1. Somatosensory Systems
June 17, 2011

2. Review Quiz
Describe the two different types of ways that taste information gets processed into electrical information that the brain can understand. (Hint: What are the two different types of receptors that we talked about in class?)
How do taste, smell, and vision interact?
Name the type of cells that process smell information.
Name one chemical or hormone important in food intake/eating.

3. Concept Mapping What do you think is important in somatosensation?
What brain regions might be involved?

4. Two-Point Discrimination

5. Somatosensation & The Skin
Skin comes in 4 different types:
Mucocutaneous: forms the boundary between the mucous membrane and the surrounding hairy skin, lips, or tongue
Mucous membrane: lines the insides of body cavities (i.e., nose, etc)
Glabrous: skin without hair
Hairy: skin with hair

6. Skin Cells Have Many Receptor Types
Fiber group
Fiber name
Modality
Cutaneous and subcutaneous mechanoreceptors
Meissner’s corpuscle
Merkel disk receptor
Pacinian corpuscle2
Ruffini ending
Hair-tylotrich, hair-guard
Hair-down
Field
Aα,β Aδ RA
SAI PC
SAII
G1, G2 D F
Touch
Stroking, fluttering
Pressure, texture
Vibration
Skin stretch
Light stroking
Thermal receptors
Cool receptors
Warm receptors
Heat nociceptors
Cold nociceptors C
III IV
Temperature
Skin cooling (25°C)
Skin warming (41°C)
Hot temperatures (>45°C)
Cold temperatures (<5°C)
Nociceptors
Mechanical
Thermal-mechanical
Polymodal
Pain
Sharp, pricking pain
Burning pain
Freezing pain
Slow, burning pain
From Lecture by Krish Sathian, Emory University , 2006

7. Mechanoreceptors Highly sensitive receptors for tactile information
Meissner’s corpuscles
Responds to lower frequency vibration
Found in the dermis of glabrous skin
Ruffini’s corpuscles
Respond to pressure on glabrous or hairy skin
Pacinian corpuscles
Respond to vibration
Found in the deeper layers of the dermis (hairy & glabrous skin)
Merkel’s disks
Responds to pressure on the epidermis of glabrous skin

8. Mechanoreceptor Transmission
Receptor type determines which sensation is felt.
Spatial distribution of the activated receptors transmits information about location and size.
Merkel cells and Meissner’s corpuscles have the smallest response fields and so are more specific.
Intensity is determined by firing rates of individual receptors.
Time course of firing indicates duration of stimulation.
From Lecture by Krish Sathian, Emory University , 2006

9. Receptive Fields & Intensity
Figure 21-1 The sensory systems encode four elementary attributes of stimuli—modality, location, intensity, and timing—which are manifested in
sensation. The four attributes of sensation are illustrated in this figure for the somatosensory modality of touch.
A. In the human hand the submodalities of touch are sensed by four types of mechanoreceptors. Specific tactile sensations occur when distinct types of receptors are
activated. Firing of all four receptors produces the sensation of contact with an object. Selective activation of Merkel cells and Ruffini endings produces sensations of
steady pressure on the skin above the receptor. When the same patterns of firing occur only in Meissner's and Pacinian corpuscles, the tingling sensation of vibration
is perceived.
B. Location and other spatial properties of a stimulus are encoded by the spatial distribution of the population of activated receptors. Each receptor fires action
potentials only when the skin close to its sensory terminals is touched, i.e., when a stimulus impinges on the receptor's receptive field (see Figure 21-5). The receptive
fields of mechanoreceptors—shown as red areas on the finger tip—differ in size and response to touch. Merkel cells and Meissner's corpuscles provide the most
precise localization of touch, as they have the smallest receptive fields and are also more sensitive to pressure applied by a small probe.
C. The intensity of stimulation is signaled by the firing rates of individual receptors, and the duration of stimulation is signaled by the time course of firing. The spike
trains below each finger indicate the action potentials evoked by pressure from a small probe at the center of the receptive field. Two of these receptors (Meissner's
and Pacinian corpuscles) adapt rapidly to constant stimulation, while the other two adapt slowly (see Figure 21-8).
Principles of Neural Science. Kandel et al, Fig 21-1

10. Sensory Pathway from Mechanoreceptors
Follows the medial lemniscal pathway
Ascend in a dorsal column to the medulla
Cuneate nucleus: arm & upper body
Gracile nucleus: leg & lower body
Axons then cross the midline and travel up the medial lemniscus
Enter the Ventral Posterior Nucleus of the thalamus
Continue to Primary Somatosensory Cortex (S1)

