Numerous studies have tried to determine whether alterations in the hormonal content of the plasma would modify pineal function. Recent evidence has indicated that the pineal is relatively insensicive to alterations in plasma hormone concentration, as contrasted with the neuroendocrine hypothalamus which is very responsive to hormonal feedback. Therefore, two types of neuroendocrine glands appear to exist based on their ablity to respond to hormonal feedback. The first type of neuroendocrine gland, exemplified by the neuroendocrine portion of the hypothalamus, is extremely sensitive to hormonal modulation and is concerned with maintaining the endogenous balance of plasma hormones. The second type of neuroendocrine gland, exemplified by the pineal gland, is principally responsible for converting its neural input, usually a environmental stimulus, into a hormonal messenger, irrespective of plasma hormone levels. These two types of neuroendocrine glands would act in synchrony to maintain homeostasis throughout a variety of external conditions.
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