1. Somatic Sensory System
Light touch, vibration, pressure, position of limbs (sense of self), pain, temperature.
Leads to the ability to identify shapes and textures of objects.
Monitors internal and external forces acting on the body.
Detects potentially harmful circumstances.

2. Overview Receptor neurons located in skin or joints receive stimuli.
Information is carried to brain via the spinal cord> brainstem> thalamus> postcentral gyrus of the parietal lobe> other cortical areas.
Projections are topographic with respect to body and the amount of cortical space allocated to various body parts is proportional to the density of sensory receptors in that area.

3. Somatic Sensory System
Touch and Pain have different
routes to the brain.

4. General Organization of the Somatic Sensory System
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5. Different receptors are classified by different properties
Function- Pain, temperature, touch, muscle length.
Morphology- free nerve endings or encapsulated. Nociceptors and thermoceptors have free nerve endings, most others encapsulated.
Conduction velocities , fast vs. slow
Location: skin, muscle, tendon, hair
Rate of adaptation: slow vs fast

6. General Properties
stimuli applied to skin change the nerve endings, produce graded receptor potentials that trigger action potentials.
quality of stimulus (what it represents and where it is) is determined by the neuron’s targets in the brain.
quantity or strength of stimulus signalled by rate of action potentials.

7. Slowly Adapting and Rapidly Adapting Mechanoreceptors Respond Differently to Stimulation

8. Low threshold (or high sensitivity) mechanoreceptors
Provide information about touch, pressure, vibration, and skin tension.
Four major types of encapsulated mechanoreceptors:
Meissner’s corpuscle
Pacinian corpuscle
Merkel’s disk
Ruffini’s corpuscle.
Called low-threshold because even weak stimulation causes action potentials. Innervated by large myelinated axons.

9. Receptive fields
Receptive field is the region of the skin within which a tactile (touch) stimulus evokes a sensory response in the cell or its axon.
Receptive fields are small in the finger tip (1-2 mm), but larger (5-10 mm) in the palms.

10. The Skin Mechanoreceptors

12. Differences in mechanosensory discrimination across the body surface
Accuracy of of touch varies over the body.
Tested with two-point discrimination.
Fingers can distinguish things 2mm apart, forearms 40 mm apart.
Mechanoreceptors are more numerous in finger tips and have smaller receptive fields.
Doesn’t explain everything. Two-point thresholds vary with practice (can be learned), and depend on the stimulus.
Phantom limbs- central processing can occur in the absence of peripheral receptors.

13. Sensitivity of Tactile Discrimination Varies with Location on the Body Surface

15. The route of sensory information
Mechanoreceptors and propioception receptors use the Dorsal-column-medial lemniscus pathway
Pain and temperature use spinothalamic (anterolateral pathway).

17. The Main Mechanosensory Pathways
Medial lemniscus
cross in the medulla
upper and lower body
use slightly different
pathways.

18. Trigeminal pathway
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info from head and face

19. Somatic sensory cortex
Located in parietal lobe-post central gyrus
Divided into regions, Broadmann’s areas
3b and 1: cutaneous
3a: proprioception
2: tactile and proprioception
Each area contains a somatotopic “map” of the body
homunculous

20. Schematic Representation of the Main Mechanosensory Pathways
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22. Somatotopic Order in Human Primary Somatosensory Cortex
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23. The Somatic Sensory Portions of the Thalamus and Cortical Targets
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Upper body medial
Lower body lateral

24. Human Primary Somatoensory Cortex
areas of high receptor density
get more cortical space
somatotopy

26. The Humunculus reflects sensory receptor density

27. receptive fields of somatosensory cortex
Area 3b- simple, responses generally from stimulations of a single finger
1 and 2- respond to stimulation of multiple fingers. 1- often a particular direction is important, 2- particular shapes.
SI is organized in columns, by receptive field and modality. Stick an electrode vertically, all neurons share same region of body and slowly adapting vs fast adapting mechanosensory neurons

28. Higher order processing
SI sends out projections to other areas of cortex.
SII, adjacent to SI. Receives info from SI and sends it to amygdala and hippocampus. Plays roles in fear conditioning and tactile learning and memory.

29. Summary - organization
First order neurons: cell bodies in dorsal root and cranial nerve ganglia
Second order: brainstem nuclei
Third order: thalamus ….project to SI
Fourth order; SI projects to SII
Topographical arrangement throughout

30. Pain
Submodality of the sense of touch, warns of injury and things that should be avoided.
More subjective that the other senses. The same stimulus can produce different responses in different individuals, or in the same individual in different circumstances.

31. Pain involves specialized neurons not just extra stimulation of touch receptors.
scheme for transcutaneous
nerve recording.
Nociceptor doesn’t
fire until pain is felt.
Other thermorecptors
fire at all temps and at
about the same frequency.
Pain threshold is about 45
degrees C

32. Detecting pain
Ion channel receptors open in response to heat as well as capsaicin, called TRP (transient receptor potential) channels.
When open allow in Ca++ and Na+ that generate action potentials.

33. Heat gated ion channels
Capsaicin receptors are nonselective cation channels opened by heat, low pH, and capsaicin (the hot in hot peppers).
Mice without VR1 (vanilloid receptor – 1 or TRPV1) have impaired sensitivity to pain. Can drink capsaicin as if it were water.

34. Nociceptors
Activation of VR1 channels by pure capsaicin and extracts of various peppers.
Nature 1997 Oct 23;389(6653):816-24

35. Two types of pain: Two types of axons
first pain (sharp), Ad fibers
second pain (dull, longer lasting) C-fibers

36. Referred pain Few if any dorsal horn neurons
are specialized solely for the
transmission of visceral pain.
It is conveyed relayed to brain via
dorsal horn neurons that also get inputs from skin.
Therefore a person may feel pain at a site far from its source.

37. Descending Systems Modulate Transmission of Ascending Pain Signals
Descending pathways from cortex and hypothalamus…

38. Descending Systems Modulate Transmission of Ascending Pain Signals
Through periaqueductal gray rostral medulla reduces spinothalamic tract activation.

39. opioids Opioid receptors (metabotropic) in descending pain pathway.
Ligands- enkephalins, endorphins, and dynorphin. Found in all descending pain areas.
Opiate antagonist naloxone blocks morphine-induced analgesia.
Opioids decrease the chance that a nociceptor will fire, cause inhibition.

40. Endogenous opioid peptides hyperpolarize cells to
Endogenous opioid peptides hyperpolarize cells to decrease action potential firing

41. Placebo Effect Sugar pills can reduce perception of pain.
The effect can be blocked by naloxone, a competitive antagonist of opioid receptors.
The placebo effect is based on a biochemical change in the brain.

42. Endogenous Opioids
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