Lecture 3: Tactile Sensation Circuits Flashcards
What is the somatosensory organization of the body?
each spinal segment receives somatosensory inputs from a specific part of the body
What is a dermatome?
area of the body sending inputs to a specific segment of the spinal cord
What is the somatosensory pathway of touch?
touch travels from receptors → DRG neurons → dorsal column nuclei (DCN) → thalamus → cortex
Where is touch information represented?
in somatosensory cortex – primary somatosensory cortex (SI or S1)
“Describe how tactile information is represented in the cerebral cortex.”
What does ‘represented’ mean?
pattern of neural activity associated with a stimulus (sensory) or action (motor)
What does cortical activity in S1 mediate?
feeling of being touched
What is the somatotopic map?
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
What is ultimately the source of the feeling of being touched?
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
What causes cortical changes?
sensory deprivation
When does the cortex remodel?
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
Why are certain body parts over-represented in the somatotopic map?
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.
What determines the precision of location coding?
receptive field size and density
What is the receptive field of a neuron?
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
What is spatial acuity measured by?
two-point discrimination test
What is spatial acuity?
ability to discriminate two stimuli close in space
What is an important question in discussion of spatial acuity?
how far apart two touches need to be in order to perceive two touches rather than one
Spatial acuity differs between body areas. Why is this a useful pattern to have? Why not just have high spatial acuity everywhere?
larger two-point discrimination → lower spatial acuity
What are the mechanisms of spatial acuity?
- tactile circuits
- convergence and divergence
- lateral inhibition
What do tactile circuits do for spacial acuity?
help preserve spatial information as it travels from periphery to cortex
What is convergence?
when a number of neurons converge on a smaller number of targets
What is divergence?
when a number of neurons project to a larger number of targets
What does convergence and divergence do to positional information?
they naturally blur this information
What do feed-forward excitatory projections do?
show extensive convergence and divergence, which on its own would blur maps and reduce spatial acuity
What does lateral inhibition do?
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
