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There are a number of biological rhythms controlled by a pacemaker near the hypothalamus. Releasing hormones are discharged episodically in pulses from the hypothalamus rather than as a continuous flow. External stimuli including darkness and light (circadian rhythm) and the sleep-wake cycles influence the releasing hormone pulse frequency. Abnormality of these biorhythms may be responsible for some types of abnormal reproductive function.
There are usually negative feed back systems that reduce stimulating hormone release. As oestrogen levels rise, the gonadotrophin, the hormones that influence the production of the sex steroids ovaries, output falls (Figure 2.6). the whole setup is known as the hypothalamic-pituitary ovarian axis.
Figure 2.6 Hypothalamic-pituitary ovarian axis
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