Principles of Sensation, Touch Flashcards
what is the difference between sensation and perception?
- usually sensation and perception are parts of one continuous process
- sensation - how the brain represents external energy/signals
- perception - organization and interpretation of sensory information
- neuroscientists argue that sound is just the interpretation of energy and wavelength changes
- if a tree falls in the woods and nobody is there, it doesn’t make any sound
what are the different types of processing?
bottom-up and top-down processing
what is bottom-up processing?
- flow of information from sensory input to higher-level cognitive processes
- starts with raw data (sensory input) that the brain organizes and interprets to form a perception or understanding
- look at external world → signal is sent to the brain → sent to specialized areas → becomes a fully perceived image
- most of our perception happens through this
what is top-down processing?
- involves using pre-existing knowledge, expectations, or experiences to interpret and organize incoming sensory information
- we form representations of the world in our brain and when we look at the world, we look for features related to our representations
- ex. we have expectations about the environment/situations
- going to a new classroom, we expect to find desks and tables
- unique feature of humans perceptual system
what is transduction?
process of turning external energy into nervous system signals
how do our senses generally work?
- we receive sensory stimulation (external energy) using specialized receptor cells
- some specialized to pick up fluctuations in air pressure, others to pick up changes in electromagnetic spectrum
- transduction turns energy into NS signal
- turn into receptor potentials, fluctuations in resting membrane potential, not necessarily at a synapse
- neural information is delivered to the brain
what are the characteristics of receptor potentials?
- similar to post-synaptic potentials, but not at synapse
- can be via ionotropic or metabotropic receptors
- ionotropic receptors are faster than metabotropic receptor
what is the process of hearing?
- hair cell is found in inner ear and has cilia on it
- sound is changes in air pressure → arrives at ear → causes eardrum to flap and bones to move → causes liquid inside cochlea to move → ripples cause cilia on hair cells to move back and forth → channels are pulled open as the cilia sways
do we have five sense?
- don’t necessarily have five senses
- many more subcategories of senses
- there are many other senses in animals
- birds can orient themselves to N/S alignment even in a room
what is an example of how our sensory systems have a restricted range of responsiveness?
- the way we sense the world is evolutionarily useful
- visible light is a small part of the electromagnetic spectrum, we cannot see all aspects of the electromagnetic spectrum
- we see a specific band of the electromagnetic spectrum because those bands of light bounce off us and is reflected
- factors such as the type of sun and the light it gives off make this possible
- gamma rays would not provide meaningful information about the world if we could sense them
why do some animals see different bands of the electromagnetic spectrum?
- some animals can see different bands of light because it is evolutionarily important for them
- even closely related species can have differences in sensory systems
- for example, cats can hear higher frequencies, which helps them hunt mice - despite differences, there are still similarities in sensory systems between mammals, fish, and birds.
how does the human eye provide proof of human’s evolution? in what ways is this not optimal?
- we started from light sensitive cells that developed based on evolution
- human eyes are full of fluid because they evolved from when we were an aquatic species
- when light travels from one medium to another (water to air), it is distorted
- our eyes would’ve been better if they weren’t filled with fluid
- axons and other cells and blood vessels are in front of photoreceptors
- light has to bounce off of all these things before reaching photoreceptors
- we use metabotropic receptors in the eye, which are slower
- the place in the eye where all the axons leave is a blindspot
how did octopus’ eyes evolve?
- octopus had a different lineage of visual evolution
- water outside the octopus and water on the inside of the octopus’ eye
- photoreceptors are the most external part of the eye
- all the blood vessels and axons are behind photoreceptor (no blindspot)
- their evolution was much more optimal than ours
if all senses are just changes in electricity, how do we know which is for what sense? what are the two theories for how we decode action potentials?
- doctrine of specific nerve endings
- labelled lines
what is the doctrine of specific nerve endings? what are its limitations?
- specialized sensory cells respond only to their specific type of energy (touch-sensitive cells respond to touch, not light or sound)
Limitation:
- if you close your eye and push on it, you may see light or dark spots even without light entering
- this occurs because light-sensitive cells interpret any stimulus (pressure) as a change in light, despite no actual light change
what is the idea of labelled lines?
