Lecture 3: Tactile Sensation Circuits

Question 1

What is the somatosensory organization of the body?

Answer 1

each spinal segment receives somatosensory inputs from a specific part of the body

Question 2

What is a dermatome?

Answer 2

area of the body sending inputs to a specific segment of the spinal cord

Question 3

What is the somatosensory pathway of touch?

Answer 3

touch travels from receptors → DRG neurons → dorsal column nuclei (DCN) → thalamus → cortex

Question 4

Where is touch information represented?

Answer 4

in somatosensory cortex – primary somatosensory cortex (SI or S1)

Question 5

“Describe how tactile information is represented in the cerebral cortex.”

What does ‘represented’ mean?

Answer 5

pattern of neural activity associated with a stimulus (sensory) or action (motor)

Question 6

What does cortical activity in S1 mediate?

Answer 6

feeling of being touched

Question 7

What is the somatotopic map?

Answer 7

map of the parts of the somatosensory cortex that correspond to specific parts of the body

representation of a particular sense in the cortex is often organized according to some continuous parameter (here, it is body position) of the stimuli – ie. where on the body touch occurs

Question 8

What is ultimately the source of the feeling of being touched?

Answer 8

cortical activity

given that the feeling of touch on your body is ultimately generated in your brain (and that activity is sufficient to generate the percept), it follows that your body isn’t actually necessary for the feeling that feeling of being touched

Question 9

What causes cortical changes?

Answer 9

sensory deprivation

Question 10

When does the cortex remodel?

Answer 10

in response to changes in sensory input

ie. if the third digit on a monkey’s hand is amputated, the representation of neighbouring digits expand into the space once occupied by neurons encoding sensations from the amputated digit

Question 11

Why are certain body parts over-represented in the somatotopic map?

Answer 11

density of sensory receptors differs in different areas of the body
- more sensory receptors means connections to more neurons in pathways up to the brian, which connects to more neurons in the cortex (takes up larger part of the cortex
- smaller receptive field

we interact with the world more with our hands, lips, ears, etc.

Question 12

What determines the precision of location coding?

Answer 12

receptive field size and density

Question 13

What is the receptive field of a neuron?

Answer 13

range of stimuli that change the activity of that particular neuron – in the case of touch, this is the area on the skin that causes a change in neuron’s activity

Question 14

What is spatial acuity measured by?

Answer 14

two-point discrimination test

Question 15

What is spatial acuity?

Answer 15

ability to discriminate two stimuli close in space

Question 16

What is an important question in discussion of spatial acuity?

Answer 16

how far apart two touches need to be in order to perceive two touches rather than one

Question 17

Spatial acuity differs between body areas. Why is this a useful pattern to have? Why not just have high spatial acuity everywhere?

Answer 17

larger two-point discrimination → lower spatial acuity

Question 18

What are the mechanisms of spatial acuity?

Answer 18

• tactile circuits
• convergence and divergence
• lateral inhibition

Question 19

What do tactile circuits do for spacial acuity?

Answer 19

help preserve spatial information as it travels from periphery to cortex

Question 20

What is convergence?

Answer 20

when a number of neurons converge on a smaller number of targets

Question 21

What is divergence?

Answer 21

when a number of neurons project to a larger number of targets

Question 22

What does convergence and divergence do to positional information?

Answer 22

they naturally blur this information

Question 23

What do feed-forward excitatory projections do?

Answer 23

show extensive convergence and divergence, which on its own would blur maps and reduce spatial acuity

Question 24

What does lateral inhibition do?

Answer 24

sharpens representation (the peak)

• activity of each neuron excites GABAergic neurons, which inhibit neighbouring neurons – this creates a ‘winner take all’ scenario, where only the most strongly activated neurons show activity
• causes net inhibition of neurons in a ring around the excited neurons

Question 25

What does lateral inhibition enhance?

Answer 25

enhances distinction between two neighbouring peaks of activity

• helps two-point discrimination threshold
• with lateral inhibition, the peaks are more discrete, and therefore allows the brain to more easily discriminate and perceive the two touches

Question 26

What else is lateral inhibition responsible for?

Answer 26

surround inhibition in touch receptive fields

these two phenomena are two sides of the same coin

Question 27

Why should there be circuits with convergence and lateral inhibition?

Answer 27

• convergence promotes detection of small signals over noise, but degrades spatial information
• convergence plus surround inhibition achieves both high sensitivity (high signal-to-noise) and good spatial localization
• convergence allows increase in signal-to-noise – filters noise, keeps signal (does NOT need inhibition to do this)
• signal is being blurred to get convergence (increase signal-to-noise) ∴ need to add lateral inhibition on top

Question 28

How is convergence in neural circuits related to signal-to-noise?

Answer 28

more convergence = more signal-to-noise

Question 29

What do two peaks with lateral inhibition look like?

Answer 29

each peak’s lateral inhibition is inhibiting the neighbouring peak, which increases spatial acuity and therefore makes the two peaks more discrete

(compared to without lateral inhibition, where the two peaks overlap more)

Question 30

Where do sensory afferent neurons from DRG of a particular spinal segment receive inputs from?

Answer 30

a localized area of the body (dermatome)

Question 31

What does the somatotopic map in S1 show?

Answer 31

body parts are under-sized or over-sized according to spatial acuity