1. Cogs 107b – Systems Neuroscience www.thiscourse.com/ucsd/cogs107b/wi16
Lecture 2: 01/07 – tactile sensation (a.k.a., touch sense or mechanoreception)
the weekly principle: ‘topographic representation’

2. proprioception and touch sense: the ‘all-axon’ ganglion cell
ganglion cell types: breakdown by conduction speed
Aα – proprioception – myelinated, very fast ( m/s)
Aβ – mechanoreception – myelinated, pretty fast (40-70 m/s)
Aδ – thermoreception, nociception, hair cell – myelinated, fast (12-36 m/s)
C – nociception – unmyelinated, slow (0.5-2 m/s)

3. Meissner corpuscle, Merkel disks are located superficially multiple connect to 1 DRG
Pacinian corpuscle, Ruffini ending (SA2) are located subcutaneously 1:1 arrangement
Ruffini endings seem to mainly be found in humans and we won’t be covering them in much detail here, but they seem to be sensitive to the stretch of the skin.

4. Forms of Response Fields
excitatory response = more AP’s
inhibitory response = fewer AP’s
baseline response = no change in AP rate
small versus large reponse fields
inhibitory surround
complete vs. incomplete
Response field = way of looking at how the nervous system responds to the world. Isolate a single DRG cell or SSctx cell and touch on the skin to see if you get a response from the cell.
Move a touch probe very accurately across the 2D surface of the skin to see where the cell fires more or fires less.
Small/ Large
Inhibitory surround/ inhibitory surround incomplete
Edge detection
Sharpen the signal? Two point discrimination?
Direction that mechanoreception is moving
Whole/ patchy
whole versus patchy

5. Transient versus Sustained Adaptation of Mechanoreceptors
Responses of a sensory system, either reflecting dynamic changes to the external world, or constant stimulation to the external world.

6. Forms of Mechanoreceptors
Meissner’s corpuscle:
Unmyelinated end of DRG cell winds through a stack of flattened skin cells that are positioned in the skin below the epidermis.
Shallow placement
Small response field (3-5 mm)
Responsive to light touch
Rapidly-adapting
Highly sensitive to low-frequency vibrations between Hz

7. Meissner’s corpuscles = low-frequency vibration sensitivity
sensitivity to slip = grip control
Whenever you hold an item in your hand there are thousands of microslips of the object in your hand.
Gravity is pulling on the object and the muscles in my hand are adjusting for these pulls
These microslips are happening at a relatively slow frequency, so Meissner’s carry information about these microslips up to the brain to the motor output to grip.

8. Forms of Mechanoreceptors
Pacinian corpuscle:
Unmyelinated portion of the DRG cell is wrapped with multiple layers of skin cells
Deep-placement
Large response field
Sensitive to vibration and pressure
Rapidly-adapting
Sensitive to pressure and high-frequency vibration (~250Hz)
May help to detect texture
Large response field, like whole hand and would get a response.

9. Pacinian corpuscles = frequency-dependent sensitivity to vibration
Experiment, vibrating bar at different frequencies, and can vibrate over short distances or long distances (the throw) on each cycle.
When do you sense the vibration?
If it is a hiFreq vibration it only needs to indent the skin 0.5 mm, If it’s slower, say 10 Hz, then the bar has to indent the skin 10 mm.
Black dashed line is the detection threshold of a DRG cell with the vibration occurring in its response field.
When might you need this, making an incision? Being able to cut through the skin versus muscle, recognizing the vibrations at the tip of the scalpel. Skin is rough and hard, if you are mostly feeling high frequency vibrations you are on skin, muscle is smooth so you will detect the difference between skin and muscle with the
freq. (Hz)
throw (mm)

10. Forms of Mechanoreceptors
Merkel Disk:
Single DRG cell contacts multiple Merkel cells embedded in the skin
Shallow-placement
Small, spotty response field (0.5 mm)
Slowly-adapting (sustained)
Sensitive to form, texture, points, edges

11. two-point discrimination = Merkl disk density
Small response field and sustained response means that MD can tell you about exactly where something is touching your skin.
Probe that has two points that we can change the distance between.
Do you feel 1 or 2 points?
For every part of the body how far apart do the points need to be for us to detect 2 points?
If you were to plot merkel density vs. distance?

