chemoreceptors? Give an example.
Energy handled: Chemical
Example: Taste and smell
What is the energy handled by nociceptors? Give an example.
Energy handled: Tissue damage
Example: Pain
Does every living organism sense the environment in the same way?
No- Animals can use different ways to sense environment around them
How do fish sense their environment?
o Fish emit electrical discharges into surrounding water via electric organs
Electrosensory organs detect changes in shape, amplitude or frequency of the electric field. Useful for:
• Hunting and defense
• Communication
• Navigation
How do snakes sense their environment?
o Snakes can sense infrared radiation by using pits on the face
Pits- contain terminals of trigeminal nerves and they contain receptors which are sensitive to heat/infrared radiation
How do bats/dolphins sense their environment?
o Bats/dolphins- echolocation
What are special senses?
o Special senses- their receptors are localised in a particular area (gustatory, olfactory, visual and auditory system)
What are general senses?
o General senses- their receptors are widely distributed in the body (pain, touch, pressure and proprioception-somatosensory system)
What is sensory transduction?
o Conversion of environmental energies into signal understood by body (change in membrane potential)
Describe the process of sensory transduction (generally)
o When the sensory receptor interacts with the adequate stimulus, it will produce a conformational change in the receptor and produces a change in conductance (G- membrane becomes more permable to specific ions)
Conformational change can lead to the opening of a channel within the receptor itself OR
Can also lead to activation of second messenger cascade that leads to either the opening or closing of a membrane channel
o Influx/eflux of the ions generates a receptor/generator potential
Chang in membrane potential is the important part- doesn’t matter if the change is a depolarisation or hyperpolarisation
o Conversion of energy of stimulus into electrical signal
o Receptors produce graded potentials
o Response will be passed to postsynaptic cell
o Neurons further down the circuit will produce action potentials that will carry information to rest of CNS
o When signals reach cerebral cortex and are interpreted, produces awareness of stimulus
What are 3 different transduction channels?
o Cyclic nucleotide gated channels o TRP (transient receptor potential) channels: o Other channels Piezo
What are two types of cyclic nucleotide gated channels and what are their respective sensory systems?
cGMP-> vision
cAMP-> pain
What stimuli do TRP channels respond to?
Temperature Touch Pain Pheromones Taste and other stimuli Osmolarity
Are piezo channels evolutionary conserved in animals and plants or are they recently developed channels?
• Piezos (Piezo1 and Piezo2) are evolutionarily conserved in animals and plants
Do piezo channels have sequence homology to other known ion channel families?
• No sequence homology to other known ion channel families
Where is Piezo 2 expressed?
- Piezo 2 expressed in a subset of DRG neurons that innervate the skin and hair follicles (low threshold mechanoreceptors- lanceolate endings, circumferential endings and Meissner corpuscles)
- Piezo2 is also expressed in Merkel cells (skin touch receptors involved in the detection of light touch)
What is the structure of Piezo proteins and what is known about their sensing mechanisms and structures?
- Sensing mechanisms and structures still unknown
* Have 18 and potentially up to 38 transmembrane spanning domains
What is known about the structure, history and activation mechanisms of TRP channels?
o Highly conserve channels- preserved in worms, flies mice and humans
o 6 transmembrane spanning domains
o Activation mechanisms still unknown
What are TRP channels sensitive to?
o Sensitive to plant derived chemicals (capsaicin, menthol, camphor, cannabinoids etc)
What are Group I TRPs?
TRPC TRPV TRPM TRPA TRPN
What are Group II TRPs?
TRPP
TRPML
Describe where TRPV is found and what it is activated by
- Present in somatosensory system in nociceptors
* Activated by temperature, pH, capsaicin, endocannabinoids…
Are vampire bat TRPV channels different than normal channels? Why/why not? Describe the experiment that found this result
• Vampire bats have splice variant of TRPV1 channel (shorter channel) that activates at lower temperatures compared to normal TRPV1 channel
• Experimental procedure: Hek cells transfected with 2 different isoforms of TRPV channel (normal and vampire bat variant) and observe the temperature at which they are activated
o Temperature range: 24 C, 29 C, 35 C, 42C
Are TRPA channels in rattle snakes the same as normal channels? Why/why not? Describe the experiment that found this
• Rattlesnake adopted TRPA1 (traditionally not sensitive to heat, instead to plant derived chemicals) and have a genetic variation that makes TRPA1 heat sensitive- has lower temperature threshold for activation
• Experimental procedure: Hek cells transfected with 2 different isoforms of TRPA channel (normal and rattlesnake variant) and observe the temperature at which they are activated
o Temperature range: 24 C, 29 C, 35 C, 42C
Why do small stimulus signals have to be amplified?
- Small signals do not have the energy to generate a behavioural response and must be amplified within the organism by its metabolic and neural machinery
- A highly sensitive amplification system would cause problems because living organisms are constantly bombarded by background stimuli such as light, odour and sound, which could saturate the sensory system
What is amplification?
