The problem of relating plasticity and skilled reaching after motor cortex stroke in the rat

Abstract

The plasticity of the nervous system is illustrated in the many new neuronal connections that are formed during the acquisition of behavioral skills, loss of function after brain injury, and subsequent recovery of function. The present review describes the acquisition of skilled reaching, the act of reaching for food with a forelimb, and the changes that take place in skilled reaching following motor cortex stroke. The review then discusses the difficulty in associating plastic changes with specific aspects of behavioral change. Skilled reaching behavior is complex and consists of a number of oppositions (stimulus response relationships), between the rat and the food target, a number of forelimb gestures (non-weight supporting movements), which are performed to obtain food, and a complex series of segmental movements (of the limb, head, and trunk), all of which influence the success of the act. Measures of these four aspects of skilled reaching behavior following motor cortex stroke reveal that there are a number of learned changes that take place at different times, including learned nonuse, learned bad-use, and forgetting. The widespread dendritic proliferation, axonal growth, and synaptic formation that take place both before and after stroke are difficult to precisely relate to these behavioral changes. Whereas plasticity is usually proposed to be associated with improved performance it is suggested that future work should attempt to better relate plastic changes to the details of behavioral changes.

Introduction

There is evidence to show that the nervous system is plastic in forming new connections between neurons as a result of experience. Plasticity can be associated with the birth of new neurons, increases in the complexity of the dendritic arbor of existing neurons, and increases in the number, size, and shape of synapses. Following brain injury, plastic changes can also be widespread and can additionally include the growth of anomalous pathways. In much of the literature on neural plasticity, it is assumed that most changes are beneficial and mediate the acquisition of new behavior as well as any functional recovery that occurs after brain damage. Nevertheless, Paillard [49] has questioned this assumption by positing that behavior can be modified in a number of ways, not all of which involve plasticity. Thus, he argues that understanding functional acquisition and the loss and recovery of function after brain injury requires distinguishing between behavioral changes that are mediated by activation of pre-existing neuronal connections and behavioral changes that are due to plastic changes. Such a distinction is theoretically reasonable, but there is challenge in determining the relationship between neuronal alteration and behavioral change. As we will describe in the first section of this paper, behavioral changes are every bit as complex as neuronal changes. We will illustrate this point by describing some aspects of a behavior that fascinated Paillard [50], prehension. We will then consider the evidence that plastic changes are associated with prehension both before and during a variety of events related to recovery of function after brain injury. The future challenge is to document more precisely the relationship between behavior and neuronal plasticity.