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?

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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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What’s under the sensory spots?

Sensory nerve terminals in the skin:

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Sensory nerve terminalsare the peripheral ends of sensory neurones

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A general somatosensory pathwayThird order neurone

Thalamus

Second order neurone(crosses midline)

Primary sensory neurone(DRG neurone)

Receptor7

A general somatosensory pathway

We’ll focus on these stages:

receptors

central processing

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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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Muscle and tendon receptors

Golgi tendon organ Muscle spindle 10

Muscle spindle

Muscle spindle: detects muscle stretch

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Muscle spindle and stretch reflex

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Golgi tendon organ

Golgi tendon organ: detects muscle contraction force

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Golgi tendon organ reflex

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Skin mechanoreceptor types

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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 terminal

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Local potential at a receptor terminalAmplitude and impulse frequency

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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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Local potential at a receptor

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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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How do we localise skin sensation?:Two-point discrimination

Density ofMerkel disks

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How does two-point discrimination work?•Lateral inhibition in spinal cord

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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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Sensory representation in other species

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Whisker barrel cortex in rodents

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Sensory contribution to a motor task:Somatosensory cortex on one side anaesthetised

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Reading for this and the next lecture:

•Purves et al chapter 9 (give particular emphasis to the part up to page 198, but please read the rest of the chapter too); chapter 10 (all)

•Nicholls et al chapter 17 pages 334-340 - see also chapter 18 pages 356-366

•Kandel et al chapters 21-24