• Amplification- the strengthening of stimulus energy by cells in sensory pathways
How does signal amplification occur in general? Give examples
• Signal amplification can take place in accessory structures (for example ossicles in middle ear) or be a part of the transduction cascade in sensory cells (for example receptor-> GPCR-> AC-> cAMP-> cellular response)
o At each step in the cascade, the number of activated products is much greater than in the preceding step- this is how change in membrane potential can be substantial)
Are receptor responses to a stimulus graded or all-or-none?
- Receptor response to a stimulus is graded (non-regenerative- die out with time and space)
- Some receptors display purely graded responses (generator potential)-
What are generator potentials and how do they different from action potentials?
purely graded responses (generator potential)- responses of varying amplitude proportional to the strength of the stimulus
-action potentials: when threshold is exceeded (all or none)- relatively the same amplitude
What is receptor adaptation? How can receptors adapt?
• Adaptation- change in the strength of a response in the continuous presence of the stimulus
Adapt by:
-Adaptation of signalling cascade
-Adaptation of accessory structure
How do photoreceptors adapt to continuous light?
o Photoreceptor adaptation- adaptation of signalling cascade
In phototransduction, there is a cGMP channel in the membrane
Allows for influx of sodium and calcium
When channel is activated for too long, there is an increase in intracellular calcium concentration
Calcium inside the cell has several targets and it produces a number of responses
• Stimulates cyclase activity that produces GMP
• Increases channel affinity for cGMP
• Affects transduction cascade
o Changes in signalling cascade that lead to adaptation
Why does the Paccinian corpuscle adapt? What stimuli does it respond to and why?
Paccinian corpuscle- respond to touch and pressure
• Pressure on capsule of Paccinian corpuscle deforms fluid-filled layers of capsule
• Stimulate stretch channels which will produce receptor potential
• If pressure is maintained, the inner layers of the capsule slip back and stop exerting the pressure on the axonal membrane and consequently the receptor potential, and hence action potentials, decay
• When stimulus is removed, the deformation produces another wave of activation of channels in membrane of axon, another receptor potential and hence another action potential
• Only responds to onset and offset of stimulus- good to transmit information about changes
What are the two ways in which a receptor can adapt? Give an example for each and the result of these two different ways
• Rapidly adapting- transmit information about changes
o Paccinian corpuscles
• Slowly adapting- transmit information about changes and maintained features of the stimulus
o Pain receptors
How does frequency coding work? Describe the response to a weak stimulus vs a strong stimulus
o Stimulus strength is encoded in the frequency of action potentials
o Weak stimulus will produce a very small receptor potential (graded responses) and produce a small frequency of action potential
o Strong stimulus will produce a much bigger receptor potentials and higher frequency of action potentials
How does population coding work? Describe in terms of the somatosensory system
o Depends on number of receptors activated
For example, in somatosensory system, light pressure will activate only a few receptors while more pressure will activate a large number of receptors
What is a receptive field?
• Area covered by receptors in a sensory unit is a receptive field
What does coding of location depend on?
• Coding of location depends on receptor location
What does acuity depend on?
• Acuity depends on the density of receptors
What is acuity?
o Acuity- ability to discriminate two separate stimuli in a sensory system
What do you need for high acuity?
High density of receptors
Small receptive fields
Low convergence
Describe how convergence of sensory neurons occurs and the result of convergence
o Each receptor picks up information from a specific area
o Sensory neurons converge on single second order neuron
o Receptive field of secondary neuron will be the sum of the receptive field of sensory neurons that converge on it
Hence, secondary neuron cannot discern from which converged sensory neuron the stimulus came from
What is the relationship between convergence, acuity and sensitivity?
o High converge=
Low acuity
High sensitivity
What is the general process for lateral inhibition?
• Stimulus-> responses to stimulus-> inhibition of neighbouring cells-> contrast enhancement
What is the olfactory system? Describe its sensitivity and ability to discriminate
• Chemoreceptive system-highly sensitive and discriminative
Is the olfactory system a new feature of eukaryotes or has it been present for a long time in evolutionary history?
• Structure and function of olfactory system conserved in many eukaryotes
o Evolved from systems designed to detect presence of chemicals in environment
In eukaryotes, what role does the olfactory system play?
o Food search
o Predator and prey detection
o Territory marking
• Role in reproduction (pheromones)
Why can smell of an odour trigger vivid memories very rapidly in a subconscious manner?
• Smell transmits information to old areas of the brain associated with emotion and memory (entorhinal cortex and amygdala)
Where are the olfactory receptors located?
• Receptors in the olfactory mucosa (deep within the nasal cavity)
What are the cells in the olfactory mucosa and what is their purpose?
• In olfactory mucosa, there are:
o Bipolar receptor cells
o Supporting cells- provide structure
o Basal cells- stem cells which replace olfactory sensory neurons when they die (every one or two months)
High specificity of replacement (olfactory receptor replaced by same olfactory receptor)