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Incoming sensory input is routed to the cerebral cortex through a structure called the thalamus. Many distinct groups of neurons comprise the thalamus, and one of these is the medial geniculate nucleus (MGN), also called the medial geniculate body. The MGN is involved in auditory processing, and in directing one's attention toward specific auditory stimuli. Conditioned fear responses involving auditory stimuli also use this structure in the brain.
Several subnuclei exist inside of the medial geniculate nucleus, each with their own specialized neurons and functions. These regions have multiple sources of input, but all of them receive information from the inferior colliculus, the initial area that auditory information is routed to from the brain. Additionally, they all send projections to the auditory cortex, and other areas of the cerebrum. The first subarea, the ventral subnucleus, contains cells that activate in response to certain sound frequencies. Information on the intensity of sounds, and how frequency and intensity differs between the ears, is also relayed to the cerebrum through these specific cells.
Another subnucleus of the medial geniculate nucleus is the dorsal subnucleus. Some cells in this area seem to respond to both sounds and other sensory input, such as touch. They may also play a role in processing complex sounds, since they respond to a variety of frequencies. This area seems to be capable of modification by learning.
The third main subarea of the medial geniculate nucleus is the medial subnucleus. Cells in this area process sound duration and loudness. They also seem to respond to small ranges of frequencies, instead of just one frequency. Other sensory stimuli, such as visual information, seems to influence how these cells respond to sound.
Many cells in the medial geniculate nucleus send projections to the amygdala, a brain region involved in both learning and expressing fear responses. Activation of cells in the MGN that produce the chemical neurotransmitter glutamate allow learned auditory fear responses to be expressed. These same cells also seem to be involved in the extinction of the fear response.
Cells in the MGN that respond to auditory stress do so in a specific pattern. These cells are involved in the creation of certain ribonucleic acid (RNA) called c-fos. This process seems to result in the production of stress hormones, called corticosteroids, by the hypothalamus. Learning fear responses seems to depend, at least in part, on this initial RNA induction and stress response.
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