Lecture 2 - neural coding in somatosensory systems Flashcards
1
Q
4 features of perceptual experience
A
- Modality = type of sensation e.g. touch
- Intensity e.g. how bright light is
- Position e.g. what part of body is in touch with the world
- Timing e.g. when does it occur and is that relevant to you in that moment
2
Q
Modality
A
- Often depends on specific receptors
- Vision, taste and touch
- Within this modalities you have sub-modal qualities = qualia e.g. in taste you have sweet, sour, bitter
3
Q
Sensory neurons have specific properties
A
- Receptive fields = region within sensory space they respond = sensory space defined by modality
- Fields vary in size and shape = depends on receptor type, how deep in skin its located and how much precision required for that body part
- Response changes over time, even if stimulus unchanged
- Response properties reflect mechanical and neural characteristics = stimulation filtered by physical structures before reaches neuron
- Sensations signalled by changes in membrane voltage in sensory neuron = if powerful enough triggers AP
4
Q
Sensory qualities
A
- Depends on perceptual coding
- Don’t have receptor for everything e.g. don’t have for water but can detect wetness
5
Q
Keeping modalities distinct
A
- AP don’t carry info about modality
- Labelled line hypothesis = idea the brain identifies modalities by specific connections
- Brain receives AP containing minimal info but doesn’t tell brain what info relates to = brain becomes aware of what it relates to as wired up in particular configuration
6
Q
Intensity
A
- Determines size of receptor potential and frequency of AP
- Determined by amount of AP receives
- Rate coding = firing frequency codes for intensity = non-linear
- Process occurs due to adaptation
- At start of contact AP fires quickly and then decreases over time as cells adapts to contact = because wants to be sensitive to environment
7
Q
Intensity and adaptation
A
- Range of intensity of neurones is low
- Range of intensity of stimuli can be huge
8
Q
Position and receptive fields
A
- Localisation ability varies within each sensory system
- -> Vision = retina and eye position
- -> Tactile = receptive fields of receptors in skin
- Variation in receptor density and specificity determines sensitivity
- Receptors with large receptive fields won’t be distinct = overlapping
- Course coding = can perceive response in high resolution due to overlapping receptive fields
- Signals transmitted to CNS and then combined to give perception
9
Q
Lateral inhibition
A
- Good for things like recognising edge of table
- Tactile inhibition = occurs upstream in spinal chord
- E.g. with cluster of cells each have own receptive field = have line of contact rather than single point of contact = activates set of receptors (overlap) = within edge region activated and those outside are not
- Benefit = extra resolution to response
- Mexican hat response = cells inhibit neighbours –> firing rates decrease –> base line firing at edges, inhibited region and strongly excited section in middle = increases amplitude from peak to trough = increases resolution
- Response can happen at any angle
10
Q
Modalities in somatosensation
A
- Touch = mechanoreceptor
- Temperature = thermoreceptor
- Proprioception = stretch receptor
- Nociception = pain receptor
11
Q
Tactile receptors
A
- Skin = huge sensory surface
- Consists of epidermis and dermis
- Important for sense of touch, kinaesthesia, temp, poor
- 5 tactile receptor types in glabrous/smooth versus hair skin
- All are mechanoreceptors responsive to mechanical distortion of sensory nerve membrane
- Receptors that start with M are superficial
- 4 receptors we are interested in:
- -> Meissner’s corpuscle
- -> Merkel’s disc
- -> Ruffini ending
- -> Pacinian corpuscle
12
Q
Pacinian corpuscle
A
- Wrapped in sheaths of bi-lipid layers
- Light touch will be absorbed physically by fatter layers
- Strong touch causes nerve ending to bend = AP
- Nerve ending rapidly adapting
- Bounces back to original shape after deformed = cell will fire less with continued force
- Having corpuscles and non-corpuscles gives 2X2 configuration:
- -> Rapidly adapting cells that are superficial
- -> Slowly adapting cells that are superficial
- -> Rapidly adaptive cells that are deep
- -> Slowly adaptive cells that are deep
- -> Combination provides us with lots of info that we combine to produce sense of world
13
Q
Tactile receptors classified according to firing properties
A
Rapidly adaptive:
- Short burst on stimulus application and removal
- Rapidly adapt if stimulus sustained
Slowly adaptive:
- Sudden onset response on stimulus application/removal
- Take longer to reduce firing rate if stimulus sustained
14
Q
Effect of skin mechanics
A
- Mechanical properties of skin filter effect of forces reaching receptor
- Thick skin spreads forces laterally
- Elastic skin = absorbs energy from stimulus
- Shearing forces = stretch skin and have directional component
- Cells near surface good for light touch
- Deep laying cells good for vibrations and indentations
- Combination = allows us to distinguish force direction and new sensations
- Detect world using at least 4 receptors: Meissner corpuscle, Merkel cells, pacinian corpuscle and Ruffini endings
15
Q
Human microneurography
A
- Place fine recording electrodes into upper arm nerve
- Search receptive field on skin of hand and single cell recordings
- Test the response properties:
- -> Free endings = heat, pain, mechanical pressure
- -> Merkely disks = light contact forces
- -> Meissner’s corpuscle = shear forces
- -> Ruffini endings = tension
- -> Pacinian corpuscle = non-directional mechanical pressure
Ways to detect:
- Put in an electrode
- Brush hand = activates superficial cells
- Try different types of contact and work out what cell is recording from
16
Q
Local anaesthetic inactivation of skin receptors
A
- Simple task
- Pick up match and light it
- Time how long process takes
- Give local anaesthetic into figures
- Repeat experiment again
- Found no sensations so can’t guide movement = sig longer to light match
17
Q
Coding of sensory events
A
- Multiple sensory fibres deliver complex code about tactile events
- Contact point near start = have fast adapting cells firing
- During contact = slow adapting cells firing throughout
- Slip in movement = signalled by combination of cells, mostly fast adapting = as lose grip, body tell you so you can make adjustments
18
Q
Central pathways: 1st order afferents
A
- Signals detected at periphery and transmitted up to spinal chord
- May make local connections
- Axons make contact in spine or slightly further up in brain stem
- First connection made
- Afferent = info coming into CNS
19
Q
Central pathways: 2nd order afferents
A
- Projection to contralateral thalamus
- Where lateral inhibition takes place
- Then goes up to thalamus = gate keeper to cortex
- Second connection made
20
Q
Central pathways: 3rd order afferents
A
- Where it goes from thalamus up to cortex
- Third connection made
21
Q
Central processing
A
-Secondary relay nuclei recode info by lateral inhibition, convergence and divergence
- Central modulation:
- -> By efferent projections from higher CNS area to lower brain stem areas
- -> By neuro-modulators
- -> By changes in arousal and threshold
- Peripheral control:
- -> Either by modulation receptor filtering properties
- -> Or by modulating transmitter relies
22
Q
Efferent control
A
- Sensation controlled, regulated, filtered, not passive
- CNS controls sensory systems through actions
- Afferent = Inward sensory signal into CNS
- Efferent = outward signal from CNS to periphery
23
Q
Sensory maps
A
- Distorted somatosensory map reflects density of peripheral receptors
- Magnification of finger, lips, tongue = high receptor density = showing we are bias towards these areas and more info coming in from these areas
- Topographical maps allows for short connections, lateral interaction and relative processing
- Critical maps represent the central processing of peripheral signals
24
Q
What happens to sensory maps when in cortex?
A
- Maps represent central processing of peripheral signals
- The image is modified to extract features and information
- Organised in serial and parallel architecture
