accomplish – n. 实现,达到
fine-tuning – n. 微调,细调
negative feedback – 负反馈
hypothalamic–pituitary axis (HPA) -- 下丘脑-垂体轴
law – n. 定律,规则
sense – v. 感知,感觉
secrete – v. 分泌
release – v. 释放
factor – n. 因子
backup – a. n. 备用(的),辅助(的)
override – v. 不顾,超越,不理
exogenous – a. 外源的,外生的
target organ – 靶器官,目标器官
somatostatin – n. 生长抑素
feedback loops – 反馈环
★ISPN Review ★
Endocrine Regulation 内分泌调节
The production and release of hormones needs to be tightly regulated within the body. Hormones are released in small amounts to accomplish what needs to be done to maintain homeostasis within the body. The fine-tuning and regulation of hormone release through the hypothalamus are often regulated by a series of negative feedback systems. Other hormones are not controlled in this fashion but respond to other direct stimuli.
Because of its position in the brain the hypothalamus is stimulated by many things, such as light, emotion, cerebral cortex activity, and a variety of chemical and hormonal stimuli. Together, the hypothalamus and the pituitary function closely to maintain endocrine activity along what is called the hypothalamic–pituitary axis (HPA) using a series of negative feedback systems.
A negative feedback system works much like the law of supply and demand in business. In business, when there is an adequate supply of a product, production of that product will slow down because there is an adequate supply and no current demand for it. When the supply is used up, demand will increase, and so production will pick up. Production continues until the supply is adequate and demand is reduced. When the hypothalamus senses a need for a particular hormone—for example, thyroid hormone—it secretes the releasing factor TRH directly into the anterior pituitary. In response to the TRH the anterior pituitary secretes TSH, which in turn stimulates the thyroid gland to produce thyroid hormone. When the hypothalamus senses the rising levels of thyroid hormone, it stops secreting TRH, resulting in decreased TSH production and subsequent reduced thyroid hormone levels. The hypothalamus, sensing the falling thyroid hormone levels, secretes TRH again. The negative feedback system continues in this fashion, maintaining the levels of thyroid hormone within a relatively narrow range of normal.
It is thought that this feedback system is more complex than once believed. The hypothalamus probably also senses TRH and TSH levels and regulates TRH secretion within a narrow range, even if thyroid hormone is not produced. The anterior pituitary may also be sensitive to TSH levels and thyroid hormone, regulating its own production of TSH. This complex system provides backup controls and regulation if any part of the HPA fails. This system also can create complications, especially when there is a need to override or interact with the total system, as is the case with hormone replacement therapy or the treatment of endocrine disorders. Supplying an exogenous hormone, for example, may increase the hormone levels in the body, but then may affect the HPA to stop production of releasing and stimulating hormones, leading to a decrease in the body’s normal production of the hormone.
Two of the anterior pituitary hormones (i.e., GH and PRL) do not have a target organ to produce hormones and so cannot be regulated by the same type of feedback mechanism. The hypothalamus in this case responds directly to rising levels of GH and PRL. When levels rise the hypothalamus releases the inhibiting factors somatostatin and PIF directly to inhibit the pituitary’s release of GH and PRL, respectively. The HPA functions through negative feedback loops or the direct use of inhibiting factors to constantly keep these hormones regulated.
1. Which statement about the hypothalamus is correct?
A. The hypothalamus is connected to the brain by the infundibulum
B. The hypothalamus is composed of glandular epithelial tissue
C. The hypothalamus secretes “releasing hormones”
D. The hypothalamus secretes epinephrine and norepinephrine
2. Which structure controls the endocrine system and integrates the activities of the nervous and endocrine systems?
A. The infundibulum
B. The pituitary gland
C. The thalamus
D. The hypothalamus
.
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答案 Answers 1. C: The hypothalamus secrets “releasing hormones” Rationale: The hypothalamus is part of the brain so is composed of neural tissue. Epinephrine and norepinephrine are released from the adrenal medulla. 2. D. The hypothalamus Rationale: Hypothalamus secretes regulatory hormones that control endocrine cells in the antepituitary gland; produces ADH and oxytocin; contains “autonomic centres” that exert neural control over endocrine cells of the adrenal medullae.