- specialized sensory cells respond only to their specific type of energy (touch cells respond only to touch) AND they stay segregated as signals travel to the brain
- each sense has separate pathways leading to specific brain areas, ensuring signals remain distinct
- crossed pathways can cause mixed sensory experiences like synesthesia (seeing sounds, hearing colors)
- if pathways are rewired to different targets, the brain can adapt and repurpose areas for new functions (sound-processing areas adapting to process taste)
what are the different layers of skin? what aspects of somatosensation do they detect?
- hypodermis (deepest layer): anchors tissue to muscles and the rest of the body; detects stretch and vibration
- dermis (middle layer): contains most sensory cells; responsible for fine touch discrimination
- epidermis (outermost layer): contributes to fine touch discrimination
what is the pacinian corpuscle (lamellated corpuscle)? where is it located?
- specialized skin receptor that detects vibrations
- input layer located in the hypodermis, the deepest layer
in what way is the pacinian corpuscle’s structure unique?
- they are pseudounipolar neurons - have a single process that splits into branches for sensory input and signal transmission to the central nervous system (output)
- doesn’t have dendrites
- it’s input layer is deep in the skin and connects directly to the output layer without a cell body in between
- axon travels to the spinal cord; the cell body is located in the dorsal root ganglion
- exception to the typical neuron structure (dendrite → soma → axon): instead, input → axon → soma
how do receptor potentials from the pacinian corpuscle work?
- receptor potentials are also called graded potentials (similar to PSPs)
- small vibrations produce small graded potentials; large vibrations produce large graded potentials
- graded potentials on the input layer can depolarize the cell, then an action potential occurs in the axon
- action potential is the same as what we know, but means of input are different
how does transduction happed in the pacinian corpuscle?
- vibration transduction occurs via stretch receptors with mechanically gated channels
- found in the skin and muscles, including muscle spindles (receptors for proprioception)
how do cells with differing thresholds contribute to sensory perception?
- neurons can transmit a max of 250 action potentials (APs) per second, limiting their ability to detect the full range of stimuli
- one neuron type cannot detect the entire range of a stimulus
- some cells are sensitive to small changes and reach their firing ceiling early, detecting only the difference between no signal and a weak signal
- these cells cannot distinguish between weak and moderate signals
- a range of cells with varying sensitivities is needed for full sensory perception
- population coding is necessary to interpret even simple touch stimuli.
what is intensity coding?
- the brain decodes sensory information by counting the total number of action potentials reaching the next neuron (summation of responses)
- multiple neurons work in parallel to convey the signal
- as the stimulus strengthens, more neurons are recruited to respond
- range fractionation: some neurons are specialized to be more sensitive to specific ranges of stimuli
what is sensory adaptation? what are the different kinds of sensory adaptation?
adaptation: progressive loss of response to maintained stimulus
- when we are touched, there is more action potentials at the beginning, even if the stimulus continues
- adaption happens at every level of sensation
- tonic receptors (slow-adapting) - persist in firing for longer
- phasic receptor (fast-adapting) - tell you about a change, then firing decreases
what is the benefit of sensory adaptation?
- adaptation is beneficial because constant pieces of information don’t provide any new information
- not going to be particularly useful to us
- nervous system is more interested in changes than absolute values of stimuli
what are receptive fields? how would they differ for types of touch?
- area on the skin in which this neuron will respond
- modality specific - receptive field for visual neurons is part of visual field, receptive field for touch is a patch of skin
- areas deeper in the skin have larger receptive fields (receptive fields for vibration are bigger than those for fine touch)
what is an “on centre, off surround” receptive field for touch? how does this increase discrimination of touch?
- if you press in the centre, it’ll stimulate the cell and cause bigger potentials
- if you press in the periphery of the receptive field, it has an inhibitory effect
- helps with discrimination and precision
- the centre of one cell is the periphery of another cell, increasing discrimination
what are the four main skin receptors? what are they responsible for? what receptors are found in different layers of skin?
