Central nervous system activation by noise.

Connections between thalamic structures of the auditory system and subcortical areas (amygdala, hippocampus, hypothalamus) had been hypothesized to act as a fast reacting "memory chain" establishing and enhancing adverse excitations during noise exposure. Recent studies prove that the lateral amygdala is an important part of a second separate pathway to the telencephalic projections of the auditory system. This fast, monosynaptic thalamo-amygdala tract is responsible for full-blown "fear responses" evoked by auditory stimuli as shown by several conditioning experiments in animals: A fear memory system. The appertaining basic processes of plasticity in the amygdala are reductions of latencies of neuronal excitations and recruiting of more elements with shorter latency, long-term potentiation causing enhancement of auditory-evoked responses by repeated stimulation, as well as sharpening of primary broad tuning curves of elements. Very recently a study using Functional-Magnetic-Resonance-Imaging (fMRI) demonstrated that an amygdalar contribution to conditioned fear learning can be revealed in normal human subjects too. These findings were supported by Positron-Emission-Tomography (PET) studies in depressive persons showing that amygdala metabolic abnormality predicted the cortisol concentration in blood. Using connections via central amygdala, lateral and medial hypothalamus to parts named nuclei paraventriculares and regio arcuata, the sound evoked excitations reach two essential components of endocrine functioning: a) the well-known hypothalamic-pituitary-adrenal (HPA) system with a subsequent rise (via Corticotropin-Releasing Hormone: CRH) in Corticotropin (Adreno-CorticoTropin Hormone: ACTH) and the corticosterone levels; b) the synthesis of ACTH and beta-endorphine-like substances in the arcuate region being axonally transported to extrahypothalamic brain regions. Longer-lasting activation of the HPA-axis, especially abnormally increased or periodically elevated levels of cortisol and the widespread extrahypothalamically distributed CRF/ACTH may lead to disturbed hormonal balance and even to severe diseases.