2. Neuronal Structure and Function

Neuron Pyramidal cell Basal Dendrites Axon Myelin sheath Apica Dendrites Postsynaptic cells Preynaptic cells Synapse

 Unit of Function Cortical Columns From

 Most synapses between pyramidal cells are located on spines.  Human cortex includes neurons (85%, pyramidal cell),. synapses  Efficient integrator of different postsynaptic signals.  Changing size and shape over timescales of seconds to minutes and of hours and days: a remarkably dynamic structure. Dendritic Spines

 Primary cortex – related to language processing Brodmann’s Area Central sulcus Sylvian fissure (lateral sulcus) (17) Primary visual cortex (41) Primary auditory cortex (1-3) Primary somatosensory cortex (4) Primary motor cortex

 Many relay neurons in Inferior frontal lobe (44,45) Superior temporal lobe (22, 42)  between-modality Associations

 Topographically ordered projection are established early in life.  They can be altered by Brain injury or repeated exercises. From Homunculus

From Action Potential

1. Afferent and efferent projections are ordered. a. They reach, or insert from, primary areas. b. Sensory and motor projections are organized topographically. 2. Intracortical connections permit mixing of afferent and efferent information. a. Adjacent neurons are heavily connected and form local clusters. b. Primary area tend not to be linked directly, but through relay area. c. Adjacent areas are connected with high probability. d. There is a lower but still good chance for connections between areas farther apart(15~30%). e. Homotopic areas of the two hemispheres tend to be connected. f. Connections between areas tend to be reciprocal. 3. Synaptic connections between neurons are modified depending on their activity. (correlation learning, between modality) a. Neurons that fire together strengthen their mutual connections. b. Neurons that fire independently of each other weaken their connections. Principles reflecting universal neuroanatomical and neurophysiological properties of the cortex

I. That are strongly connected to each other II. That are distributed over a specific set of cortical areas III. That work together as a functional unit IV. Whose major parts are functionally dependent on each other so that each of them is necessary for the optimal functioning of the web.  Established terms : cell assembly, neuronal assembly, neuronal ensemble.  Neurons in both areas should a. Share specific response features b. Show these response feature only if the respective other area is intact. Functional Webs

 The participating neurons are being bound into strongly connected webs of neurons, functional units that represent cognitive entities with sensory and action aspects. Action-related : frontal areas Sensory-related : posterior areas  Questions (answers in latter chapter) 1. What are the functional dynamics of these distributed neuronal representations? (ch10,12) 2. Where exactly are they localized, or, formulating the question in a slightly more adequate manner? (ch 4,6) 3. How can the internal wiring of the functional webs be specified? ( 4. Is it sufficient to assume that the formation of these network is driven by associative learning principles, or do genetically determined factors play a role as well? A view of cortical Function

 Ignition : stimulation of a fraction of its neurons can lead to a full activation of the entire population.  Memory and ignition. Think of Gestalt principle. Temporal Dynamics in Functional Webs: Ignition and Reverberation

 From Filtering based on Gestalt Principles

 Proposed that activation of a distributed cortical neuron ensemble is reflected by coherent oscillatory activity of participating neuron. Short-term(active) Memory Spikes/sec Time(sec) SMBaselineDelayPost-Choice Short-term Memory Operation