- pacinian corpuscle - vibration
- meissner’s corpuscle - light touch
- merkel’s discs - fine touch
- ruffini’s ending - stretch
- hypodermis (deepest): pacinian corpuscle and ruffini’s ending
- dermis (middle): meissner’s corpuscle
- epidermis: merkel’s discs
what types of receptors have the largest receptive fields? what type have the smallest?
in order from largest to smallest:
1. pacinian corpuscle
2. ruffini’s ending
3. meissner’s corpuscle
4. merkel’s discs
- areas deepers in the skin have larger receptive fields
which skin receptors are fast adapting and which are slow adapting?
fast adapting:
pacinian corpuscle - vibration
meissner’s corpuscle - light touch
slow adapting:
merkel’s discs - fine touch
ruffini’s ending - stretch
how do sensory axons differ from each other?
- there are different sizes and thickness of axons
- in order of thickest to thinnest: A alpha, A beta, A delta, C
- A axons are myelinated and C axons are not
- conduction speed is extremely fast in axons that are thick and myelinated
- can’t have this for every cell, reserved for most important signals
- proprioception has the thickest myelinated axons because they are important for knowing where your body is in space
what types of sensation use what types of axons?
A alpha: used for proprioception (muscle spindle)
A beta: used for touch (pacinian corpuscle, ruffini’s endings, merkel’s discs, meissner’s corpuscle)
A delta: pain, temperature (free nerve endings)
C: pain, temperature, itch (free nerve endings)
what is the dorsal column system?
- axons comes in from skin receptors → travels past dorsal root ganglion → travels into spinal cord
- touch from the right side of the body moves to the right side of the spinal cord (ipsilateral)
- the signal goes into white matter → reaches medulla in the brain
- in the medulla, the axons switch sides to the other side of the brain
what are dermatomes?
- dermatomes are specific areas of the skin that are mainly supplied by a single spinal nerve
- organization of spinal cord is based on our lineage as a four-legged creature
- nerve bundles (dorsal roots) in the spine represent different patches of skin (dermatones)
- there is overlap with adjacent dermatomes
what is the sensory pathway?
- spinal cord → brainstem (medulla) → thalamus → primary sensory cortex
- arrive via nerves to spinal cord or brain stem - only sense that doesn’t go through the thalamus before primary sensory cortex is smell (because it’s the oldest sense”
- segregated pathways (labelled lines) - somatosensory information is segregated from other sensory information
where is the primary somatosensory cortex located?
- primary somatosensory cortex is the post central gyrus
- immediately posterior to the central gyrus/sulcus
what are the characteristics of the primary somatosensory cortex?
- perceptual processing begins because we are organizing information and making sense of it
- contralateral organization - left side of the brain represents touch from the right side of the body
- somatotopic organization - we maintain the map of our body inside the brain
what are the characteristics of the secondary somatosensory cortex?
- more sophisticated processing - senses bigger patterns, directional movement, hardness of object on skin
- there is some bilateral information - sometimes when someone is touched on one side, it’ll stimulate both sides
- makes it easier to learn how to do something on the other side of the body, after you know how to do it on one side
what is topic organization?
- refers to the way in which information or content is structured and arranged
- cortical magnification: relative increase in the amount of cortical area dedicated to body parts that have high tactile acuity and sensory specialization
- disproportionately large somatosensory patch in the brain because it’s evolutionary important
what special sensory systems do animals have and how is this seen in cortical representation?
- some animals have whiskers, they tell animals whether or not they will fit in small places
-
barrel cortex - disproportionately large cortical area representing whiskers
- cortical magnification is also based on relevance for the species
what is cortical representation?
- the mapping of sensory and motor functions to specific regions of the brain’s cortex, reflecting the organization of body parts
- neurons that are in D1 (brain area) are most receptive to touch from thumb
- can be considered as receptive fields
- still get signals from other fingers but they’re weak
- size of region in the brain is based on use
how does neuroplasticity affect cortical representation when a sensory organ is removed?
- neurons in somatosensory cortex for that digit has no input
- loses synapses from that digit
- strengthens connections for touch in adjacent digits
how does neuroplasticity affect cortical representation when a sensory organ is used more?
receptive fields related to digits that are used more become larger
how does neuroplasticity affect cortical representation when the somatosensory cortex is damaged?
