history, part iii anatomical and neurochemical correlates of neuronal plasticity two primary goals...

History, Part IIIAnatomical and neurochemical correlates of

neuronal plasticity• Two primary goals of learning and memory studies are

– i. neural centers involved in specific behaviours– ii. kinds of changes that underly the observed plasticity

• The changes in neural functions (neuronal plasticity) might involve– i. chemistry (the number of transmitter chemicals released),– ii. morphology (the number or types of connections made

between nerve cells)– iii. electrical activity (how rapidly, or in what sequence, nerve

cells fire)

Ramón y Cajal (1894) suggested that neurons extend their axons and dentrites

• to make new connections with other neurons in both development and learning.

• He further stated that the higher one looked in

the vertebrate scale, the more the neural terminals and collaterals ramified.

Charles Sherrington (1897) stated the synapses was likely to be strategic for learning.

Donald O. Hebb

• conditions that might be required to produce changes at synapses

• i. He proposed that functional relationship between a presynatic neuron (A) and a postsynaptic neuron (B) could change if A frequently took part in exciting B.

• (1) when an axon of cell A is near enough to excite a cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells suchthat A’s efficacy, as one of the cells firing B, is increased.

• ii. Extension of the hypothesis:

• (1) any two cells that are repeated active at the same time will tend to become ‘associated’, so that activity in one facilitates activity in the other.

• Summary: “Neurons that fire together wire together”

Findings by Rosenzweig and associates

• a. Formal training• i. Rats trained and tested for spatial problem task• ii. Measured AchE activity in the cerebral cortex• iii. Found the groups that had been trained and

tested on more difficult problems > those given easier problems > groups given no training and testing.

• b. Informal learning, enriched experience,• i. increased acetylcholinesterase (AchE)• ii. increased weights of regions of neocortex.

• Further systematic testing• i. Enriched condition (EC), Standard condition

(SC), Isolated condition (IC)• ii. training and differential experience could

produce measurable changes in• the brain Fig 0-8, Fig 0-9• iii. Changes can be produced throughout life

span and rather rapidly• iv. Change were not uniformly distributed

throughout cortex, largest change in occipital cortex

• Other measures, Fig 0-10• (1) cortical thickness,• (2) increase in cortical volume and intracortical

connections• (3) sizes of neuronal cell bodies,• (4) count of dendritic spines Fig 0-11• (5) increase in extent and branching of dendrites• (6) number of synapses per neuron • (7) size of the synaptic contact areas, Fig 0-12, Fig 0-13