Ch 23 Modification of Brain Circuits as a

Ch. 23 Modification of Brain Circuits as a Result of Experience

Activity mediated influence on the developing brain • During temporal windows called critical periods.

During critical periods • Environmental factors influence the development of the nervous system. • Birds - imprinting only during a restricted period • Human language - long period that is not well delineated.

Critical Periods in Visual System Development • In cats and monkeys, visual deprivation during a specific period of development alters neural connections and functions in the visual cortex

• Hubel and Wiesel assigned visual cortical neurons into 1 of 7 dominance categories. • Group 1 cells are driven only by stimulating the contralateral eye. • Group 7 cells are driven only by the ipsilateral eye • Group 4 cells are driven equally by either eye. 23. 6

Hebb’s hypothesis • Co-ordinated activity of presynaptic and postsynaptic terminals strengthen the synaptic connection.

Apply Hebb’s hypothesis to the visual cortex • Mono-ocular deprivation alters levels and patterns of activity • Cutting an oculomotor nerve both eyes are active, but the activity is not coordinated (strabismus)

In Strabismic Animals • The activity of neurons is not coordinated. • Prevents binocular interaction • Few visual cortex neurons are directed by both eyes.

Activity Dependent Plasticity during Critical Periods • How? • Calcium in post synaptic cells is particularly important. • - calcium drives kinases • - Ca++/calmodulin kinase II leads to phosphorylation of cytoskeleton (dendritic and axonal branching)

Activity Dependent Plasticity during Critical Periods • Calcium --> kinases, nuclear kinases activate transcription factors (CREB) through phosphorylation. • These DNA binding proteins alter transcription during experience-driven functional changes.