- adjacent areas sometimes give up neurons so that damaged brain areas can function
- neuroplastic and flexible process, changes how neurons are firing
what is constraint-induced therapy?
- when we constrain the functioning limb and use the less functioning limb
- used when areas of the somatosensory cortex are damaged
- will see that the more functioning brain area will give up neurons so that damaged area can function better
what is phantom limb? why do we think this happens?
- people who lose a hand sometimes report that they can feel their hand still
- caused by an incomplete neuroplastic process
- if you lose your whole hand, the neurons then go on to represent other touch areas (arm and face)
- then, when the person is touched on their arm or face, they can mistake it as someone touching their hand
are cortical sensory neurons specialized for their type of sensation?
- yes, but wiring also seems to be an essential feature
- ex. they are somatosensory because they’re in the skin
- neurons can make sense of nearly any input, even artificial
what is an example of how neurons can make sense of any input?
- can put an electrode in someone’s scalp and send someone a magnetic electrical signal when they move
- makes humans able to detect magnetic fields, north and west
- cochlear implants pick up auditory information because electrical signal stimulates auditory nerve
- neurons are designed to make sense of signals and patterns
what is the benefit of flexibility in the brain while we are young, and inflexibility in the brain as we age?
- most plastic brain is the young brain, makes them better at learning
- their brains are flexible, they forget things they’ve learned sometimes
- there is a benefit to inflexibility, we hold onto skills that we learn and valuable things
- the brain abhors an unused neuron
what is attention?
- spending more of our brain on some processing than others
- selective attention is our filtering mechanism for consciousness
- a way of guiding or determining what we will spend our limited resources on
- we have guiding mechanisms that tell sensory cortex to process things differently
do things that we don’t pay attention to reach the brain?
- unattended information, that you are not paying attention to, still reaches the brain
- things that are strong stimuli
- ex. someone says your name, catches your attention
- we get bits and pieces of unattended information, reaches the cortex but is processed less
what regions are important for attention?
- posterior parietal cortex - important in guiding attention (main top-down guiding mechanism)
- cingulate cortex - important in egocentric attention, interested in imaging ourselves in the past and future
what is the “association” cortex?
also called the tertiary cortex
- pieces of cortex that are multimodal, respond to touch and sound and vision (multiple senses)
- allows us to see two simultaneous sensory experiences as one experience
- richest aspects of cognition is represented by activity in association cortex
- organization, thinking, schemas
- a lot of the brain is association cortex, motor association cortex, sensory association cortex
- a lot more related to executive function and thoughts than motor
- this is where we take in all inputs and outputs and plan what we are going to do
what is synesthesia (cross-modal stimulation)?
- when someone experiences one sensory modality, they also experience another sensory modality
- grapheme-colour synesthesia: whenever they see a letter or number, they see it in a specific colour
what are some ideas and theories behind synesthesia?
- are they actually perceiving it as a different colour or are they just thinking it’s a different colour (association)
- thought to be driven by learning rules rather than specific memories
- a is for apple, apples are red
- pre-existing multi-modality that is in all people but more emphasized in some people
- may be related to other processes that have been evolutionarily conserved
in what ways might multi-modality have been conserved for evolutionary purposes?
we are all multi-modal in some ways
- kiki vs. bouba - cross-culturally, everybody agrees on what shape is what name
- kiki is the name of the object with sharp points, bouba is the name of the object that has round corners
- we think of sounds as “sharp” or “round” even though these aren’t really qualities of sound
how can we test grapheme-colour synesthesia?
- show a picture of lots of 5s and 2s on a page in black ink
- if they see the numbers 5 and 2 as different colours, they should be able to point them out more easily
what is pain? how is it triggered?
- associated with tissue damage, an unpleasant experience
- signal sent to the brain to trigger pain to stop the tissue damage
- works like other senses, pain has specialized cells with axons (nociceptors) that fire APs
how does congenital insensitivity to pain work?
