Evidence for Rapid Functional Reorganization in Inferior Colliculus and Cochlear Nucleus

Abstract

During development, the neural circuits in the central auditory pathway are laid down and fine tuned; however, once adulthood is reached, it has generally been assumed that the neural circuits remain stable throughout the life span. Recent studies in the somatosensory and visual system have challenged this hard wired view of the nervous system since extensive remodeling and reorganization occur in both somatosensory and visual pathways when their peripheral inputs are either temporarily or permanently eliminated (Eysel et al., 1980, 1981; Merzenich and Kaas, 1982; Kaas et al., 1990). Because of the relative ease of deafferenting a specific segment of the peripheral receptor surface, the somatosensory system has been one of the most popular systems for studying the reorganization of the central nervous system. A striking example of the rapid reorganization that can occur in the somatosensory cortex is illustrated by studies in which the peripheral inputs from a specific digit or peripheral nerve is temporarily or permanently eliminated by chemical or surgical means (Dostrovsky et al., 1976; Devor and Wall, 1981; Calford and Tweedale, 1988). For example, some neurons in the somatosensory cortex have excitatory receptive fields that are only activated by mechanical stimulation delivered to a specific digit. If the peripheral inputs that activate the neuron are eliminated by amputating or anesthetizing the digit, the neuron does not become “silent” or inactive, but instead, rapidly shifts it excitatory receptive field to an adjacent region of the body surface to which it was originally unresponsive (Calford and Tweedale, 1988). This rapid reorganization is presumably due to the unmasking of the system’s intrinsic neural networks which were originally “covered over” by powerful inhibitory circuits. Functional reorganization is not unique to the somatosensory cortex. Indeed, it has been observed at the level of the brainstem and spinal cord following peripheral damage (Dostrovsky et al., 1976; Devor and Wall, 1981).