2. Neuronal Structure and Function

1. 2. Neuronal Structure and Function

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

3. Cortical Columns • Unit of Function From http://www.iscarecrow.com/wp-content/uploads/myrvel/myrvelcolumns.html

4. Dendritic Spines • 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.

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

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

7. Homunculus • Topographically ordered projection are established early in life. • They can be altered by Brain injury or repeated exercises. From http://faculty.etsu.edu/currie/images/homunculus1.JPG

8. Action Potential From http://www.wikipedia.org

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

10. Functional Webs • That are strongly connected to each other • That are distributed over a specific set of cortical areas • That work together as a functional unit • 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 • Share specific response features • Show these response feature only if the respective other area is intact.

11. A view of cortical Function • 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) • What are the functional dynamics of these distributed neuronal representations? (ch10,12) • Where exactly are they localized, or, formulating the question in a slightly more adequate manner? (ch 4,6) • How can the internal wiring of the functional webs be specified? ( • 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?

12. Temporal Dynamics in Functional Webs: Ignition and Reverberation • 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.

13. Filtering based on Gestalt Principles • From http://rubenswieringa.com/nhl/the-secret-sales-pitch

14. Short-term(active) Memory Short-term Memory Operation • Proposed that activation of a distributed cortical neuron ensemble is reflected by coherent oscillatory activity of participating neuron. Spikes/sec 0 5 10 15 Baseline S Delay M Post-Choice Time(sec)