- it is a voltage-gated sodium channel mutation
- sodium channels are used in the action potentials
- cells have proteins to transduce the information, but can’t generate AP
- individuals frequently damage themselves and don’t realize, they don’t live too long
- can’t feel physical pain but can still feel psychological pain
do people with congenital insensitivity to pain still have reflexes?
people with inability to feel pain are still have the reflex to remove themselves from noxious stimuli
- noxious stimulus - something hot or sharp
- sensory info moves to spinal cord → synapses at an interneuron in the spinal cord
- bridges the gap between dorsal and ventral sides of spinal cord
- impacts a motor neuron that causes us to remove our hand/feet
what are some advantages to the subjective feeling of pain?
-
recuperation - pain is a relatively slow signal that is useful in the long run
- if you hurt a part of your body and there is lingering pain, you will take care of that part and protect it
-
learning - don’t want to just move away from noxious stimuli, we want to never experience it again
- learn what is dangerous so that we don’t do it again
-
signal to others - changes in facial expression when we experience pain provide information to other humans
- humans are prosocial creatures, signal to others regarding pain so that they don’t do the same thing
what are nociceptors?
- receptors for pain, found on cells called free nerve endings
- pick up change in temperature or have receptors that bind to chemicals
- if you damage skin, some cells get damaged spill chemicals
- free nerve endings are trying to pick up these chemicals and detect damaged cells
in what cases do cells deliberately release chemicals to evoke nociception?
- histamine is released deliberately when you are exposed to something that is potentially bad
- allergies also cause the release of more histamines
- anti-histamine binds to even receptors in the skin, reduce inflammation and itchiness
- we have histamine receptors on some free nerve endings
in what way can free nerve endings detect damaged cells?
- they have potassium receptors on free nerve endings to detect damaged cells
- there is normally more potassium on the inside of the cell, so if there is an increase of K+ outside the cell, that means there has been some damage
what are the different types of receptors the detect temperature?
- TRPV1 - detect warmth
- TRPM3 - detect HEAT
- CMR1 - detect cold
what are the characteristics of TRPV1 receptors?
- change shape when the temperature rises, which causes change in voltage or signals GCPRs
- will let you know when something is warm (pleasantly)
- axons are C fibres, small unmyelinated axons (AP is slower)
- binds to capsacin, the stuff that makes food spicy
- we think spicy food = hot because capsacin binds to heat receptors
what are the characteristics of TRPM3 receptors?
- change shape when the temperature rises, which causes change in voltage or signals GCPRs
- not as sensitive to lower heats, activated when things are dangerously hot
- A delta receptors, smallest myelinated axons (AP is faster)
- if smth is dangerously hot, we need to get the signal fast and remove ourselves immediately
what are the characteristics of CMR1 receptors?
- detects lower body temp by changing shape which changes receptive potentials
- axons are C fibres, small unmyelinated axons (AP is slower)
- binds to menthol, which is found in mint
- mint has a cooling effect because it binds to our cold receptors
at baseline, what does activity in our temperature receptors look like? how does this change as temperature gets colder then warmer?
- at our baseline body temp, none of the receptors have much activity (don’t allow ions through)
- when it is cold, only CMR1 receptors have activity
- as it gets warmer, TRPV1 opens first because it detects warm stimuli
- then, TRPM3 opens last because it detects hot stimuli
what are some characteristics of cells that cause the sensation of itchiness?
- also have free nerve endings
- some itch cells have histamine receptors, and some don’t
- they all have TRPV1 receptors (detect temperature, warmth)
- there is an association between feeling warm and feeling itchy, but not a clear connection why
- itch cells release a large peptide neurotransmitter - natriuretic polypeptide B (NPPB)
- unique neurotransmitter system
what is the spinothalamic system?
also called the anterolateral system
- sensory pathway in CNS responsible for transmitting pain, temperature, and itch signals
- signal crosses the midline in the spinal cord (rather than crossing after the medulla) before ascending to the thalamus
- axons of the first neuron synapse as soon as it gets to the spinal cord
- axons of the second neuron in the spinal cord immediately crosses over, then travels to the thalamus
- if you damage the right half of dorsal spinal cord, it will affect touch differently from pain and temperature
- lose right side of touch, but left side of pain and temperature
why do we think phantom limb pain happens?
also called neuropathic pain
- feel tightness and immense pain in a limb that is no longer there
- caused by dysregulated peripheral neurons, where free nerve endings are lost but cell bodies in dorsal root ganglion are intact
- damaged neurons may become leaky, which can lead to dysregulation and hyperexcitability
- genetic predisposition may make some individuals more likely to experience this dysregulation.