12. small Merkl disk receptive field = fine texture discrimination (e. g
small Merkl disk receptive field = fine texture discrimination (e.g., Braille)
Merkel Disks
Meissner C.
Pacinian C.
Put braille bar on response field of a DRG cell. Put each portion of the bar onto the response field of the finger. Where there is a bump, it will indent the skin on the response field. Where there is flat there will be no indentation. As you record from the DRG cell, put black dots to indicate APs fired and the corresponding portion of the bar that was pressed on the response field. Merkel Disk gives best decoding so to speak of the braille.

13. Mechanoreceptors summary
type RA / SA depth response field sensitivity info. processed/best stimulus
Pacinian RA deep very large (hand) very high (10 nm) high-freq. vibration
Meissner RA shallow 3-5 mm slip / low-freq. vibration
Merkl SA shallow spotty 2-3 mm broad depth range form, texture / points, edges
(0.5 mm)
hair RA deep 10 mm 1 micron hair displacement

14. dorsal root ganglion pathways to the brain
medulla
DRG
Ventral Posterolateral Nucleus (VPN) of the Thalamus
Somatosensory ctx. (S1)
Mechano information streams remain separate until cortex. DRG cells carrying specific types of mechano information can synapse on the same medullar cell, but there is no convergence of different types of mechano information until SSctx.

15. Somatosensory cortex is a homunculus
Somatosensory cortex is a homunculus. Known as S1, neurons from this region project down to S2 which is smaller. So information begins to converge even more in S2.
Posterior parietal is responsible for putting together all of the sensory information.

16. topographic representation:
In many cases, neurons in the brain are anatomically (i.e., spatially) arranged in a systematic fashion such that those responding to (or ‘representing’) similar features of a single sensory modality (e.g., vision or audition) are grouped into the same space in the brain. An important feature of such groupings is the interconnectivity of its members. Multiple such groups are, in turn, organized in a systematic fashion.
Sensory inputs that are topographically represented in the space of the brain may reflect actual space, as in the space of the retina or skin surface, or may reflect stimulus space as, for example, type of odor or sound frequency.
related concepts: surround inhibition, the cortical columns, the homunculus
Penfield.
Found when he stimulated particular portions of the SSCtx you feel touch on particular parts of your body. Same for motor.
Topographic is a more general term, not necessarily the space of the body, but rather the space of some variable. I.e. Auditory or Visual space
Putting neurons together allows other processes to take place such as the surround inhibition or cortical columns which we will talk about later.

17. topographic representation
1 barrel = 1 whisker
Rat has a rat homunculus.
Barrel cortex is a great example of a topographic representation

18. primary somatosensory cortex: within-region (column) processing
the cortical column
layer-specific inputs
Cortex seems to divide up into 6 layers.
Explain stain
Topography, Vertical axons mean that neurons with same response fields in different parts of the brain are connected to each other, also horizontal axons connect regions of the topographic map that respond to different parts of the body.
Merkl Disk inputs, synapsed in medulla and thalamus. Thalamic projection for SA (MD) goes to layers 2-3
Corpuscles with transient responses project into layer 4.
Layer 4 projects up to layer 2-3, where slow and rapid responses are first merged.
However, fast adapting inputs are a synapse behind with respect to thalamic inputs, means there is a delay in transient information reaching layer 2-3.
Means that Merkel Disc information is more accurately portraying what is happening right now, and is receiving information about how that stimulus got there because it has info about what changed from the previous time step.
Then, this information is sent back down to layer 6 to go to other regions of cortex.
merging RA info. into SA info.?
dynamical into static?
Pacinian’s/Meissner’s into Merkl’s?
low-resolution into high-resolution?

19. S1: direction-selective surround inhibition
Recording from a DRG cell. Hold hand still and drag a stimulus across the response field of the neuron. Black corresponds to more AP, white corresponds to a decrease in AP relative to the baseline (gray). What you see is incomplete surround inhibition. Surround field is different for different direction of motion. If I move a probe in one direction I get a series of activations and inactivations that will be informative about the direction of motion that the stimulus moved in.

20. S1: direction-selective surround inhibition
incomplete
discriminated
APs/sec
baseline
not discriminated
complete
APs/sec
not discriminated
baseline
excitation
inhibition

21. S2 – response fields expand across digits, but maintain directional selectivityJ S2 S1 J A B C D E F G
(+)
Convergence: multiple neurons in one region synapse on a single neuron in another region