11. Somatosensory Cortex
Once again, we see topographic mapping! Has anyone else noticed a theme here?
This time, our map has a special name: the Homunculus
The homunculus has 2 key features:
Discontinuities: areas that are not adjacent in body are adjacent in map
Distortions: Cortical magnification map—larger areas are more sensitive

12. Mapping Your Homunculus

13. Somatosensation

14. Make Your Own Braille Alphabet

15. Backyard Experiments Video
Temperature & Pain
Backyard Experiments Video

16. Pain and Temperature are sensed using the same types of receptors.
Temperature & Pain
Receptor Type
Fiber group
Fiber name
Modality
Thermal receptors
Cool receptors
Warm receptors
Heat nociceptors
Cold nociceptors Aδ C
III IV
Temperature
Skin cooling (25°C)
Skin warming (41°C)
Hot temperatures (>45°C)
Cold temperatures (<5°C)
Nociceptors
Mechanical
Thermal-mechanical
Polymodal
Pain
Sharp, pricking pain
Burning pain
Freezing pain
Slow, burning pain
Pain and Temperature are sensed using the same types of receptors.

17. Types of Pain Receptors (Nociceptors)
Mechanical nociceptors:
Activated by mechanical stimuli (stabbing, pinching, etc of the skin)
Sharp or pricking pain.
Thermal nociceptors:
Activated by extreme heat or cold
Polymodal nociceptors:
Activated by multiple types of assaults: mechanical stimuli, heat, cold, irritating chemicals, etc
Cause a slow, burning pain that continues after the end of the stimulus

18. Pain Sensation
Injury causes inflammation and pain to the site of injury to induce withdrawal from noxious stimuli.
Also increases sensitivity to pain to prevent further re-injury to same area
Can be affected by environment (ex: soldiers)
Involves both:
Pain sensations (sensory component)
Unpleasantness (emotional consequences)
These act through two separate pathways

19. Pain Sensation: Spinothalamic Tract
Nocioceptors send pain info through the dorsal root ganglion to the dorsal horn of the spinal cord.
Information passes to the medulla, through the midbrain, and to the ventral posterior thalamic nucleus.
From here, information is delivered to the primary & secondary somatosensory cortices (S1 & 2), and may also be passed to the limbic system and cortex.

20. Unpleasantness of Pain
Nocioceptive information from the spinal cord is relayed to the dorsomedial thalamic nucleus…
Passed to the Anterior cingulate cortex…
And then relayed to the prefrontal cortex.
Unpleasantness information can also come from the spinothalamic path: information from the secondary somatosensory cortex is related through the insular cortex to the anterior cingulate.

21. But, No One Wants Pain to Last Forever!
Descending pathway begins in the Periaqueductal Gray (PAG) to decrease pain response.
Fibers pass to the reticular formation of the medulla
Serotonergic and noradrenergic projections connect to the neurons in spinal cord to cause the release of enkephalin.
Fibres pass from PAG to the reticular formation of the medulla (the nucleus raphe magnus or "NRM", and the closely associated nucleus reticularis gigantocellularis pars alpha, and nucleus reticularis paragigantocellularis, all together called the ventromedian medulla or "VMM") where connections are serotoninergic, and from there axons descend in the "dorsolateral funiculus" of the spinal cord, to end up (surprise, surprise) on interneurones right next to the substantia gelatinosa (lamina II) in the cord. The synapses here are enkephalinergic. Stimulation of this system causes inhibition of incoming pain impulses. Thus, although serotonin applied peripherally augments pain, its action centrally is important in descending inhibition of incoming painful impulses! New evidence suggests that GABA is also important in inhibition of pain pathways by the VMM [Neuroscience Jul (2) pp].
To make matters more complex, quite apart from the above inhibitory pathway that works mainly on opiates, there is a separate, noradrenaline-based pathway that inhibits pain. Noradrenaline-based projections appear to come from the NRM, and also from the locus coeruleus in the pons. This is one explanation why antidepressants (which inhibit noradrenaline uptake) may be effective in controlling pain, but remember that antidepressants also inhibit serotonin uptake, so they may also increase activity in the fibres that pass from PAG to medulla!
"On" and "Off" cells: There are probably two main classes of neurones in the PAG and VMM:
"On" cells increase pain transmission;
"Off" cells decrease it.
Opioids inhibit "on" neurones and increase transmission in "off" neurones, and noradrenaline also inhibits "on" neurons.

22. Current Clinical Trials