Memory and Learning - 13.3

The Road Ahead

In this part of the chapter, we will look at some of the ways in which new learning involves changes in synapses. Reading this material should enable you to:

13.3.1List the possible ways in which changes in neural function and structure could encode memories.

13.3.2Review evidence that exposure to an enriched environment can affect brain structure and affect future behavior.

13.3.3Describe how a circuit involving the cerebellum mediates certain types of conditioning.

In introducing the term synapse, Charles Sherrington (1897) speculated that synaptic alterations might be the basis of learning. Sherrington’s notion anticipated what remains one of the most intensive efforts in all of neuroscience, since most theories of learning focus on neuroplasticity (or neural plasticity), changes in the structure and function of synapses.

Plastic changes at synapses can be physiological or structural

Synaptic changes that may store information can be measured physiologically. The changes can be presynaptic, postsynaptic, or both. They can include changes in the amount of neurotransmitter released and/or changes in the number or sensitivity of the postsynaptic receptors, resulting in larger (or smaller) postsynaptic potentials. Inhibiting inactivation of the transmitter (by altering reuptake or enzymatic degradation) can produce a similar effect (FIGURE 13.15A). Synaptic activity can also be influenced by inputs from other neurons, causing extra depolarization or hyperpolarization of the axon terminals and therefore changes in the amount of neurotransmitter released (FIGURE 13.15B).

FIGURE 13.15 Synaptic Changes That May Store Memories View larger image

Long-term memories may require changes in the nervous system so substantial that they can be directly observed (with the aid of a microscope, of course). After all, structural changes resulting from use are apparent in other parts of the body, as when exercise tones and shapes muscle. In a similar way, new synapses can form (or old synapses may die back) as a result of use (FIGURE 13.15C).

Training can also lead to the reorganization of synaptic connections. For example, it can cause a more active pathway to take over sites formerly occupied by a less active competitor (FIGURE 13.15D).

Varied experiences and learning cause the brain to change and grow

The remarkable plasticity of the brain is easy to demonstrate. Classic studies found that simply living in a complex environment, with its many opportunities for new learning, produces pronounced biochemical and anatomical changes in the brains of rats (Renner and Rosenzweig, 1987).

In studies of environmental enrichment, rats are randomly assigned to one of three housing conditions:

Impoverished condition (IC) Animals are housed individually in standard lab cages (FIGURE 13.16A).

Standard condition (SC) Animals are housed in small groups in standard lab cages (FIGURE 13.16B).

Enriched condition (EC) Animals are housed in large social groups in special cages containing various toys and other interesting features (FIGURE 13.16C). This condition provides enhanced opportunities for learning perceptual and motor skills, social learning, and so on.

FIGURE 13.16 Experimental Environments to Test the Effects of Enrichment on Learning and Brain Measures View larger image

In dozens of studies over several decades, a variety of changes in the brain were linked to environmental enrichment. For example, compared with IC animals:

EC animals have a heavier, thicker cortex, especially in somatosensory and visual cortical areas (M. C. Diamond, 1967).

EC animals show enhanced cholinergic activity throughout the cortex (Rosenzweig et al., 1961).

EC animals have more dendritic branches on cortical neurons, and many more dendritic spines on those branches (FIGURE 13.17) (Greenough, 1976).