lecture 3: Functional organization of the cerebral cortex .pdf

lecture 3: Functional organization of the cerebral cortex The thalamus relays sensory information & emotional state: ● Acts as an active relay station between the sensory input & the cortex ○ There are some processes that happen within the thalamus that can modify the nature, intensity, value of the...

lecture 3: Functional organization of the cerebral cortex The thalamus relays sensory information & emotional state: ● Acts as an active relay station between the sensory input & the cortex ○ There are some processes that happen within the thalamus that can modify the nature, intensity, value of the sensory input or block it before it goes to the cortex ○ The action without sensation ○ Nuclei receive info specific to are is no sensory modality & project it to a specific area of the cortex ● Modulates passage of specific information, depending on behavioral state (hunger, thirst, etc) ○ ex: You see water but ur not thirsty, so this info is not very imp to you. if Thirsty → thalamus will amplify the signal to the cortex. So, modulation of input in the thalamus depends on: ■ Your behavior ■ Importance of this stimulus to you ● Plays a significant role in the initial processing of info. Each of sensory systems, except olfaction, has similar processing step within a distinct region of thalamus: ● Why? Bcz there is no nucleus in the thalamus for Olfaction (smell) ● The thalamus has 4 major nuclei: ● ● ● ● ● Every somatosensory information goes through the ventral posterior lateral Sensory information are transferred from the retina to the visual cortex through lateral geniculate nucleus Auditory info are transferred from the ear to the cortex through the medial geniculate nucleus Somatosensory information of the LL are the 1st to set into the sc & reach a specific area in sensory cortex Non specific nuclei (diffuse projection) → involved in awareness, arousal & awakness ○ Midline nuclei ○ Intralaminar nuclei Reticular nucleus of the thalamus: a filtering system: ● Outer coat of the thalamus is formed by the reticular nucleus (RN) ● RN is not interconnected with the cortex, its axons terminate on the other thalamic nuclei ● RN is mostly inhibitory (GABA) ● Most other thalamic nuclei use the excitatory neurotransmitter → Glutamate ● RN modulates the stream of information between the thalamus & cortex ● Reticular nucleus is different from the reticular information ○ Reticular nucleus: coating of the thalamus that is going to be involved in filtering information ○ Reticular information: in BS; many diff function: ex: correlated to the awareness & arousal or consciousness ● Stimulatory nts are sent to cortex, but the inhibitory are filtering depending on status (if it is impt it will let it go & if it is not, the RN will stop it or slow it down. ● Since the RN is not connected to the cortex, when it gets the information from the thalamus, it goes back to the thalamus to go to cortex ● It either ↑ or ↓ information within thalamus & then the projection from thalamic neurons to cortex are going to be either strong or weak The higher perceptual and cognitive functions are performed in the cortex: ● The cortex is organized into vertical columns, spanning all 6 layers from cortical surface to white matter ● The brain is compacted in a small area (cranium) & this allows having a large amount of columns ● The importance of the cerebral cortex in evolution is based on the number of columns ● So the higher the amount, the better the cognitive function ● So columns give the brain its computational power (powerhouse of the brain) Functional organization of the cortical columns: ● Columns are fundamental computational modules of neocortex → upper cortex, not hippocampus or amygdala ● All of the neurons within a column receive inputs from the same local area of skin & respond to a single class of receptors ○ In the same place, diff modalities will activate diff columns, ex: if we touch a skin area we activate a specific column, but if we introduce pain to that same area, a diff column will be activated in the cortex but in the same area for that specific skin area ● All of the neurons in a column usually respond to only one modality: touch, pressure, temperature, or pain ● A column provides an anatomical structure that preserves the properties of location & modality ● Columns give an idea of where the information is coming from, so they provide 2 types of info: ○ Type of input ○ Location Examining the receptive fields of adjacent cortical neurons in sequence: ● The neuron recorded at location “a” in area 1 has a sustained SAR to pressure applied to wrist & ceases firing when no pressure is applied to the the skin. It does not respond to pressure on