1. 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

2. 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

3. 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

4. Sensory spots and sensory receptors Skin sensations are localised: e.g. 2 x 2.5 cm on wrist Cold Warm Light touch 4

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

6. Sensory nerve terminals are the peripheral ends of sensory neurones 6

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

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

9. 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

10. Muscle and tendon receptors Golgi tendon organMuscle spindle 10

11. Muscle spindle Muscle spindle: detects muscle stretch 11

12. Muscle spindle and stretch reflex 12

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

14. Golgi tendon organ reflex 14

15. Skin mechanoreceptor types 15

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

17. 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

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

19. Local potential at a receptor terminal 19

20. Local potential at a receptor terminal Amplitude and impulse frequency 20

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

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

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

24. Local potential at a receptor 24

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

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

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

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

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

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

31. Sensory representation in other species 31

32. Whisker barrel cortex in rodents 32

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

34. 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