- continuous dysregulated signals from peripheral neurons can influence the organization of pain pathways further along the nervous system
- dorsal horn neurons in the spinal cord can also become dysregulated and hyperexcitable after peripheral nerve damage, amplifying pain sensations
what is the most effective pain management technique?
- placebo is very effective for pain but has weak/no effect in other modalities
- works for pain because it is likely top-down processed, meaning expectations play a significant role in its management
- small intervention (pill) - less effective placebo
- large intervention (injection) - more effective placebo
- likely mediated by endogenous opioids because we expect pain relief
- when we have placebo effects, there are strong BOLD activity in the basal ganglia
- people say “must be endogenous opioids” “must be dopamine release” “must be top-down processing”
- essentially, we use top-down perception to organize information, presuming that the placebo should relieve pain
what are two other common ways that we manage pain?
- attention and focus can modify experience of pain (psychogenic interventions)
- we also use pharmalogical interventions and sometimes surgical interventions
in what way is pain not just a physical sensation? what types of pain are experienced the same as physical pain?
- irrespective of modality, we see common activation in the brain for every kind of pain
- social rejection experienced and described as pain
- all pain types seem to activate the anterior cingulate cortex
- ACC isn’t necessarily the pain center, it is involved in other roles
- when people feel physical, emotional and social pain, we see BOLD responses in the ACC
- painkillers can lessen the pain of social rejection and emotional pain
- also lessen pain of existential pain
- suggests that when we take something that we know will impact pain, it can work as a placebo and lessen emotional pain
- placebo still changes the way that our brain functions
how do we use drugs for pain management? what are the two types of drugs we use?
analgesia - the absence or reduction of pain, two types of analgesic drugs (NSAIDS and opioids)
- non-steroidal anti-inflammatory drugs (NSAIDS)
- opioids - endogenous and exogenous
how do non-steroidal anti-inflammatory drugs (NSAIDS) work?
- work by targeting a specific pain pathway, distinct from other mechanisms previously discussed
- NSAIDs inhibit the COX pathway and reduce prostaglandin production, thus alleviating pain
- pain causes certain cells to activate the COX (cyclooxygenase) metabolic pathway, which produces prostaglandins
- prostaglandins are chemicals that bind to pain receptors and amplify the sensation of pain
- reducing the production of prostaglandins decreases the sensation of pain
- pain caused by prostaglandins can result from both external and internal stimuli:
- for example, menstrual pain is a result of prostaglandins acting on receptors during uterine contractions
how do endogenous and exogenous opioids work to manage pain?
Endogenous Opioids: we naturally produce these neurotransmitters that reduce pain in response to stress or danger, allowing focus on survival
- Example: runner’s high happens because we are misattributing the running to a dangerous situation, causes the production/release of endogenous opioids
Exogenous Opioids: external substances (e.g., morphine, fentanyl) that mimic endogenous opioids by binding to opioid receptors
- effective in pain relief when they cross the blood-brain barrier, with stronger permeability increasing potency and risk
- act on the spinal cord, brain regions (movement, motivation, reward), and descending pain pathways to block or modulate pain signals
what is the periacqueductal gray, and how is it involved in pain managment?
- periacqueductal gray (PAG) is in the tegmentum in the midbrain
- one of the main inputs is from the amygdala, the PAG is activated when amygdala activity is really high (ex. really scared)
- when PAG is activated, we see very intense pain relieving effects (analgesia)
- PAG is rich in opioid receptors and endogenous opioids
- through the descending pain pathway, the PAG modulates pain signals all the way down to the spinal cord
- activation of the PAG inhibits the first synapse in the pain pathway, reducing pain transmission
- signal travels from skin to spinal cord, and opioids inhibit this synapse through heterosynaptic inhibition
- stopping pain signals as they come into the spinal cord
