Senses Aristotle: classical “five senses”: Sight Hearing Taste Smell Touch This is not all: what did Aristotle leave out? There are other somatosensory.
Post on 13-Dec-2015
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SensesAristotle: classical “five senses”:•Sight•Hearing•Taste•Smell•Touch
•This is not all: what did Aristotle leave out?
•There are other somatosensory modalities - temperature, pain, proprioception - which could be included in a broader “sense of touch”•These modalities are handled by the somatosensory system
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The somatosensory system: what does it tell us?
Information we need about external stimuli:•what is it? (modality)•where is it? (location)•when did it happen? (timing)•how strong was the stimulus? (intensity)
From Descartes “De Homine” (1662)
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Sensory qualities in terms of receptors
•Modality: which receptor type is stimulated?
•Location: which area of skin (or body interior) is innervated by the receptor?
•Timing/Intensity: action potential frequency
•First: where do our ideas about receptors originate?
3
Sensory spots and sensory receptors
Skin sensations are localised: e.g. 2 x 2.5 cm on wrist
Cold
Warm
Light touch
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A general somatosensory pathwayThird order neurone
Thalamus
Second order neurone(crosses midline)
Primary sensory neurone(DRG neurone)
Receptor7
Low-threshold mechanoreceptors:•Pressure )•Vibration ) cutaneous “touch”•Tapping )•Muscle stretch )•Tendon stretch ) deep “proprioception”•Joint position )
High-threshold mechanoreceptors:•Pain (tomorrow’s lecture)
Mechanoreceptors and modalities in the somatosensory system
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Skin mechanoreceptor types
Receptor type
Receptive field
Spike activity
Stimulus
Fast Slow Fast SlowAdaptation
Meissner’s Merkel Pacinian Ruffinicorpuscle disk corpuscle ending
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•Merkel disk: accurate light pressure (Braille)
•Meissner’s corpuscle: light tapping
•Pacinian corpuscle: coarse tapping/vibration
•Ruffini ending: skin stretch
•Free nerve endings: warm, cool, pain
Skin mechanoreceptor types
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A real stimulus: Braille dot pattern
Skin mechanoreceptor types
Reconstructing stimulus dot pattern from spike activity: Merkel disks are the most accurate
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Local potential at a receptor: where does it originate?
Recording the local potential in a Pacinian corpuscle
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Pacinian corpuscle: the end organ is not the receptor: the axon is directly mechanosensitive
Local potential at a receptor: where does it originate?
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The end organ is a mechanical “cushion” that gives the receptor its rapid adaptation
So what’s the “onion” for?
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•We don’t know what ion channel(s) underlie the mechanoreceptor potential!•Hypothetical scheme:
How does mechanotransduction work?
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How do we localise skin sensation?
Receptive fields overlap:the smaller they are, the more receptors per unit area, the finer the discrimination
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Somatosensory cortex
Brodmann areas 1, 2, 3: primary somatosensory cortexBrodmann areas 5, 7: somatosensory association cortex
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Primary somatosensory cortex
Area devoted to each part of skin surface corresponds closely to the density of receptors
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