Senses Aristotle: 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 1

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) 2

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 4

What’s under the sensory spots? Sensory nerve terminals in the skin: 5

Sensory nerve terminals are the peripheral ends of sensory neurones 6

A general somatosensory pathway Third order neurone Thalamus Second order neurone (crosses midline) Primary sensory neurone (DRG neurone) Receptor 7

A general somatosensory pathway We’ll focus on these stages: receptors central processing 8

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 9

Muscle and tendon receptors Golgi tendon organMuscle spindle 10

Muscle spindle Muscle spindle: detects muscle stretch 11

Muscle spindle and stretch reflex 12

Golgi tendon organ Golgi tendon organ: detects muscle contraction force 13

Golgi tendon organ reflex 14

Skin mechanoreceptor types 15

Skin mechanoreceptor types Receptor type Receptive field Spike activity Stimulus Fast Slow Adaptation Meissner’s Merkel Pacinian Ruffini corpuscle disk corpuscle ending 16

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 17

A real stimulus: Braille dot pattern Skin mechanoreceptor types Reconstructing stimulus dot pattern from spike activity: Merkel disks are the most accurate 18

Local potential at a receptor terminal 19

Local potential at a receptor terminal Amplitude and impulse frequency 20

Local potential at a receptor: where does it originate? Recording the local potential in a Pacinian corpuscle 21

Pacinian corpuscle: the end organ is not the receptor: the axon is directly mechanosensitive Local potential at a receptor: where does it originate? 22

The end organ is a mechanical “cushion” that gives the receptor its rapid adaptation So what’s the “onion” for? 23

Local potential at a receptor 24

We don’t know what ion channel(s) underlie the mechanoreceptor potential! Hypothetical scheme: How does mechanotransduction work? 25

How do we localise skin sensation? Receptive fields overlap: the smaller they are, the more receptors per unit area, the finer the discrimination 26

How do we localise skin sensation?: Two-point discrimination Density of Merkel disks 27

How does two-point discrimination work? Lateral inhibition in spinal cord 28

Somatosensory cortex Brodmann areas 1, 2, 3: primary somatosensory cortex Brodmann areas 5, 7: somatosensory association cortex 29

Primary somatosensory cortex Area devoted to each part of skin surface corresponds closely to the density of receptors 30

Sensory representation in other species 31

Whisker barrel cortex in rodents 32

Sensory contribution to a motor task: Somatosensory cortex on one side anaesthetised 33

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 see also chapter 18 pages Kandel et al chapters 21-24