the forearm. ● The neuron recorded at location b in area 3b (cell b) responds vigorously to pressure on the forearm but not on the wrist ● This shows the specificity of areas in columns Feature-detection neurons in area 2 of primary somatosensory cortex respond to ↑ specific fts of stimulus: ● Orientation-sensitive neuron distinguishes horizontal and vertical edges pressed on the palm ● Direction-sensitive neurons respond most vigorously to movement across the hand toward the thumb & index finger. ● The neuron displays its strongest response in the radial area rather than the ulnar side. Progressive higher perception of somatosensory stimulus: ● Area 1 → touch information & more advanced touch info ex: tactile discrimination ● Area 2 → touch info from area 1 is combined with info from muscles and joints (proprioception) ● Area 3a → Muscles & joints “proprioception” (GTO nerves & muscle spindles) ● Area 3b → Specific classes of touch receptors in the skin (mechanoreceptors for touch) ● This is why if we have a stroke in area 1 → you lose sensation of touch but not proprioception (Area 2) ● Area 1 & 3b, have some sort level of sensory info of touch on skin, that is either slow adapting or quickly adapting & the info are combined in area 2 to give information that is for sensation towards the orientation & location/direction or the movement of sensory, and they are more complex neurons ● Basically, we have primary sensory cortex that let me know a little bit about where I’m touched & how I’m touched & I’m going to combine “how” with the sensory input coming from the limbs (proprioception) ● We go from simple to complex in within same primary cortex Inhibitory interneurons within each relay nucleus help sharpen contrast between stimuli: ● Different mechanisms underlie lateral inhibition to enhances the contrast between stimuli ● Feedback inhibitory process: occurs when excitatory principal neurons synapse onto inhibitory interneurons, which project back to the principal neurons & inhibit them (negative-feedback loop) ● Feed-forward inhibitory process: occurs when axons synapse directly onto inhibitory interneurons, inhibiting downstream principal neurons ● Distal inhibitory process: from sensory cortex, from higher cognitive function depending whether I like to do this action or not, interested in the sensory input or not, awake or sleepy, my sensory cortex also can send information down to the sc to modulate the ascending information. If I’m in coma, these ones are not going to be active, but If I’m interested in what type of cell (I want to know the touch), I’m going to depress interneurons by inhibiting them. So, I can modulate the sensory information from higher cortical areas ○ On terminals of primary sensory neurons ○ On cell bodies of projection neurons ● How are we modulating this sensory information? ○ Sensory input → activate inhibitory interneuron (Feed-Forward) ○ Once this is highly active it send collaterally to interneurons that is going to add more inhibition (feedback inhibition) ○ Depend on my motivation, I have cognitive inputs that coming from the cortex to control (distal inhibition) ● Long story short: inhibitory neurons within each relay nucleus help sharpen contrast between stimuli Cortical column: sensory cortex function (visual cortex): ● V1 - V2 → PRIMARY SENSORY CORTEX (only sensation/location “photon sensitive”) ○ Respond optimally to bars with specific orientation and location in the visual field ● VE2 - V4 → SECONDARY SENSORY CORTEX (angle “orientation sensitive”) ● V4 - IT → ASSOCIATION AREA (perceives sharpens) ○ Respond optimally to more complex shapes (ex: triangle) ● Recap: ○ V1 (primary) gives me information about whether I have a light or not & what type of light. ○ V2 telling me about the shape (horizontal, vertical, ...) ○ & then it starts to become a little more complicated angle shape and then the triangle Cortical column: Motor cortex function: ● Cells in the same column influence common synergistic muscle ○ Ex: muscles that contract to hold a cup, they cooperate in a given movement ● Pyramidal neurons in layer 5 will activate multiple digits in your hand ○ In Sensory cortex: change modality ○ In Motor cortex: change strategy ● ​1 muscle can be activated by many sets of columns bcz 1 muscle may be synergistic in variety of movements ○ Ex: to pick up a cup the thumb may be used with digit 1 or with digits 1 & 2 Sequential information processing in the cortex: ● Successively higher-order processing of sensory input information ● The reverse in the motor hierarchy leading to output responses that interact with the environment (from complex to simple) ● Sensory-motor hierarchy: 1ry sensory → 2ndry sensory → post. Ass. → ant. Ass. → premotor → 1ry motor