Experience-driven brain plasticity: beyond the synapse

Abstract

The brain is remarkably responsive to its interactions with the environment, and its morphology is altered by experience in measurable ways. Histological examination of the brains of animals exposed to either a complex ('enriched') environment or learning paradigm, compared with appropriate controls, has illuminated the nature of experience-induced morphological plasticity in the brain. For example, this research reveals that changes in synapse number and morphology are associated with learning and are stable, in that they persist well beyond the period of exposure to the learning experience. In addition, other components of the nervous system also respond to experience: oligodendrocytes and axonal myelination might also be permanently altered, whereas changes in astrocytes and cerebrovasculature are more transient and appear to be activity- rather than learning-driven. Thus, experience induces multiple forms of plasticity in the brain that are apparently regulated, at least in part, by independent mechanisms.

Fig. 1. Persistence of the EC-induced increase in the number of synapses per neuron in the adult rat visual cortex

The EC-induced increase in the number of synapses per neuron in the adult rat visual cortex persists for at least 30 days after animals are removed from EC. ICIC animals, which were individually caged (IC) for 60 days, were significantly different (*, P<0.05) from each of the three other groups: ICEC animals (housed in IC for 30 days followed by EC housing for 30 days); ECIC animals (housed in EC for 30 days followed by IC housing for 30 days); and ECEC animals (housed in EC for 60 days). Modified, with permission, from Briones et al. (2004).

Fig. 2. The glial response to motor-skill learning

The glial response to motor-skill learning (A) is transient and requires persistent motor skill training for maintenance, whereas the increase in synapses per neuron (B) is stable in the absence of continued training. AC (acrobat) rats were trained on a motor skill learning task, whereas MC (motor control) animals ran on a treadmill but were not given an opportunity for learning. Animals in the Early group either participated in training (AC) or exercised (MC) for 10 days, animals in the Continuous group participated for 38 days, and animals in the Delay group participated for 10 days and then training (or exercise) was discontinued for the following 28 days before histological examination. * indicates P<0.05 for the comparison between the MC and AC animals of a particular group (Early, Continuous and Delay). Modified, with permission, from Kleim et al. (1997) and from Kleim et al. (in preparation).

Fig. 3. Astrocytic processes in layer IV of the rat visual cortex

Astrocytic processes (dashed outline) in layer IV of the rat visual cortex revealed by electron microscopy (A), and tracing of these processes (B). Processes in direct apposition to synaptic elements are indicated (arrows, A; solid lines, B). Scale bar, 0.2 μm. Reprinted, with permission, from Jones and Greenough (1996).

Fig. 4. Exercise increases changes in the vascular volume fraction in the cortex of mature, but not young, monkeys

Exercise increases the vascular volume fraction in the cortex of mature (15–17 years) but not young (10–12 years) monkeys. In mature animals, Runners (exercised for 24 weeks) had greater capillary volume fraction compared with both Run/Stop animals (that had a 12 week period of inactivity following 24 weeks of exercise) and sedentary Control animals. Reprinted, with permission, from Rhyu et al. (2003).