Physiology 3120 Lecture 5: Neuro1
9 May 2018
School
Department
Course
Professor
Physiology 3120
Dr. Pruszynski
Lecture 5
Somatosensory system 2
Somatosensory Cortex
- Last lecture
o Peripheral receptors deconstruct environmental energy in the somatosensory system
on the body surface into AP that then can be propagated to the CNS
- This lecture
o How somatosensory cortex processes and reconstructs (decodes)
o Need to reconstruct to turn the population of spikes into a
representation of an object in the hand or the texture on a surface
- when we talk about reconstruction of stimuli, we are focusing on the
cerebral cortex (outside coating of the brain)
- somatosensory cortex (behind the central sulcus) is one part of the cerebral
cortex(grey matter)
- in front of the central sulcus is the motor cortex
- we are focusing on cortex as an example of where reconstruction occurs but its not the first
place where reconstruction occurs (its already been happening at the level of the brain stem (ex:
for certain types of reflex responses) or spinal cord)
- it is clear that somatosensory cortex is important for your conscious appreciation of your
somatosensory stimuli so for you to record how something feels, somatosensory cortex is really
important
Somatotopic Organization
- How do we know?
o Recording – experimental approaches (sticking electrodes in brain)
o Lesions
o Stimulation
- Why is it important?
o Source of seizure can be determined by getting an idea of where the dysfunctional
neural population are based on what parts of body
▪ Seizures are usually in the temporal lobe but this helps us find out what parts of
body are being affected
o Location of tumor can be discovered based on the behavioral responses
the person will give
▪ Example: if they are unable to perceive things from one hand
you can get a good idea of the tumor being on the contralateral
side
- homunculus - map of body surface roughly laid out on the somatosensory cortex
(parts of body are located on a particular spot)
o leg and foot located medial
o face is lateral
o hand is in bw
- left hand is largely represented in the right somatosensory cortex and vice versa
o you can also find SOME ipsilateral
- this is not a simple map, its distorted
- diff parts of body surface get diff amount of homunculus that is out of proportion to their
physical size
o parts of the body surface that are more heavily innervated (more sensory acuity) is
more heavily represented in the somatosensory cortex (ex: there are more receptors on
find more resources at oneclass.com
find more resources at oneclass.com
the surface of the hand than there are in the back) and in turn there is more need to get
fine somatosensory info from the hand than there is for the back and this is represented
in the amount of neural tissue devoted for processing info from that part of the body’s
surface
- diff creatures have diff representations related to their use of those somatosensory signals
o ex: mouse/rat: whiskers will have larger level of acuity and so it will have a larger spot
in the somatosensory cortex called the barrel cortex (corresponds to the individual
whiskers that the rat has)
Plasticity
- Somatotopic organization is present at birth
o Has both mental and genetic aspects
- Size of region is modified with use
o ex. Braille – increase size of region for fingers in homunculus
▪ you will also see the visual cortex begin to respond to tactile inputs
o ex: music
o ex: juggling
- Possible mechanism = dormant synapses
o Remove inhibition / increase excitation
- Size of these regions can vary depending on how much practice you do
- Extremity amputation
o Limb removal or no limb at birth
o The chunk of the somatosensory cortex (Ex: the arm) is no longer used for the arm
o That part is used for other things
o The space begins to be taken over by the flanking region – ex: eye and nose will begin to
infiltrate into that part of the somatosensory cortex
o You get this relatively organized take over from the flanking parts of the representation
- How does this take over happen?
o There is always a synaptic overlap bw diff areas
o Even in normal person, there are synapses in the ex: hand that are bringing info from
the eye and nose
▪ So there are always synapses ready, they’re just dormant
o When you remove this dominant driver for those neurons (meaning those from the
hand) they become sensitized, or released from inhibition
o Via GABA inhibition, pushing that those inputs down
o But when you lose that limb and its no longer being used for that primary purpose that
inhibition is removed and it can be used for a secondary purpose
o
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Last lecture: peripheral receptors deconstruct environmental energy in the somatosensory system on the body surface into ap that then can be propagated to the cns. This lecture: how somatosensory cortex processes and reconstructs (decodes, need to reconstruct to turn the population of spikes into a representation of an object in the hand or the texture on a surface. How do we know: recording experimental approaches (sticking electrodes in brain, lesions, stimulation. Somatotopic organization is present at birth: has both mental and genetic aspects. Size of region is modified with use: ex. Braille increase size of region for fingers in homunculus: you will also see the visual cortex begin to respond to tactile inputs, ex: music, ex: juggling. Possible mechanism = dormant synapses: remove inhibition / increase excitation. Size of these regions can vary depending on how much practice you do.
